Antireflection film, and optical element and optical system that include the same
An antireflection film formed on a substrate having a refractive index between 1.40 and 1.85 inclusive for light having a wavelength of 550 nm includes an intermediate layer formed on the substrate, and a concavo-convex layer formed on the intermediate layer, the concavo-convex layer includes a part has a refractive index that continuously increases from a side closer to a surface toward a side closer to the substrate, the intermediate layer includes a plurality of thin film layers having different refractive indices, and at least one of the thin film layers has a refractive index higher than the refractive index of the substrate, and the thin film layers include a first layer formed on the substrate and having a physical thickness d 1 , and a second layer formed on the first layer and having a physical thickness d 2 , and satisfy predetermined conditional expressions.
1. An optical element comprising:
a substrate having a refractive index between 1.40 and 1.85 inclusive for light having a wavelength of 550 nm, and
an antireflection film including an intermediate layer arranged on the substrate and a concavo-convex layer arranged on the intermediate layer, wherein:
the concavo-convex layer includes a part having a refractive index that continuously increases from a side closer to a surface toward a side closer to the substrate,
the intermediate layer includes a plurality of thin film layers having different refractive indices, and at least one of the thin film layers has a refractive index higher than the refractive index of the substrate,
the thin film layers include:
a first layer arranged on the substrate, and
a second layer arranged on the first layer and having a physical thickness d 2 (nm), and
the following conditional expressions are satisfied:
ns<n2,
1.72≦n2≦1.95, and
230≦n2d2≦290,
where ns is the refractive index of the substrate at a wavelength of 550 nm, and n 2 is a refractive index of the second layer at a wavelength of 550 nm.
2. The optical element according to claim 1 , wherein the following conditional expressions are satisfied:
1.55≦n1≦1.76, and
100≦n1d1≦300,
where n 1 is a refractive index of the first layer at a wavelength of 550 nm, and and d 1 (nm) is a physical thickness of the first layer.
3. The optical element according to claim 1 , wherein:
the concavo-convex layer includes a homogeneous layer having a refractive index that does not change in a thickness direction, and
the thin film layers further include a third layer formed on the second layer and having a physical thickness d 3 (nm), and
the following conditional expressions are satisfied:
1.35≦na≦1.58, and
70 ≦n 3 d 3 +nada≦ 100,
where na is a refractive index of the homogeneous layer at a wavelength of 550 nm, da (nm) is a physical thickness of the homogeneous layer, and n 3 is a refractive index of the third layer at a wavelength of 550 nm.
4. The optical element according to claim 1 , wherein the concavo-convex layer consists of plate crystal including aluminum oxide as a primary component.
5. The optical element according to claim 1 , wherein:
the concavo-convex layer is formed by performing a hot-water treatment on a layer including aluminum oxide as a primary component, and
the thin film layers are formed by an evaporation method or sputtering method.
6. The optical element according to claim 1 , wherein the concavo-convex layer has an average pitch smaller than a shortest wavelength in a use wavelength range.
7. The optical element according to claim 1 , wherein a use wavelength range is between 400 nm and 700 nm inclusive.
8. The optical element according to claim 1 , wherein the concavo-convex layer has an average pitch smaller than 400 nm.
9. The optical element according to claim 1 , wherein the concavo-convex layer has an optical thickness between 180 nm and 300 nm inclusive.
10. An optical system comprising a plurality of optical elements, wherein:
at least one of the optical elements includes a substrate having a refractive index between 1.40 and 1.85 inclusive for light having a wavelength of 550 nm, and an antireflection film including an intermediate layer arranged on the substrate and a concavo-convex layer arranged on the intermediate layer,
the concavo-convex layer includes a part having a refractive index that continuously increases from a side closer to a surface toward a side closer to the substrate,
the intermediate layer includes a plurality of thin film layers having different refractive indices, and at least one of the thin film layers has a refractive index higher than the refractive index of the substrate, and
the thin film layers include:
a first layer arranged on the substrate, and
a second layer arranged on the first layer and having a physical thickness d 2 (nm), and the following conditional expressions are satisfied:
ns<n2,
1.72≦n2≦1.95, and
230≦n2d2≦290,
where ns is the refractive index of the substrate at a wavelength of 550 nm, and n 2 is a refractive index of the second layer at a wavelength of 550 nm.
11. The optical element according to claim 1 , wherein the thin film layers further include a third layer arranged on the second layer and having a physical thickness d 3 (nm), and the following conditional expression are satisfied:
1.36≦n3≦1.55, and
n3d3≦100,
where n 3 is a refractive index of the third layer at a wavelength of 550 nm.