Diffractive optical element, optical system, and optical apparatus
A diffractive optical element includes a first diffractive grating including a first grating surface and a first grating wall surface, a light shielding member disposed on the first grating wall surface, and a second diffractive grating including a second grating surface and a second grating wall surface, disposed so that the second grating surface contacts the first grating surface and the second grating wall surface contacts the light shielding member. An extinction coefficient k of a material that constitutes the light shielding member meets the expression of 0.001<k<0.5.
1. A diffractive optical element comprising:
a first diffractive grating including a first grating surface and a first grating wall surface;
a light shielding member disposed on the first grating wall surface; and
a second diffractive grating including a second grating surface and a second grating wall surface, the second diffractive grating being disposed so that the second grating surface contacts the first grating surface and the second grating wall surface contacts the light shielding member,
wherein the following expression is met:
0.001 <k< 0.5,
where k is an extinction coefficient of a material that constitutes the light shielding member, and
wherein a thickness of the light shielding member increases from a valley of one of the first or second diffracting grating that receives an incident light to an apex of the one diffractive grating.
2. The diffractive optical element according to claim 1 , wherein:
the first diffractive grating has a first refractive index,
the second diffractive grating has a second refractive index different from the first refractive index, and
the light shielding member reduces total reflection light and transmitted light of incident light on the first grating wall surface and the second grating wall surface.
3. The diffractive optical element according to claim 1 , wherein the light shielding member has a film-shaped structure that is disposed between the first grating wall surface and the second grating wall surface.
4. The diffractive optical element according to claim 1 , wherein a refractive index of the material constituting the light shielding member is higher than a refractive index of a material having the lower refractive index of materials constituting the first diffractive grating and the second diffractive grating.
5. The diffractive optical element according to claim 1 , wherein the following condition is met:
| nd 3 −nd 2|<0.2,
where nd3 is a refractive index of the material constituting the light shielding member for d-line, and nd2 is a refractive index of a material having the higher refractive index of materials constituting the first diffractive grating and the second diffractive grating.
6. The diffractive optical element according to claim 1 , wherein the following condition is met:
W/P< 0.07,
where W is a width of whole of the light shielding member, and P is a grating pitch.
7. The diffractive optical element according to claim 1 , wherein the light shielding member has a plurality of film-shaped structures configured by laminating members made of a plurality of materials different from each other.
8. The diffractive optical element according to claim 1 , wherein a fine structure whose size is smaller than a wavelength is provided in an interface between the light shielding member and a grating wall surface of a diffractive grating having the higher refractive index of materials of the first diffractive grating and the second diffractive grating.
9. The diffractive optical element according to claim 8 , wherein the following condition is met:
p< 0.98×λ/ n 2,
where p is a pitch of the fine structure, n2 is a diffractive index of the material having the higher refractive index of the materials of the first diffractive grating and the second diffractive grating for the minimum wavelength in a use wavelength range, and λ is the minimum wavelength in the use wavelength range.
10. The diffractive optical element according to claim 1 , wherein a fine structure whose size is smaller than a wavelength is provided in a medium between the light shielding member and a grating wall surface of a diffractive grating having the higher refractive index of materials of the first diffractive grating and the second diffractive grating.
11. The diffractive optical element according to claim 1 , wherein the light shielding member includes a plurality of absorption materials having different extinction coefficients.
12. The diffractive optical element according to claim 11 , wherein the light shielding member includes a layer having a refractive index of 1.55, an extinction coefficient of k=0.03 and a width of 0.2 μm, a layer having a refractive index of 1.40, an extinction coefficient of k=0.09 and a width of 0.25 μm, and a layer having a refractive index of 1.53, an extinction coefficient of K=0.03 and a width of 0.15 μm.
13. The diffractive optical element according to claim 1 , wherein the material that constitutes the light shielding member is an absorption material having a refractive index of 1.55 and an extinction constant of k=0.1.
14. The diffractive optical element according to claim 13 , wherein the light shielding member has a maximum thickness of 0.2 μm.
15. The diffractive optical element according to claim 14 , wherein the first diffractive grating has a refractive index for a d-line of 1.504, an Abbe's number of 16.3, a partial dispersion ratio of 0.390 and a refractive index for wavelength 550 nm of 1.511, and the second diffractive grating has a refractive index for a d-line of 1.567, an Abbe's number of 47.0, a partial dispersion ratio of 0.569 and a refractive index for wavelength 550 nm of 1.570.
16. The diffractive optical element according to claim 1 , wherein the light shielding member is smaller in height than the first grating wall surface and the second grating wall surface.
17. The diffractive optical element according to claim 1 , wherein the first diffractive grating is larger in diffractive index than the second diffractive grating.
18. The diffractive optical element according to claim 17 , wherein the thickness of the light shielding member increases from the valley of the first diffractive grating to the apex of the first diffractive grating.
19. An optical system comprising:
a refractive optical element; and
a diffractive optical element including:
a first diffractive grating including a first grating surface and a first grating wall surface;
a light shielding member disposed on the first grating wall surface; and
a second diffractive grating including a second grating surface and a second grating wall surface, the second diffractive grating being disposed so that the second grating surface contacts the first grating surface and the second grating wall surface contacts the light shielding member,
wherein the following expression is met:
0.001< k≦ 0.5,
where k is an extinction coefficient of a material that constitutes the light shielding member, and
wherein a thickness of the light shielding member increases from a valley of one of the first or second diffractive grating that receives an incident light to an apex of the first diffractive grating.
20. An optical apparatus comprising:
an optical system including:
a refractive optical element; and
a diffractive optical element including:
a first diffractive grating including a first grating surface and a first grating wall surface;
a light shielding member disposed on the first grating wall surface; and
a second diffractive grating including a second grating surface and a second grating wall surface, the second diffractive grating being disposed so that the second grating surface contacts the first grating surface and the second grating wall surface contacts the light shielding member,
wherein the following expression is met:
0.001 ≦k≦ 0.5,
where k is an extinction coefficient of a material that constitutes the light shielding member, and
wherein a thickness of the light shielding member increases from a valley of one of the first or second diffractive grating that receives an incident light to an apex of the one diffractive grating.