JPH09288211A - Polarizing optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a small-sized element which can be easily produced and can divide near infrared or visible rays according to polarization directions by specifying the period of a metal grating and the refractive indices of a substrate and the metal grating. SOLUTION: This polarizing optical element consists of an almost planer substrate 11 and a metal grating 12 formed on the substrate 11. This element has a function to separate light of wavelengths large than n0 ×d and smaller than n1 ×d into transmitted light and reflected flight according to its polarizing direction, wherein n1 is the refractive index of the substrate 11, n0 is the refractive index of a material which covers the top of the metal grating 12, and (d) is the grating period of the metal grating. For example, when the element is irradiated with polarized light of 780nm wavelength having the electric field component in the same direction as the grating vector of the metal grating 12, about 82% of the incident light is reflected while about 2.8% enters the substrate 11. As for polarized light having the electric field component in the perpendicular direction, about 1.5% of the incident light is reflected while about 80% of the light is made incident on the substrate 11.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarizing optical element that uses a metal grating to separate incident light into transmitted light and reflected light according to the polarization direction.

[0002]

2. Description of the Related Art A polarization optical element for separating light according to a polarization direction is used, for example, in an optical pickup of a drive unit for a magneto-optical disk.

As such a polarization optical element, an element utilizing Brewster reflection of a thin film is widely known. An element using Brewster reflection of a thin film is usually used by allowing light to enter the element formed in a cube perpendicularly, or at a predetermined angle with respect to an element formed in a plate. Light is incident and used.

However, the element utilizing the Brewster reflection of a thin film has a drawback that precise alignment is required and a large volume. for that reason,
In such an element, for example, when it is used for an optical pickup, it requires precise alignment with other components such as a light-receiving element that constitutes the optical pickup, and therefore, there is a problem that man-hours are required for manufacturing. there were. In addition, since such an element has a large volume, it is also a big limitation for miniaturization of an optical pickup or the like.

Therefore, in order to solve such a problem, attempts are being made to obtain a thin film or thin plate element capable of separating light according to the polarization direction.
Specifically, as the thin-film or thin-plate element, the following elements have been reported.

For example, "SPIE proceedings, vol.1116, p4
46 (1989) ”, a device in which a metal colloid is dispersed in glass and polarization detection is performed by utilizing its resonance absorption. With this device, a relatively large extinction ratio can be obtained for transmitted light. However, this element does not have a polarization splitting function in principle because it absorbs one polarized light, so this element is not a polarization optical element that splits light according to the polarization direction. Not suitable.

Further, for example, "Applied Physics Letter"
s, vol.61, No.22, p2633 (1992) ”, a thin film made of a high refractive index material and a thin film made of a low refractive index material are laminated, and light is obliquely incident on the laminated film. , An element that separates light according to the polarization direction has been proposed.However, this element requires a thin film laminating process, and it is necessary to cut it obliquely to form a thin plate. The device itself is very complicated to fabricate, and it is very difficult to fabricate it in a large area.In addition, the thickness of the device is limited, and a large polarization separation angle can be obtained. It also has the drawback of not being.

Further, for example, an element called a wire grid or a wire grating has been proposed in which polarized light is separated by a metal grating in which a metal is formed in a fine grid shape. However, conventionally, in such an element,
The grating period of the metal grating is required to be sufficiently smaller than the wavelength of incident light. Specifically, for example, when the metal grating has a grating period of 600 nm, it has polarization separation characteristics for light having a wavelength about 5 times or more, that is, light having a wavelength of about 3 μm or more. . Therefore, the wavelength is 800 nm like near infrared light or visible light.
In order to apply such an element to light of a certain level or less, the grating period of the metal grating must be about 150 nm or less. However, it is extremely difficult and impractical to produce a metal grating having such a grating period with the current technology.

[0009]

As described above, a polarizing optical element having a thin film or thin plate shape, easy to manufacture, and applicable to near-infrared light or visible light has not yet been known. The current situation is that development is awaited.

By the way, in an optical pickup or the like, the light separated by the polarization optical element according to the polarization direction is detected by a light receiving element such as a photodiode. However, as described above, when the light separated by the polarization optical element is to be detected, conventionally, the polarization optical element and the light receiving element are separate elements, so that the overall size becomes large. There was a problem of being lost. Moreover, conventionally, it is necessary to arrange the polarization optical element and the light receiving element at appropriate positions and integrate them.

Therefore, an element used for an optical pickup or the like has a function of separating light according to the polarization direction,
An element having a function of detecting separated light is preferable. However, at the moment,
An element that is small in size, easy to manufacture, and has a function of detecting near infrared light or visible light after separating it according to the polarization direction has not yet been known.

The present invention has been proposed in view of the above conventional circumstances, is small in size and easy to manufacture, and separates near-infrared light or visible light according to the polarization direction. It is an object of the present invention to provide a polarizing optical element capable of Further, the present invention is small and easy to manufacture,
Moreover, it is also an object to provide a polarization optical element having a function of detecting near infrared light or visible light after separating it according to the polarization direction.

[0013]

Polarizing optical element according to the present invention has been accomplished in order to achieve the above object, according to an aspect of, along with the refractive index is formed on a substrate of n _1, a refractive index of n ₀ material A polarization optical element having a metal grating whose upper part is covered with, separating light incident on the metal grating into transmitted light and reflected light according to a polarization direction, wherein the grating period of the metal grating is d. At this time, the wavelength of the light is larger than n ₀ × d and smaller than n ₁ × d.

In such a polarization optical element according to the present invention, near infrared light or visible light can be separated according to the polarization direction. Further, this polarizing optical element can be formed into a thin film shape or a thin plate shape, and can be manufactured very easily.

In the polarization optical element, the substrate may have a light detecting function and output the amount of transmitted light that has passed through the metal grating as an electric signal. At this time, it is preferable that the main component of the material of the electrode formed on the substrate for extracting the electric signal and the main component of the material of the metal grid are the same.

As described above, in the polarization optical element in which the substrate has the light detecting function, the incident light can be detected after being separated according to the polarization direction. Further, in this polarization optical element, since the substrate is provided with the light detecting function, it is not necessary to newly provide a light receiving element, and even if the light detecting function is provided, it can be downsized, and it is very easy. Can be manufactured.

[0017]

Embodiments of the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the following examples, and it goes without saying that the shape, material, and the like can be arbitrarily changed without departing from the gist of the present invention.

First, the polarization optical element according to the first embodiment to which the present invention is applied will be described.

The polarization optical element according to this embodiment is a polarization optical element that separates incident light into transmitted light and reflected light according to the polarization direction, and as shown in FIG. 1, a substantially plate-shaped substrate. 11 and a metal grid 12 formed on the substrate 11.

Here, the substrate 11 is made of GaAs,
Its refractive index n ₁ is about 3.7. Also, the metal grid 1
No. 2 is made of a material containing Al as a main component, and its upper portion is covered with air, which is a substance having a refractive index n ₀ of 1. In this embodiment, the metal grating 12 has a grating period d of about 600 nm and a width b of each grating of about 400 n.
m, and the height h of each lattice was about 120 nm.

In this polarization optical element, light having a wavelength larger than n ₀ × d and smaller than n ₁ × d is separated into transmitted light and reflected light according to the polarization direction.

Specifically, for example, when light having a wavelength of 780 nm enters the metal grating 12 perpendicularly, the metal grating 1
About 82% of the incident light of polarized light having an electric field component in the same direction as the grating vector of 2 (hereinafter referred to as P wave) is reflected, and about 2.8% is incident on the substrate 11 side. On the other hand, polarized light (hereinafter referred to as S wave) having an electric field component in the direction perpendicular to the grating vector of the metal grating 12 reflects about 1.5% of the incident light, and about 8%.
0% is incident on the substrate 11 side.

As described above, the polarization optical element mainly reflects P waves and mainly transmits S waves. That is, the polarization optical element has a polarization separation function of separating incident light into transmitted light and reflected light according to the polarization direction.

However, the above-mentioned transmittance is a value at the stage when the light is transmitted through the metal grid 12, that is, a value on the surface of the substrate 11. Then, GaAs, which is the material of the substrate 11,
Since it is an absorber for light with a wavelength of 780 nm, the amount of light transmitted through the metal grating 12 is attenuated as it passes through the substrate 11. Therefore, the polarization optical element according to the present embodiment is particularly suitable for applications in which only reflected light is used among the lights separated according to the polarization direction.

In the polarization optical element according to the present embodiment as described above, the metal grating 12 having a large grating period d on the substrate 11.
Can be obtained simply by forming. Therefore, this polarizing optical element can be manufactured very thinly and compactly and easily.

The reason why the above polarized light separating action is obtained will be described below.

FIG. 2 shows the S-wave reflectivity and transmissivity of the polarizing optical element as a function of the incident light wavelength. In addition, FIG. 3 shows the reflectance and transmittance of P waves in the polarization optical element as a function of the incident light wavelength. Note that FIG.
3 and FIG. 3 show the total transmittance and the total reflectance including the high-order diffracted light.

As shown in FIG. 2, the S-wave transmittance has a minimum value when the wavelength of the incident light is about 600 nm, and then increases as the wavelength increases, so that the wavelength of the incident light is about 7 nm.
It reaches a maximum at 80 nm. Then, the transmittance of the S wave exceeds the wavelength range shown in FIG. 2 when the wavelength of the incident light is further increased, but becomes minimum again when the wavelength of the incident light is about 2210 nm.

[0029] Here, two singular points at which the transmittance of the S-wave is minimized, the grating period of the metal grating is d, the refractive index of the substrate and the n _1, the refractive index of the material covering the top of the metal grid n ₀
It is known that they are given as n ₀ × d and n ₁ × d, respectively, and these are called Rayleigh anomaly.

On the other hand, as shown in FIG. 3, for the P wave, neither the reflectance nor the transmittance is much dependent on the wavelength of the incident light,
It is almost constant. And, for example, a wavelength of 780n
For incident light of m, the reflectance is about 82% and the transmittance is about 2.8%.

By the way, the diffraction phenomenon with respect to the light of wavelength λ can be classified into the following three with reference to the Rayleigh anomaly. The first diffraction phenomenon is a diffraction phenomenon when λ <n ₀ × d, and at this time, the reflected light and the transmitted light both have 0
Higher-order diffracted light higher than the second order is generated. Then, a general diffraction grating utilizes such a first diffraction phenomenon.
The second diffraction phenomenon is a diffraction phenomenon when n ₀ × d <λ <n ₁ × d. At this time, high-order diffracted light is generated in transmitted light, but 0th-order diffracted light is generated in reflected light. Only light is produced. The third diffraction phenomenon is a diffraction phenomenon when λ> n ₁ × d, and at this time, only 0th-order diffracted light occurs in both reflected light and transmitted light.

Conventionally, the polarization optical element using the metal grating utilizes the diffraction phenomenon in which only the 0th order diffracted light is generated, that is, the third diffraction phenomenon. Therefore, in the conventional polarization optical element, the wavelength λ of the incident light is limited to the range that satisfies λ> n ₁ × d. Moreover, in the conventional polarization optical element, in order to avoid the influence of the fluctuation of the transmittance and the reflectance in the vicinity of the Rayleigh anomaly, it is usually conditioned that the wavelength is sufficiently larger than n ₁ × d.

In a typical conventional polarizing optical element, a metal grating is formed on a silica substrate having a refractive index of 1.5. Here, the upper part of the metal grid is usually in its original state, that is,
It is in a state of being covered with air, which is a substance having a refractive index of 1. In such a polarization optical element, for example, when the grating period d of the metal grating is 600 nm, Rayleigh anomaly appears when the wavelengths of incident light are 600 nm and 900 m. Therefore, such a polarization optical element can be applied only to infrared light having a wavelength sufficiently longer than 900 nm.

On the other hand, in order to use such a conventional polarization optical element for near infrared light having a wavelength of 780 nm, the grating period d of the metal grating must be about 150 nm. However, it is extremely difficult to manufacture a metal grating having a fine structure with a grating period d of about 150 nm at the current technical level, and such a polarization optical element has not been realized yet.

On the other hand, the inventor of the present invention optimizes the structure of the polarization optical element even in the region where the diffraction phenomenon in the wavelength region between the two Rayleigh anomalies, that is, the second diffraction phenomenon occurs. It was found that a good polarized light separating effect can be obtained by using all diffracted light including high-order diffracted light as the transmitted light.

That is, in the present invention, the wavelength of the light incident on the metal grating is larger than n ₀ × d and smaller than n ₁ × d so as to utilize the second diffraction phenomenon. To be In addition, in order to utilize the second diffraction phenomenon as described above, it is necessary to dispose the metal grating on the substrate. Further, a substrate whose refractive index n ₁ is larger than the refractive index n _{0 of the} substance covering the upper part of the metal lattice is used.

By utilizing the second diffraction phenomenon as described above, as shown in the above embodiment, for example,
Even with a polarization optical element that produces a polarization separation action also for near-infrared light having a wavelength of 780 nm, the grating period d of the metal grating can be set to about 600 nm. That is,
In the polarization optical element to which the present invention is applied, the grating period d of the metal grating can be made several times larger than the grating period d of the metal grating in the conventional polarizing optical element.

In the polarization optical element to which the present invention is applied, since the grating period d of the metal grating can be increased as described above, a polarization separation effect is generated for near infrared light or visible light. Even if it is, it can be easily manufactured by the already established current processing technology.

In the above-described embodiment, the P-wave reflectance and the S-wave transmittance can be set to about 80% or more, respectively, because the Al-based material having high conductivity is used. This is partly due to the formation of the metal grid. That is, by forming the metal grating with a material having high conductivity, the loss of light in the metal grating is reduced, and the polarization separation characteristic is improved. As a material of such a metal lattice, for example, in addition to a material containing Al as a main component, a material containing Au as a main component and the like can be cited.

In the polarizing optical element to which the present invention is applied, the substrate is preferably made of a material having a large refractive index n ₁ . This is because the wavelength region between the Rayleigh anomalies is expanded by forming the substrate with a material having a large refractive index n ₁ , and as a result, the wavelength range applicable as the polarization optical element is expanded.

As a material having a large refractive index n ₁ , for example, a semiconductor material containing GaAs, Si, InP or the like as a main component can be cited. These semiconductor materials have a very large refractive index of 3.0 or more. Therefore, by using such a semiconductor material, the wavelength region in which the polarization separation action is obtained is widened, and the wavelength range applicable as the polarization optical element is widened. In addition to such semiconductor materials, for example, various dielectric materials and the like can be used as the material of the substrate.

By the way, if the substrate is made of a semiconductor material and the substrate is made to have a light detecting function, the transmitted light transmitted through the metal grating can be converted into a higher-order diffracted light while maintaining the above-mentioned polarization separation action. It becomes possible to detect easily including the above.

Therefore, a polarization optical element having such a light detecting function will be described below as a second embodiment of the present invention.

The polarization optical element according to the present embodiment is a polarization optical element that separates incident light into transmitted light and reflected light according to the polarization direction and detects the polarized and separated transmitted light. 4, a substantially plate-shaped substrate 21, a metal grid 22 formed on the front surface of the substrate 21, an upper electrode 23 formed on the front surface of the substrate 21, and a lower electrode formed on the back surface of the substrate 21. 24 and.

The metal grid 22 is formed in the same manner as the metal grid 12 in the first embodiment. On the other hand, the substrate 21 is made of GaAs and has a so-called pn
It has a structure or a pin structure. And the substrate 21,
The upper electrode 23 and the lower electrode 24 constitute a photodiode as a whole. That is, in the present embodiment, the substrate 21 has a light detection function, and this polarization optical element determines the amount of transmitted light that has passed through the metal grating 22.
The electric signal is output from the upper electrode 23 and the lower electrode 24.

Also in this polarizing optical element, a polarization separating action can be obtained as in the polarizing optical element according to the first embodiment. That is, this polarizing optical element also has the above-mentioned first
Similar to the polarization optical element according to the embodiment, the P wave is reflected and the S wave is transmitted.

In particular, in the polarization optical element according to the present embodiment, as described above, the transmitted light transmitted through the metal grating 22 is made of the substrate 21 made of a semiconductor material having a light detecting function.
Is detected by That is, this polarization optical element
It is possible to detect the S wave transmitted through the metal grating 22 in the incident light.

By the way, GaAs which is the material of the substrate 21
Is an absorber for light having a wavelength of 780 nm, for example, the amount of light transmitted through the metal grating 22 is attenuated as it passes through the substrate 21. However, in the polarization optical element according to the present embodiment, the transmitted light that has passed through the metal grating 22 is directly detected by the substrate 21. That is, in this polarization optical element, the transmitted light that has passed through the metal grating 22 is received and detected by the substrate 21 with almost no attenuation. Therefore, in this polarization optical element, such attenuation of transmitted light does not cause any particular problem.

In the polarization optical element according to this embodiment as described above, the upper electrode 23 on the surface of the substrate 21 and the grating period d.
With a large metal grid 22 and a substrate 21
It can be obtained only by forming the lower electrode 24 on the back surface of. Therefore, this polarizing optical element can be manufactured very thinly and compactly and easily.

In the polarization optical element according to this embodiment, the upper electrode 23 is preferably made of the same material as the metal grid 22. As described above, when the upper electrode 23 is formed of the same material as the metal grid 22, the metal grid 22 to which the present invention is applied can be formed at the same time when the pattern of the upper electrode 23 is formed.
Therefore, except that the upper electrode 23 is formed of the same material as the metal grid 22 to form the metal grid 22 together with the upper electrode 23, the polarization separation function is exactly the same as the manufacturing procedure of the conventional photodiode. It is possible to easily form the polarization optical element according to the present embodiment having both the function of detecting light and the function of detecting light.

In the present embodiment, the upper electrode 23 is provided on the surface of the substrate 21 in addition to the metal grid 22, but the metal grid 22 has a high electric conductivity, and therefore the metal grid 22 has a high conductivity. May function as an electrode for extracting an electric signal. In this way, by making the metal grating 22 also function as an electrode, it is not necessary to provide the upper electrode 23 on the surface of the substrate 21 separately from the metal grating 22, and it has both a polarization separation function and a light detection function. The polarizing optical element can be formed more easily.

When the substrate 21 has a light detecting function as in the present embodiment, the material of the substrate 21 is GaA.
It is not limited to s, but GaAs, Si or In
It goes without saying that semiconductor materials containing P as a main component can be widely used.

Next, an example in which the polarizing optical element according to the second embodiment is applied to an optical pickup will be described with reference to FIG.

The optical pickup 50 is used in a drive device for a magneto-optical disk, and has a light source 51 for irradiating a laser beam, a light from the light source 51, and a reflected light from the magneto-optical disk 52 in a predetermined optical path. Guide body 53
A beam splitter film 54 arranged at a predetermined position on one main surface of the waveguide 53, and a polarization optical element 55 and a photodiode 56 arranged at a predetermined position on another main surface of the waveguide 53.
And a total reflection film 57 arranged at a predetermined position on the other main surface of the waveguide 53, and a support portion 58 for supporting them.

The waveguide 53 is processed into a substantially trapezoidal cross section so that the surface irradiated with the light from the light source 51 is inclined with respect to the light source 51. The beam splitter film 54 is disposed on this inclined surface, reflects the light from the light source 51 so that the light from the light source 51 enters the magneto-optical disk 52, and reflects the light from the magneto-optical disk 52. Transmits the reflected light from the magneto-optical disk 52 so that the light enters the waveguide 53.

The light reflected by the beam splitter film 54 is further reflected by the mirrors 59 and 60 so as to be incident on a predetermined position on the magneto-optical disk 52, and is focused on the recording surface of the magneto-optical disk 52. After being focused by the objective lens 61 so as to be connected, the light is incident on the magneto-optical disk 52. Then, the light incident on the magneto-optical disk 52 is reflected by the magneto-optical disk 52, and the reflected light is guided through the beam splitter film 54 to the waveguide 53.
Incident on.

The polarization optical element 55 is arranged in the area irradiated by this reflected light. The polarization optical element 55 has the same structure as the polarization optical element according to the second embodiment, and has both the polarization separation function and the light detection function. Therefore, the polarization optical element 55 reflects the P wave and detects the S wave in the reflected light from the magneto-optical disk 52.

The total reflection film 57 is arranged in a region irradiated with the P wave reflected by the polarization optical element 55.
The total reflection film 57 totally reflects the P wave reflected by the polarization optical element 55, and the P wave is reflected by the photodiode 56.
Lead to. The photodiode 56 is arranged in a region irradiated with the P wave totally reflected by the total reflection film 57, and detects the P wave.

In the optical pickup 50, as described above, of the reflected light from the magneto-optical disk 52, the S wave is detected by the polarization optical element 55 and the P wave is detected by the photodiode 56. Then, the optical pickup 50 detects the signal from the magneto-optical disk 52 by obtaining the sum signal or the difference signal of the S wave and the P wave detected in this way.

Since the optical pickup 50 as described above uses the thin polarizing optical element 55 having a thin plate shape, it can be miniaturized. Moreover, in the conventional optical pickup, it was necessary to align the polarization separation element and the light receiving element, but in this optical pickup 50, the polarization optical element 55 having both the polarization separation function and the light detection function is used. Therefore, it is not necessary to perform such alignment. Therefore, this optical pickup 50 can significantly reduce the number of manufacturing steps as compared with the conventional optical pickup.

The application of the polarization optical element according to the present invention is not limited to such an optical pickup, and the light is separated according to the polarization direction, or the light is separated according to the polarization direction. Needless to say, the present invention can be widely applied to devices and the like that are required to be detected after the above.

[0062]

As is apparent from the above description, according to the present invention, it is small and easy to manufacture, and the near infrared light or visible light can be separated according to the polarization direction. A polarizing optical element can be provided.

Further, according to the present invention, it is possible to provide a polarizing optical element which is small in size, easy to manufacture, and has a function of detecting near infrared light or visible light after separating it according to the polarization direction. it can.

[Brief description of drawings]

FIG. 1 is a perspective view showing an example of a polarization optical element to which the present invention is applied.

FIG. 2 is a characteristic diagram showing wavelength dependence of transmittance and reflectance of S wave in the polarization optical element shown in FIG.

FIG. 3 is a characteristic diagram showing wavelength dependence of transmittance and reflectance of P wave in the polarization optical element shown in FIG.

FIG. 4 is a perspective view showing another example of a polarization optical element to which the present invention is applied.

FIG. 5 is a side view showing an example of an optical pickup using a polarization optical element to which the present invention is applied.

[Explanation of symbols]

11, 21 substrate, 12, 22 metal grid, 23
Upper electrode, 24 Lower electrode

Claims (6)

[Claims] 1. A metal grating, which is formed on a substrate having a refractive index of n ₁ and whose upper portion is covered with a material having a refractive index of n ₀ , comprises: In a polarizing optical element that separates transmitted light and reflected light, the wavelength of the light is greater than n ₀ × d and smaller than n ₁ × d, where d is the grating period of the metal grating. A polarizing optical element characterized by being present. 2. The polarizing optical element according to claim 1, wherein the metal grating is made of a material containing Al or Au as a main component. 3. The polarizing optical element according to claim 1, wherein the substrate is made of a semiconductor material. 4. The substrate made of the semiconductor material is GaA.
The polarizing optical element according to claim 3, wherein s, Si or InP is a main component. 5. The polarization optical element according to claim 1, wherein the substrate has a light detecting function, and outputs the amount of transmitted light that has passed through the metal grating as an electric signal. 6. The main component of the material of the electrode formed on the substrate for extracting the electric signal and the main component of the material of the metal grid are the same.
The polarizing optical element described.