Diffractive optical element, optical system, and optical apparatus which reduce generation of unnecessary light

Abstract

A diffractive optical element () includes a first diffraction grating () including a first grating surface () and a first grating wall surface (), a second diffraction grating () including a second grating surface () and a second grating wall surface (), and a thin film () provided between the first grating wall surface and the second grating wall surface and being in contact with both of the first and second grating wall surfaces, an extinction coefficient of the thin film with respect to a wavelength λ in a use wavelength band is not greater than 0.0005, and with respect to the wavelength λ, refractive indices of materials of the thin film, the first diffraction grating, and the second diffraction grating, a relative diffractive index difference of the thin film and the first diffraction grating, and a width of the thin film satisfy predetermined conditions.