CN109406454B - An Improved Z Scanning Device - Google Patents
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- CN109406454B CN109406454B CN201811057856.7A CN201811057856A CN109406454B CN 109406454 B CN109406454 B CN 109406454B CN 201811057856 A CN201811057856 A CN 201811057856A CN 109406454 B CN109406454 B CN 109406454B
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Abstract
本发明公开了一种改进的Z扫描装置。利用本发明装置对不同的待测样品进行预扫描可以自动确定对应待测样品最优的测量光强。在此最优测量光强下可以有效的避免：(1)因测量光强过强导致的高阶非线性光学效应的影响；(2)因光测量强过弱导致的噪声信号相对过大带来的影响，从而使得测量结果准确可靠。在装置中设置多个反射镜及小孔光阑，对入射光束进行整形。在装置中添加了相应的器件来监测测量过程中所遇到的激光器能量波动、激光器锁模不稳、样品击穿等所产生的影响，添加了相应的器件来避免测量过程中所遇到的热效应和偏振态的影响。在此基础上本装置还可以实现入射光源可更换、偏振态可调节、闭孔数据和开孔数据同时测量等功能。
The invention discloses an improved Z scanning device. Using the device of the present invention to perform pre-scanning on different samples to be tested can automatically determine the optimal measuring light intensity corresponding to the samples to be tested. Under this optimal measurement light intensity, it can be effectively avoided: (1) the influence of high-order nonlinear optical effects caused by the measurement light intensity is too strong; (2) the noise signal caused by the light measurement intensity is too weak relatively large band Therefore, the measurement results are accurate and reliable. A plurality of mirrors and aperture diaphragms are arranged in the device to shape the incident beam. Corresponding devices are added to the device to monitor the effects of laser energy fluctuations, unstable laser mode locking, sample breakdown, etc. encountered during the measurement process, and corresponding devices are added to avoid the problems encountered during the measurement process. Thermal effects and effects of polarization states. On this basis, the device can also realize the functions of replaceable incident light source, adjustable polarization state, simultaneous measurement of closed-hole data and open-hole data, and the like.
Description
Technical Field
The invention relates to an improved Z scanning device, belonging to the field of nonlinear optics and optical detection.
Background
The three-order optical nonlinearity of the material enables the refractive index and the absorption coefficient not to be in linear proportion with the light intensity any more, and by utilizing the special property, the application of two-photon spectroscopy, high-resolution fluorescence microscopy, photodynamic therapy, up-conversion laser, micro-nano manufacturing, three-dimensional optical data storage, optical amplitude limiting and the like can be realized. Has been developed in the last decades. The development of this field has been greatly driven by the emergence of new technologies and new applications based on nonlinear optics, particularly in recent years. The nonlinear effect of the material is utilized, and the measurement of the nonlinear optical parameter is very critical. The traditional optical nonlinear measuring method comprises the following steps: nonlinear interference, degenerate four-wave mixing, near-degenerate three-wave mixing, ellipsometry, and beam distortion measurement, among others. The first four methods have high measurement sensitivity, but cannot measure the sign of the nonlinear refractive index, and the measurement device is very complex and difficult to implement.
In 1990, the single-beam Z-scan technique first proposed by M.Sheik-Bahae et al had the advantages of simple device, high measurement sensitivity, etc., and could detect the wavefront distortion of lambda/300, and this method could measure the magnitude and sign of the third-order nonlinear refractive index and the nonlinear absorption coefficient at the same time (see SHEIK-BAHAE, M.; SAID, A.A.; WEI, T.H.; HAGAN, D.J.; STRYLAND, E.W.V., Sensitive measurement of optical nonlinear indexes using a single beam of IEEE J.Quantum Electron.1990,26(4), 760-769.). Based on this, J.Wang et al have proposed a Time-resolved two-tone Z-scan technique that can measure non-degenerate nonlinear absorption and non-degenerate nonlinear refractive index at different Time delays (see Wang, J.; Sheik-Bahae, M.; Said, A.A.; Hagan, D.J.; Stryland, E.W.V., Time-resolved Z-scan measurements of optical nonlinear optics. J.Opt.Soc.Am.B 1994,11(6), 1009) 1017.). Since many researchers have proposed many improved methods, the Z-scan technique has been developed as a widely used experimental method with important practical application value in the study of nonlinear optical properties of materials.
The Z-scan technique is so named because it requires the sample to be measured to move along the optical axis of the light beam transmission during the measurement process. The Z scanning technology is based on the principle of spatial beam distortion, and measures the third-order nonlinear refractive index and the third-order nonlinear absorption coefficient, namely chi, of a medium through closed-hole (porous) and open-hole (non-porous) Z scanning experiments(3)Real and imaginary parts of (c). The experimental setup of the method is schematically shown in FIG. 1. In recent years, Z-scan technology has been continuously improved and developed, and many new and improved methods have emerged. But the basic principles are largely the same.
In a closed-cell Z-scan experiment, the excitation light is focused by a lens, the sample is scanned in the Z direction and passes through the focal point of the lens, which causes convergence or divergence of the light beam due to the nonlinear action of the medium, thereby causing a change in the intensity of the light passing through the diaphragm. The change in the beam due to the non-linear effect is schematically shown in fig. 2. In connection with the schematic diagram of the Z-scan shown in fig. 1: a Gaussian laser beam is focused by a convergent lens, passes through a sample to a far field and reaches a detector D through a small aperture diaphragm2The nonlinear medium sample to be measured is placed near the focal point. Lens Forward A Beam splitter BS (Beam splitterer)，D1Measuring the variation of the input light, D2Measuring the transmitted light intensity, D, after passing through the aperture2/D1Defined as Z-scan normalized transmittance. The movement of the sample in the direction of propagation of the light (Z direction) near the focal point causes the beam to diverge or converge due to the nonlinear action of the medium, thus normalizing the transmittance D2/D1There will be a one-to-one correspondence with the sample position (Z). The nonlinear refractive index of the medium can be obtained by fitting a relation curve of the normalized transmittance and the coordinate Z, namely a Z-scan curve.
The normalized transmittance t (z) is related to the material properties and is also closely related to many experimental parameters, such as the non-linear refractive index γ, the non-linear absorption coefficient β, the beam intensity distribution, the beam time characteristic, the light wave frequency ω, the Aperture Size S (or called Aperture Size,
wherein r isaIs the radius of the diaphragm, waRadius of beam waist at stop when sample is in linear region far from focus), focal length f of lens, confocal length z of beam0Girdling radius w0Optical power density I0Sample thickness L, etc. It is difficult to obtain an analytical expression of t (z) with the above parameters without some assumptions and approximations in theoretical calculations. This hardly allows the non-linear refractive index γ and the non-linear absorption coefficient β to be solved. In general, it is required that the incident laser beam should have a Gaussian distribution, a low incident light intensity, and the like (see SHEIK-BAHAE, M.; SAID, A.A.; WEI, T.H.; HAGAN, D.J.; STRYLAND, E.W.V., Sensitive measurement of optical nonlinearities using a single beam, IEEE J.Quantum Electron.1990,26(4),760- "769.). Only when the incident light intensity is small and only three-order optical nonlinear effects occur in the experiment, the nonlinear coefficient measured by the experiment is the three-order nonlinear coefficient of the medium; if the incident light intensity is too high, other non-linear effects will occur, leading to inaccuracies in the resulting values. However, how to reasonably determine the incident light intensity and satisfy the condition of smaller incident light intensity is not mentioned in the prior art at present. And for measuringThe corresponding reasonable incident light intensities are different for different samples. How to automatically and rapidly determine the optimal incident light intensity is not mentioned in the prior art. Meanwhile, the Z scanning technology is only limited to measuring the energy change condition of the sample, and can not accurately distinguish whether only three-order nonlinear refraction and three-order nonlinear absorption processes occur in the sample.
Disclosure of Invention
In response to the deficiencies of the prior art, the present invention provides an improved Z-scan apparatus. The device can automatically determine the corresponding optimal incident light intensity for different samples to be measured through the pre-scanning process so as to reliably and accurately measure the samples. A plurality of reflectors and small aperture diaphragms are arranged in the device to shape incident beams, so that the beam quality of the incident beams is ensured. Meanwhile, an optical multichannel analyzer probe is arranged near a sample to be measured, so that the up-conversion fluorescence measurement of the luminescent material can be realized, and the two-photon absorption cross section of the material can be determined by a two-photon fluorescence method. The accuracy and reliability of the experimental result can be mutually verified with the Z scanning result. Meanwhile, corresponding devices are added in the measuring device to monitor the influences of laser energy fluctuation, unstable laser mode locking, sample breakdown and the like in the measuring process, and the corresponding devices are added to avoid the influences of thermal effect and polarization state in the measuring process. On the basis, the device can also realize the functions of replaceable incident light source, adjustable polarization state, simultaneous measurement of closed hole data and open hole data and the like.
The technical solution of the invention is as follows:
an improved Z-scan apparatus comprising: the laser comprises a laser for outputting laser wavelength lambda, wherein a first small hole diaphragm, a first reflector, a second reflector, a third reflector, a fourth reflector, a fifth reflector, a sixth reflector, a seventh reflector, an eighth reflector, a ninth reflector, a tenth reflector, a eleventh reflector, a tenth reflector, a second small hole diaphragm, a chopper, a first polarizer, a second polarizer, a first laser spectroscope, a second laser spectroscope, an electric control laser attenuation sheet, a first convergent lens, a sample to be detected, a third laser spectroscope, a third small hole diaphragm, a second lens, a first adjustable attenuation sheet and a photomultiplier tube are sequentially arranged along a main optical axis formed by a main optical path of laser output of the laser; the photomultiplier is connected with a computer; the first laser spectroscope and the main optical axis form a 45-degree angle, a first photoelectric detector is arranged in the output direction of the reflected light of the first spectroscope, the first photoelectric detector is connected with an oscilloscope, and the oscilloscope is connected with a computer; the second laser spectroscope and the main optical axis form a 45-degree angle, a second photoelectric detector is arranged in the reflected light output direction of the second spectroscope, and the second photoelectric detector is connected with a computer; the electrically controlled laser attenuation sheet is connected with an attenuation sheet controller, and the attenuation sheet controller is connected with a computer; arranging an optical multi-channel analyzer probe near a sample to be detected, wherein the optical multi-channel analyzer probe is connected with an optical multi-channel analyzer, and the optical multi-channel analyzer is connected with a computer; the sample to be tested is placed on an electric platform, and the electric platform is connected with a computer; the third laser spectroscope and the main optical axis form a 45-degree angle, a second adjustable attenuation sheet, a third converging lens and a third photoelectric detector are sequentially arranged in the reflected light output direction of the third spectroscope, and the third photoelectric detector is connected with a computer;
the diameters of the first small aperture diaphragm and the second small aperture diaphragm are both 5mm, and the diameter of the third aperture diaphragm is 30 mm; the adjustable range of the chopper frequency is 4Hz to 10 KHz; the polarization direction of the second polaroid is horizontal; the first laser spectroscope, the second laser spectroscope and the third laser spectroscope are spectroscopes with the transmittance of 90% and the reflectivity of 10% for laser with the wavelength lambda; the focal length of the first convergent lens is 15cm, and the focal lengths of the second convergent lens and the third convergent lens are both 10 cm. The thickness of the sample to be detected is 1 mm.
The improved Z scanning device for measuring the third-order nonlinear absorption coefficient and the third-order nonlinear refractive index comprises the following steps:
firstly, system initialization:
firstly, according to actual measurement needs, selecting a proper laser, adjusting a first polaroid according to parameters (such as output power, pulse frequency and the like) of the laser to control the energy range of the sample to be measured, and adjusting a first adjustable attenuation sheet and a second adjustable attenuation sheet simultaneously to ensure that the sample to be measured, a photomultiplier and a third photoelectric detector are not damaged under the condition that the transmittance of an electric control laser attenuation sheet is 100%. The sampling frequency, the number of sampling points and the laser of the photomultiplier, the second photoelectric detector, the electric platform and the third photoelectric detector are set by a computer to synchronously work. And setting the spectrum acquisition range of the optical multi-channel analyzer through a computer according to the actual sample luminescence wavelength.
Two, prescan
And secondly, placing the sample to be measured on an electric platform, adjusting the measuring surface of the sample to be measured to be vertical to the main optical axis, namely the z axis, and setting the focus of the first converging lens to be z equal to 0. The computer simultaneously starts the electric platform, the attenuation sheet controller and the third photoelectric detector, the electric platform is used for changing the position of a sample, the forward direction of the laser is positive, the reverse direction of the laser is negative, the attenuation sheet controller continuously changes the transmittance of the electric control laser attenuation sheet, and a light intensity signal output by the third photoelectric detector is transmission closed hole data. The computer controls the attenuation sheet controller to continuously change the transmittance of the electric control laser attenuation sheet at the initial position where z is 0, the transmittance is continuously changed from 0% to 100%, and meanwhile, the computer synchronously acquires a light intensity signal curve I output by the third photoelectric detector1(x) Wherein x is 0,1,2,3 … …,100, corresponding to a transmittance of 0%, 1%, 2% · 100%, respectively; the computer controls the electric platform to move the sample to be detected to a negative endpoint of-10 z0Where, define z0＝πω0 2λ is the confocal length of the beam, where λ is the wavelength of the incident laser, ω 02 λ f/pi d is the laser beam waist radius, f is the focal length of the first converging lens, and d is the spot diameter at the first converging lens. The computer controls the attenuation sheet controller to control the electric control laserThe attenuation sheet continuously changes the transmittance from 0% to 100%, and the computer synchronously acquires the light intensity signal curve I output by the third photodetector2(x) Wherein x is 0,1,2,3 … …,100, corresponding to a transmittance of 0%, 1%, 2% · 100%, respectively; let T1(x)＝I1(x)/I2(x) Where x is 0,1,2,3 … …,100, finds T1(x) Point x corresponding to 0.95 ═ x0The transmittance of the electrically controlled laser attenuation sheet is adjusted to x by the computer-controlled attenuation sheet controller0Percent, the light intensity incident on the sample to be measured at the moment is the optimal incident light intensity of the sample.
Measuring data of open pores and closed pores
And thirdly, the sample to be measured is placed on an electric platform, the measuring surface of the sample to be measured is adjusted to be vertical to the main optical axis, namely the z axis, and the focus of the first convergent lens is z 0. The computer controls to start the photomultiplier, the electric platform and the third photoelectric detector at the same time, and the sample to be detected is from a negative end point of-10 z0Moves forward along the main optical axis, passes through the focal point of the first converging lens (z is 0), and has a motion range of 20z0. The photomultiplier and the third photodetector transmit the detected light intensity signal to a computer. The collected output light intensity signals of the photomultiplier and the third photodetector are respectively closed hole data and open hole data. The collected light intensity value is taken as the ordinate, z is the abscissa, and the closed pore curve I is recordedca(zn) And opening curve Ioa(zn) Wherein N is 1,2,3 … …, N, znIs the abscissa, z, of each sample point1～znIs corresponding to-10 z0～+10z0The abscissa value at the focus is znAnd N is the number of sampling points as 0.
Fourth, monitor the part
Fourthly, the laser pulse waveform emitted by the first photoelectric detector is input into an oscilloscope, and the oscilloscope feeds back the state information to the computer; whether the mode locking of the pulse laser emitted by the laser is good or not can be monitored according to the signal presented by the oscilloscope, so that whether the laser emitted by the laser meets the experimental conditions or not is judged, and if the mode locking is abnormal, the Z scanning experimental data is judged to be unreliable; and the second photoelectric detector transmits the detected light intensity signal to a computer in real time, in the Z scanning process, the output light intensity signal of the second photoelectric detector is collected as a power monitoring curve of the incident laser, and if the fluctuation range of the curve exceeds a set threshold value, the Z scanning experimental data is judged to be unreliable. If the sample to be detected is a luminescent material, the two-photon fluorescence spectrum can be monitored simultaneously in the Z scanning process, the optical multichannel analyzer transmits detected spectrum data to a computer, the acquired spectrum data has the abscissa as the wavelength and the ordinate as the light intensity, and if the two-photon fluorescence spectrum has obvious spectrum type change in the Z scanning process, particularly when the sample is scanned to a position near a position where Z is equal to 0, the Z scanning experimental data is judged to be unreliable; and if the abnormity occurs in the monitoring process, the system automatically abandons the Z scanning data and restarts a new Z scanning experiment.
Fifthly, data processing
Determination of test parameters: incident light power P of the sample to be measuredsThe optical power P measured by the second photodetector2Conversion is carried out: ps＝P2*9*x0% of the total weight of the composition. For closed pore curve Ica(zn) And opening curve Ioa(zn) (where N is 1,2,3 … …, N) is normalized. Dividing the ordinate values in the two curves by the corresponding z values1The longitudinal coordinate value of the position is respectively used for obtaining the normalized closed hole transmission curve T of the sampleca(zn) And Toa(zn) Where N is 1,2,3 … …, N. Take n0Is an integer part of N/2, let Tca(n0) And Toa(n0) The corresponding abscissa is 0, i.e. at the focus.
The normalized transmittance t (z) is related to the material properties and is also closely related to many experimental parameters, such as the non-linear refractive index γ, the non-linear absorption coefficient β, the beam intensity distribution, the beam time characteristic, the light wave frequency ω, the Aperture Size S (or called Aperture Size,
wherein r isaIs the radius of the diaphragm, waRadius of beam waist at stop when sample is in linear region far from focus), focal length f of lens, confocal length z of beam0Girdling radius w0Optical power density I0Sample thickness L, etc. Normalization of transmittance T under the condition of satisfying reasonable approximationca(zn) This can be written as follows:
wherein x is zn/z0According to a confocal length z0Normalized dimensionless position parameter, Δ Φ ═ k γ I0LeffFor phase shifts induced by non-linear refraction, k is 2 pi n0λ is the vector of light waves, γ is the non-linear refractive index, I0Is the incident light intensity, Leff＝[1-exp(-α0L)]/α0Is the effective length of the sample, alpha0Is the linear absorption coefficient of the material. Only one γ in the formula (1) is an unknown quantity, and the value of γ can be obtained by fitting.
Transmission curve T of open pore by normalization under open pore conditionoa(zn) Taking a focal point z 00 open pore transmittance value Toa(0) And substituting the following formula to calculate the nonlinear absorption coefficient beta of the sample to be detected:
in the above formula, Leff＝[1-exp(-α0L)]/α0Is the effective length of the sample, alpha0Is the linear absorption coefficient of the material, L is the sample thickness, I0Is the incident light intensity.
As an optimization scheme, the third step, the fourth step and the fifth step can be programmed in a computer to realize one-key control, so that the automation of the measurement process is realized.
The invention has the technical effects that:
1. the invention adopts a pre-scanning mode, and can automatically determine the corresponding optimal incident light intensity for different samples to be measured so as to reliably and accurately measure the samples.
2. The invention adopts a plurality of reflectors and small-hole diaphragms which are combined according to a specific sequence and specific parameters to shape the incident beam, thereby ensuring the beam quality of the incident beam.
3. According to the invention, an optical multichannel analyzer probe is arranged near a sample to be detected, so that the up-conversion fluorescence measurement of the luminescent material can be realized, and whether sample breakdown and other abnormal phenomena occur in the Z scanning process can be monitored.
4. The measuring device is added with corresponding devices (a second photoelectric detector, an oscilloscope and an optical multichannel analyzer) to monitor the influences of laser energy fluctuation, unstable laser mode locking, sample breakdown and the like in the measuring process, so that the reliability and the accuracy of measurement are improved.
5. The invention adds corresponding devices (chopper, second polarizer) to avoid the effects of thermal effects and polarization states encountered during the measurement. The reliability and accuracy of the measurement are improved.
6. The invention can also realize the functions of replaceable incident light source, adjustable polarization state, simultaneous measurement of closed hole data and open hole data and the like.
Drawings
FIG. 1 is a schematic diagram of a prior art Z-scan experimental setup;
FIG. 2 is a schematic diagram of a conventional sample showing a self-focusing phenomenon occurring before and after a lens focus;
FIG. 3 is an optical path block diagram of an improved Z-scan apparatus of the present invention;
FIG. 4 is a graph of normalized closed cell transmission curve experimental data and theoretical fit versus Z-scan position for an improved Z-scan apparatus of the present invention;
FIG. 5 is a graph of normalized aperture transmission curve experimental data and theoretical fit versus Z-scan position for an improved Z-scan apparatus of the present invention.
Detailed Description
The present invention will be further described with reference to the following examples and drawings, but the scope of the present invention should not be limited thereto.
As shown in fig. 3, fig. 3 is a light path structure diagram of an embodiment of the improved Z scanning device implemented by the present invention, and it can be seen from the diagram that an improved Z scanning device proposed by the present invention includes a laser 1 outputting a laser wavelength λ, and a first aperture diaphragm 2, a first reflector 3, a second reflector 4, a third reflector 5, a fourth reflector 6, a fifth reflector 7, a sixth reflector 8, a seventh reflector 9, an eighth reflector 10, a ninth reflector 11, a tenth reflector 12, an eleventh reflector 13, a tenth reflector 14, a second aperture diaphragm 15, a chopper 16, a first polarizer 17, a second polarizer 18, a first laser spectroscope 19, a second laser spectroscope 20, an electrically controlled laser attenuator 21, a first converging lens 22, a sample to be measured 23, and a second reflector 3, a second reflector 4, a third reflector 5, a fourth reflector 6, a fifth reflector 7, a sixth reflector 8, a seventh reflector 9, a eighth reflector 10, a ninth reflector 10, a tenth reflector 11, a tenth reflector 12, A third laser spectroscope 24, a third aperture diaphragm 25, a second converging lens 26, a first adjustable attenuation sheet 27 and a photomultiplier 28, wherein the photomultiplier 28 is connected with a computer 38; the angle between the first laser spectroscope 19 and the main optical axis is 45 degrees, a first photoelectric detector 29 is arranged in the output direction of the reflected light of the first spectroscope 19, the first photoelectric detector 29 is connected with an oscilloscope 39, and the oscilloscope 39 is connected with a computer 38; the second laser spectroscope 20 and the main optical axis form an angle of 45 degrees, a second photoelectric detector 30 is arranged in the output direction of the reflected light of the second spectroscope 20, and the second photoelectric detector 30 is connected with a computer 38; the electrically controlled laser attenuation sheet 21 is connected with an attenuation sheet controller 31, and the attenuation sheet controller 31 is connected with a computer 38; arranging an optical multi-channel analyzer probe 32 near a sample to be measured, wherein the optical multi-channel analyzer probe 32 is connected with an optical multi-channel analyzer 33, and the optical multi-channel analyzer 33 is connected with a computer 38; the sample 23 to be tested is placed on an electric platform 34, and the electric platform is connected with a computer 38; the third laser spectroscope 24 and the main optical axis form a 45-degree angle, a second adjustable attenuation sheet 35, a third converging lens 36 and a third photoelectric detector 37 are sequentially arranged in the output direction of the reflected light of the third spectroscope 24, and the third photoelectric detector 37 is connected with a computer 38;
the diameters of the first small aperture diaphragm 2 and the second small aperture diaphragm 15 are both 5mm, and the diameter of the third aperture diaphragm 25 is 30 mm; the adjustable range of the frequency of the chopper 16 is 4Hz to 10 KHz; the polarization direction of the second polarizer 17 is horizontal; the first laser spectroscope 19, the second laser spectroscope 20 and the third laser spectroscope 24 are spectroscopes having a transmittance of 90% and a reflectance of 10% for laser light of a wavelength λ; the focal length of the first converging lens 22 is 15cm, and the focal lengths of the second converging lens 26 and the third converging lens 36 are both 10 cm. The thickness of the sample 23 to be measured is 1 mm.
In this embodiment, the laser 1 is a pulsed femtosecond laser with a laser wavelength of 800nm and a repetition frequency of 1 KHz.
The specific operation steps of the embodiment are as follows:
firstly, system initialization: setting initial values of devices in the Z scanning device according to actual measurement requirements and laser parameters, which is specifically as follows:
firstly, according to actual measurement requirements, the laser wavelength is selected to be 800nm, the first polaroid 16 is adjusted to control the energy range of the incident light to the sample 23 to be measured, and meanwhile, the first adjustable attenuation sheet 27 and the second adjustable attenuation sheet 35 are adjusted to ensure that the sample 23 to be measured, the photomultiplier 28 and the third photoelectric detector 37 cannot be damaged under the condition that the transmittance of the electric control laser attenuation sheet 21 is 100%. The sampling frequency and the number of sampling points of the photomultiplier 28, the second photodetector 30, the electric platform 34 and the third photodetector 37 are set by the computer 38 to synchronously work with the laser 1. The spectral collection range of the optical multi-channel analyzer 33 is set by the computer 38 according to the actual sample luminescence wavelength.
Secondly, pre-scanning: in order to determine the optimal measurement light intensity, the sample 23 to be measured needs to be pre-scanned, which is as follows:
secondly, the sample 23 to be measured is placed on the electric platform 34, and the measurement of the sample 23 to be measured is adjustedThe measuring plane is perpendicular to the main optical axis, i.e. the z-axis, and z is equal to 0 at the focal point of the first converging lens 22. The computer 38 simultaneously starts the electric platform 34, the attenuation sheet controller 31 and the third photoelectric detector 37, the electric platform is used for changing the position of the sample, the forward direction of the laser is positive, the reverse direction of the laser is negative, the attenuation sheet controller 31 continuously changes the transmittance of the electric control laser attenuation sheet 21, and the light intensity signal output by the third photoelectric detector 37 is transmission closed hole data. When the initial position of the sample 23 to be measured is z-0, the computer 38 controls the attenuation sheet controller 31 to continuously change the transmittance of the electrically controlled laser attenuation sheet 21 from 0% to 100%, and simultaneously the computer 38 synchronously acquires the light intensity signal curve I output by the third photodetector 371(x) Wherein x is 0,1,2,3 … …,100, corresponding to a transmittance of 0%, 1%, 2% · 100%, respectively; the computer 38 controls the electric platform 34 to move the sample 23 to be measured to a negative end point of-10 z0Where, define z0＝πω0 2λ is the confocal length of the beam, where λ is the wavelength of the incident laser, ω 02 λ f/pi d is the laser beam waist radius, f is the focal length of the first condenser lens 22, and d is the spot diameter at the first condenser lens 22. The computer 38 controls the attenuation sheet controller 31 to continuously change the transmittance of the electrically controlled laser attenuation sheet 21 from 0% to 100%, and simultaneously the computer 38 synchronously acquires the light intensity signal curve I output by the third photodetector 372(x) Wherein x is 0,1,2,3 … …,100, corresponding to a transmittance of 0%, 1%, 2% · 100%, respectively; let T1(x)＝I1(x)/I2(x) Where x is 0,1,2,3 … …,100, finds T1(x) The computer 38 controls the transmittance of the electrically controlled laser attenuation sheet 21 by the attenuation sheet controller 31 to be adjusted to 55% at the point 55 corresponding to 0.95, and the light intensity incident on the sample 23 to be measured is the optimal incident light intensity of the sample.
Measuring data of open pores and closed pores: the Z-scan experiment requires measurement of a corresponding open-cell transmission curve and closed-cell transmission curve, so as to calculate a nonlinear absorption coefficient and a nonlinear refraction coefficient of a sample to be measured, specifically as follows:
placing the sample 23 to be measured on the electric platform 34, adjusting the measuring plane of the sample 23 to be measured to be perpendicular to the main optical axis, i.e. the z axis, and setting the focus of the first converging lens 22 to be z equal to 0. The computer 38 simultaneously starts the photomultiplier 28, the electric platform 34 and the third photoelectric detector 37, and the sample 23 to be measured is from a negative end point of-10 z0Moves forward along the main optical axis, passes through the focal point of the first converging lens 22 (z is 0), and has a movement range of 20z0. The photomultiplier 28 and the third photodetector 37 transmit the detected light intensity signal to the computer 38. The collected output light intensity signals of the photomultiplier 28 and the third photodetector 37 are respectively closed hole data and open hole data. The collected light intensity value is taken as the ordinate, z is the abscissa, and the closed pore curve I is recordedca(zn) And opening curve Ioa(zn) Wherein n is 1,2,3 … …,200, znIs the abscissa, z, of each sample point1～z200Is corresponding to-10 z0～+10z0The abscissa value at the focus is znThe number of samples is 200 at 0.
Fourthly, a monitoring part: monitoring influence factors possibly encountered in the measurement process, and timely processing the abnormal condition, wherein the method specifically comprises the following steps:
fourthly, the laser pulse waveform emitted by the first photodetector 29 is input to an oscilloscope 39, and the oscilloscope 39 feeds back the state information to the computer 38; whether the mode locking of the pulse laser emitted by the laser 1 is good or not can be monitored according to the signal presented by the oscilloscope 39, so as to judge whether the laser emitted by the laser 1 meets the experimental conditions or not, and if the mode locking is abnormal, the Z scanning experimental data is judged to be unreliable; the second photodetector 30 transmits the detected light intensity signal to the computer 38 in real time, and during the Z scanning process, the output light intensity signal of the second photodetector 30 is collected as a power monitoring curve of the incident laser, and if the fluctuation range of the curve exceeds a set threshold, it is determined that the Z scanning experimental data is unreliable. If the sample 23 to be measured is a luminescent material, the two-photon fluorescence spectrum can be monitored simultaneously in the Z scanning process, the optical multichannel analyzer 33 transmits the detected spectrum data to the computer 38, the acquired spectrum data has the abscissa as the wavelength and the ordinate as the light intensity, and if the two-photon fluorescence spectrum undergoes an obvious spectrum type change in the Z scanning process, particularly when the sample is scanned to a position near a position where Z is 0, the Z scanning experimental data is judged to be unreliable; and if the abnormity occurs in the monitoring process, the system automatically abandons the Z scanning data and restarts a new Z scanning experiment.
Fifthly, data processing: and carrying out corresponding processing according to the data obtained by the experiment to obtain the required nonlinear absorption coefficient and nonlinear refraction coefficient of the sample to be detected. The method comprises the following specific steps:
determination of test parameters: incident light power P at sample 23 to be measuredsThe optical power P that can be measured by the second photodetector 302Conversion is carried out: ps＝P29 x 55%. For closed pore curve Ica(zn) And opening curve Ioa(zn) (where n is 1,2,3 … …,200) is normalized. Dividing the ordinate values in the two curves by the corresponding z values1The longitudinal coordinate value of the position is respectively used for obtaining the normalized closed hole transmission curve T of the sampleca(zn) And Toa(zn) Wherein n is 1,2,3 … …, 200. Take n0Is 100, let Tca(100) And Toa(100) The corresponding abscissa is 0, i.e. at the focus. Normalized closed cell transmission curve Tca(zn) And Toa(zn) As shown in fig. 4 and 5.
The normalized transmittance t (z) is related to the material properties and is also closely related to many experimental parameters, such as the non-linear refractive index γ, the non-linear absorption coefficient β, the beam intensity distribution, the beam time characteristic, the light wave frequency ω, the Aperture Size S (or called Aperture Size,
wherein r isaIs the radius of the diaphragm, waThe light beam is in a linear region when the sample is far away from the focal pointBeam waist radius at diaphragm), lens focal length f, beam confocal length z0Girdling radius w0Optical power density I0Sample thickness L, etc. Normalization of transmittance T under the condition of satisfying reasonable approximationca(zn) This can be written as follows:
here, x is zn/z0According to a confocal length z0Normalized dimensionless position parameter, Δ Φ ═ k γ I0LeffFor phase shifts induced by non-linear refraction, k is 2 pi n0λ is the vector of light waves, γ is the non-linear refractive index, I0Is the incident light intensity, Leff＝[1-exp(-α0L)]/α0Is the effective length of the sample, alpha0Is the linear absorption coefficient of the material. Only one γ in the formula (1) is an unknown quantity, and the value of γ can be obtained by fitting.
Transmission curve T of open pore by normalization under open pore conditionoa(zn) Taking a focal point z 00 open pore transmittance value Toa(0) And substituting the following formula to calculate the nonlinear absorption coefficient beta of the sample 23 to be detected:
β＝2.83[1-Toa(0)]/I0Leff (2)
in the above formula, Leff＝[1-exp(-α0L)]/α0Is the effective length of the sample, alpha0Is the linear absorption coefficient of the material, L is the sample thickness, I0Is the incident light intensity.
The third step, the fourth step, the fifth step can realize one-key control in software, thereby realizing the automation of the measurement process.
The above-listed detailed description is only a specific description of a possible embodiment of the present invention, and they are not intended to limit the scope of the present invention, and equivalent embodiments or modifications made without departing from the technical spirit of the present invention should be included in the scope of the present invention.
Claims (8)
1. An improved Z-scan apparatus, comprising: the laser (1) for outputting laser wavelength lambda sequentially comprises a first aperture diaphragm (2), a first reflector (3), a second reflector (4), a third reflector (5), a fourth reflector (6), a fifth reflector (7), a sixth reflector (8), a seventh reflector (9), an eighth reflector (10), a ninth reflector (11), a tenth reflector (12), an eleventh reflector (13), a tenth reflector (14), a second aperture diaphragm (15), a chopper (16), a first polarizing film (17), a second polarizing film (18), a first laser beam splitter (19), a second laser beam splitter (20), a laser electric control attenuation sheet (21), a first converging lens (22), a sample to be detected (23), a third laser beam splitter (24) and a second polarizing film (18) along a main optical axis formed by a main optical path of laser output of the laser (1), A third aperture diaphragm (25), a second converging lens (26), a first adjustable attenuation sheet (27) and a photomultiplier tube (28); the photomultiplier (28) is connected with a computer (38); the first laser spectroscope (19) and a main optical axis form an angle of 45 degrees, a first photoelectric detector (29) is arranged in the output direction of reflected light of the first laser spectroscope (19), the first photoelectric detector (29) is connected with an oscilloscope (39), and the oscilloscope (39) is connected with a computer (38); the second laser spectroscope (20) and the main optical axis form a 45-degree angle, a second photoelectric detector (30) is arranged in the output direction of the reflected light of the second laser spectroscope (20), and the second photoelectric detector (30) is connected with a computer (38); the electric control laser attenuation sheet (21) is connected with an attenuation sheet controller (31), and the attenuation sheet controller (31) is connected with a computer (38); arranging an optical multi-channel analyzer probe near a sample to be measured, wherein the optical multi-channel analyzer probe (32) is connected with an optical multi-channel analyzer (33), and the optical multi-channel analyzer (33) is connected with a computer (38); the sample (23) to be tested is arranged on an electric platform (34), and the electric platform is connected with a computer (38); the third laser spectroscope (24) and the main optical axis form a 45-degree angle, a second adjustable attenuation sheet (35), a third converging lens (36) and a third photoelectric detector (37) are sequentially arranged in the output direction of reflected light of the third laser spectroscope (24), and the third photoelectric detector (37) is connected with a computer (38).
2. An improved Z scanning device as claimed in claim 1, characterized in that the first aperture stop (2), the second aperture stop (15) are 5mm in diameter and the third aperture stop (25) is 30mm in diameter.
3. An improved Z scanning device as claimed in claim 1, characterized in that said chopper (16) has a frequency adjustable range of 4HZ to 10 KHZ.
4. An improved Z scanning device as claimed in claim 1, characterized in that the polarization direction of said second polarizer (17) is horizontal.
5. An improved Z scanning device as set forth in claim 1, characterized in that the first laser beam splitter (19), the second laser beam splitter (20) and the third laser beam splitter (24) are beam splitters having a laser transmissivity of 90% and a reflectivity of 10% for each wavelength λ.
6. An improved Z scanning device as claimed in claim 1, characterized in that said first converging lens (22) has a focal length of 15cm, and said second (26) and third (36) converging lenses have a focal length of 10 cm.
7. An improved Z-scan apparatus as claimed in claim 1, characterised in that the sample (23) to be measured is 1mm thick.
8. An improved Z-scan apparatus as claimed in any of claims 1 to 7, wherein the operation of the Z-scan apparatus can be automated by a single key programmed by a computer.
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