Optical device

Provided is an optical device that includes a substrate, a plurality of pillars arrayed on the surface of the substrate at intervals of a pitch that is equal to or less than the wavelength of incident light, and a medium that fills the gaps between the pillars and that has a different refractive index from the pillars. Each of the pillars has one or more steps that discontinuously change the transverse cross-sectional area of the pillar, along the direction perpendicular to the surface of the substrate. The transverse cross-sectional area of at least one of step portions formed by the steps of the pillar differs from the transverse cross-sectional area of an adjacent pillar.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of Japanese Application No. 2010-152208 filed in Japan on Jul. 2, 2010, the contents of which is hereby incorporated by its reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical device.

2. Description of Related Art

A half-mirror that can adjust the reflectance by using dielectric multilayer films is conventionally known (for example, see Japanese Unexamined Patent Application, Publication No. Sho-64-72102). Further, a anti-reflection technology by using multilayer films is also known (for example, see Japanese Unexamined Patent Application, Publication No. 2003-98312).

In these cases, the reflectance is accurately controlled with multilayer films by adjusting the thickness and the refractive index of each layer of the multilayer films.

However, in the case of the multilayer films described in Japanese Unexamined Patent Application, Publication No. Sho-64-72102 and Japanese Unexamined Patent Application, Publication No. 2003-98312, it is difficult to change the thickness and the refractive index of each layer of the multilayer films toward an in-plane direction that is perpendicular to the thickness direction in each layer. As a result, it is difficult to configure an optical device in which refractive power is accurately controlled with multilayer films.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in view of the above-described circumstances and provides an optical device capable of accurately controlling the in-plane refractive index while accurately controlling the reflectance of incident light.

According to the present invention, there is provided an optical device including: a substrate; a plurality of pillars arrayed on a surface of the substrate at intervals of a pitch that is equal to or less than the wavelength of incident light; and a medium that fills gaps between the pillars and that has a different refractive index from the pillars, in which each of the pillars has one or more steps that discontinuously change the transverse cross-sectional area of the pillar along the direction perpendicular to the surface of the substrate; and the transverse cross-sectional area of at least one of step portions formed by the steps of the pillar differs from the transverse cross-sectional area of an adjacent pillar.

According to the present invention, a subwavelength structure is configured by a plurality of pillars arrayed on the surface of the substrate and the medium filling gaps between the pillars, and incident light can be transmitted without diffraction. In each of the pillars, a plurality of step portions are formed by steps that discontinuously change the transverse cross-sectional area of the pillar. Each of the step portions functions in the same way as a layer having an effective refractive index determined by the volume ratio of the pillar and the medium around the pillar, and, as a result, each pillar having the steps is equivalent to a multilayer film.

Then, when the transverse cross-sectional areas of adjacent pillars are made different at respective step portions, the effective refractive index can be changed in a direction along the surface of the substrate, and, as a result, the optical device has refractive power.

Specifically, in the subwavelength structure formed by the plurality of pillars each having the steps, the transverse cross-sectional areas at respective step portions are adjusted to change the effective refractive index in the in-plane direction to give refractive power, and the position of each step and the thickness dimension of each step portion are adjusted to form a structure equivalent to a multilayer film and to accurately control the reflectance of incident light.

In the above-described invention, one or more step portions having thicknesses equal to or less than the wavelength of the incident light may be provided on the tip surface of each of the pillars.

By doing so, one or more layers that cause the interference effect can be formed by the step portions formed on the tip surface of each of the pillars.

Furthermore, in the above-described invention, the step portions having thicknesses equal to or less than the wavelength of the incident light may have an antireflection function due to an interference effect.

Furthermore, in the above-described invention, the step portions having thicknesses equal to or less than the wavelength of the incident light may have a reflection increasing function due to an interference effect.

Furthermore, in the above-described invention, the medium may be air.

By doing so, the effective refractive index can be easily set by the volume ratio of the pillars and the air that is in the gaps between the pillars.

Furthermore, in the above-described invention, the medium may be made of a solid material.

By doing so, it is possible to prevent dust etc. from entering the gaps between the pillars because the gaps between the pillars are filled with the solid material.

Furthermore, in the above-described invention, the effective refractive indices determined by the volume ratios of the pillars and the medium in unit pitches that are formed by the pillars and the gaps between the pillars may have the largest value at the center of the optical device and may be gradually reduced toward the peripheries of the optical device.

By doing so, an optical device having positive refractive power can be easily configured.

Furthermore, in the above-described invention, the effective refractive indices determined by the volume ratios of the pillars and the medium in unit pitches that are formed by the pillars and the gaps between the pillars may have the smallest value at the center of the optical device and may be gradually increased toward the peripheries of the optical device'.

By doing so, an optical device having negative refractive power can be easily configured.

According to the present invention, an advantage is afforded in that the refractive index in the in-plane direction can be accurately controlled while accurately controlling the reflectance of incident light.

DETAILED DESCRIPTION OF THE INVENTION

An optical device1according to an embodiment of the present invention will be described below with reference to the drawings.

As shown inFIG. 1, the optical device1of this embodiment includes a substrate2made of a transparent material and a plurality of pillars3arrayed on one surface of the substrate2.

As shown inFIG. 1, each of the pillars3has a plurality of steps3aat intervals in the direction perpendicular to the surface of the substrate2. Each of the transverse cross-sectional areas of a plurality of step portions3bthat are formed by the steps3ais substantially constant, and the transverse cross-sectional areas are changed discontinuously at the positions of the steps3a.

The pillars3are arrayed at intervals of a constant pitch P that is equal to or less than the wavelength of incident light.

Since the pillars3are arrayed at intervals of the pitch P, which is equal to or less than the wavelength of incident light, a subwavelength structure is configured.

As shown inFIGS. 2 and 3, since each of the pillars3has one or more steps3a, the pillar3is regarded as a structure equivalent to a multilayer film A.

When the transverse cross-sectional areas are made different in the step portions3bcorresponding to the respective layers of the multilayer film A, the effective refractive indices are different in the unit periodic spaces repeated by the pitch P.

The effective refractive index can be calculated using Expression 1.

Here, nTEindicates the effective refractive index for TE polarized light, nTMindicates the effective refractive index for TM polarized light, n1indicates the refractive index of each of the step portions3bof the pillar3, n2indicates the refractive index of a medium that exists around the step portion3b, such as air B, and w indicates the width dimension of each step portion3b.

Furthermore, the TE polarization indicates that the electric field component of light oscillates in a direction parallel to a stripe structure (in the direction of an arrow S inFIG. 6), and the TM polarization indicates that the electric field component of light oscillates in a direction perpendicular to the stripe structure (in a direction perpendicular to the arrow S inFIG. 6).

According to the thus-configured optical device1of this embodiment, as shown inFIG. 3, it is possible to configure structures that are equivalent to different multilayer films A, in each pillar3. As a result, an advantage is afforded in that the light reflectance can be accurately controlled with the multilayer films A, and the optical device1has refractive power by arraying the different multilayer films A along the surface of the substrate.

Although it is difficult to realize these structures with the multilayer films A, the structures can be easily configured in the optical device1of this embodiment.

For example, as shown inFIG. 4, when the pillars3are configured such that the effective refractive indices of the pillars3have the largest value at the center of the optical device1and are gradually reduced toward the peripheries thereof, the optical device1can have positive refractive power for converging light L, as shown by arrows, as in a convex lens.

Furthermore, for example, as shown inFIG. 5, when the pillars3are configured such that the effective refractive indices of the pillars3have the smallest value at the center of the optical device1and are gradually increased toward the peripheries thereof, the optical device1can have negative refractive power for diverging the light L, as shown by arrows, as in a concave lens.

Note that, in the optical device1of this embodiment, the thickness dimension of a step portion3bthat is located at the tip of each pillar3may be set to the wavelength of the incident light L or less, thereby making the step portion3bfunction as an antireflection layer or a reflection increasing layer due to interference effects.

The antireflection layer or the reflection increasing layer may be configured by a single step portion3bor may be configured by a plurality of step portions3b.

Furthermore, the thickness dimensions of all step portions3bmay be set to 1 μm or less. With this structure, reflectance control and refractive power control can be executed even for broadband light, such as natural light.

Furthermore, the thicknesses of all step portions3bmay be set to 100 nm or less. In designing the multilayer film for visible light, it is known that setting the thickness of each layer to 100 nm or less is effective. By doing so, the same effect as that of a multilayer film can be obtained.

Furthermore, in the optical device1of this embodiment, a description has been given of a case where the pillars3are arrayed at intervals of the constant pitch P; however, the arrangement is not limited thereto, and the pitch P may be changed along the surface direction as long as the pitch P is equal to or less than the wavelength of incident light and the volume ratio of the pillars3and gaps between the pillars is changed.

Furthermore, a description has been given of a case where air is provided, as a medium B, in the gaps between the pillars3; however, the medium B is not limited thereto, and another medium B that has a refractive index different from that of a substance constituting the pillars3may fill the gaps. The medium B is not limited to gases, such as air, and a solid medium B may fill the gaps. In this case, there is an advantage in that dust etc. does not enter the gaps because the gaps are filled with the medium B.

Calculation examples of the reflectance of the optical device1of this embodiment are shown below.

A description will be given of a case where light having a polarization plane indicated by the arrow S is made to enter a stripe-type subwavelength structure in which the pillars3are arrayed in one dimensional direction at intervals of the constant pitch P, as shown inFIG. 6.

FIG. 7shows a pillar3in which an antireflection layer formed of a single step portion3bis provided on the tip surface. Specifically, the pillar3is constituted by a first layer and a second layer provided on the substrate2, and the second layer serves as an antireflection layer.FIG. 8shows the reflectance-wavelength characteristic obtained with and without the antireflection layer when light is made to enter the pillar3from the direction perpendicular to the tip surface of the pillar3. According toFIG. 8, it is clear that the reflectance is sufficiently reduced by the antireflection layer.

FIG. 9shows a pillar3in which antireflection layers formed of two step portions3bare provided on the tip surface. Specifically, the pillar3is constituted by a first layer to a third layer provided on the substrate2, and the second layer and the third layer serve as two antireflection layers.FIG. 10shows the reflectance-wavelength characteristic obtained with and without the antireflection layers when light is made to enter the pillar3from the direction perpendicular to the tip surface of the pillar3. According toFIG. 10, it is clear that the reflectance is sufficiently reduced by the antireflection layers.

FIG. 11shows a pillar3in which antireflection layers formed of five step portions3bare provided on the tip surface. Specifically, the pillar3is constituted by a first layer to a sixth layer provided on the substrate2, and the second layer to the sixth layer serve as multiple antireflection layers.FIG. 12shows various values of each layer, andFIG. 13shows the reflectance-wavelength characteristic obtained with and without the antireflection layers when light is made to enter the pillar3from the direction perpendicular to the tip surface of the pillar3. According toFIG. 13, it is clear that the reflectance is sufficiently reduced by the antireflection layers.

FIG. 14shows a pillar3in which reflection increasing layers formed of five step portions3bare provided on the tip surface. Specifically, the pillar3is constituted by a first layer to a sixth layer provided on the substrate2, and the second layer to the sixth layer serve as multiple reflection increasing layers.FIG. 15shows various values of each layer, andFIG. 16shows the reflectance-wavelength characteristic when light is made to enter the pillar3from a direction inclined by 45° with respect to the tip surface of the pillar3. According toFIG. 16, it is clear that the reflectance is sufficiently increased by the reflection increasing layers.