JP2004020504A

JP2004020504A - Evaluating method and system for electrooptical crystal or magnetooptical crystal, terahertz light measuring method and system therefor

Info

Publication number: JP2004020504A
Application number: JP2002179324A
Authority: JP
Inventors: Mamoru Usami; 宇佐見　護
Original assignee: Tochigi Nikon Corp; Nikon Corp
Current assignee: Tochigi Nikon Corp; Nikon Corp
Priority date: 2002-06-20
Filing date: 2002-06-20
Publication date: 2004-01-22

Abstract

<P>PROBLEM TO BE SOLVED: To make an electrooptical crystal or the like able to be properly evaluated. <P>SOLUTION: A bias voltage is appied from a voltage applying part 13 on a light conduction antenna 9, and pump pulse light is irradiated, thereby terahertz light generates from the light conduction antenna 9. The terahertz pulse light is irradiated onto a two-dimensional region of an electrooptical crystal 17 of an evaluated object. Probe pulse light polarized with a polarizer 22 is irradiated onto the two-dimensional region of the crystal 17. A two-dimensional CCD camera 24 images a two-dimensional light intensity distribution image of the probe pulse light passing the two-dimensional region of the crystal 17. Image signals are obtained from the camera 24 at each bias voltage while changing the bias voltage. Detection efficiency and a residual birefringence rate of the crystal 17 and each distribution of influence of scattering of the probe pulse light are evaluated from the relation of the bias voltage and the image signal every pixel. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、電気光学結晶及び磁気光学結晶の評価方法及び装置、並びに、これを利用したテラヘルツ光計測方法及び装置に関するものである。
【０００２】
【従来の技術】
近年、テラヘルツ技術が注目を集めている。特に、テラヘルツパルス光を用いた計測では、従来の技術では検出が困難であった「位相」の情報を得ることができるため、その応用に高い関心が集まっている。テラヘルツパルス光を検出する代表的な方法には、光伝導アンテナを用いる手法と、電気光学（ＥＯ）サンプリング法とがある。ＥＯサンプリング法の利点としては、（１）２次元データを一括して取得することが可能であること、（２）高周波数（数十ＴＨｚ以上）領域の検出が可能であること、などを挙げることができる。
【０００３】
テラヘルツ光の応用として、テラヘルツイメージング技術の開発が世界中で進められている。イメージングの手法としては、テラヘルツ光をサンプル上の一点に集光し、その集光点を移動させて（もしくは、サンプリングを相対的に移動させて）像を得る「走査型イメージング法」と、２次元テラヘルツ画像を一時に取得する「リアルタイムイメージング法」の２つに、大きく分類される。後者を実現するためには、検出器として、（１）光伝導アンテナをアレイ化する、（２）ＥＯサンプリング法を用いる、の２つの手法が考えられるが、現在は（２）が主流となっており、（１）の成功例は未だに報告されていない。
【０００４】
ＥＯサンプリング法を用いたテラヘルツ光検出には、（１）バランス検出、（２）クロスポーラリゼーション、のどちらかの手法が用いられている。
【０００５】
なお、ＥＯサンプリング法の変形として、電気光学結晶に代えて磁気光学結晶を用いる磁気光学サンプリング法がある。
【０００６】
【発明が解決しようとする課題】
前記バランス検出手法では、電気光学結晶にテラヘルツパルス光を導くとともに前記テラヘルツパルス光と同期しかつ直線偏光にされたプローブパルス光を１／４波長板を介して前記電気光学結晶に導き、前記電気光学結晶を通過して前記テラヘルツパルス光により偏光状態が変化した前記プローブパルス光を偏光ビームスプリッタで２つの光束に分離し、分離された２つの光束を２つの光電変換手段でそれぞれ検出し、両者の出力信号の差分を取り、この差分信号をテラヘルツ光の検出信号（テラヘルツ光の電場の強度を示す信号）とする。したがって、バランス検出手法では、電気光学結晶の残留複屈折率（理想的な電気光学結晶では電場が印加されていない状態での屈折率の異方性はゼロであるが、現実には結晶内に残っている応力などにより屈折率の異方性がある。）や、電気光学結晶におけるプローブパルス光の散乱の影響などを効果的に取り除いた、テラヘルツパルス光の検出を行うことができるが、２つの光電変換手段が必要になる。
【０００７】
一方、前記クロスポーラリゼーション手法では、電気光学結晶にテラヘルツパルス光を導くとともに前記テラヘルツパルス光と同期しかつ直線偏光されたプローブパルス光を前記電気光学結晶に導き、前記電気光学結晶を通過して前記テラヘルツパルス光により偏光状態が変化した前記プローブパルス光を偏光板からなる検光子で検光し、この検光後のプローブパルス光を１つの光電変換手段で検出し、その出力信号をテラヘルツ光の検出信号（テラヘルツ光の電場の強度を示す信号）とする。したがって、クロスポーラリゼーション手法では、光電変換手段の数は１つですむが、光検出器の出力信号には、電気光学結晶の残留複屈折率や電気光学結晶におけるプローブパルス光の散乱の影響などが反映されてしまい、正確にテラヘルツパルス光を計測することができない。
【０００８】
走査型イメージング法の場合、光電変換手段としてポイントセンサを用いることができるので、光電変換手段の数が２つになってもさほどコストが増大しない。したがって、走査型イメージング法の場合、クロスポーラリゼーション手法に比べて正確にテラヘルツパルス光を計測することができるバランス検出手法が用いられることが多い。
【０００９】
一方、リアルタイムイメージング法では、光電変換手段としてＣＣＤカメラなどの２次元センサを用いるので、光電変換手段の数が２つになると光電変換手段の数が１つの場合に比べてコストが著しく増大してしまう。したがって、リアルタイムイメージング法では、クロスポーラリゼーション手法を用いるが主流である。しかしながら、この場合には、電気光学結晶の残留複屈折率や電気光学結晶におけるプローブパルス光の散乱の影響などで、各部位のテラヘルツパルス光を正確に計測することはできず、しかも、実際には電気光学結晶の残留複屈折率、散乱の影響及び検出効率が各部位でばらつくので、バランス検出手法を用いる場合に比べて、得られる画像の画質は低下していた。
【００１０】
前述したように、テラヘルツ光の検出において、電気光学結晶の残留複屈折率、散乱の影響及び検出効率が与える影響が大きいにも拘わらず、未だにそれらを評価する手法が確立していない。電気光学結晶の評価手法が確立すれば、例えば、空間的に均一性の高い特性を持った電気光学結晶の開発や研究を促進させることができる。
【００１１】
以上、電気光学結晶に関して説明したが、磁気光学結晶についても同様である。また、電気光学結晶や磁気光学結晶は、テラヘルツ光検出以外の種々の用途においても重要である。
【００１２】
本発明は、前述したような事情に鑑みてなされたもので、電気光学結晶や磁気光学結晶を適切に評価することができる電気光学結晶又は磁気光学結晶の評価方法及び装置を提供することを目的とする。
【００１３】
また、本発明は、電気光学結晶又は磁気光学結晶を用いてテラヘルツパルス光を計測することができ、しかも、バランス検出を行うことなく、テラヘルツパルス光を正確に計測することができるテラヘルツ光計測方法及び装置を提供することを目的とする。
【００１４】
さらに、本発明は、テラヘルツ光検出に用いる電気光学結晶又は磁気光学結晶の空間的均一性が低くても、高画質の対象物体の像を得ることができるテラヘルツ光計測装置を提供することを目的とする。
【００１５】
【課題を解決するための手段】
前記課題を解決するため、本発明の第１の態様による電気光学結晶又は磁気光学結晶の評価方法は、発生するテラヘルツパルス光の電場又は磁場の強度を変え得るテラヘルツ光発生部を用いて、前記テラヘルツ光発生部から発したテラヘルツパルス光を評価対象結晶である電気光学結晶又は磁気光学結晶に導くとともに、前記テラヘルツパルス光と同期しかつ偏光されたプローブパルス光を前記評価対象結晶に導き、前記評価対象結晶を通過して前記テラヘルツパルス光により偏光状態が変化した前記プローブパルス光を検光し、前記検光後の前記プローブパルス光を光電変換し、前記テラヘルツパルス光の電場又は磁場の強度を変えることで、前記テラヘルツパルス光の電場又は磁場の強度と前記光電変換により得られる信号の強度との関係を得、前記関係に基づいて、前記評価対象結晶の検出効率、前記評価対象結晶の残留複屈折率、及び、前記評価対象結晶における前記プローブパルス光の散乱の影響のうちの、少なくとも１つを評価するものである。
【００１６】
テラヘルツパルス光の電場又は磁場の前記強度は、符号を含むものでもよいし、符号を含まないものでもよい。この点は、後述する各態様についても同様である。
【００１７】
本発明の第２の態様による電気光学結晶又は磁気光学結晶の評価方法は、前記第１の態様において、前記テラヘルツ光発生部は、（ａ）光伝導部と、該光伝導部の所定の面上に形成され互いに分離された２つの導電部とを有し、前記２つの導電部の少なくとも一部同士が前記所定の面に沿った方向に所定間隔をあけるように配置されたテラヘルツ光発生素子と、（ｂ）前記テラヘルツ光発生素子の所定箇所に励起パルス光を照射する励起パルス光照射部と、（ｃ）前記２つの導電部間に可変のバイアス電圧を印加する電圧印加部と、を含み、前記バイアス電圧を変えることで前記テラヘルツパルス光の電場又は磁場の強度を変え、前記関係として、前記バイアス電圧と前記光電変換により得られる信号の強度との関係を得るものである。
【００１８】
本発明の第３の態様による電気光学結晶又は磁気光学結晶の評価方法は、発生するテラヘルツパルス光の電場又は磁場の強度を変え得るテラヘルツ光発生部を用いて、前記テラヘルツ光発生部から発したテラヘルツパルス光を評価対象結晶である電気光学結晶又は磁気光学結晶の２次元領域に導くとともに、前記テラヘルツパルス光と同期しかつ偏光されたプローブパルス光を前記評価対象結晶の前記２次元領域に導き、前記評価対象結晶を通過して前記テラヘルツパルス光により偏光状態が変化した前記プローブパルス光を検光し、前記検光後の前記プローブパルス光を、前記評価対象結晶の前記２次元領域の各部位に対応するもの毎にそれぞれ光電変換し、前記テラヘルツパルス光の電場又は磁場の強度を変えることで、前記テラヘルツパルス光の電場又は磁場の強度と前記光電変換により得られる信号の強度との関係を、前記評価対象結晶の前記２次元領域の各部位に対応するもの毎にそれぞれ得、前記関係に基づいて、前記評価対象結晶の前記各部位の検出効率、前記評価対象結晶の前記各部位の残留複屈折率、及び、前記評価対象結晶の前記各部位における前記プローブパルス光の散乱の影響のうちの、少なくとも１つを評価するものである。
【００１９】
本発明の第４の態様による電気光学結晶又は磁気光学結晶の評価方法は、前記第３の態様において、前記テラヘルツ光発生部は、（ａ）光伝導部と、該光伝導部の所定の面上に形成され互いに分離された２つの導電部とを有し、前記２つの導電部の少なくとも一部同士が前記所定の面に沿った方向に所定間隔をあけるように配置されたテラヘルツ光発生素子と、（ｂ）前記テラヘルツ光発生素子の所定箇所に励起パルス光を照射する励起パルス光照射部と、（ｃ）前記２つの導電部間に可変のバイアス電圧を印加する電圧印加部と、を含み、前記バイアス電圧を変えることで前記テラヘルツパルス光の電場又は磁場の強度を変え、前記関係として、前記バイアス電圧と前記光電変換により得られる信号の強度との関係を、前記評価対象結晶の前記２次元領域の各部位に対応するもの毎にそれぞれ得るものである。
【００２０】
本発明の第５の態様による電気光学結晶又は磁気光学結晶の評価装置は、発生するテラヘルツパルス光の電場又は磁場の強度を変え得るテラヘルツ光発生部を有し、前記テラヘルツ光発生部から発したテラヘルツパルス光を評価対象結晶である電気光学結晶又は磁気光学結晶に照射するテラヘルツパルス光照射部と、前記テラヘルツパルス光と同期しかつ偏光されたプローブパルス光を、前記評価対象結晶に照射するプローブパルス光照射部と、前記評価対象結晶を通過して前記テラヘルツパルス光により偏光状態が変化した前記プローブパルス光を検光する検光部と、前記検光部による検光後の前記プローブパルス光を光電変換する光電変換部と、前記テラヘルツパルス光の電場又は磁場の強度を変えることで得られる、前記テラヘルツパルス光の電場又は磁場の強度と前記光電変換部の出力信号の強度との関係を、記憶する記憶部と、前記関係に基づいて、前記評価対象結晶の検出効率を示す値、前記評価対象結晶の残留複屈折率を示す値、及び、前記評価対象結晶における前記プローブパルス光の散乱の影響を示す値のうちの、少なくとも１つを得る演算部と、を備えたものである。
【００２１】
本発明の第６の態様による電気光学結晶又は磁気光学結晶の評価装置は、前記第５の態様において、前記テラヘルツ光発生部は、（ａ）光伝導部と、該光伝導部の所定の面上に形成され互いに分離された２つの導電部とを有し、前記２つの導電部の少なくとも一部同士が前記所定の面に沿った方向に所定間隔をあけるように配置されたテラヘルツ光発生素子と、（ｂ）前記テラヘルツ光発生素子の所定箇所に励起パルス光を照射する励起パルス光照射部と、（ｃ）前記２つの導電部間に可変のバイアス電圧を印加する電圧印加部と、を含み、前記記憶部は、前記関係として、前記バイアス電圧を変えることで得られる、前記バイアス電圧と前記光電変換部の出力信号の強度との関係を、記憶するものである。
【００２２】
本発明の第７の態様による電気光学結晶又は磁気光学結晶の評価装置は、発生するテラヘルツパルス光の電場又は磁場の強度を変え得るテラヘルツ光発生部を有し、前記テラヘルツ光発生部から発したテラヘルツパルス光を評価対象結晶である電気光学結晶又は磁気光学結晶の２次元領域に照射するテラヘルツパルス光照射部と、前記テラヘルツパルス光と同期しかつ偏光されたプローブパルス光を、前記評価対象結晶の前記２次元領域に照射するプローブパルス光照射部と、前記評価対象結晶を通過して前記テラヘルツパルス光により偏光状態が変化した前記プローブパルス光を検光する検光部と、前記検光部による検光後の前記プローブパルス光を、前記評価対象結晶の前記２次元領域の各部位に対応するもの毎にそれぞれ光電変換する光電変換部と、前記テラヘルツパルス光の電場又は磁場の強度を変えることで得られる、前記テラヘルツパルス光の電場又は磁場の強度と前記光電変換部の出力信号の強度との関係を、前記評価対象結晶の前記２次元領域の各部位に対応するもの毎にそれぞれ記憶する記憶部と、前記関係に基づいて、前記評価対象結晶の前記各部位の検出効率を示す値、前記評価対象結晶の前記各部位の残留複屈折率を示す値、及び、前記評価対象結晶の前記各部位における前記プローブパルス光の散乱の影響を示す値のうちの、少なくとも１つを得る演算部と、を備えたものである。
【００２３】
本発明の第８の態様による電気光学結晶又は磁気光学結晶の評価装置は、前記第７の態様において、前記テラヘルツ光発生部は、（ａ）光伝導部と、該光伝導部の所定の面上に形成され互いに分離された２つの導電部とを有し、前記２つの導電部の少なくとも一部同士が前記所定の面に沿った方向に所定間隔をあけるように配置されたテラヘルツ光発生素子と、（ｂ）前記テラヘルツ光発生素子の所定箇所に励起パルス光を照射する励起パルス光照射部と、（ｃ）前記２つの導電部間に可変のバイアス電圧を印加する電圧印加部と、を含み、前記記憶部は、前記関係として、前記バイアス電圧を変えることで得られる、前記バイアス電圧と前記光電変換部の出力信号の強度との関係を、前記評価対象結晶の前記２次元領域の各部位に対応するもの毎にそれぞれ記憶するものである。
【００２４】
本発明の第９の態様によるテラヘルツ光計測方法は、テラヘルツパルス光を電気光学結晶又は磁気光学結晶である検出用結晶に導びくとともに、前記テラヘルツパルス光と同期しかつ偏光されたプローブパルス光を前記検出用結晶に導き、前記検出用結晶を通過して前記テラヘルツパルス光により偏光状態が変化した前記プローブパルス光を検光し、前記検光後の前記プローブパルス光を光電変換し、前記テラヘルツパルス光の電場又は磁場の強度と前記光電変換により得られる信号の強度との予め得られた関係であって、前記検出用結晶の検出効率、前記検出用結晶の残留複屈折率、及び、前記検出用結晶における前記プローブパルス光の散乱の影響を反映した関係を用いて、前記光電変換により得られた信号の強度に応じた前記テラヘルツパルス光の電場又は磁場の強度を得るものである。
【００２５】
本発明の第１０の態様によるテラヘルツ光計測方法は、前記第９の態様において、前記検出用結晶に導かれる前記テラヘルツパルス光は、対象物体を透過又は反射したものである。
【００２６】
本発明の第１１の態様によるテラヘルツ光計測方法は、前記第９又は第１０の態様において、前記検出用結晶は残留複屈折率を持つものである。
【００２７】
本発明の第１２の態様によるテラヘルツ光計測装置は、テラヘルツパルス光を対象物体に照射するテラヘルツパルス光照射部と、前記対象物体を透過又は反射したテラヘルツパルス光を受光する電気光学結晶又は磁気光学結晶である検出用結晶と、前記テラヘルツパルス光と同期しかつ偏光されたプローブパルス光を、前記検出用結晶に照射するプローブパルス光照射部と、前記検出用結晶を通過して前記テラヘルツパルス光により偏光状態が変化した前記プローブパルス光を検光する検光部と、前記検光部による検光後の前記プローブパルス光を、光電変換する光電変換部と、前記テラヘルツパルス光の電場又は磁場の強度と前記光電変換により得られる信号の強度との予め得られた関係であって、前記検出用結晶の検出効率、前記検出用結晶の残留複屈折率、及び、前記検出用結晶における前記プローブパルス光の散乱の影響を反映した関係を、記憶する記憶部と、前記関係に基づいて、前記光電変換により得られた信号の強度に応じた前記テラヘルツパルス光の電場又は磁場の強度を得る処理部と、を備えたものである。
【００２８】
本発明の第１３の態様によるテラヘルツ光計測装置は、前記第１２の態様において、前記検出用結晶は残留複屈折率を持つものである。
【００２９】
本発明の第１４の態様によるテラヘルツ光計測装置は、テラヘルツパルス光を対象物体の２次元領域に一括照射するテラヘルツパルス光照射部と、前記対象物体の前記２次元領域を透過又は反射したテラヘルツパルス光を、前記対象物体の前記２次元領域に対応する２次元領域で一括受光する電気光学結晶又は磁気光学結晶である検出用結晶と、前記テラヘルツパルス光と同期しかつ偏光されたプローブパルス光を、前記検出用結晶の前記２次元領域に一括照射するプローブパルス光照射部と、前記検出用結晶を通過して前記テラヘルツパルス光により偏光状態が変化した前記プローブパルス光を検光する検光部と、前記検光部による検光後の前記プローブパルス光を、前記検出用結晶の前記２次元領域の各部位に対応するもの毎に、それぞれ光電変換する光電変換部と、前記テラヘルツパルス光の電場又は磁場の強度と前記光電変換により得られる信号の強度との予め得られた関係であって、前記検出用結晶の検出効率、前記検出用結晶の残留複屈折率、及び、前記検出用結晶における前記プローブパルス光の散乱の影響を反映した関係を、前記検出用結晶の前記２次元領域の各部位に対応するもの毎に記憶する記憶部と、前記関係に基づいて、前記光電変換により得られた信号の強度に応じた前記テラヘルツパルス光の電場又は磁場の強度を、前記検出用結晶の前記２次元領域の各部位に対応するもの毎に得る処理部と、を備えたものである。
【００３０】
本発明の第１５の態様によるテラヘルツ光計測装置は、前記第１４の態様において、前記検出用結晶は前記各部位においてそれぞれ残留複屈折率を持つものである。
【００３１】
なお、前記電気光学結晶や前記磁気光学結晶は、通常は板状に構成される。また、前記電気光学結晶としては、ＺｎＴｅ、ＣｄＴｅ、ＺｎＳｅ、ＧａＡｓ、ＣｄＺｎＴｅ、ＢＧＯ、ＢＴＯ、ＧａＰ、ＢａＴａＯ_３などを挙げることができる。前記磁気光学結晶としては、ＴＧＧ、ＫＴｂ_３Ｆ_１０、ＧａＰなどを挙げることができる。
【００３２】
【発明の実施の形態】
以下、本発明による電気光学結晶又は磁気光学結晶の評価方法及び装置、並びに、テラヘルツ光計測方法及び装置について、図面を参照して説明する。
【００３３】
［第１の実施の形態］
【００３４】
図１は、本発明の第１の実施の形態によるテラヘルツ光計測装置を模式的に示す概略構成図である。図２は、図１に示すテラヘルツ光計測装置において用いられているラヘルツ光発生素子としての光伝導アンテナテ９を示す概略斜視図である。
【００３５】
本実施の形態によるテラヘルツ光計測装置は、対象物体としての試料１００をイメージ化するイメージ化装置として構成されている。
【００３６】
本実施の形態によるテラヘルツ光計測装置では、フェムト秒パルス光源１から放射された超短パルスレーザ光としてのフェムト秒パルス光Ｌ１が、平面反射鏡２を経た後に、ビームスプリッタ３で２つのパルス光Ｌ２，Ｌ３に分割される。本実施の形態では、フェムト秒パルス光源１は、レーザ光源等からなり、例えば、フェムト秒パルス光Ｌ１として、中心波長が近赤外領域のうちの７８０〜８００ｎｍ程度、パルス幅が１０〜１００ｆｓ程度のパルス光を、所定の繰り返し周期で発する。
【００３７】
ビームスプリッタ３で分割された一方のパルス光Ｌ２は、テラヘルツ光発生素子としての光伝導アンテナ９を励起してテラヘルツパルス光を発生させるためのポンプパルス光（励起パルス光）となる。このポンプパルス光Ｌ２は、ビームエキスパンダ４でその断面が拡張された後、平面反射鏡５，６、２枚もしくは３枚の平面反射鏡が組み合わされてなる可動鏡７、及び平面反射鏡８を経て、光伝導アンテナ９へ導かれる。
【００３８】
ここで、光伝導アンテナ９の一例を図２に示す。この光伝導アンテナ９は、光伝導部としての基板１０と、該基板１０の一方の表面に形成された互いに分離された２つの導電部としての導電膜１１，１２とを備えている。導電膜１１，１２の少なくとも一部同士が、基板１０の図２中の手前側の平面に沿った方向に所定間隔ｇをあけるように配置されている。本実施の形態では、導電膜１１，１２の全体同士が間隔ｇをあけている。この間隔ｇが２ｍｍ以上、例えば５ｍｍに設定されており、基板１０及び導電膜１１，１２によっていわゆる大口径の光スイッチ素子が構成されている。もっとも、光伝導アンテナ９は、例えば、ダイポールアンテナ等を用いた光伝導アンテナとして構成してもよい。
【００３９】
基板１０の材質としては、例えば、抵抗率が高い半導体（例えば、半絶縁性ＧａＡｓ）を用いることができる。導電膜１１，１２の材質としては、例えば、金などの金属を用いることができ、例えば蒸着等により基板１４の表面に形成することができる。
【００４０】
本実施の形態では、前述したように基板１０自体が光伝導部として用いられているが、例えば、基板１０上に光伝導部として光伝導膜を形成し、この光伝導膜上に導電膜１１，１２を形成してもよい。この場合、例えば、基板１０の材質としてＧａＡｓを用いるとともに、光伝導膜として低温成長ＧａＡｓを用いることができる。
【００４１】
前記ポンプパルス光（超短パルスレーザ光）Ｌ２は、図２に示すように、光伝導アンテナ９の間隔ｇに相当するハッチングを付した領域Ｒに照射される。
【００４２】
図１及び図２に示すように、２つの導電膜１１，１２間には、電圧印加部１３からバイアス電圧が印加される。電圧印加部１３としては、例えば、直流電源を用いることができる。このような直流電源としては、例えば、商用電源からの交流を直流に変換する電源回路で構成することができる。本実施の形態では、電圧印加部１３は、後述する制御・演算処理部２６から指令された値のバイアス電圧を印加するように構成されており、印加するバイアス電圧は可変となっている。もっとも、電圧印加部１３は、制御・演算処理部２６によりバイアス電圧の値が指令されるのではなく、オペレータ等が手動でバイアス電圧を変え得るように構成しておいてもよい。
【００４３】
光伝導アンテナ９は、導電膜１１，１２間に電圧印加部１３によりバイアス電圧が印加され、ポンプパルス光Ｌ２が領域Ｒに照射されることにより、テラヘルツ光Ｌ４を発生する。ポンプパルス光Ｌ２の照射により励起された光励起キャリアがバイアス電圧による印加電場によって加速されることで、テラヘルツ光が発生する。発生するテラヘルツ光Ｌ４の電場及び磁場の強度（符号も含む）は、一般的に知られているように、電圧印加部１３から印加されているバイアス電圧に比例する。光伝導アンテナ９で発生するテラヘルツパルス光Ｌ４としては、概ね０．１×１０^１２から１００×１０^１２ヘルツまでの周波数領域の光が望ましい。そして、測定対象物としての試料１００が図１に示すように設置されている場合には、発生したテラヘルツパルス光Ｌ４が、試料１００の所定の２次元領域を一括照射する。なお、本装置の通常の使用時には試料１００が図１に示す位置に設置されるが、後述する初期動作時には、試料１００は通常は設置されない。
【００４４】
このように、本実施の形態では、フェムト秒パルス光源１、平面反射鏡２、ビームスプリッタ３、ビームエキスパンダ４、平面鏡５，６、可動鏡７及び平面鏡８が、テラヘルツ光発生素子としての光伝導アンテナ９の所定箇所に励起パルス光Ｌ２を照射する励起パルス光照射部を構成している。そして、試料１００を図１に示す位置に設置した場合には、前記励起パルス光照射部及び光伝導アンテナ９が、テラヘルツパルス光Ｌ４を試料１００に照射するテラヘルツパルス光照射部を構成する。
【００４５】
試料１００が図１に示す位置に設置されている場合、試料１００の２次元領域を透過したテラヘルツパルス光Ｌ５は、結像光学系を構成するレンズ１４，１５を経てビームスプリッタ１６を透過した後に、ＺｎＴｅ等の電気光学結晶１７に入射する。本実施の形態では、レンズ１４，１５の焦点距離は共にｆであり、前段のレンズ１４は試料１００から焦点距離ｆだけ離れた位置に配置され、後段のレンズ１５は電気光学結晶１７から焦点距離ｆだけ離れた位置に配置されている。したがって、レンズ１４，１５によって、試料１００を透過したテラヘルツパルス光Ｌ５による試料１００の２次元領域の像が、電気光学結晶１７の対応する２次元領域に結像される。このように、本実施の形態では、試料１００が図１に示す位置に設置されている場合には、電気光学結晶１７は、試料１００の２次元領域を透過したテラヘルツパルス光Ｌ５を、試料１００の前記２次元領域に対応する２次元領域で一括受光する。また、試料１００が図１に示す位置に設置されていない場合には、電気光学結晶１７は、光伝導アンテナ９から発したテラヘルツパルス光Ｌ４を、レンズ１１４，１５を介して２次元領域で一括受光する。なお、本発明では、電気光学結晶１７が、試料１００の２次元領域で反射されたテラヘルツパルス光を、試料１００の前記２次元領域に対応する２次元領域で一括受光するように、構成してもよい。
【００４６】
ビームスプリッタ３で分割された他方のパルス光Ｌ３は、テラヘルツパルス光を検出するためのプローブパルス光となる。このプローブパルス光Ｌ３は、平面反射鏡１８〜２０を経た後にビームエキスパンダ２１でテラヘルツパルス光の断面に応じて拡張され、偏光子２２を通過した後に直線偏光光となり、更に、ビームスプリッタ１６で反射された後に電気光学結晶１７に入射する。電気光学結晶１７に入射した直線偏光光であるプローブパルス光は、電気光学結晶１７を透過する。その透過光の偏光状態は、テラヘルツパルス光の電場強度により生じた電気光学結晶１７の複屈折変化に応じて、楕円偏光に変化する。すなわち、プローブパルス光は、電気光学結晶１７を通過して、テラヘルツパルス光により、偏光状態が変化する。電気光学結晶１７を透過したプローブパルス光は、検光子２３で検光された後に、光電変換部としての２次元ＣＣＤカメラ２４により光強度分布が検出される。前記光強度分布を示す２次元ＣＣＤカメラ２４からの画像信号は、Ａ／Ｄ変換器２５でＡ／Ｄ変換された後に、コンピュータ等からなる制御・演算処理部２６に取り込まれ、メモリ２７に記憶される。すなわち、検光されたプローブパルス光の光強度分布が、一括してデータとして制御・演算処理部２５によりメモリ２７に取り込まれる。なお、メモリ２７は、通常は制御・演算処理部２６の内部のメモリとして構成されるが、図１では、理解を容易にするため、メモリ２７を制御・演算処理部２６の外部に配置している。勿論、メモリ２７は制御・演算処理部２６の内部に構成する必要はない。
【００４７】
なお、ポンプパルス光Ｌ２の光路上に配置された可動鏡７は、制御・演算処理部２６による制御下で、移動機構２９により矢印Ｘ方向に移動可能となっている。可動鏡７の移動量に応じて、ポンプパルス光Ｌ２の光路長が変わり、ポンプ光Ｌ２が光伝導アンテナ９へ到達する時間が遅延する。すなわち、本実施の形態では、可動鏡７及び移動機構２９が、ポンプ光Ｌ２の時間遅延装置を構成している。
【００４８】
ここで、２次元ＣＣＤカメラ２４からの画像信号が含む情報について、説明する。この画像信号の各画素信号（各画素の光検出信号）Ｉは、Ｚｈｉｐｉｎｇ　Ｊｉａｎｇらの論文（Ａｐｐｌｉｅｄ　Ｐｈｙｓｉｃｓ　Ｌｅｔｔｅｒｓ，　Ｖｏｌ．７４，　Ｎｏ．９，　１　Ｍａｒｃｈ　１９９９，　ｐｐ．１１９１−１１９３）の記載からもわかるように、次の数１で表される。この論文には、数１及び後述する数３と同様の式が開示されている。
【００４９】
【数１】
Ｉ＝Ｉ_０［η＋ｓｉｎ^２（Γ_０＋Γ）］
【００５０】
数１において、Ｉ_０は当該画素に対応する部位のプローブパルス光の強度、Γは電気光学結晶１７における当該画素に対応する部位のテラヘルツパルス光の電場により誘起された複屈折からの寄与、Γ_０は電気光学結晶１７における当該画素に対応する部位の残留複屈折率によるバイアス、ηは電気光学結晶１７における当該画素に対応する部位でのプローブパルス光の散乱からの寄与である。電気光学結晶１７における当該画素に対応する部位のテラヘルツパルス光検出効率をαとし、電気光学結晶１７における当該画素に対応する部位のテラヘルツパルス光の電場強度をＥとすると、Γは次の数２で表すことができる。数２は、前記論文に開示されたものではないが、電気光学係数の空間依存性（電気光学係数が各部位ごとに異なる可能性があること）を考慮して導入したものであり、数２を導入する方が現実に即していると考えられる。η、Γ_０、αは、当該電気光学結晶１７に固有の定数であるが、各画素毎に（すなわち、各部位毎に）定まる定数である。Ｉ_０もフェムト秒パルス光源１等によって定まる定数であるが、各画素毎に定まる。
【００５１】
【数２】
Γ＝αＥ
【００５２】
今、｜Γ_０＋Γ｜≪１とすると、数１は次の数３の通りに書き直せる。数３に数２を代入すると、数４が得られる。
【００５３】
【数３】
Ｉ＝Ｉ_０［η＋（Γ_０＋Γ）^２］
【００５４】
【数４】
Ｉ＝Ｉ_０［η＋（Γ_０＋αＥ）^２］
【００５５】
数４からわかるように、２次元ＣＣＤカメラ２４からの画像信号の各画素信号Ｉには、電気光学結晶１７の各部位の検出効率や残留複屈折率やプローブパルス光の散乱の影響が含まれている。そして、数４からわかるように、各画素信号Ｉは、当該部位のテラヘルツパルス光の電場強度Ｅの２次式で記述され、この２次式は、各画素信号Ｉと当該画素に対応する部位のテラヘルツパルス光の電場強度Ｅとの関係を示している。
【００５６】
このような各画素信号Ｉと当該画素に対応する部位のテラヘルツパルス光の電場強度Ｅとの関係を、各画素毎に予め得ておけば、その関係を用いることによって、各画素信号Ｉから、当該部位のη、Γ_０、αの影響を排除した真のテラヘルツパルス光の電場強度Ｅを求めることができる。なお、ここでは、例えばＩ_０＝１として相対強度のみを問題とする。本実施の形態によるテラヘルツパルス光計測装置は、以下に説明する初期動作により、前記関係を自動的に得て、メモリ２７に格納しておくことができるように構成されている。
【００５７】
次に、この初期動作について説明する。ここで、テラヘルツ光源として光伝導アンテナ９を使用している点に注目する。前述したように、光伝導アンテナ９からの放射電磁波（テラヘルツパルス光）の強度（電場強度及び磁場強度）は、光伝導アンテナ９に印加したバイアス電圧Ｖに比例することが一般的に知られている。よって、その比例定数をβとすると、Ｅ＝βＶとなる。したがって、図１に示す位置に試料１００を設置しない状態（試料１００を設置した状態でもよいが、試料１００を設置しない方が好ましい。）で、光伝導アンテナ９に印加するバイアス電圧Ｖを変化させながら、各バイアス電圧Ｖにおいて２次元ＣＣＤカメラ２４から画像信号をそれぞれ得ると、各画素について、バイアス電圧Ｖと画素信号Ｉとの関係は、数４においてＥ＝βＶを代入した関係となり、画素信号Ｉはバイアス電圧Ｖの２次式で表される。
【００５８】
本発明者は、実際に、バイアス電圧Ｖと画素信号（ＣＣＤ出力値）Ｉとの関係を測定した。その測定結果の一例を図３中に四角の点で示す。図３において、横軸がバイアス電圧Ｖ、縦軸がある特定の画素におけるＣＣＤ出力値Ｉとなっている。図３中の実線は、測定点に最も適合するようにフィッティングした、バイアス電圧Ｖの２次式を示している。前述したようにＥ＝βＶであるので、図３に示す横軸は、比例定数βのためにスケールは異なるものの、当該画素に対応する部位のテラヘルツパルス光の電場強度Ｅであると読み替えることができる。換言すれば、バイアス電圧Ｖと画素信号Ｉとの関係は、テラヘルツパルス光の電場強度Ｅと画素信号Ｉとの関係を示している。なお、通常は、電場強度Ｅの絶対的な値を問題とせずに相対的な値を問題とするので、β＝１として、Ｅ＝Ｖとすればよい。そして、前述したフィッティングにより得た２次式は、数４においてＥ＝βＶを代入した２次式と等しいので、各次数の項の係数が互いに等しいという方程式を立て、この方程式を解くことにより、電気光学結晶１７における当該画素に対応する部位のη、Γ_０、αを求めることができる。なお、η、Γ_０、αの絶対的な値を問題とせずに相対的な値を問題とする場合には、例えば、β＝１、Ｉ_０＝１としてη、Γ_０、αを求めればよい。ただし、本実施の形態では、必ずしもη、Γ_０、αを求める必要はない。
【００５９】
このように、光伝導アンテナ９に印加するバイアス電圧Ｖを変化させながら、各バイアス電圧Ｖにおいて２次元ＣＣＤカメラ２４から画像信号をそれぞれ得ることによって、各画素信号Ｉと当該画素に対応する部位のテラヘルツパルス光の電場強度Ｅとの関係を、各画素毎に得ることができる。
【００６０】
そこで、本実施の形態によるテラヘルツ光計測装置では、例えば当該装置の出荷時や使用開始時などに、次の初期動作を行う。この初期動作は、図１に示す位置に試料１００を設定しない状態で行うことが好ましい。まず、制御・演算処理部２６は、テラヘルツパルス光とプローブパルス光が電気光学結晶１７に到達するタイミングを調整するために、移動機構２９を制御して、可動鏡７を、２次元ＣＣＤカメラ２４からのある特定の画素の画素信号が例えば最大となる位置に位置させる。次に、制御・演算処理部２６は、電圧印加部１３に順次異なる指令値を与えて、電圧印加部１３が光伝導アンテナ９に印加するバイアス電圧を順次変化させる。そして、制御・演算処理部２６は、各バイアス電圧が光伝導アンテナ９に印加されているときに、ＣＣＤカメラ２４から得られる各画素信号の値を当該バイアス電圧（具体的には、バイアス電圧指令値でもよいし、バイアス電圧検出器を用いる場合にはその検出値でもよい。）と関連づけてメモリ２７に格納させる。その後、制御・演算処理部２６は、各画素毎に、各バイアス電圧とその際に得られた画素信号の値とに基づいて、フィッティング処理により画素信号をバイアス電圧の２次式で記述する当該２次式を求め、各画素毎に得られた当該２次式をメモリ２７に格納させる。この各画素の２次式が、当該画素に対応する部位のテラヘルツパルス光の電場強度Ｅと画素信号Ｉとの関係に相当することは、前述した通りである。もっとも、この関係は、２次式の形式ではなく、当該２次式に相当するルックアップテーブルの形式で、メモリ２４に格納してもよい。このように、前記２次式又は前記ルックアップテーブルをメモリに格納する場合には、各バイアス電圧とその際に得られた画素信号の値は、最終的に不要であるのでメモリ２７から削除してもよい。もっとも、フィッティング処理を行わずに、前記関係として、各バイアス電圧とその際に得られた画素信号自体を、例えばルックアップテーブルの形式で、メモリ２７に格納しておくだけでもよい。ただし、誤差を低減するため、フィッティング処理を行うことが好ましい。なお、前記２次式の保存形式としては、例えば、各次数の項の係数をメモリ２７に格納してもよいし、予め前述した演算によりη、Γ_０、αを求めてこれらをメモリ２７に格納してもよい。
【００６１】
このように、本実施の形態では、各画素について、当該画素に対応する部位のテラヘルツパルス光の電場強度Ｅと画素信号Ｉとの関係を、自動的に取得してメモリ２７に格納するように構成されているが、本発明では必ずしもこの構成に限定されるものではない。例えば、オペレータがバイアス電圧を計測する電圧計等を見ながらバイアス電圧を変えて、各バイアス電圧の時に得られる各画素信号の値をメモリ２７に格納させるように構成してもよい。また、本装置とは別の装置等により既に前記関係を得ていて前記関係が予め既知であれば、その関係をメモリ２７に予め格納しておくだけでもよく、その場合には、前記初期動作は不要であり、電圧印加部１３が印加するバイアス電圧は必ずしも可変である必要はないし、また、光伝導アンテナ９に代えて電気光学結晶や磁気光学結晶などの他のテラヘルツ光発生器を用いてもよい。
【００６２】
次に、前述した初期動作の後に行われる試料１００をイメージ化する通常動作について、説明する。勿論、この際には、試料１００を図１に示す位置に設置しておく。
【００６３】
通常動作では、制御・演算処理部２６は、ＣＣＤカメラ２４からの画像信号をメモリ２７に取り込んだ後、各画素毎に、前記初期動作により既に格納されている当該画素に関する画素信号と電場強度Ｅとの関係を用いて、通常動作時に取り込まれた画素信号に対応する電場強度Ｅを得る。このとき、前記関係として前記２次式がメモリ２７に格納されていれば、通常動作時に取り込まれた画素信号を当該２次式に代入して当該２次式を解くことによって電場強度Ｅを得ることができ、前記関係として前記ルックアップテーブルがメモリ２７に格納されていれば、当該ルックアップテーブルを参照することによって電場強度Ｅを得ることができる。次に、制御・演算処理部２６は、前述したようにして各画素について得た電場強度Ｅを新たな画素値として画像を再構成し、その画像を表示部２８に表示させる。なお、以上の動作は、可動鏡７の位置やバイアス電圧の値は、所定の位置や所定の値に設定された状態で行われるが、これらは、図示しない入力装置からのオペレータによる指令に応じて、適宜変え得るようにしてもよいことは、言うまでもない。
【００６４】
このように、本実施の形態では、各画素毎に、当該画素に関する画素信号Ｉと電場強度Ｅとの関係（電気光学結晶１７の当該部位のη、Γ_０、α、Ｉ_０を反映した関係）を用いて、試料１００の対応する部位を透過したテラヘルツパルス光に対応する画像信号から真の電場強度を得、これによる画像を表示部２８に表示させている。したがって、本実施の形態によれば、電気光学結晶１７の各部位のη、Γ_０、α、Ｉ_０のばらつきが大きくても、その影響が排除され、高画質の画像を得ることができる。
【００６５】
なお、前述した例では、電場強度Ｅによる画像をそのまま表示部２８に表示させているが、各部位ごとに電場強度Ｅの時系列波形を得て、各部位毎に当該時系列波形から所定の特性値（例えば、複素誘電率や複素屈折率）等を演算し、当該特性値による画像を表示部２８に表示させてもよい。このとき、電場強度Ｅの時系列波形の取得については、制御・演算処理部２６は、いわゆるポンプ−プローブ法に従って、移動機構２９を制御して可動鏡７の位置を順次変え、各位置でそれぞれＣＣＤカメラ２４から画像信号を取り込み、可動鏡７の各位置における各画素信号に関して、各画素毎に前記関係を用いて電場強度Ｅを得ればよい。
【００６６】
ところで、電気光学結晶１７が各部位において残留複屈折率がほとんどない理想的なものであるとすれば、本実施の形態によるテラヘルツ光計測装置では、電場強度Ｅの符号を区別することができない。これは、数４においてΓ_０＝０とすると明らかなように、画像信号Ｉには電場強度Ｅの絶対値しか反映されないからである。
【００６７】
そこで、電場強度Ｅの符号を区別する必要がある場合には、電気光学結晶１７として残留複屈折率を有するものを用いることが好ましい。残留複屈折率がある場合には、数４においてΓ_０≠０であるので、残留複屈折率の存在を積極的に利用することにより、電場強度の符号の検出が可能になる。例えば、前述した図３は、残留複屈折率がある場合の例であるが、この例では、例えば、図３中のバイアス電圧Ｖ（電場強度Ｅに相当）がＶ１（Ｅ１に相当）の場合と−Ｖ１（−Ｅ１に相当）の場合とを区別することが可能になる。
【００６８】
電場強度Ｅの符号が区別できないと、時間領域分光の実現は困難である。理想的なクロスポーラリゼーション手法では、電場強度Ｅの符号を区別することができなかったので、時間領域分光を実現することができなかった。
【００６９】
これに対し、電気光学結晶１７として残留複屈折率を有するものを用いれば、電場強度Ｅの符号を区別することができるので、時間領域の分光が可能となり、本実施の形態を次のように変形すれば、２次元分光イメージングシステムを構成することができる。すなわち、この場合には、制御・演算処理部２６は、電場強度Ｅによる画像をそのまま表示部２８に表示させず、各部位ごとに電場強度Ｅの時系列波形を得る。このとき、電場強度Ｅの時系列波形の取得については、制御・演算処理部２６は、いわゆるポンプ−プローブ法に従って、移動機構２９を制御して可動鏡７の位置を順次変え、各位置でそれぞれＣＣＤカメラ２４から画像信号を取り込み、各可動鏡７の位置における各画素信号に関して、各画素毎に前記関係を用いて電場強度を得ればよい。このとき、電気光学結晶１７として残留複屈折率を有するものを用いることにより、符号を含む電場強度を得ることができる。その後、制御・演算処理部２６は、各部位ごとに、電場強度Ｅの時系列波形（符号も含む時系列波形）をフーリエ変換することによりスペクトルを得る。
【００７０】
なお、本実施の形態は、試料１００の２次元領域における各部位を透過したテラヘルツパルス光をそれぞれ一括して計測するテラヘルツ光計測装置の例であるが、試料１００の所定の部位のみを透過したテラヘルツパルス光を計測する場合には、本実施の形態を次のように変形してもよい。すなわち、この場合には、テラヘルツパルス光を試料１００の所定の部位のみに集光させ、試料１００を透過したテラヘルツパルス光を電気光学結晶１７の所定の部位のみに集光させるとともに、プローブパルス光を電気光学結晶１７の当該部位のみに集光させ、ＣＣＤカメラ２４に代えてポイントセンサを用いればよい。
【００７１】
［第２の実施の形態］
【００７２】
次に、本実施の形態の第２の実施の形態による電気光学結晶の評価装置について、説明する。
【００７３】
本実施の形態による評価装置は、前記第１の実施の形態によるテラヘルツ光計測装置を以下に説明するように変形したものである。そこで、ここでも図１を参照することとし、前記第１の実施の形態と重複する説明は省略する。
【００７４】
本実施の形態による評価装置が前記第１の実施の形態によるテラヘルツ光計測装置と異なる所は、試料１００は図１に示す位置に設定されない点と、電気光学結晶１７が評価対象とされる点と、制御・演算処理部２６の動作である。
【００７５】
本実施の形態による評価装置では、前記第１の実施の形態によるテラヘルツ光計測装置の初期動作と同様に、制御・演算処理部２６は、テラヘルツパルス光とプローブパルス光が電気光学結晶１７に到達するタイミングを調整するために、移動機構２９を制御して、可動鏡７を、２次元ＣＣＤカメラ２４からのある特定の画素の画素信号が例えば最大となる位置に位置させる。次に、制御・演算処理部２６は、電圧印加部１３に順次異なる指令値を与えて、電圧印加部１３が光伝導アンテナ９に印加するバイアス電圧を順次変化させる。そして、制御・演算処理部２６は、各バイアス電圧が光伝導アンテナ９に印加されているときに、ＣＣＤカメラ２４から得られる各画素信号の値を当該バイアス電圧（具体的には、バイアス電圧指令値でもよいし、バイアス電圧検出器を用いる場合にはその検出値でもよい。）と関連づけてメモリ２７に格納させる。その後、制御・演算処理部２６は、各画素毎に、各バイアス電圧とその際に得られた画素信号の値とに基づいて、フィッティング処理により画素信号をバイアス電圧の２次式で記述する当該２次式を求め、各画素毎に得られた当該２次式をメモリ２７に格納させる。この各画素の２次式が、当該画素に対応する部位のテラヘルツパルス光の電場強度Ｅと画素信号Ｉとの関係に相当することは、前述した通りである。
【００７６】
次に、制御・演算処理部２６は、各画素毎に、当該画素に対応する前記２次式から、前記第１の実施の形態に関連して既に説明した演算（フィッティングにより得た２次式が、数４においてＥ＝βＶを代入した２次式と等しいので、各次数の項の係数が互いに等しいという方程式を立て、この方程式を解く演算）を行い、各画素毎にη、Γ_０、αを求め、これらをメモリ２７に格納する。
【００７７】
その後、制御・演算処理部２６は、各画素のηを画素値とした２次元データ（電気光学結晶１７のηの分布画像に相当）、各画素のΓ_０を画素値とした２次元データ（電気光学結晶１７のΓ_０の分布画像に相当）、各画素のαを画素値とした２次元データ（電気光学結晶１７のαの分布画像に相当）を、表示部２８に画像表示させる。
【００７８】
したがって、本実施の形態によれば、電気光学結晶１７のα、η、Γ_０の各分布画像が表示されるので、評価者は、この表示を見て、電気光学結晶１７の検出効率、残留複屈折率、及びプローブパルス光の散乱の影響の均一性等を評価することができる。このため、電気光学結晶の開発や研究の重要な指標を得ることができる。
【００７９】
本実施の形態では、電気光学結晶１７のα、η、Γ_０の各分布画像を表示しているが、制御・演算処理部２６は、各画素毎の前記２次式を示すグラフを表示部２８に表示させるか、あるいは、各バイアス電圧が光伝導アンテナ９に印加されているときに、ＣＣＤカメラ２４から得られる各画素信号の値を、当該バイアス電圧と関連づけてそのまま表示部２８に表示させてもよい。これらの表示によっても、評価者は、電気光学結晶１７の検出効率、残留複屈折率、及びプローブパルス光の散乱の影響の均一性等を評価することは、可能である。
【００８０】
また、本実施の形態は、本発明による電気光学結晶１７の評価方法を自動的に実現する評価装置の例であるが、本発明による評価方法は評価者がいわば人手により実現してもよい。この場合には、例えば、オペレータは、バイアス電圧を計測する電圧計等を見ながらバイアス電圧を変えて、各バイアス電圧の時に得られる各画素信号の値を知り、各画素毎に、バイアス電圧と画素信号の値との関係から、電気光学結晶１７の検出効率、残留複屈折率、及びプローブパルス光の散乱の影響の均一性等を評価してもよい。
【００８１】
なお、本実施の形態は、電気光学結晶１７の２次元領域における各部位の検出効率等の評価を行うための装置の例であるが、電気光学結晶１７の所定の部位のみの検出効率等の評価を行う場合には、本実施の形態を次のように変形してもよい。すなわち、この場合には、テラヘルツパルス光を電気光学結晶１７の当該部位のみに集光させるとともに、プローブパルス光を電気光学結晶１７の当該部位のみに集光させ、ＣＣＤカメラ２４に代えてポイントセンサを用いればよい。
【００８２】
以上、本発明の各実施の形態及びその変形例について説明したが、本発明はこれらの実施の形態や変形例に限定されるものではない。
【００８３】
例えば、前述した各実施の形態では光伝導アンテナ９に印加されるバイアス電圧を変えることで、発生するテラヘルツパルス光の電場強度を変えていたが、他のパラメータを変えることで、発生するテラヘルツパルス光の電場強度を変えてもよい。
【００８４】
また、前述した各実施の形態やその変形例において、電気光学結晶１７に代えて磁気光学結晶を用いてもよい。すなわち、前記第１の実施の形態において検出用結晶として磁気光学結晶を用いてもよいし、前記第２の実施の形態において評価対象結晶として磁気光学結晶を用いてもよい。この場合、以上の説明において、電気光学を磁気光学と読み替え、電場を磁場と読み替えれば、以上の説明がそのまま適合する。したがって、ここでは、電気光学結晶に代えて磁気光学結晶を用いた場合の説明は省略する。
【００８５】
【発明の効果】
以上説明したように、本発明によれば、電気光学結晶や磁気光学結晶を適切に評価することができる電気光学結晶又は磁気光学結晶の評価方法及び装置を提供することができる。
【００８６】
また、本発明によれば、電気光学結晶又は磁気光学結晶を用いてテラヘルツパルス光を計測することができ、しかも、バランス検出を行うことなく、テラヘルツパルス光を正確に計測することができるテラヘルツ光計測方法及び装置を提供することができる。
【００８７】
さらに、本発明によれば、テラヘルツ光検出に用いる電気光学結晶又は磁気光学結晶の空間的均一性が低くても、高画質の対象物体の像を得ることができるテラヘルツ光計測装置を提供することができる。
【図面の簡単な説明】
【図１】本発明の第１の実施の形態によるテラヘルツ光計測装置を模式的に示す概略構成図である。
【図２】図１に示すテラヘルツ光計測装置で用いられる光伝導アンテナの一例を模式的に示す概略斜視図である。
【図３】特定の画素におけるＣＣＤ出力値とバイアス電圧との関係を示す測定結果の一例を示す図である。
【符号の説明】
１　フェムト秒パルス光源
４，２１　ビームエキスパンダ
９　光伝導アンテナ
１３　電圧印加部
１７　電気光学結晶
２２　偏光子
２３　検光子
２４　２次元ＣＣＤカメラ（光電変換部）
２６　制御・演算処理部
２７　メモリ
２８　表示部
１００　試料[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for evaluating an electro-optical crystal and a magneto-optical crystal, and a method and an apparatus for measuring terahertz light using the same.
[0002]
[Prior art]
In recent years, terahertz technology has attracted attention. In particular, in the measurement using terahertz pulsed light, it is possible to obtain “phase” information, which is difficult to detect with the conventional technology, and therefore, there is a great interest in its application. Representative methods for detecting terahertz pulsed light include a method using a photoconductive antenna and an electro-optic (EO) sampling method. The advantages of the EO sampling method include (1) that two-dimensional data can be obtained collectively, and (2) that high-frequency (tens of THz or more) regions can be detected. be able to.
[0003]
As an application of terahertz light, development of terahertz imaging technology is being promoted worldwide. As a technique of imaging, a “scanning imaging method” in which terahertz light is focused on one point on a sample and the focused point is moved (or the sampling is relatively moved) to obtain an image, The method is broadly classified into two types, a “real-time imaging method” for obtaining a two-dimensional terahertz image at a time. In order to realize the latter, two methods of (1) arraying photoconductive antennas and (2) using the EO sampling method can be considered as detectors, but (2) is currently the mainstream. The successful case of (1) has not been reported yet.
[0004]
Either (1) balance detection or (2) cross-polarization is used for terahertz light detection using the EO sampling method.
[0005]
As a modification of the EO sampling method, there is a magneto-optic sampling method using a magneto-optic crystal instead of an electro-optic crystal.
[0006]
[Problems to be solved by the invention]
In the balance detection method, the terahertz pulse light is guided to the electro-optic crystal, and the probe pulse light synchronized with the terahertz pulse light and linearly polarized is guided to the electro-optic crystal via a quarter-wave plate. The probe pulse light, the polarization state of which has been changed by the terahertz pulse light after passing through the optical crystal, is separated into two light beams by a polarization beam splitter, and the separated two light beams are respectively detected by two photoelectric conversion units. And the difference signal is used as a terahertz light detection signal (a signal indicating the intensity of the electric field of the terahertz light). Therefore, in the balance detection method, the residual birefringence of the electro-optic crystal (in an ideal electro-optic crystal, the anisotropy of the refractive index when no electric field is applied is zero, but in reality, Terahertz pulsed light can be detected by effectively removing the influence of scattering of the probe pulsed light in the electro-optic crystal and the like, since the remaining index causes anisotropy in the refractive index. Two photoelectric conversion means are required.
[0007]
On the other hand, in the cross-polarization method, the terahertz pulse light is guided to the electro-optic crystal and the probe pulse light that is synchronized with the terahertz pulse light and linearly polarized is guided to the electro-optic crystal, and passes through the electro-optic crystal. The probe pulse light, the polarization state of which has been changed by the terahertz pulse light, is analyzed by an analyzer composed of a polarizing plate, and the probe pulse light after the analysis is detected by one photoelectric conversion unit, and the output signal is terahertz. A light detection signal (a signal indicating the intensity of the electric field of the terahertz light) is used. Therefore, in the cross-polarization method, only one photoelectric conversion unit is required, but the output signal of the photodetector is affected by the residual birefringence of the electro-optic crystal and the scattering of probe pulse light in the electro-optic crystal. Are reflected, and the terahertz pulse light cannot be measured accurately.
[0008]
In the case of the scanning imaging method, since a point sensor can be used as the photoelectric conversion unit, the cost does not increase so much even if the number of the photoelectric conversion units is two. Therefore, in the case of the scanning imaging method, a balance detection method that can measure terahertz pulsed light more accurately than the cross-polarization method is often used.
[0009]
On the other hand, in the real-time imaging method, since a two-dimensional sensor such as a CCD camera is used as the photoelectric conversion means, if the number of the photoelectric conversion means is two, the cost is significantly increased as compared with the case where the number of the photoelectric conversion means is one. I will. Therefore, in the real-time imaging method, a cross-polarization method is mainly used. However, in this case, due to the residual birefringence of the electro-optic crystal and the scattering of the probe pulse light in the electro-optic crystal, the terahertz pulse light at each site cannot be measured accurately, and moreover, it is not actually possible. Since the residual birefringence of the electro-optic crystal, the influence of scattering, and the detection efficiency vary at each site, the image quality of the obtained image is lower than in the case of using the balance detection method.
[0010]
As described above, in the detection of terahertz light, although the effects of the residual birefringence, scattering, and detection efficiency of the electro-optic crystal have a large effect, a method for evaluating them has not yet been established. If an evaluation method for an electro-optic crystal is established, for example, development and research of an electro-optic crystal having high spatially uniform characteristics can be promoted.
[0011]
The electro-optical crystal has been described above, but the same applies to the magneto-optical crystal. Further, the electro-optic crystal and the magneto-optic crystal are important in various uses other than the terahertz light detection.
[0012]
The present invention has been made in view of the circumstances described above, and has as its object to provide a method and an apparatus for evaluating an electro-optical crystal or a magneto-optical crystal that can appropriately evaluate an electro-optical crystal or a magneto-optical crystal. And
[0013]
Further, the present invention provides a terahertz light measurement method capable of measuring terahertz pulse light using an electro-optic crystal or a magneto-optic crystal, and capable of accurately measuring terahertz pulse light without performing balance detection. And an apparatus.
[0014]
Still another object of the present invention is to provide a terahertz light measurement device capable of obtaining a high-quality image of a target object even if the spatial uniformity of an electro-optic crystal or a magneto-optic crystal used for terahertz light detection is low. And
[0015]
[Means for Solving the Problems]
In order to solve the above problem, the method for evaluating an electro-optical crystal or a magneto-optical crystal according to the first aspect of the present invention employs a terahertz light generation unit capable of changing the intensity of an electric field or a magnetic field of a generated terahertz pulse light, While guiding the terahertz pulse light emitted from the terahertz light generation unit to the electro-optic crystal or the magneto-optic crystal that is the crystal to be evaluated, the probe pulse light synchronized with the terahertz pulse light and polarized is guided to the crystal to be evaluated. The probe pulse light whose polarization state has been changed by the terahertz pulse light passing through the crystal to be evaluated is analyzed, the probe pulse light after the analysis is photoelectrically converted, and the electric field or magnetic field intensity of the terahertz pulse light is analyzed. By changing the intensity of the electric field or the magnetic field of the terahertz pulse light and the intensity of the signal obtained by the photoelectric conversion, And, based on the relationship, the detection efficiency of the evaluation target crystal, the residual birefringence of the evaluation target crystal, and at least one of the effects of the scattering of the probe pulse light in the evaluation target crystal. To evaluate.
[0016]
The intensity of the electric or magnetic field of the terahertz pulse light may include a sign or may not include a sign. This is the same for each embodiment described later.
[0017]
In the method for evaluating an electro-optical crystal or a magneto-optical crystal according to a second aspect of the present invention, in the first aspect, the terahertz light generating section may include: (a) a photoconductive section; and a predetermined surface of the photoconductive section. A terahertz light generating element having two conductive portions formed thereon and separated from each other, at least a part of the two conductive portions being arranged so as to be spaced apart from each other by a predetermined distance in a direction along the predetermined surface. (B) an excitation pulse light irradiating unit that irradiates a predetermined portion of the terahertz light generation element with excitation pulse light, and (c) a voltage application unit that applies a variable bias voltage between the two conductive units. And changing the bias voltage to change the intensity of the electric field or magnetic field of the terahertz pulse light, and obtaining the relationship between the bias voltage and the intensity of a signal obtained by the photoelectric conversion.
[0018]
The method for evaluating an electro-optical crystal or a magneto-optical crystal according to the third aspect of the present invention uses the terahertz light generator that can change the intensity of the electric or magnetic field of the generated terahertz pulse light, and emits the light from the terahertz light generator. The terahertz pulse light is guided to the two-dimensional region of the electro-optical crystal or the magneto-optical crystal, which is the crystal to be evaluated, and the probe pulse light synchronized and polarized with the terahertz pulse light is guided to the two-dimensional region of the crystal to be evaluated. Analyzing the probe pulse light, which has passed through the crystal to be evaluated and changed in polarization state by the terahertz pulse light, and analyzes the probe pulse light after the analysis, in each of the two-dimensional regions of the crystal to be evaluated. The terahertz pulse light is subjected to photoelectric conversion, and the intensity of the electric field or magnetic field of the terahertz pulse light is changed. The relationship between the intensity of the electric field or magnetic field of the luminescence light and the intensity of the signal obtained by the photoelectric conversion is obtained for each of the portions of the two-dimensional region of the crystal to be evaluated, based on the relationship, At least one of the detection efficiency of each part of the crystal to be evaluated, the residual birefringence of each part of the crystal to be evaluated, and the influence of the scattering of the probe pulse light at each part of the crystal to be evaluated. Evaluate one.
[0019]
In the method for evaluating an electro-optic crystal or a magneto-optic crystal according to a fourth aspect of the present invention, in the third aspect, the terahertz light generating section may include: (a) a photoconductive section; and a predetermined surface of the photoconductive section. A terahertz light generating element having two conductive portions formed thereon and separated from each other, at least a part of the two conductive portions being arranged so as to be spaced apart from each other by a predetermined distance in a direction along the predetermined surface. (B) an excitation pulse light irradiating unit that irradiates a predetermined portion of the terahertz light generation element with excitation pulse light, and (c) a voltage application unit that applies a variable bias voltage between the two conductive units. Including, the intensity of the electric field or the magnetic field of the terahertz pulsed light is changed by changing the bias voltage, and as the relationship, the relationship between the bias voltage and the intensity of the signal obtained by the photoelectric conversion is determined. Serial those obtained respectively in each one corresponding to each part of the two-dimensional region.
[0020]
The electro-optical crystal or the magneto-optical crystal evaluation apparatus according to the fifth aspect of the present invention has a terahertz light generation unit capable of changing the intensity of an electric field or a magnetic field of a generated terahertz pulse light, and is emitted from the terahertz light generation unit. A terahertz pulse light irradiating unit that irradiates the terahertz pulse light to an electro-optical crystal or a magneto-optical crystal as an evaluation target crystal, and a probe that irradiates the evaluation target crystal with a probe pulse light synchronized and polarized with the terahertz pulse light. A pulsed light irradiator, an analyzer for analyzing the probe pulsed light that has passed through the crystal to be evaluated and whose polarization state has been changed by the terahertz pulsed light, and the probe pulsed light after the analysis by the analyzer A photoelectric conversion unit that performs photoelectric conversion of the terahertz pulse light, and the terahertz pulse obtained by changing the intensity of an electric field or a magnetic field of the terahertz pulse light. A storage unit that stores the relationship between the intensity of the electric field or magnetic field of the light and the intensity of the output signal of the photoelectric conversion unit, based on the relationship, a value indicating the detection efficiency of the crystal to be evaluated, and the crystal to be evaluated. And a calculation unit that obtains at least one of a value indicating the residual birefringence index and a value indicating the influence of scattering of the probe pulse light on the crystal to be evaluated.
[0021]
The apparatus for evaluating an electro-optic crystal or a magneto-optic crystal according to a sixth aspect of the present invention is the above-described fifth aspect, wherein the terahertz light generation section comprises: (a) a light conduction section; and a predetermined surface of the light conduction section. A terahertz light generating element having two conductive portions formed thereon and separated from each other, at least a part of the two conductive portions being arranged so as to be spaced apart from each other by a predetermined distance in a direction along the predetermined surface. (B) an excitation pulse light irradiating unit that irradiates a predetermined portion of the terahertz light generation element with excitation pulse light, and (c) a voltage application unit that applies a variable bias voltage between the two conductive units. The storage unit stores the relationship between the bias voltage and the intensity of the output signal of the photoelectric conversion unit, which is obtained by changing the bias voltage, as the relationship.
[0022]
The electro-optical crystal or the magneto-optical crystal evaluation apparatus according to the seventh aspect of the present invention has a terahertz light generation unit capable of changing the intensity of an electric field or a magnetic field of a generated terahertz pulse light, and is emitted from the terahertz light generation unit. A terahertz pulse light irradiating section for irradiating the terahertz pulse light to a two-dimensional region of the electro-optical crystal or the magneto-optical crystal as the crystal to be evaluated, and a probe pulse light synchronized and polarized with the terahertz pulse light, A probe pulse light irradiating unit for irradiating the two-dimensional region, a light analyzing unit for analyzing the probe pulse light that has passed through the crystal to be evaluated and whose polarization state has been changed by the terahertz pulse light, and the light analyzing unit The probe pulse light after the analysis by the above is photoelectrically converted for each of the portions corresponding to each part of the two-dimensional region of the crystal to be evaluated. The electrical conversion unit, obtained by changing the intensity of the electric field or magnetic field of the terahertz pulse light, the relationship between the intensity of the electric field or magnetic field of the terahertz pulse light and the intensity of the output signal of the photoelectric conversion unit, the evaluation target A storage unit for storing each one corresponding to each part of the two-dimensional region of the crystal, a value indicating the detection efficiency of each part of the crystal to be evaluated based on the relationship, A calculation unit that obtains at least one of a value indicating a residual birefringence index of a part, and a value indicating an influence of scattering of the probe pulse light in each part of the crystal to be evaluated. is there.
[0023]
The evaluation apparatus for an electro-optic crystal or a magneto-optic crystal according to an eighth aspect of the present invention is the apparatus for evaluating a terahertz light according to the seventh aspect, wherein the terahertz light generation section comprises: (a) a light conduction section; A terahertz light generating element having two conductive portions formed thereon and separated from each other, at least a part of the two conductive portions being arranged so as to be spaced apart from each other by a predetermined distance in a direction along the predetermined surface. (B) an excitation pulse light irradiating unit that irradiates a predetermined portion of the terahertz light generation element with excitation pulse light, and (c) a voltage application unit that applies a variable bias voltage between the two conductive units. The storage unit includes, as the relationship, a relationship between the bias voltage and the intensity of the output signal of the photoelectric conversion unit, which is obtained by changing the bias voltage, for each of the two-dimensional regions of the evaluation target crystal. Corresponding to the part It is for storing each for each of the.
[0024]
The terahertz light measurement method according to the ninth aspect of the present invention guides the terahertz pulse light to a detection crystal that is an electro-optic crystal or a magneto-optic crystal, and outputs a probe pulse light synchronized and polarized with the terahertz pulse light. The probe pulse light whose polarization state has been changed by the terahertz pulse light passing through the detection crystal after being guided to the detection crystal is analyzed, and the probe pulse light after the analysis is photoelectrically converted, and the terahertz is detected. A previously obtained relationship between the intensity of the electric or magnetic field of the pulsed light and the intensity of the signal obtained by the photoelectric conversion, the detection efficiency of the detection crystal, the residual birefringence of the detection crystal, and the Using the relationship reflecting the influence of the scattering of the probe pulse light in the detection crystal, the terahertz corresponding to the intensity of the signal obtained by the photoelectric conversion is used. It is intended to obtain an electric or magnetic field strength of the pulse light.
[0025]
In the terahertz light measurement method according to a tenth aspect of the present invention, in the ninth aspect, the terahertz pulsed light guided to the detection crystal is transmitted or reflected by a target object.
[0026]
In the terahertz light measurement method according to an eleventh aspect of the present invention, in the ninth or tenth aspect, the detection crystal has a residual birefringence.
[0027]
A terahertz light measurement apparatus according to a twelfth aspect of the present invention includes a terahertz pulse light irradiation unit that irradiates a terahertz pulse light to a target object, and an electro-optic crystal or a magneto-optic that receives the terahertz pulse light transmitted or reflected by the target object. A detection crystal that is a crystal, a probe pulse light irradiation unit that irradiates the detection crystal with a probe pulse light synchronized and polarized with the terahertz pulse light, and the terahertz pulse light that passes through the detection crystal. An analyzer for analyzing the probe pulse light whose polarization state has changed, a photoelectric converter for photoelectrically converting the probe pulse light after the analysis by the analyzer, and an electric or magnetic field of the terahertz pulse light And the intensity of the signal obtained by the photoelectric conversion is obtained in advance, the detection efficiency of the detection crystal, the detection efficiency And a storage unit for storing a relationship reflecting the influence of the scattering of the probe pulse light on the detection crystal, and the intensity of the signal obtained by the photoelectric conversion based on the relationship. And a processing unit for obtaining the intensity of the electric or magnetic field of the terahertz pulse light according to the above.
[0028]
In a terahertz optical measurement apparatus according to a thirteenth aspect of the present invention, in the twelfth aspect, the detection crystal has a residual birefringence.
[0029]
A terahertz light measuring apparatus according to a fourteenth aspect of the present invention includes a terahertz pulse light irradiating unit that collectively irradiates terahertz pulse light to a two-dimensional region of a target object, and a terahertz pulse transmitted or reflected through the two-dimensional region of the target object. A detection crystal that is an electro-optic crystal or a magneto-optic crystal that collectively receives light in a two-dimensional area corresponding to the two-dimensional area of the target object, and a probe pulse light synchronized and polarized with the terahertz pulse light. A probe pulse light irradiation unit that collectively irradiates the two-dimensional region of the detection crystal, and a light detection unit that detects the probe pulse light that has passed through the detection crystal and has been changed in polarization state by the terahertz pulse light. And transmitting the probe pulse light after the light detection by the light detection unit to each of the portions of the two-dimensional region of the detection crystal corresponding to the respective portions. A photoelectric conversion unit that performs photoelectric conversion, a relationship obtained in advance between the intensity of an electric field or a magnetic field of the terahertz pulse light and the intensity of a signal obtained by the photoelectric conversion, and the detection efficiency of the detection crystal and the detection efficiency. A storage unit for storing a relationship that reflects the residual birefringence of the crystal and the effect of scattering of the probe pulse light on the detection crystal for each of the two-dimensional regions of the detection crystal. And, based on the relationship, the intensity of the electric or magnetic field of the terahertz pulsed light according to the intensity of the signal obtained by the photoelectric conversion, for each one corresponding to each part of the two-dimensional region of the detection crystal. And a processing unit to be obtained.
[0030]
In a terahertz optical measurement apparatus according to a fifteenth aspect of the present invention, in the fourteenth aspect, the detection crystal has a residual birefringence at each of the portions.
[0031]
The electro-optic crystal and the magneto-optic crystal are usually formed in a plate shape. Further, as the electro-optic crystal, ZnTe, CdTe, ZnSe, GaAs, CdZnTe, BGO, BTO, GaP, BaTaO ₃ And the like. As the magneto-optical crystal, TGG, KTb ₃ F ₁₀ , GaP and the like.
[0032]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a method and an apparatus for evaluating an electro-optical crystal or a magneto-optical crystal and a method and an apparatus for measuring terahertz light according to the present invention will be described with reference to the drawings.
[0033]
[First Embodiment]
[0034]
FIG. 1 is a schematic configuration diagram schematically illustrating a terahertz optical measurement device according to a first embodiment of the present invention. FIG. 2 is a schematic perspective view showing a photoconductive antennae 9 as a lahertz light generating element used in the terahertz light measuring device shown in FIG.
[0035]
The terahertz light measurement device according to the present embodiment is configured as an imaging device that images a sample 100 as a target object.
[0036]
In the terahertz light measurement device according to the present embodiment, the femtosecond pulse light L1 as the ultrashort pulse laser light emitted from the femtosecond pulse light source 1 passes through the plane reflecting mirror 2, and then is split into two pulse lights by the beam splitter 3. It is divided into L2 and L3. In the present embodiment, the femtosecond pulse light source 1 is composed of a laser light source or the like. For example, the femtosecond pulse light L1 has a center wavelength of about 780 to 800 nm in the near infrared region and a pulse width of about 10 to 100 fs. Are emitted at a predetermined repetition period.
[0037]
One of the pulse lights L2 split by the beam splitter 3 becomes pump pulse light (excitation pulse light) for exciting the photoconductive antenna 9 as a terahertz light generation element to generate terahertz pulse light. After the pump pulse light L2 has its cross section expanded by the beam expander 4, the movable mirror 7, which is a combination of two or three plane reflecting mirrors, and the plane reflecting mirror 8, , And guided to the photoconductive antenna 9.
[0038]
Here, an example of the photoconductive antenna 9 is shown in FIG. The photoconductive antenna 9 includes a substrate 10 as a photoconductive portion, and two conductive films 11 and 12 formed on one surface of the substrate 10 and separated from each other. At least some of the conductive films 11 and 12 are arranged so as to have a predetermined interval g in a direction along a plane on the near side of the substrate 10 in FIG. In the present embodiment, the whole of the conductive films 11 and 12 is spaced from each other by a distance g. The distance g is set to 2 mm or more, for example, 5 mm, and the substrate 10 and the conductive films 11 and 12 constitute a so-called large-diameter optical switch element. However, the photoconductive antenna 9 may be configured as a photoconductive antenna using a dipole antenna or the like, for example.
[0039]
As a material of the substrate 10, for example, a semiconductor having high resistivity (for example, semi-insulating GaAs) can be used. As a material of the conductive films 11 and 12, for example, a metal such as gold can be used, and the conductive films 11 and 12 can be formed on the surface of the substrate 14 by, for example, vapor deposition.
[0040]
In the present embodiment, the substrate 10 itself is used as a photoconductive portion as described above. For example, a photoconductive film is formed as a photoconductive portion on the substrate 10 and the conductive film 11 is formed on the photoconductive film. , 12 may be formed. In this case, for example, GaAs can be used as the material of the substrate 10 and GaAs grown at a low temperature can be used as the photoconductive film.
[0041]
The pump pulse light (ultra-short pulse laser light) L2 is applied to a hatched region R corresponding to the interval g of the photoconductive antenna 9, as shown in FIG.
[0042]
As shown in FIGS. 1 and 2, a bias voltage is applied between the two conductive films 11 and 12 from the voltage application unit 13. As the voltage applying unit 13, for example, a DC power supply can be used. As such a DC power supply, for example, a power supply circuit that converts AC from commercial power into DC can be used. In the present embodiment, the voltage applying unit 13 is configured to apply a bias voltage having a value instructed from the control / arithmetic processing unit 26 described later, and the applied bias voltage is variable. However, the voltage applying unit 13 may be configured so that the bias voltage value is not commanded by the control / arithmetic processing unit 26 but an operator or the like can manually change the bias voltage.
[0043]
The photoconductive antenna 9 generates a terahertz light L4 by applying a bias voltage between the conductive films 11 and 12 by the voltage applying unit 13 and irradiating the region R with the pump pulse light L2. Terahertz light is generated by accelerating the photoexcited carriers excited by the irradiation of the pump pulse light L2 by the applied electric field due to the bias voltage. The intensity (including the sign) of the electric field and the magnetic field of the generated terahertz light L4 is proportional to the bias voltage applied from the voltage application unit 13 as is generally known. The terahertz pulse light L4 generated by the photoconductive antenna 9 is approximately 0.1 × 10 ¹² From 100 × 10 ¹² Light in the frequency range up to Hertz is desirable. Then, when the sample 100 as the measurement target is installed as shown in FIG. 1, the generated terahertz pulse light L4 irradiates a predetermined two-dimensional area of the sample 100 at a time. Note that the sample 100 is installed at the position shown in FIG. 1 during normal use of the apparatus, but the sample 100 is not normally installed during an initial operation described later.
[0044]
As described above, in the present embodiment, the femtosecond pulse light source 1, the plane reflecting mirror 2, the beam splitter 3, the beam expander 4, the plane mirrors 5, 6, the movable mirror 7, and the plane mirror 8 serve as a terahertz light generating element. An excitation pulse light irradiation unit configured to irradiate a predetermined portion of the conductive antenna 9 with the excitation pulse light L2 is configured. When the sample 100 is placed at the position shown in FIG. 1, the excitation pulse light irradiation unit and the photoconductive antenna 9 constitute a terahertz pulse light irradiation unit that irradiates the sample 100 with the terahertz pulse light L4.
[0045]
When the sample 100 is set at the position shown in FIG. 1, the terahertz pulse light L5 transmitted through the two-dimensional area of the sample 100 passes through the beam splitter 16 via the lenses 14 and 15 constituting the imaging optical system, and , ZnTe or the like. In the present embodiment, the focal lengths of the lenses 14 and 15 are both f, the front lens 14 is disposed at a position away from the sample 100 by the focal distance f, and the rear lens 15 is located at a focal distance from the electro-optic crystal 17. It is arranged at a position separated by f. Therefore, the images of the two-dimensional area of the sample 100 by the terahertz pulse light L5 transmitted through the sample 100 are formed on the corresponding two-dimensional area of the electro-optic crystal 17 by the lenses 14 and 15. As described above, in the present embodiment, when the sample 100 is installed at the position shown in FIG. 1, the electro-optic crystal 17 transmits the terahertz pulse light L5 transmitted through the two-dimensional region of the sample 100 to the sample 100. And collectively receives light in a two-dimensional area corresponding to the two-dimensional area. When the sample 100 is not placed at the position shown in FIG. 1, the electro-optic crystal 17 collectively transmits the terahertz pulse light L4 emitted from the photoconductive antenna 9 in the two-dimensional region via the lenses 114 and 15. Receive light. In the present invention, the electro-optic crystal 17 is configured to collectively receive the terahertz pulse light reflected on the two-dimensional region of the sample 100 in a two-dimensional region corresponding to the two-dimensional region of the sample 100. Is also good.
[0046]
The other pulse light L3 split by the beam splitter 3 becomes probe pulse light for detecting terahertz pulse light. The probe pulse light L3 is extended according to the cross section of the terahertz pulse light by the beam expander 21 after passing through the plane reflecting mirrors 18 to 20, becomes a linearly polarized light after passing through the polarizer 22, and further passes through the beam splitter 16. After being reflected, it enters the electro-optic crystal 17. The probe pulse light that is linearly polarized light that has entered the electro-optic crystal 17 passes through the electro-optic crystal 17. The polarization state of the transmitted light changes to elliptically polarized light according to the birefringence change of the electro-optic crystal 17 caused by the electric field strength of the terahertz pulse light. That is, the probe pulse light passes through the electro-optic crystal 17 and changes its polarization state due to the terahertz pulse light. After the probe pulse light transmitted through the electro-optic crystal 17 is analyzed by the analyzer 23, the light intensity distribution is detected by the two-dimensional CCD camera 24 as a photoelectric conversion unit. An image signal from the two-dimensional CCD camera 24 showing the light intensity distribution is A / D converted by an A / D converter 25, and then taken into a control / arithmetic processing unit 26 such as a computer and stored in a memory 27. Is done. That is, the light intensity distribution of the detected probe pulse light is fetched into the memory 27 by the control / arithmetic processing unit 25 as data collectively. Although the memory 27 is usually configured as a memory inside the control / arithmetic processing unit 26, in FIG. 1, the memory 27 is arranged outside the control / arithmetic processing unit 26 for easy understanding. I have. Needless to say, the memory 27 does not need to be configured inside the control / arithmetic processing unit 26.
[0047]
The movable mirror 7 arranged on the optical path of the pump pulse light L2 can be moved in the direction of the arrow X by the moving mechanism 29 under the control of the control / arithmetic processing unit 26. The optical path length of the pump pulse light L2 changes according to the amount of movement of the movable mirror 7, and the time for the pump light L2 to reach the photoconductive antenna 9 is delayed. That is, in the present embodiment, the movable mirror 7 and the moving mechanism 29 constitute a time delay device for the pump light L2.
[0048]
Here, information included in the image signal from the two-dimensional CCD camera 24 will be described. Each pixel signal (light detection signal of each pixel) I of this image signal can be obtained from the description of a paper by Zhipping Jiang et al. (Applied Physics Letters, Vol. 74, No. 9, 1 March 1999, pp. 1191-1193). As can be seen, it is represented by the following equation 1. In this paper, equations similar to Equation 1 and Equation 3 described later are disclosed.
[0049]
(Equation 1)
I = I ₀ [Η + sin ² (Γ ₀ + Γ)]
[0050]
In Equation 1, I ₀ Is the intensity of the probe pulse light at the portion corresponding to the pixel, Γ is the contribution from the birefringence induced by the electric field of the terahertz pulse light at the portion corresponding to the pixel in the electro-optic crystal 17, Γ. ₀ Is the bias due to the residual birefringence of the portion of the electro-optic crystal 17 corresponding to the pixel, and η is the contribution from the scattering of the probe pulse light at the portion of the electro-optic crystal 17 corresponding to the pixel. Assuming that α is the terahertz pulse light detection efficiency of the portion corresponding to the pixel in the electro-optic crystal 17 and E is the electric field intensity of the terahertz pulse light of the portion corresponding to the pixel in the electro-optic crystal 17, Γ is given by the following equation (2). Can be represented by Although Equation 2 is not disclosed in the above-mentioned paper, it is introduced in consideration of the spatial dependence of the electro-optic coefficient (the electro-optic coefficient may be different for each part). It is thought that introducing is more realistic. η, Γ ₀ , Α are constants specific to the electro-optic crystal 17 and are constants determined for each pixel (that is, for each part). I ₀ Is also a constant determined by the femtosecond pulse light source 1 and the like, but is determined for each pixel.
[0051]
(Equation 2)
Γ = αE
[0052]
Now, | Γ ₀ If + Γ | ≪1, Equation 1 can be rewritten as Equation 3 below. By substituting Equation 2 into Equation 3, Equation 4 is obtained.
[0053]
[Equation 3]
I = I ₀ [Η + (Γ ₀ + Γ) ² ]
[0054]
(Equation 4)
I = I ₀ [Η + (Γ ₀ + ΑE) ² ]
[0055]
As can be seen from Equation 4, each pixel signal I of the image signal from the two-dimensional CCD camera 24 includes the detection efficiency of each part of the electro-optic crystal 17, the residual birefringence, and the influence of the scattering of the probe pulse light. ing. As can be seen from Equation 4, each pixel signal I is described by a quadratic expression of the electric field intensity E of the terahertz pulse light at the relevant portion. 3 shows the relationship with the electric field intensity E of the terahertz pulse light.
[0056]
If such a relationship between each pixel signal I and the electric field intensity E of the terahertz pulse light at the portion corresponding to the pixel is obtained in advance for each pixel, the relationship is used to obtain the relationship from each pixel signal I. Η, Γ ₀ , The electric field intensity E of the true terahertz pulse light excluding the influence of α can be obtained. Here, for example, I ₀ = 1 and only the relative strength is considered. The terahertz pulse light measurement device according to the present embodiment is configured so that the above relationship can be automatically obtained by the initial operation described below and stored in the memory 27.
[0057]
Next, this initial operation will be described. Here, it is noted that the photoconductive antenna 9 is used as the terahertz light source. As described above, it is generally known that the intensity (electric field intensity and magnetic field intensity) of the electromagnetic wave (terahertz pulse light) radiated from the photoconductive antenna 9 is proportional to the bias voltage V applied to the photoconductive antenna 9. I have. Therefore, if the proportional constant is β, E = βV. Therefore, the bias voltage V applied to the photoconductive antenna 9 is changed in a state where the sample 100 is not installed at the position shown in FIG. 1 (the sample 100 may be installed, but it is preferable not to install the sample 100). However, when an image signal is obtained from the two-dimensional CCD camera 24 at each bias voltage V, the relationship between the bias voltage V and the pixel signal I for each pixel is a relationship obtained by substituting E = βV in equation (4). I is represented by a quadratic expression of the bias voltage V.
[0058]
The inventor actually measured the relationship between the bias voltage V and the pixel signal (CCD output value) I. An example of the measurement result is shown by a square point in FIG. In FIG. 3, the horizontal axis represents the bias voltage V, and the vertical axis represents the CCD output value I at a specific pixel. The solid line in FIG. 3 shows the quadratic expression of the bias voltage V fitted so as to be most suitable for the measurement point. Since E = βV as described above, the horizontal axis shown in FIG. 3 can be read as the electric field intensity E of the terahertz pulse light at the portion corresponding to the pixel, although the scale is different due to the proportionality constant β. it can. In other words, the relationship between the bias voltage V and the pixel signal I indicates the relationship between the electric field strength E of the terahertz pulse light and the pixel signal I. Normally, the relative value of the electric field intensity E does not matter, but the relative value does not matter, so that it is sufficient to set β = 1 and E = V. Since the quadratic equation obtained by the above-described fitting is equal to the quadratic equation obtained by substituting E = βV in Equation 4, an equation is established in which the coefficients of the terms of the respective orders are equal to each other, and by solving this equation, Η, の of a portion corresponding to the pixel in the electro-optic crystal 17 ₀ , Α. Note that η, Γ ₀ , Α, but not relative values, for example, β = 1, I ₀ = 1 and η, Γ ₀ , Α may be obtained. However, in the present embodiment, η, Γ ₀ , Α need not be determined.
[0059]
As described above, by changing the bias voltage V applied to the photoconductive antenna 9 and obtaining image signals from the two-dimensional CCD camera 24 at each bias voltage V, each pixel signal I and the portion corresponding to the pixel are obtained. The relationship with the electric field intensity E of the terahertz pulse light can be obtained for each pixel.
[0060]
Therefore, in the terahertz optical measurement device according to the present embodiment, the following initial operation is performed, for example, when the device is shipped or used. This initial operation is preferably performed in a state where the sample 100 is not set at the position shown in FIG. First, in order to adjust the timing at which the terahertz pulse light and the probe pulse light reach the electro-optic crystal 17, the control / arithmetic processing unit 26 controls the moving mechanism 29 to move the movable mirror 7 to the two-dimensional CCD camera 24. Is located at a position where the pixel signal of a specific pixel from the pixel becomes maximum, for example. Next, the control / arithmetic processing unit 26 sequentially gives different command values to the voltage application unit 13 to sequentially change the bias voltage applied to the photoconductive antenna 9 by the voltage application unit 13. Then, when each bias voltage is applied to the photoconductive antenna 9, the control / arithmetic processing unit 26 converts the value of each pixel signal obtained from the CCD camera 24 into the bias voltage (specifically, the bias voltage command). Value, or the detected value when a bias voltage detector is used). Thereafter, the control / arithmetic processing unit 26 describes, for each pixel, the pixel signal by a fitting equation based on the bias voltage and the value of the pixel signal obtained at that time, using a quadratic expression of the bias voltage. A quadratic equation is obtained, and the quadratic equation obtained for each pixel is stored in the memory 27. As described above, the quadratic equation of each pixel corresponds to the relationship between the electric field intensity E of the terahertz pulse light at the portion corresponding to the pixel and the pixel signal I. However, this relationship may be stored in the memory 24 not in the form of a quadratic expression but in the form of a look-up table corresponding to the quadratic expression. As described above, when the quadratic expression or the look-up table is stored in the memory, the bias voltage and the value of the pixel signal obtained at that time are finally unnecessary and are deleted from the memory 27. You may. Of course, the bias voltage and the pixel signal itself obtained at that time may be stored in the memory 27 in the form of a look-up table, for example, without performing the fitting process. However, it is preferable to perform a fitting process in order to reduce errors. As the storage format of the quadratic expression, for example, the coefficient of each order term may be stored in the memory 27, or η, Γ ₀ , Α may be obtained and stored in the memory 27.
[0061]
As described above, in the present embodiment, for each pixel, the relationship between the electric field intensity E of the terahertz pulse light at the portion corresponding to the pixel and the pixel signal I is automatically acquired and stored in the memory 27. Although it is configured, the present invention is not necessarily limited to this configuration. For example, the configuration may be such that the operator changes the bias voltage while looking at a voltmeter or the like that measures the bias voltage, and stores the value of each pixel signal obtained at each bias voltage in the memory 27. Further, if the relationship has already been obtained by another device other than the present device and the relationship is known in advance, the relationship may simply be stored in the memory 27 in advance. Is unnecessary, and the bias voltage applied by the voltage applying unit 13 does not necessarily have to be variable. Further, instead of the photoconductive antenna 9, another terahertz light generator such as an electro-optic crystal or a magneto-optic crystal is used. Is also good.
[0062]
Next, a normal operation for imaging the sample 100 performed after the above-described initial operation will be described. Of course, in this case, the sample 100 is set at the position shown in FIG.
[0063]
In the normal operation, the control / arithmetic processing unit 26 fetches the image signal from the CCD camera 24 into the memory 27, and then, for each pixel, a pixel signal and an electric field intensity E for the pixel already stored by the initial operation. The electric field intensity E corresponding to the pixel signal taken in the normal operation is obtained using the relationship At this time, if the quadratic equation is stored in the memory 27 as the relation, the electric field intensity E is obtained by substituting the pixel signal taken in during the normal operation into the quadratic equation and solving the quadratic equation. If the lookup table is stored in the memory 27 as the relationship, the electric field intensity E can be obtained by referring to the lookup table. Next, the control / arithmetic processing unit 26 reconstructs an image using the electric field strength E obtained for each pixel as a new pixel value as described above, and causes the display unit 28 to display the image. The above operation is performed in a state where the position of the movable mirror 7 and the value of the bias voltage are set to a predetermined position and a predetermined value. These operations are performed in response to a command from an input device (not shown) by an operator. Needless to say, it can be changed as appropriate.
[0064]
As described above, in the present embodiment, for each pixel, the relationship between the pixel signal I and the electric field intensity E for the pixel (η, Γ ₀ , Α, I ₀ ), The true electric field strength is obtained from the image signal corresponding to the terahertz pulse light transmitted through the corresponding portion of the sample 100, and the image based on this is displayed on the display unit 28. Therefore, according to the present embodiment, η, Γ ₀ , Α, I ₀ Is large, the effect is eliminated and a high-quality image can be obtained.
[0065]
In the above-described example, the image based on the electric field strength E is displayed on the display unit 28 as it is. However, a time-series waveform of the electric field strength E is obtained for each part, and a predetermined sequence is obtained from the time-series waveform for each part. A characteristic value (for example, a complex dielectric constant or a complex refractive index) may be calculated, and an image based on the characteristic value may be displayed on the display unit 28. At this time, regarding the acquisition of the time-series waveform of the electric field intensity E, the control / arithmetic processing unit 26 controls the moving mechanism 29 to sequentially change the position of the movable mirror 7 according to the so-called pump-probe method, An image signal is taken in from the CCD camera 24, and the electric field intensity E may be obtained for each pixel signal at each position of the movable mirror 7 using the above relationship for each pixel.
[0066]
By the way, if the electro-optic crystal 17 is an ideal one having almost no residual birefringence in each part, the terahertz optical measurement device according to the present embodiment cannot distinguish the sign of the electric field intensity E. This is given by ₀ As is clear when = 0, the image signal I reflects only the absolute value of the electric field strength E.
[0067]
Therefore, when it is necessary to distinguish the sign of the electric field strength E, it is preferable to use an electro-optic crystal 17 having a residual birefringence. If there is a residual birefringence, ₀ Since ≠ 0, the sign of the electric field intensity can be detected by positively utilizing the existence of the residual birefringence. For example, FIG. 3 described above is an example in the case where there is a residual birefringence. In this example, for example, when the bias voltage V (corresponding to the electric field intensity E) in FIG. 3 is V1 (corresponding to E1). And -V1 (corresponding to -E1).
[0068]
If the sign of the electric field intensity E cannot be distinguished, it is difficult to realize time-domain spectroscopy. In the ideal cross-polarization method, the sign of the electric field intensity E could not be distinguished, so that time-domain spectroscopy could not be realized.
[0069]
On the other hand, if a crystal having a residual birefringence is used as the electro-optic crystal 17, the sign of the electric field intensity E can be distinguished, so that time-domain spectroscopy becomes possible. If deformed, a two-dimensional spectral imaging system can be configured. That is, in this case, the control / arithmetic processing unit 26 obtains a time-series waveform of the electric field intensity E for each part without displaying the image based on the electric field intensity E on the display unit 28 as it is. At this time, regarding the acquisition of the time-series waveform of the electric field intensity E, the control / arithmetic processing unit 26 controls the moving mechanism 29 to sequentially change the position of the movable mirror 7 according to the so-called pump-probe method, An image signal may be captured from the CCD camera 24, and the electric field strength may be obtained for each pixel signal at the position of each movable mirror 7 using the above-described relationship for each pixel. At this time, by using the electro-optic crystal 17 having a residual birefringence, an electric field strength including a sign can be obtained. After that, the control / arithmetic processing unit 26 obtains a spectrum by performing a Fourier transform on the time-series waveform (time-series waveform including a sign) of the electric field intensity E for each part.
[0070]
Note that this embodiment is an example of a terahertz light measurement device that collectively measures terahertz pulsed light transmitted through each part in a two-dimensional region of the sample 100, but transmits only a predetermined part of the sample 100. When measuring terahertz pulse light, the present embodiment may be modified as follows. That is, in this case, the terahertz pulse light is focused only on a predetermined portion of the sample 100, the terahertz pulse light transmitted through the sample 100 is focused only on a predetermined portion of the electro-optic crystal 17, and the probe pulse light is Is focused only on the relevant portion of the electro-optic crystal 17, and a point sensor may be used instead of the CCD camera 24.
[0071]
[Second embodiment]
[0072]
Next, an electro-optical crystal evaluation apparatus according to a second embodiment of the present embodiment will be described.
[0073]
The evaluation device according to the present embodiment is a modification of the terahertz optical measurement device according to the first embodiment as described below. Therefore, FIG. 1 will be referred to again, and description overlapping with the first embodiment will be omitted.
[0074]
The difference between the evaluation device according to the present embodiment and the terahertz optical measurement device according to the first embodiment is that the sample 100 is not set at the position shown in FIG. 1 and that the electro-optic crystal 17 is evaluated. And the operation of the control / arithmetic processing unit 26.
[0075]
In the evaluation device according to the present embodiment, the control / arithmetic processing unit 26 allows the terahertz pulse light and the probe pulse light to reach the electro-optic crystal 17 as in the initial operation of the terahertz light measurement device according to the first embodiment. In order to adjust the timing of the movement, the moving mechanism 29 is controlled to position the movable mirror 7 at a position where the pixel signal of a specific pixel from the two-dimensional CCD camera 24 becomes maximum, for example. Next, the control / arithmetic processing unit 26 sequentially gives different command values to the voltage application unit 13 to sequentially change the bias voltage applied to the photoconductive antenna 9 by the voltage application unit 13. Then, when each bias voltage is applied to the photoconductive antenna 9, the control / arithmetic processing unit 26 converts the value of each pixel signal obtained from the CCD camera 24 into the bias voltage (specifically, the bias voltage command). Value or, when a bias voltage detector is used, the detected value may be used). Thereafter, the control / arithmetic processing unit 26 describes, for each pixel, the pixel signal by a fitting equation based on the bias voltage and the value of the pixel signal obtained at that time, using a quadratic expression of the bias voltage. A quadratic equation is obtained, and the quadratic equation obtained for each pixel is stored in the memory 27. As described above, the quadratic equation of each pixel corresponds to the relationship between the electric field intensity E of the terahertz pulse light at the portion corresponding to the pixel and the pixel signal I.
[0076]
Next, for each pixel, the control / arithmetic processing unit 26 calculates, from the quadratic equation corresponding to the pixel, the arithmetic (the quadratic equation obtained by fitting) already described in relation to the first embodiment. Is equal to the quadratic equation in which E = βV is substituted in Equation 4, so that an equation is established in which the coefficients of the terms of the respective orders are equal to each other, and an operation for solving this equation is performed, and η, Γ ₀ , Α and store these in the memory 27.
[0077]
After that, the control / arithmetic processing unit 26 generates two-dimensional data (corresponding to the distribution image of η of the electro-optic crystal 17) using η of each pixel as a pixel value, ₀ Is the two-dimensional data with the pixel value (（of the electro-optic crystal 17). ₀ ), And two-dimensional data (corresponding to the distribution image of α of the electro-optic crystal 17) using α of each pixel as a pixel value are displayed on the display unit 28 as an image.
[0078]
Therefore, according to the present embodiment, α, η, Γ ₀ Is displayed, the evaluator can look at this display and evaluate the detection efficiency of the electro-optic crystal 17, the residual birefringence, and the uniformity of the influence of the scattering of the probe pulse light. it can. Therefore, an important index for the development and research of the electro-optic crystal can be obtained.
[0079]
In the present embodiment, α, η, Γ ₀ Are displayed on the display unit 28, or the control / arithmetic processing unit 26 causes the display unit 28 to display a graph indicating the quadratic expression for each pixel, or displays the bias voltage on the photoconductive antenna 9 When the voltage is applied, the value of each pixel signal obtained from the CCD camera 24 may be directly displayed on the display unit 28 in association with the bias voltage. Even with these displays, the evaluator can evaluate the detection efficiency of the electro-optic crystal 17, the residual birefringence, the uniformity of the influence of the scattering of the probe pulse light, and the like.
[0080]
Although the present embodiment is an example of an evaluation apparatus that automatically realizes the method for evaluating the electro-optic crystal 17 according to the present invention, the evaluation method according to the present invention may be realized manually by an evaluator. In this case, for example, the operator changes the bias voltage while looking at the voltmeter or the like for measuring the bias voltage, knows the value of each pixel signal obtained at each bias voltage, and sets the bias voltage and the From the relationship with the value of the pixel signal, the detection efficiency of the electro-optic crystal 17, the residual birefringence, and the uniformity of the influence of the scattering of the probe pulse light may be evaluated.
[0081]
Note that the present embodiment is an example of an apparatus for evaluating the detection efficiency and the like of each part in the two-dimensional region of the electro-optic crystal 17. When performing an evaluation, the present embodiment may be modified as follows. That is, in this case, the terahertz pulse light is focused only on the relevant portion of the electro-optic crystal 17, and the probe pulse light is focused only on the relevant portion of the electro-optic crystal 17. May be used.
[0082]
The embodiments of the present invention and the modifications thereof have been described above, but the present invention is not limited to these embodiments and the modifications.
[0083]
For example, in each of the above-described embodiments, the electric field strength of the generated terahertz pulse light is changed by changing the bias voltage applied to the photoconductive antenna 9, but the generated terahertz pulse is changed by changing other parameters. The electric field intensity of light may be changed.
[0084]
Further, in each of the above-described embodiments and the modifications thereof, a magneto-optic crystal may be used instead of the electro-optic crystal 17. That is, a magneto-optical crystal may be used as the detection crystal in the first embodiment, and a magneto-optical crystal may be used as the crystal to be evaluated in the second embodiment. In this case, in the above description, if the electro-optic is read as the magneto-optic and the electric field is read as the magnetic field, the above description is directly applicable. Therefore, here, description of the case where a magneto-optical crystal is used instead of the electro-optical crystal will be omitted.
[0085]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a method and an apparatus for evaluating an electro-optical crystal or a magneto-optical crystal that can appropriately evaluate an electro-optical crystal or a magneto-optical crystal.
[0086]
Further, according to the present invention, the terahertz pulse light can be measured using the electro-optic crystal or the magneto-optic crystal, and the terahertz light can be accurately measured without performing the balance detection. A measurement method and device can be provided.
[0087]
Further, according to the present invention, there is provided a terahertz light measurement device capable of obtaining a high-quality image of a target object even when the spatial uniformity of an electro-optic crystal or a magneto-optic crystal used for terahertz light detection is low. Can be.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram schematically showing a terahertz optical measurement device according to a first embodiment of the present invention.
FIG. 2 is a schematic perspective view schematically showing an example of a photoconductive antenna used in the terahertz optical measurement device shown in FIG.
FIG. 3 is a diagram illustrating an example of a measurement result indicating a relationship between a CCD output value and a bias voltage in a specific pixel.
[Explanation of symbols]
1 Femtosecond pulse light source
4,21 beam expander
9 Photoconductive antenna
13 Voltage application part
17 Electro-optic crystal
22 Polarizer
23 Analyzer
24 2D CCD camera (photoelectric conversion unit)
26 Control / arithmetic processing unit
27 memory
28 Display
100 samples

Claims

Using a terahertz light generation unit that can change the intensity of the electric field or magnetic field of the generated terahertz pulse light, while guiding the terahertz pulse light emitted from the terahertz light generation unit to an electro-optical crystal or a magneto-optical crystal that is a crystal to be evaluated, Guide the probe pulse light synchronized and polarized with the terahertz pulse light to the crystal to be evaluated,
The probe pulse light whose polarization state has been changed by the terahertz pulse light passing through the crystal to be evaluated is analyzed,
The probe pulse light after the analysis is photoelectrically converted,
By changing the intensity of the electric field or magnetic field of the terahertz pulse light, obtain the relationship between the intensity of the electric field or magnetic field of the terahertz pulse light and the intensity of the signal obtained by the photoelectric conversion,
Based on the relationship, evaluating at least one of the detection efficiency of the crystal to be evaluated, the residual birefringence of the crystal to be evaluated, and the influence of the scattering of the probe pulse light in the crystal to be evaluated. A method for evaluating an electro-optical crystal or a magneto-optical crystal, comprising:

The terahertz light generation unit includes: (a) a photoconductive portion; and two conductive portions formed on a predetermined surface of the photoconductive portion and separated from each other, and at least a part of the two conductive portions is A terahertz light generating element arranged so as to have a predetermined interval in a direction along the predetermined surface, and (b) an excitation pulse light irradiating unit that irradiates a predetermined portion of the terahertz light generating element with excitation pulse light; (C) a voltage application unit that applies a variable bias voltage between the two conductive units,
By changing the bias voltage to change the intensity of the electric or magnetic field of the terahertz pulse light,
2. The method for evaluating an electro-optic crystal or a magneto-optic crystal according to claim 1, wherein a relationship between the bias voltage and the intensity of a signal obtained by the photoelectric conversion is obtained as the relationship.

Using a terahertz light generator capable of changing the intensity of an electric field or a magnetic field of the generated terahertz pulse light, the terahertz pulse light emitted from the terahertz light generator is converted into a two-dimensional region of an electro-optical crystal or a magneto-optical crystal to be evaluated. And guiding the probe pulse light synchronized and polarized with the terahertz pulse light to the two-dimensional region of the crystal to be evaluated,
The probe pulse light whose polarization state has been changed by the terahertz pulse light passing through the crystal to be evaluated is analyzed,
The probe pulse light after the analysis is photoelectrically converted for each corresponding to each part of the two-dimensional region of the crystal to be evaluated,
By changing the intensity of the electric field or the magnetic field of the terahertz pulse light, the relationship between the intensity of the electric field or the magnetic field of the terahertz pulse light and the intensity of the signal obtained by the photoelectric conversion, the two-dimensional region of the crystal to be evaluated Obtain for each one corresponding to each part,
Based on the relationship, the detection efficiency of each part of the crystal to be evaluated, the residual birefringence of each part of the crystal to be evaluated, and the scattering of the probe pulse light at each part of the crystal to be evaluated. A method for evaluating an electro-optical crystal or a magneto-optical crystal, wherein at least one of the effects is evaluated.

The terahertz light generation unit includes: (a) a photoconductive portion; and two conductive portions formed on a predetermined surface of the photoconductive portion and separated from each other, and at least a part of the two conductive portions is A terahertz light generating element arranged so as to have a predetermined interval in a direction along the predetermined surface, and (b) an excitation pulse light irradiating unit that irradiates a predetermined portion of the terahertz light generating element with excitation pulse light; (C) a voltage application unit that applies a variable bias voltage between the two conductive units,
By changing the bias voltage to change the intensity of the electric or magnetic field of the terahertz pulse light,
4. The relation between the bias voltage and the intensity of a signal obtained by the photoelectric conversion is obtained for each of the parts corresponding to each part of the two-dimensional region of the crystal to be evaluated, as the relation. The method for evaluating an electro-optical crystal or a magneto-optical crystal according to the above.

A terahertz that has a terahertz light generator that can change the intensity of an electric field or a magnetic field of the generated terahertz pulse light, and irradiates the terahertz pulse light emitted from the terahertz light generator to an electro-optic crystal or a magneto-optic crystal to be evaluated. A pulse light irradiation unit,
Probe pulse light irradiation unit that irradiates the crystal to be evaluated with a probe pulse light synchronized and polarized with the terahertz pulse light,
An analyzer for analyzing the probe pulse light having a polarization state changed by the terahertz pulse light passing through the evaluation target crystal,
A photoelectric conversion unit that photoelectrically converts the probe pulse light after the light detection by the light detection unit,
A storage unit that stores the relationship between the intensity of the electric field or magnetic field of the terahertz pulse light and the intensity of the output signal of the photoelectric conversion unit, which is obtained by changing the intensity of the electric field or the magnetic field of the terahertz pulse light,
Based on the relationship, a value indicating the detection efficiency of the crystal to be evaluated, a value indicating the residual birefringence of the crystal to be evaluated, and a value indicating the influence of scattering of the probe pulse light in the crystal to be evaluated. An operation unit for obtaining at least one of
An evaluation device for an electro-optic crystal or a magneto-optic crystal, comprising:

The terahertz light generation unit includes: (a) a photoconductive portion; and two conductive portions formed on a predetermined surface of the photoconductive portion and separated from each other, and at least a part of the two conductive portions is A terahertz light generating element arranged so as to have a predetermined interval in a direction along the predetermined surface, and (b) an excitation pulse light irradiating unit that irradiates a predetermined portion of the terahertz light generating element with excitation pulse light; (C) a voltage application unit that applies a variable bias voltage between the two conductive units,
The storage unit according to claim 5, wherein the storage unit stores, as the relationship, a relationship between the bias voltage and an intensity of an output signal of the photoelectric conversion unit, which is obtained by changing the bias voltage. Evaluation device for electro-optic crystal or magneto-optic crystal.

A two-dimensional region of an electro-optic crystal or a magneto-optic crystal, which has a terahertz light generator capable of changing the intensity of an electric field or a magnetic field of the generated terahertz pulse light, and converts the terahertz pulse light emitted from the terahertz light generator into a crystal to be evaluated. A terahertz pulse light irradiation unit for irradiating the
A probe pulse light irradiation unit that irradiates the probe pulse light synchronized and polarized with the terahertz pulse light to the two-dimensional region of the crystal to be evaluated,
An analyzer for analyzing the probe pulse light having a polarization state changed by the terahertz pulse light passing through the evaluation target crystal,
A photoelectric conversion unit that performs photoelectric conversion on the probe pulse light after the light analysis by the light analysis unit for each of the portions of the two-dimensional region of the crystal to be evaluated, which corresponds to each site.
The relationship between the intensity of the electric field or magnetic field of the terahertz pulse light and the intensity of the output signal of the photoelectric conversion unit, which is obtained by changing the intensity of the electric field or the magnetic field of the terahertz pulse light, is expressed by the two-dimensional A storage unit for storing for each one corresponding to each part of the region,
Based on the relationship, a value indicating the detection efficiency of each part of the crystal to be evaluated, a value indicating a residual birefringence of each part of the crystal to be evaluated, and the value of each part of the crystal to be evaluated. An arithmetic unit for obtaining at least one of values indicating the influence of scattering of the probe pulse light;
An evaluation device for an electro-optic crystal or a magneto-optic crystal, comprising:

The terahertz light generation unit includes: (a) a photoconductive portion; and two conductive portions formed on a predetermined surface of the photoconductive portion and separated from each other, and at least a part of the two conductive portions is A terahertz light generating element arranged so as to have a predetermined interval in a direction along the predetermined surface, and (b) an excitation pulse light irradiating unit that irradiates a predetermined portion of the terahertz light generating element with excitation pulse light; (C) a voltage application unit that applies a variable bias voltage between the two conductive units,
The storage unit stores the relationship between the bias voltage and the intensity of the output signal of the photoelectric conversion unit, which is obtained by changing the bias voltage, as the relationship, in each part of the two-dimensional region of the crystal to be evaluated. 8. The apparatus for evaluating an electro-optic crystal or a magneto-optic crystal according to claim 7, wherein each of the corresponding ones is stored.

Along with guiding the terahertz pulse light to the detection crystal that is an electro-optic crystal or a magneto-optic crystal, the probe pulse light synchronized with the terahertz pulse light and polarized is guided to the detection crystal,
The probe pulse light whose polarization state has been changed by the terahertz pulse light passing through the detection crystal is analyzed,
The probe pulse light after the analysis is photoelectrically converted,
A previously obtained relationship between the intensity of the electric or magnetic field of the terahertz pulse light and the intensity of the signal obtained by the photoelectric conversion, the detection efficiency of the detection crystal, the residual birefringence of the detection crystal, and Obtaining the intensity of the electric field or the magnetic field of the terahertz pulse light according to the intensity of the signal obtained by the photoelectric conversion using a relationship reflecting the influence of the scattering of the probe pulse light on the detection crystal. Terahertz light measurement method.

10. The terahertz light measurement method according to claim 9, wherein the terahertz pulse light guided to the detection crystal is transmitted or reflected by a target object.

The terahertz light measurement method according to claim 9, wherein the detection crystal has a residual birefringence.

A terahertz pulse light irradiation unit that irradiates the target object with terahertz pulse light,
A detection crystal that is an electro-optic crystal or a magneto-optic crystal that receives the terahertz pulse light transmitted or reflected by the target object,
A probe pulse light irradiator that irradiates the detection crystal with a probe pulse light that is synchronized and polarized with the terahertz pulse light,
An analyzer for analyzing the probe pulse light having a polarization state changed by the terahertz pulse light passing through the detection crystal,
The probe pulse light after the light detection by the light detection unit, a photoelectric conversion unit that performs photoelectric conversion,
A previously obtained relationship between the intensity of the electric or magnetic field of the terahertz pulse light and the intensity of the signal obtained by the photoelectric conversion, the detection efficiency of the detection crystal, the residual birefringence of the detection crystal, and A storage unit that stores a relationship reflecting the influence of the scattering of the probe pulse light in the detection crystal,
A terahertz light measurement device, comprising: a processing unit that obtains an electric field or a magnetic field intensity of the terahertz pulse light according to the intensity of the signal obtained by the photoelectric conversion based on the relationship.

13. The terahertz light measurement device according to claim 12, wherein the detection crystal has a residual birefringence.

A terahertz pulse light irradiation unit that collectively irradiates the two-dimensional region of the target object with the terahertz pulse light,
A detection crystal that is an electro-optic crystal or a magneto-optic crystal that collectively receives the terahertz pulse light transmitted or reflected by the two-dimensional region of the target object in a two-dimensional region corresponding to the two-dimensional region of the target object,
A probe pulse light irradiation unit that collectively irradiates the two-dimensional region of the detection crystal with a probe pulse light that is synchronized and polarized with the terahertz pulse light,
An analyzer for analyzing the probe pulse light having a polarization state changed by the terahertz pulse light passing through the detection crystal,
The probe pulse light after the light detection by the light detection unit, for each one corresponding to each part of the two-dimensional region of the detection crystal, a photoelectric conversion unit for photoelectric conversion,
A previously obtained relationship between the intensity of the electric or magnetic field of the terahertz pulse light and the intensity of the signal obtained by the photoelectric conversion, the detection efficiency of the detection crystal, the residual birefringence of the detection crystal, and A storage unit that stores a relationship reflecting the influence of the scattering of the probe pulse light in the detection crystal for each of the portions of the two-dimensional region of the detection crystal,
Based on the relationship, an electric field or magnetic field intensity of the terahertz pulse light corresponding to the intensity of the signal obtained by the photoelectric conversion is obtained for each of the portions of the two-dimensional region of the detection crystal corresponding to each portion. A terahertz optical measurement device comprising: a processing unit.

The terahertz light measurement device according to claim 14, wherein the detection crystal has a residual birefringence in each of the portions.