Optical element, method for manufacturing optical element, optical system, and optical apparatus
An optical element includes a substrate, and an antireflection film formed on the substrate and including a plurality of layers. The optical element satisfies predetermined conditions.
1. An optical element comprising:
a substrate; and
an antireflection film formed on the substrate and including a plurality of layers,
wherein the following conditions are satisfied:
1.10≤ n m ≤1.30
when λ=430, ( a− 1.25) 2 +( b− 0.28) 2 ≤0.45 2
when λ=1000, ( a− 1.34) 2 +( b+ 0.28) 2 ≥0.30 2
when λ=1800, ( a− 1.14) 2 +( b+ 0.24) 2 ≤0.30 2
where n m is a refractive index for a wavelength λ (nm) of a top layer most distant from the substrate among the plurality of layers, and a base layer except the top layer among the plurality of layers has an optical admittance of Y=a+ib.
2. The optical element according to claim 1 , wherein the following conditions are satisfied:
when λ=430, ( a− 1.25) 2 +( b− 0.28) 2 ≤0.41 2
when λ=1800, ( a− 1.14) 2 +( b+ 0.24) 2 ≤0.20 2 .
3. The optical element according to claim 1 , wherein the following conditions are satisfied:
when 430≤λ≤700 or 1450≤λ≤1800, ( a− 1.28) 2 +( b ) 2 ≤0.55 2 , and
when 900≤λ≤1100, ( a− 1.30) 2 +( b+ 0.28) 2 ≥0.25 2 .
4. The optical element according to claim 1 , wherein the following expression is satisfied:
125≤ n m d m ≤250
where d m (nm) is a film thickness of the top layer.
5. The optical element according to claim 1 , wherein a reflectance of the antireflection film for a light ray with an incident angle on the antireflection film of 0° to 15° inclusive is 1.0% or less where 430≤λ≤700 or 1450≤λ≤1800.
6. The optical element according to claim 1 , wherein the antireflection film has seven layers or more.
7. The optical element according to claim 1 , wherein the base layer includes a plurality of layers made of different materials, and the following conditional expression is satisfied:
0.4≤ n H −n L ≤0.9
where n H is the highest refractive index in the materials in the base layer, and n L is the lowest refractive index of the materials in the base layer.
8. The optical element according to claim 1 , wherein the top layer has a void.
9. The optical element according to claim 1 , wherein the top layer is made of silicon oxide or magnesium fluoride.
10. The optical element according to claim 1 , wherein the layer in the base layer is made of oxide of titanium, tantalum, zirconium, chromium, niobium, cerium, hafnium, aluminum, silicon, and yttrium singularly or in mixture.
11. A manufacturing method of an optical element that includes a substrate, and an antireflection film formed on the substrate and including a plurality of layers, the manufacturing method comprising the steps of:
producing a base layer except a top layer most distant from the substrate among the plurality of layers; and
producing the top layer among the plurality of layers,
wherein the following conditions are satisfied:
1.10≤ n m ≤1.30
when λ=430, ( a− 1.25) 2 +( b− 0.28) 2 ≤0.45 2
when λ=1000, ( a− 1.34) 2 +( b+ 0.28) 2 ≥0.30 2
when λ=1800, ( a− 1.14) 2 +( b+ 0.24) 2 ≤0.30 2
where n m is a refractive index for a wavelength λ (nm) of the top layer, and the base layer has an optical admittance of Y=a+ib.
12. The manufacturing method according to claim 11 , wherein the step of producing the top layer produces the top layer by a sol-gel method.
13. The manufacturing method according to claim 11 , wherein the step of producing the base layer produces the base layer by a vacuum evaporation method or a sputtering method.
14. An optical system comprising a plurality of optical elements, at least one of which includes an optical system according to claim 1 .
15. An optical apparatus comprising an optical element according to claim 1 , and a holder configured to hold the optical element.