Optical element and optical device

1. An optical element comprising:

a transparent substrate;

a subwavelength structure layer disposed on the transparent substrate, the subwavelength structure layer having a refractive index different from a refractive index of the transparent substrate;

a minute concave and convex structure of one-dimensional grating formed on the subwavelength structure layer with a subwavelength period smaller than a wavelength to be used, where a concave portion reaches a boundary surface between the transparent substrate and the subwavelength structure layer; and

an open hole portion formed on a subwavelength structure layer side of the transparent substrate so as to communicate with the concave portion of the minute concave and convex structure and to be arranged with the same period as in the minute concave and convex structure of one-dimensional grating,

wherein at least at the open hole portion, a refractive index relative to an incident light is changed in a direction orthogonal to the boundary surface, and

wherein the optical element further includes a first portion located in the concave portion, and having a refractive index lower than that of any of the transparent substrate and the subwavelength structure layer.

2. The optical element according to claim 1 , wherein a cross-sectional shape of the minute concave and convex structure of one-dimensional grating in a periodic direction formed on the subwavelength structure layer has a shape for changing a width of a convex portion in a height direction of the convex portion.

3. The optical element according to claim 2 , wherein the cross-sectional shape of the minute concave and convex structure of one-dimensional grating in the periodic direction formed on the subwavelength structure layer includes a trapezoidal shape, a triangular shape, a partially circular shape, or a partially elliptical shape.

4. The optical element according to claim 1 , wherein the refractive index of the subwavelength structure layer is higher than the refractive index of the transparent substrate.

5. The optical element according to claim 1 , wherein the optical element has an optical function as a wave plate.

6. The optical element according to claim 1 , wherein the minute concave and convex structure of the subwavelength structure layer and a periodic structure due to the periodic arrangement of the open hole portion are periodically arranged, via a flat portion of a surface of the transparent substrate, in a direction of concavity and convexity arranged in the minute concave and convex structure, thereby forming a polarization selective diffraction grating.

7. The optical element according to claim 1 , wherein the optical element has an optical function as a polarization beam splitter.

8. The optical element according to claim 1 , further including a second portion located in the open hole portion, and having the refractive index lower than that of any of the transparent substrate and the subwavelength structure layer.

9. The optical element according to claim 8 , wherein the second portion is formed so as to fill the open hole portion in a depth direction of the open hole portion.

10. An optical device comprising:

an optical element including:

a transparent substrate;

a subwavelength structure layer disposed on the transparent substrate, the subwavelength structure layer having a refractive index different from a refractive index of the transparent substrate;

a minute concave and convex structure of one-dimensional grating formed on the subwavelength structure layer with a subwavelength period smaller than a wavelength to be used, where a concave portion reaches a boundary surface between the transparent substrate and the subwavelength structure layer; and

an open hole portion formed on a subwavelength structure layer side of the transparent substrate so as to communicate with the concave portion of the minute concave and convex structure and to be arranged with the same period as in the minute concave and convex structure of one-dimensional grating,

wherein at least at the open hole portion, a refractive index relative to an incident light is changed in a direction orthogonal to the boundary surface, and

wherein the optical element further includes a first portion located in the concave portion, and having a refractive index lower than that of any of the transparent substrate and the subwavelength structure layer.

11. The optical device according to claim 10 , wherein the optical device is constructed as a projector device for guiding a beam of light emitted from a light source to a liquid crystal display element and projecting a display image of the liquid crystal display element onto a display surface using a projector lens, and the optical element is disposed on an optical path between the light source and the projector lens.

12. The optical device according to claim 10 , wherein the optical device is constructed as an optical pickup device for condensing a beam of light emitted from a light source and irradiating the beam of light onto an optical recording medium via an objective lens so as to record and/or reproduce information, and the optical element is disposed on an optical path between the light source and the objective lens.