Anti-reflection structure with graded refractive index layer and optical apparatus including same
The anti-reflection structure includes a graded refractive index layer that is disposed on a substrate and whose refractive index decreases as a distance from the substrate increases, and a homogeneous layer that is disposed on the graded refractive index layer and whose refractive index is homogeneous. The structure satisfies a condition of nb−na>0.10 where na represents the refractive index of the homogenous layer, and nb represents a homogenous layer side effective refractive index of the graded index refractive index layer.
1. An anti-reflection structure comprising:
a first layer that is a graded refractive index layer which is disposed on a substrate and whose refractive index decreases as a distance from the substrate increases;
a second layer that is disposed on the first layer and whose refractive index is homogeneous, wherein refractive index of the second layer is na and 1.10<na<1.35, and
an interface between the first layer and the second layer, wherein the following condition is satisfied at the interface:
nb−na> 0.10
where nb represents refractive index of the first layer at the interface between the first layer and the second layer.
2. An anti-reflection structure according to claim 1 , wherein the second layer is a porous layer.
3. An anti-reflection structure according to claim 2 , wherein the second layer contains silica or magnesium fluoride.
4. An anti-reflection structure according to claim 2 , wherein the second layer is a mesoporous layer.
5. An anti-reflection structure according to claim 1 , wherein the following condition is satisfied:
| ns−nc|< 0.10
where ns represents a refractive index of the substrate, and nc represents refractive index of a part of the first layer that is adjacent to the substrate.
6. An anti-reflection structure according to claim 1 , further comprising:
a third layer that is disposed between the first layer and the substrate and whose refractive index is homogeneous.
7. An anti-reflection structure according to claim 1 , wherein the first layer is constituted by (a) a base layer having roughness including convex portions whose pitch is equal to or smaller than a wavelength of light entering the anti-reflection structure, and (b) a first medium with which a concave area around the convex portions in the base layer is filled and whose refractive index is lower than the base layer,
wherein a filling rate of the first medium increases as a distance from the substrate increases.
8. An anti-reflection structure according to claim 7 , wherein the base layer is formed of a same medium as that of the substrate.
9. An anti-reflection structure according to claim 7 , wherein the first medium is a same medium as that of the second layer.
10. An anti-reflection structure according to claim 1 , wherein the following condition is satisfied:
0.1< na·Da/λo< 0.5
where Da represents a thickness of the second layer, and λo is 550 nm.
11. An anti-reflection structure according to claim 1 , wherein the following condition is satisfied:
nb−na> 0.17.
12. An anti-refection structure according to claim 1 , wherein the interface is configured so that light reflected by the interface and light reflected by an interface between the second layer and air cancel each other.
13. An anti-reflection structure according to claim 1 , wherein nb is the minimum effective refractive index in the first layer.
14. An optical element comprising:
a substrate;
a first layer that is a graded refractive index layer which is disposed on a substrate and whose refractive index decreases as a distance from the substrate increases;
a second layer that is disposed on the first layer and whose refractive index is homogeneous, wherein refractive index of the second layer is na and 1.10<na<1.35, and
an interface between the first layer and the second layer, wherein the following condition is satisfied at the interface:
nb−na> 0.10
where nb represents refractive index of the first layer at the interface between the first layer and the second layer.
15. An optical apparatus comprising:
an optical element; and
an image pickup element
wherein the optical element comprises:
a substrate;
a first layer that is a graded refractive index layer which is disposed on a substrate and whose refractive index decreases as a distance from the substrate increases;
a second layer that is disposed on the first layer and whose refractive index is homogeneous, wherein refractive index of the second layer is na and 1.10<na<1.35, and
an interface between the first layer and the second layer, wherein the following condition is satisfied at the interface:
nb−na> 0.10
where nb represents refractive index of the first layer at the interface between the first layer and the second layer.