Optical element and optical system

Abstract

The optical element includes a base member configured to have an optical surface, a concave-convex structure configured to have an average pitch smaller than a shortest wavelength of a use wavelength range, and an intermediate layer formed between the optical surface and the concave-convex structure, made of a material different from that of the concave-convex structure, and having a refractive index between those of the base member and the material of the concave-convex structure. The optical surface is formed into a shape having a rotational symmetry axis. A thickness of the intermediate layer or each of thicknesses of the intermediate layer and the concave-convex structure varies so as to increase as a distance from the rotational symmetry axis increases. The optical element has good anti-reflection performance not only at a central part of the optical surface having a small curvature radius but also at a peripheral part thereof.

1 . An optical element comprising:
 a base member configured to have an optical surface;   a concave-convex structure configured to have an average pitch smaller than a shortest wavelength of a use wavelength range; and   an intermediate layer formed between the optical surface and the concave-convex structure, made of a material different from that of the concave-convex structure, and having a refractive index between those of the base member and the material of the concave-convex structure,   wherein the optical surface is formed into a shape having a rotational symmetry axis, and   wherein a thickness of the intermediate layer or each of thicknesses of the intermediate layer and the concave-convex structure varies so as to increase as a distance from the rotational symmetry axis increases.   
     
     
         2 . The optical element according to  claim 1 , wherein the concave-convex structure varies in structure in its thickness direction. 
     
     
         3 . The optical element according to  claim 1 , wherein the following condition is satisfied:
   λ/(8× n )≦ Dc ≦(2×λ)/ n  
   
       where Dc represents the thickness of the intermediate layer at a position of the rotational symmetry axis, n represents the refractive index of the intermediate layer, and λ represents the shortest wavelength of the used wavelength range. 
     
     
         4 . The optical element according to  claim 1 , wherein the following conditions are satisfied:
   0.3 ≦n ×sin(θ( h ))<1
       Dc<D ( h )< Dc /cos(θ( h ))
   
       where Dc represents the thickness of the intermediate layer at a position of the rotational symmetry axis, n represents the refractive index of the intermediate layer, D(h) represents the thickness of the intermediate layer at a h-position away from the rotational symmetry axis by a distance h, and θ(h) represents an angle that a light ray reaching the intermediate layer at the h-position in parallel with the rotational symmetry axis forms in the intermediate layer with a normal to a concave-convex structure-side most superficial surface of the intermediate layer at the h-position. 
     
     
         5 . An optical system comprising:
 an optical element;   wherein the optical element comprising:   a base member configured to have an optical surface;   a concave-convex structure configured to have an average pitch smaller than a shortest wavelength of a use wavelength range; and   an intermediate layer formed between the optical surface and the concave-convex structure, made of a material different from that of the concave-convex structure, and having a refractive index between those of the base member and the material of the concave-convex structure,   wherein the optical surface is formed into a shape having a rotational symmetry axis, and   wherein a thickness of the intermediate layer or each of thicknesses of the intermediate layer and the concave-convex structure varies so as to increase as a distance from the rotational symmetry axis increases.   
     
     
         6 . The optical system according to  claim 5 , wherein the following conditions are satisfied:
   0.3 ≦n ×sin(ψ( h ))<1
       Dc<D ( h )< Dc /cos(ψ( h ))
   
       where Dc represents the thickness of the intermediate layer at a position of the rotational symmetry axis, n represents the refractive index of the intermediate layer, D(h) represents the thickness of the intermediate layer at a h-position away from the rotational symmetry axis by a distance h, and ψ(h) represents an angle that a light ray having an average incident angle among all light rays passing through the h-position of the intermediate layer in the optical system forms in the intermediate layer with a normal to a concave-convex structure-side most superficial surface of the intermediate layer at the h-position. 
     
     
         7 . The optical system according to  claim 5 , further comprising:
 an aperture stop;   wherein the following conditions are satisfied:
   0.3 ≦n ×sin(ζ( h ))<1
 
     Dc<D ( h )< Dc /cos(ζ( h ))
 
   
       where Dc represents the thickness of the intermediate layer at a position of the rotational symmetry axis, n represents the refractive index of the intermediate layer, D(h) represents the thickness of the intermediate layer at a h-position away from the rotational symmetry axis by a distance h, and ζ(h) represents an angle that a light ray passing through the h-position of the intermediate layer and a center of the aperture stop in the optical system forms in the intermediate layer with a normal to a concave-convex structure-side most superficial surface of the intermediate layer at the h-position. 
     
     
         8 . An optical apparatus comprising:
 an optical system configured to include an optical element;   wherein the optical element comprising:   a base member configured to have an optical surface;   a concave-convex structure configured to have an average pitch smaller than a shortest wavelength of a use wavelength range; and   an intermediate layer formed between the optical surface and the concave-convex structure, made of a material different from that of the concave-convex structure, and having a refractive index between those of the base member and the material of the concave-convex structure,   wherein the optical surface is formed into a shape having a rotational symmetry axis, and   wherein a thickness of the intermediate layer or each of thicknesses of the intermediate layer and the concave-convex structure varies so as to increase as a distance from the rotational symmetry axis increases.