Patent ID: 8092929

Claim:
An optical element which includes a base composed of a LuAg substrate having a refractive index of 2.14±0.01 for a wavelength of 193 nm and an antireflection film formed on the base, and the optical element being configured to contact a liquid having a refractive index of 1.64±0.01 for the wavelength of 193 nm, said the antireflection film comprising: a high refractive index layer that is formed on the base, contains Al 2 O 3 having a refractive index of 1.87 to 1.92 for the wavelength of 193 nm, and has an optical film thickness of 0.21λ to 0.34λ for a design center wavelength λ of 193 nm; and a low refractive index layer that is closer to the liquid than the high refractive index layer, contains Al 2 O 3 having a refractive index equal to or smaller than 1.78 but larger than the refractive index of the liquid for the wavelength of 193 nm, and has an optical film thickness of 0.29λ to 0.52λ at the design center wavelength λ.