Transmissive diffraction grating, and spectral separation element and spectroscope using the same
A transmissive diffraction grating includes a substrate and a plurality of ridges provided in a mutually parallel manner at constant periodicity p on the substrate. The ridges include a first layer, a second layer (refractive index n 2 : 2.0-2.5), and a third layer with non-continuous refractive indices, arranged in that order from the substrate outward. The first layer adjacent the substrate, in terms of its refractive index, exhibits a difference of 0.1 or less relative to the substrate. The second layer has a higher refractive index than the first layer and third layer and satisfies the following conditions. For a single ridge, the cross-sectional area S of a cross-section of the second layer perpendicular to the longitudinal direction of said ridge is in the range of 0.75p 2 k 1 θ 2 /(n 2 −1)<S<1.20p 2 k 1 θ 2 /(n 2 −1), were, θ is the angle of incidence onto the diffraction grating face, expressed in radian units, and the constant k 1 is 1.1. The thickness d 2 of the second layer is in the range of 0.70pk 2 θn 2 /(n 2 −1)<d 2 <1.30pk 2 θn 2 /(n 2 −1), where, the constant k 2 is 0.69.
1. A transmissive diffraction grating comprising a substrate and a plurality of ridges provided parallel to one another at constant periodicity p on the substrate,
wherein the ridges include a first layer, a second layer, and a third layer with non-continuous refractive indices arranged in that order from the substrate outward,
the first layer adjacent the substrate, in terms of its refractive index, exhibits a difference of 0.1 or less relative to the substrate,
the second layer has a higher refractive index than the first layer and third layer and satisfies the following conditions (A) to (C):
(A) the refractive index n 2 of the second layer is 2.0 to 2.5;
(B) for a single ridge, a cross-sectional area S of a cross-section of the second layer perpendicular to the longitudinal direction of said ridge is in the range of 0.75p 2 k 1 θ 2 /(n 2 −1)<S<1.20p 2 k 1 θ 2 /(n 2 −1), where, θ is the angle of incidence onto the diffraction grating face, expressed in radian units, and the constant k 1 is 1.1; and
(C) a thickness d 2 of the second layer is in the range of 0.70pk 2 θn 2 /(n 2 −1)<d 2 <1.30pk 2 θn 2 /(n 2 −1), where, the constant k 2 is 0.69.
2. The transmissive diffraction grating according to claim 1 , wherein a cross-section of the ridge perpendicular to the longitudinal direction of said ridge comprises a substantially rectangular first portion adjacent the substrate and an outwardly converging tapered second portion adjacent the first portion, and
a height h 1 of the first portion from the surface of the substrate is in the range of 0.2h<h 1 <0.7h, where the height of the ridge from the surface of the substrate is designated as h, and
an angle φ 1 between the normal to the surface of the substrate and the side face of the ridge in the first portion and an angle φ 2 between the normal to the surface of the substrate and the side face of the ridge in the second portion is in the range of 0°<φ 1 <5°, φ 1 <φ 2 <15°.
3. The transmissive diffraction grating according to claim 1 , wherein a plurality of mutually parallel second ridges are provided periodically on the face of the substrate on the side opposite the diffraction grating face.
4. The transmissive diffraction grating according to claim 1 , wherein the refractive index of the first layer is the same as the refractive index of the substrate.
5. The transmissive diffraction grating according to claim 1 , wherein the refractive index of the first layer is the same as the refractive index of the third layer.
6. The transmissive diffraction grating according to claim 1 , wherein the first layer is made of silicon dioxide and the substrate is made of quartz glass.
7. The transmissive diffraction grating according to claim 1 , wherein the first layer comprises a portion of the substrate.
8. The transmissive diffraction grating according to claim 7 , wherein the substrate is made of quartz glass.
9. The transmissive diffraction grating according to claim 1 , wherein the second layer is made of at least one type of material selected from the group consisting of Ta 2 O 5 , TiO 2 , and Nb 2 O 5 .
10. The transmissive diffraction grating according to claim 9 , wherein the second layer is made of Ta 2 O 5 .
11. The transmissive diffraction grating according to claim 1 , wherein the second layer, which is a high refractive index layer, is adapted so as to be divided into two portions sandwiching a low refractive index layer.
12. The transmissive diffraction grating according to claim 1 , wherein the angle of incidence θ during reception of light normal to the longitudinal direction of the ridges satisfies the condition |θ−θ 0 |<10°, where, θ 0 is defined by n i ·sinθ 0 (p/λ 0 )=0.5, where λ 0 is the center value of a wavelength band of incident light in a vacuum, and n i is the refractive index of a medium on the entrance side.
13. A spectral separation element utilizing a plurality of transmissive diffraction gratings according to claim 1 .
14. A spectroscope utilizing the spectral separation element according to claim 13 .
15. A spectroscope utilizing the transmissive diffraction grating according to claim 1 .