Optical element having periodic structure and optical apparatus using the same

An optical element comprising:

• a base portion having translucency;
• a first periodic structure portion being formed on the base portion and having a one-dimensional periodic structure in which a first medium and a second medium with a smaller refractive index than that of the first medium are alternately disposed in a first direction at a smaller period than a wavelength of incident light;
• and
• a second periodic structure portion being formed on the first periodic structure portion on an opposite side to the base portion and having a periodic structure in which a third medium and a fourth medium with a smaller refractive index than that of the third medium are alternately disposed in the first direction at a smaller period than the wavelength of the incident light,
• wherein the optical element functions as a phase plate,
• wherein, at an interface between the first periodic structure portion and the second periodic structure portion, a proportion of the first medium to the first periodic structure portion in the first direction and a proportion of the third medium to the second periodic structure portion in the first direction are different from each other, and
• wherein the second periodic structure portion has a two-dimensional periodic structure in which the third medium and the fourth medium are periodically disposed in the first direction and a direction different from the first direction.