JPH0776815B2 - Light modulator - Google Patents

Description

TECHNICAL FIELD The present invention relates to a light modulation element, in particular, a combination of a diffraction grating and a liquid crystal, and diffracts incident light by controlling the refractive index of the liquid crystal.
The present invention relates to a light modulation element that causes a non-diffraction phenomenon.

<Prior Art> Conventionally well-known light modulation elements include a pair of polarizing plates arranged so that their polarization directions are orthogonal to each other, and a pair of transparent substrates disposed between the pair of polarizing plates and facing each other. A so-called TN (twist) that modulates incident light by switching the orientation of the liquid crystal between a twisted state and a state perpendicular to the substrate surface. There is a nematic type liquid crystal display device.

This liquid crystal display element is widely used due to its simple structure and easy driving. However, since light is transmitted and blocked by using two polarizing plates, it is possible to erase or discolor light. Has low transmittance.

On the other hand, in U.S. Pat. No. 4,251,137, there is disclosed a variable subtractive color filter element in which a transmissive phase type (diffraction) grating and a nematic liquid crystal are combined. This element has a high transmittance when achromatic.

However, when modulating light polarized in the direction of the grating groove of the phase-type grating, this device keeps the refractive index n _g of the phase-type grating and the minimum refractive index n _{min of the} liquid crystal in agreement and does not apply an electric field. In this state, the liquid crystal molecules are aligned parallel to the substrate surface along the groove of the phase-type grating to generate the maximum refractive index n _max in the liquid crystal, and an electric field is applied perpendicular to the substrate surface to orient the liquid crystal molecules in the direction of the electric field. Since the minimum refractive index n _min is generated in the liquid crystal,
The amount of rotation of liquid crystal molecules was large, and there was a problem in the response of switching between the colored state and the achromatic state.

<Summary of the Invention> An object of the present invention is to provide an optical modulator having improved responsiveness.

In order to achieve this object, the light modulation element of the present invention has a diffraction grating structure in which transparent members and liquid crystals are alternately arranged, and means for controlling the refractive index of the liquid crystal. by equal approximately refractive index nθ and the refractive index n _g of the transparent member existing between the maximum refractive index n _max and the minimum refractive index n _min, the liquid crystal molecules at the time of switching the colored state and the achromatic state than the conventional The rotation amount of is reduced to improve responsiveness.

In the present invention, for example, positive and negative nematic liquid crystals and ferroelectric liquid crystals are used.

Further, a method of forming a grating, which is an array of transparent members, is a method combining photolithography and dry etching, a replica method using a thermosetting resin or an ultraviolet curable resin, cutting using a ruling engine. Law or embossing method.

<Example> FIG. 1 shows a basic configuration of an optical modulator of the present invention. In the figure,
Reference numeral 1 is a liquid crystal, and 2 is a diffraction grating made of a material transparent to a used wavelength. Each part of the liquid crystal 1 and each part of the diffraction grating 2 alternately arranged along the surface of the substrate form a diffraction grating structure. . Reference numeral 3 is a transparent electrode, 4 is a transparent substrate made of a transparent optical member, 5 is incident light having an arbitrary polarization characteristic, 6 and 6'are polarization components of the incident light 5 orthogonal to each other, and 6 is a polarization component perpendicular to the paper surface. , 6'indicate polarization components parallel to the plane of the drawing.

In the present light modulation element, a transparent electrode 3 is formed on substrate surfaces of a pair of transparent substrates 4 facing each other, and a diffraction grating 2 made of a transparent material is formed on the transparent electrodes 3 on one substrate surface of the pair of transparent substrates 4.
Is provided and the liquid crystal 1 is filled in the groove (recess) of the diffraction grating, and an electric field is applied to the liquid crystal 1 through the transparent electrode to change the refractive index of the liquid crystal 1.

The principle of light modulation of the present light modulator will be described below.

As shown in FIG. 1, in a static state in which no electric field is applied, the liquid crystal 1 is aligned in the groove direction of the diffraction grating 2, that is, in the direction perpendicular to the paper surface in a plane parallel to the surface of the substrate 4, and is in a homogeneous alignment state. Has become.

Of the polarization components 6 and 6 ′ of the incident light 5 that is incident on the present light modulation element in this static state, the polarization component 6 ′ that is a component orthogonal to the alignment direction of the liquid crystal 1 is the ordinary refractive index n _o of the liquid crystal 1. feel n _min), the polarization component which is oriented parallel to the direction component of the liquid crystal 1 6 feel crystal 1 of the extraordinary refractive index n _e (n _max).

If the refractive index of the substance forming the diffraction grating 2 is n _g , the wavelength of the incident light 5 is λ, and the thickness of the diffraction grating 2 is T, in the case of a rectangular diffraction grating, the polarization components 6, 6 of the incident light 5 are The diffraction efficiency η _o of the zero-order transmitted (diffracted) light for each of ′ is approximately represented by the following equation (1).

η _o (1/2) {1 + cos [(2πΔnT) / λ]} (1) where Δn is the difference between the refractive index n _g of the diffraction grating 2 and the refractive index (n _e or n _o ) of the liquid crystal 1. In the static state, the incident light 5
Δn = | n _e −n _g | for the polarization component 6 of the above, and Δn = | n _o −n _g | for the polarization component 6 ′ of the incident light 5.

Therefore, from the equation (1), when Δn = 0, that is, n _e = n _g and n _o = n _g , the diffraction efficiency η _o of zero-order transmitted (diffracted) light is 1.

The diffraction efficiency η _o becomes zero when ΔnT = (m + 1/2) λ (m = 1,2,3, ...).

Next, when an electric field is applied to the liquid crystal 1 through the transparent electrode 3,
The alignment direction (optical axis direction) of the liquid crystal 1 begins to tilt with respect to the surface of the substrate 4 while being parallel to the surface of the substrate 4, and the tilt angle θ gradually increases. At this time, the polarization component 6 of the incident light 5 'felt ordinary refractive index n _o of the irrespective of the tilt angle θ LCD 1,
Polarized component 6 of the incident light 5 is equivalent to that the extraordinary refractive index n _e of the liquid crystal 1 according to the inclination angle of the electric field applied amount, or a liquid crystal 1 theta and ordinary refractive index n _o is synthesized at a ratio that depends on the inclination of the liquid crystal 1 Feel the combined refractive index nθ.

When the amount of applied electric field is further increased, the liquid crystal 1 tilts at a large angle close to a state perpendicular to the surface of the substrate 4 and is finally aligned perpendicularly to the surface of the substrate 4 to be in a homeotropic alignment state. The polarization components 6 and 6'are both the ordinary refractive index of the liquid crystal.
feel n _o. Even in this state, the incident light 5 is modulated according to the equation (1).

FIG. 2 shows (1) for the polarization component 6 of the incident light 5 when n _o = n _g is set in the entire wavelength region of the incident light 5 in the optical modulator having the configuration shown in FIG. It is a figure which shows the voltage (electric field amount) dependence of the diffraction efficiency (eta) _o shown by a formula.

In FIG. 2, the horizontal axis represents the voltage (V) applied to the electrode 3 to change the refractive index, the vertical axis represents the diffraction efficiency η _o of the zero-order light, and γ ₁ and γ ₂ are V-η _o curves. It represents the main slope. As can be seen from FIG. 2, the slope of the V-η _o curve has two slope components γ ₁ and γ ₂ , the curve changes slowly, and the polarization component 6 of the incident light 5 is completely transmitted. It takes time to do.

Therefore, in the present light modulation element, in the state in which the liquid crystal 1 is tilted at a large angle with respect to the surface of the substrate 4 and is in front of being oriented perpendicularly to the surface of the substrate 4, the combined refractive index nθ of the liquid crystal 1 and the diffraction grating 2 are provided.
Is set so that the refractive index n _{g of the} substance forming the same becomes almost equal in the entire wavelength region of the incident light 5, and the slope of the V-η _o curve is made steep to improve the responsiveness of the element, so that it can be driven at a low voltage. I have to.

Hereinafter, a specific configuration example of the present light modulation element will be described. Using a pair of glass substrates, a transparent electrode such as ITO is formed on one surface of each glass substrate, and SE is formed on the transparent electrode of one glass substrate.
A diffraction grating is formed with L-N (manufactured by Somaru Kogyo Co., Ltd., refractive index _ng = 1.57), and the two substrates are laminated so that the transparent electrodes face each other, and RO-TN403 (Roche nematic liquid crystal, (n _g = 1.79, n _o = 1.53) was filled to prepare the optical modulator shown in FIG.

Fig. 3 is a diagram showing the wavelength dispersion characteristics of the refractive index of SEL-N and the ordinary refractive index and extraordinary refractive index of RO-TN403 with respect to white light at room temperature, where the horizontal axis is the wavelength and the vertical axis is the refractive index. Shows the rate. As seen from FIG. 3, the refractive index n _g of the material constituting the diffraction grating 2 is dispersed is small, since the ordinary index of refraction n _o of the liquid crystal 1 dispersion is small, although the absolute values are different, the material constituting the diffraction grating 2 The shape of the wavelength component characteristic of the refractive index n _g and the wavelength dispersion characteristic of the ordinary refractive index n _o of the liquid crystal 1 are the same in all wavelength regions of the incident light.

Therefore, in the present embodiment, the material forming the diffraction grating 2 and the combined refractive index nθ of the liquid crystal 1 in a state in which the liquid crystal 1 is tilted at a large angle with respect to the surface of the substrate 4 and is in front of being oriented perpendicularly to the surface of the substrate 4. The refractive indices n _g are set to be equal. In this state, the value of the composite refractive index nθ is equal to the extraordinary refractive index n _{e of the} liquid crystal 1.
To greatly depends on the value of even ordinary refractive index n _o than, the wavelength dispersion characteristics of the complex index of refraction nθ becomes as dispersed like the ordinary refractive index n _o is small, the refractive index of the material constituting the thus diffraction grating 2
It is almost the same as the wavelength dispersion characteristic of n _g (small dispersion).

Therefore, according to the present embodiment, the composite refractive index nθ of the liquid crystal 1 can be made substantially equal to the refractive index _{ng of the} substance forming the diffraction grating 2 in the entire wavelength range.

FIG. 4 is a diagram showing the voltage dependence of the diffraction efficiency η _o shown in the equation (1) with respect to the polarization component 6 of the incident light 5 in the light modulation element of the present embodiment. As can be seen from FIG. 4, the V-η _o curve in this example shows a steep rise,
The gradient component is also mainly composed of γ ₁ , and is superior in responsiveness to the light modulation element having the V-η _o curve of FIG. 2 and can be driven at a low voltage.

In this embodiment, a transmissive optical modulator is shown, but it is also possible to provide a light reflective film on one of the substrates to make it a reflective type.

5 to 7 are schematic views showing another embodiment of the light modulation element of the present invention, in which the same members as those in FIG. 1 are designated by the same reference numerals. Reference numerals 7 and 7'indicate directions of the optical axis of the liquid crystal 1, 7 indicates a direction perpendicular to the paper surface, and 7'indicates a direction parallel to the paper surface and orthogonal to the direction 7. 2 '
Is a diffraction grating (not shown) and 4'is a transparent substrate.

FIG. 5 shows an element formed by superposing diffraction gratings 2 and 2'so that the arrangement directions thereof are orthogonal to each other, and is formed by using a pair of the light modulation elements of FIG. With this configuration,
It is possible to simultaneously modulate the polarization components 6, 6'of the incident light 5 shown in FIG.

FIGS. 6 and 7 show an element in which the shape of the diffraction grating is changed in the light modulation element shown in FIG. 1, FIG. 6 shows a triangular wave shape, and FIG. A modulation element is shown. As described above, the shape of the diffraction grating of the present light modulation element is not limited to the rectangular shape, but can be various shapes. However,
The equation of diffraction efficiency as shown in equation (1) differs depending on the shape of the diffraction grating.

As the liquid crystal, those difference in refractive index of an ordinary refractive index n _o and the extraordinary refractive index n _e is large, preferably the refractive index difference is better of 0.2 or more.

<Effects of the Invention> As described above, according to the present invention, it is possible to provide an optical modulator having excellent responsiveness.

[Brief description of drawings]

FIG. 1 is a diagram showing a basic configuration of an optical modulator of the present invention. FIG. 2 is a diagram showing the relationship between the voltage and the diffraction efficiency of the light modulation element of FIG. FIG. 3 is a diagram showing wavelength dispersion characteristics of the refractive index of the diffraction grating and the liquid crystal of the light modulation element of the present invention. FIG. 4 is a diagram showing the relationship between the voltage and the diffraction efficiency of the light modulation element of the present invention. 5 to 7 are views showing another embodiment of the optical modulator of the invention. 1 ... Liquid crystal 2, 2 '... Diffraction grating 3, 3' ... Transparent electrode 4, 4 '... Transparent substrate 5 ... Incident light 6,6' ... Polarization components of incident light orthogonal to each other 7, 7 ′ …… Optical axis of liquid crystal

 ─────────────────────────────────────────────────── --Continued from the front page (56) References JP-A-48-22047 (JP, A) JP-A-51-7947 (JP, A) US Pat. No. 4251137 (US, A)

Claims (1)

[Claims] 1. A diffraction grating structure in which transparent members and liquid crystals are alternately arranged, and means for controlling the refractive index of the liquid crystal, wherein the maximum refractive index n _max and the minimum refractive index n _{min of the} liquid crystal are provided. An optical modulation element characterized in that the refractive index nθ existing between the two is substantially equal to the refractive index n _{g of the} transparent member.