JPH0799402B2 - Wave plate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a quarter-wave plate, a half-wave plate, a full-wave plate, etc., which causes a phase difference between two orthogonal linearly polarized lights. It relates to a wave plate.

[Conventional technology]

Conventionally, the wave plate is made by polishing a crystal of quartz, and the phase difference between ordinary and extraordinary light is (N + 1/4) wavelength (N is an integer) in the 1/4 wave plate and (N + 1/2) in the 1/2 wave plate. ) Wavelength, N for all wave plates
It is manufactured by adjusting the thickness to the wavelength. In addition to such a method by crystal polishing, a method using a grating has been proposed because a high-density surface relief grating formed on a dielectric exhibits birefringence. A proposal and experiment of a wave plate using a surface relief grating is made by DC Flanders in Applied Physics Letter, Vol. 42, No. 6 (published on Mar. 15, 1983), pages 492-494. , And Applied-Optic
s) Magazine Vol. 22 No. 20 (Published October 15, 1983) No. 3220-3
It is described in a paper by RC Enger and SK Case on page 228.

A wave plate using a grating has a grating pitch of d ₁
Then, in the region where λ is sufficiently larger than d, the fact that the refractive index n _{II in the} direction parallel to the groove of the lattice and the refractive index n _{⊥ in the} direction orthogonal to the groove of the lattice are different is utilized.
According to the paper by nders, if the lattice is rectangular, then n _II , n _⊥
Is given by

n _II = [n ₁ ² q + n ₂ ² (1-q)] ^1/2 (1) n _⊥ = [(1 / n ₁ ) ² q + (1 / n ₂ ) ² (1-q)] ^{- 1/2} ......
(2) Here, n ₁ is the refractive index of the medium 1, n ₂ is the refractive index of the medium 2, and q is the ratio of the medium 1 in one period of the grating, which is 1q0. The birefringence magnitude Δn is given by the following equation.

Δn = | n _II −n _⊥ | (3) Further, the phase difference ΔΦ received by the light incident on the grating having the magnitude of birefringence is Δn is given by the following equation.

Here, D is the groove depth of the lattice. From the equation (4), in order to obtain a large phase difference ΔΦ, the groove depth D may be increased or the birefringence size Δn may be increased. This relationship is not limited to the case where the lattice shape is rectangular, and holds even when the shape is sinusoidal, triangular, or the like.

A wave plate with a surface relief grating can be manufactured mainly by the following two methods.

The first method is to form a surface relief grating on a photoresist by interference exposure method, and then manufacture a mold by nickel electroforming from the grating, and transfer it to a thermoplastic resin by a hot press method or an injection molding method, or photocuring. It is a method of transferring to a resin.

The second method is to form a photoresist grid on the dielectric substrate by the same method as the first method, and use the photoresist as a mask to etch the dielectric substrate by an ion etching method, a reactive ion etching method, an ion beam etching method or It is a method of obtaining a surface relief lattice by etching by a reactive ion etching method.

[Problems to be solved by the invention]

The above-mentioned conventional technique has a problem that the groove depth becomes extremely large with respect to the groove width of the grating. For example, if the used wavelength λ is He
-632.8 mm for Ne laser. For this wavelength, the thermoplastic resin used in the first manufacturing method described above, such as an acrylic resin, a photocurable resin such as UV manufactured by ThreeBond Co., Ltd.
The refractive index of silica glass mainly used in X-SS-89-1 and the second manufacturing method is approximately 1.5 to 1.6. In the following, the thermoplastic resin, the photocurable resin and quartz glass are used as medium 1, and the refractive index n ₁ thereof is 1.55. Further, the medium 2 is air, and its refractive index n ₂ is 1.00. When the lattice shape is rectangular, the ratio q of the medium 1 to one period of the lattice is 0.5.
Then, the birefringence magnitude Δn is (1), (2),
It becomes 0.116 from the formula (3). Therefore, from equation (4), 1/4
The groove depths D required for the wave plate, the half wave plate, and the whole wave plate are 1.36 μm, 2.73 μm, and 5.46 μm, respectively. For the grating pitch d, to obtain birefringence based on high density, λ
Since it needs to be /d1.475, the condition of dμm must be satisfied. Since q = 0.5, the groove width W of the lattice is W0.21 μm. Therefore, the groove width is 0.21 μm
Hereinafter, a grating having a groove depth of 1.36 μm to 5.46 μm must be manufactured.

First, a grid having a groove depth extremely large with respect to the groove width
In the case of manufacturing by the manufacturing method of 1, the effective contact surface area between the medium 1 and the electroformed mold remarkably increases, so that the tensile shearing force at the time of peeling from the mold surface becomes large. For this,
There is a problem that the medium 1 which is hardened at the time of peeling is peeled off from the substrate and remains on the mold surface, making it difficult to transfer the surface relief grating.

Further, in the second manufacturing method, the time required for etching is several hours, and the photoresist mask that can withstand etching has a thickness of several μm. Therefore, it is difficult to form the photoresist mask. In addition, when the lattice formed on the photoresist is transferred to a substance having strong etching resistance, for example, chromium, and etching is performed using the substance as a mask, the substrate surface of the dielectric substrate once etched is increased as the lattice groove depth increases. It is difficult to form a desired lattice due to re-deposition onto the groove and reduction of the number of active particles, ions and neutral particles reaching the bottom of the groove. Such a problem occurs regardless of the shape of the lattice.

As described above, the surface relief grating type wave plate according to the prior art has a drawback that it is difficult to manufacture.

An object of the present invention is to solve the above problems of the prior art and to provide a phase grating type wave plate which is easy to manufacture.

[Means for solving problems]

The wave plate of the present invention comprises a substrate dielectric provided on the surface thereof with a substrate relief grating having a relationship between the wavelength λ used and the grating pitch d of λ / d1.472, and a surface coated or filled on the substrate relief grating. A surface relief grating having the same phase and the same pitch d as the substrate relief grating is formed, and includes a dielectric medium having a refractive index sufficiently higher than that of the substrate dielectric.

[Action]

 The operation of the present invention will be described in detail with reference to the drawings.

The phase difference ΔΦ received by the light incident on the grating is proportional to the groove depth D of the grating and the magnitude Δn of the birefringence.

In the present invention, the birefringence magnitude Δn can be obtained without increasing the groove depth D.
It is intended to solve the above-mentioned problems by increasing.

Consider a case where a rectangular lattice is formed on a dielectric substrate having a refractive index n ₁ . Sectional view FIG. 2 showing a case in which a rectangular grating 4 formed on the surface of the dielectric substrate 1 having a refractive index n ₁ is covered with a dielectric medium 3 having a refractive index n ₂ greater than the refractive index n ₁ Is. In this state, the lattice has a first region 5 composed of the dielectric medium 3 and air, a second region 6 composed of the dielectric substrate 1 and the dielectric medium 2 and air, and a dielectric substrate 1 and the dielectric substrate 1 from the top of the lattice along the cross section. 3 of the third region 7 composed of the body medium 3
It can be divided into two areas. The ratio occupied by the dielectric medium 1 in one period of the lattice is q ₁ , and the ratio occupied by the dielectric medium 3 is
q ₂ , and the ratio of air is q air. Here, q ₁ + q ₂ + q air = 1 (5). If the refractive index of air is 1.00, (1), (2),
From the equation (3), the magnitude of birefringence Δn _{1 in} the first region 5 is Δn ₁ = [▲ n ² ₂ ▼ q ₂ + q air] ^1/2 -[(1 / n ₂ ) ² q ₂ + q air]
^-1/2 ... (6) By expanding the equations (1) and (2), the birefringence magnitude Δn ₂ of the second region 6 is Δn ₂ = [n ₁ ² q ₁ + n ₂ ² q ₂ + q air] ^1/ _2- [ (1 / n ₁ ) ² q ₁ + (1 / n ₂ ) ²
+ q air] ^-1/2 ... (7). The birefringence magnitude Δn ₃ of the third region 7 is Δn ₃ = [n ₁ ² q ₁ + n ₂ ² q ₂ ] ^1/2 -[(1 / n ₁ ) ² q ₁ + (1 / n ₂ ) ² + q ₂ )
^-1/2 ... (8) The magnitude of birefringence Δn greatly depends on the difference in refractive index between adjacent media, and is Δn ₁ > Δn ₂ > Δn ₃ . The layer thicknesses of the first region 5, the second region 6, and the third region 7 are respectively set to D ₁ , D ₂ ,
D ₃ is the phase difference ΔΦ that the wavelength λ passing through the grating receives.
Is from equation (3) Becomes FIG. 3 shows the result of the relationship between the phase difference ΔΦ and the thickness of the dielectric medium 3 obtained from the equation (9) using the refractive index of the dielectric medium as a parameter. At this time, the grating formed on the dielectric substrate 1 has a pitch d = λ / 2, a groove depth D = λ, and a refractive index n ₁ = 1.55, and the thickness of the dielectric medium 3 is the top, bottom, and side surfaces of the grating. It was assumed that the division was the same. The left end of the figure is the dielectric medium 3
This is the case where there is only the grating formed on the dielectric substrate 1, and the right end corresponds to the case where the groove of the grating is completely filled with the dielectric medium.

From FIG. 3, when the grating surface of the dielectric substrate 1 is covered with the dielectric medium 3 having a refractive index sufficiently higher than that of the substrate 1, the phase difference received by the incident light is not covered, or the groove portion of the grating is formed. It can be seen that can be increased as compared with the case where is completely filled with the dielectric medium 3.

FIG. 4 is a sectional view showing a lattice shape when the film thickness of the dielectric medium 3 is increased and the second region 6 shown in FIG. 2 disappears. The grooves of the lattice are filled with the dielectric medium 3.
Since it is composed of the first region 5 and the third region 7, the phase difference ΔΦ received by the light of wavelength λ entering the grating is Given in. In this case, D ₃ is equal to the groove depth D of the surface relief grating formed on the dielectric substrate 1. Dielectric medium 3
The phase difference can be further increased by the phase difference generated at the thickness D ₁ as compared with the case where the interface between air and air is made flat. Refractive index of substrate 1 n ₁ = 1.55, groove depth D = λ, D ₁ = λ /
5. If the refractive index n ₂ of the dielectric medium is 2.0, 2.2, 2.4, ΔΦ
Are 0.753, 1.227, and 1.793 [rad], respectively, and ΔΦ = when there is no dielectric medium even with such a configuration.
It will be larger than 0.728 [rad].

Therefore, the groove depth of the grating manufactured on the dielectric substrate can be reduced, and a phase grating type wave plate which is easy to manufacture can be obtained.

The same applies when the grating is not rectangular but sinusoidal or triangular, and a large birefringence can be obtained by coating the grating surface with a dielectric medium that has a refractive index sufficiently higher than that of the dielectric substrate. Thus, a wave plate that is easy to manufacture can be obtained.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a cross-sectional view showing a first embodiment of the present invention, in which a lattice is enlarged much more than the actual size for the sake of clarity. The sinusoidal grating 2 is formed on the dielectric substrate 1,
The surface of the grating 2 is covered with a high-refractive-index dielectric medium 3 to increase the magnitude of birefringence. For the actual fabrication, the UV light manufactured by ThreeBond, which is a photo-curable resin, is used as the substrate dielectric 1.
X-SS89-1 was used as the high-refractive-index dielectric medium 3 made of polysilastyrene PSS75 manufactured by Shin Nisso Kako. The refractive index of the former is
The refractive index of the latter is 1.52, which is about 2.5. The wavelength used is 632.8 mm for He-Ne laser. To transfer the grating pattern to the photo-curable resin, first configure an interferometer using the He-Cd laser light beam with a wavelength of 441.6 mm, and holographically display λ / d21.472.
A grating having a pitch d of 0.3 μm that satisfies the above condition was formed in a photoresist, and a mold was manufactured from a sinusoidal surface relief grating after the photoresist phenomenon by a nickel electroforming method, and this mold was used. Liquid polysilastyrene was applied onto the lattice formed on the substrate dielectric 1 which was a photocurable resin, and the solvent was dried to form the wave plate shown in FIG.

FIG. 5 is a cross-sectional view showing a second embodiment of the present invention, in which the lattice is enlarged much more than it actually is. The actual production is the first
This is performed by forming a lattice on the photocurable resin by the same method as in the case of the first embodiment shown in the figure and then increasing the polysilastyrene film thickness as compared with the first embodiment. Since the layer thickness D ₁ of the first region having the largest birefringence becomes large, the phase difference ΔΦ can be made larger than that of the first embodiment.

[Effects of the Invention] According to the present invention, since the groove depth of the grating formed on the dielectric substrate can be made small, the grating can be easily manufactured, and thus, the wavelength plate which is easy to manufacture, inexpensive and highly producible can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view schematically showing an embodiment of the present invention, and FIG.
FIGS. 4 and 5 are schematic sectional views of a dielectric substrate coated or filled with a dielectric medium for explaining the principle of the present invention, and FIG. 3 is a thickness of the dielectric medium 3 shown in FIG. And FIG. 5 is a cross-sectional view schematically showing the relationship between the phase difference between the incident light and the incident light, and FIG. In the figure, 1 is a dielectric substrate, 2 is a lattice part, 3 is a dielectric medium, 4 is a rectangular lattice, 5 is a first region of the lattice, 6 is a second region of the lattice, and 7 is a third region of the lattice. .

Claims (1)

[Claims] 1. The relationship between the used wavelength λ and the grating pitch d is λ / d.
1.472 a substrate dielectric provided with a substrate relief grating on its surface, and a substrate dielectric coated or filled on the substrate relief grating and having a surface relief grating formed on the surface at the same phase as the substrate relief grating and at the same pitch d. And a dielectric medium having a refractive index sufficiently higher than that of the wave plate.