Optical filter and method for manufacturing optical filter

1. An optical filter, comprising:

a transparent substrate; and

an optical multilayer film formed on a surface of the transparent substrate and comprising a low-refractive index film and a high-refractive index film such that the low-refractive index film and the high-refractive index film are alternately stacked,

wherein the high-refractive index film is higher in refractive index than the low-refractive index film, and the low-refractive index film includes silicon oxide and has a low density portion formed closer to an interface between the low-refractive index film and the high-refractive index film than a middle portion of the low-refractive index film such that a density of the low density portion is lower than a density of the middle portion of the low-refractive index film.

2. The optical filter according to claim 1 , wherein the low-refractive index film comprises a first low-refractive index film stacked on the high-refractive index film and forming the low density portion and a second low-refractive index film stacked on the first low-refractive index film and forming the middle portion higher in density than the first low-refractive index film.

3. The optical filter according to claim 2 , wherein the first low-refractive index film has a physical film thickness of 3 nm or more and equal to or lower than half a physical film thickness of the low-refractive index film.

4. The optical filter according to claim 2 , wherein the transparent substrate is fluorophosphoric acid glass.

5. The optical filter according to claim 1 , wherein the low-refractive index film becomes higher in density with increasing distance from the interface with the high-refractive index film in a direction vertical to the surface of the transparent substrate.

6. The optical filter according to claim 5 , wherein the transparent substrate is fluorophosphoric acid glass.

7. The optical filter according to claim 1 , wherein the high-refractive index film includes an oxide material selected from the group consisting of titanium, niobium, and tantalum.

8. The optical filter according to claim 7 , wherein the low-refractive index film comprises a first low-refractive index film stacked on the high-refractive index film and forming the low density portion and a second low-refractive index film stacked on the first low-refractive index film and forming the middle portion higher in density than the first low-refractive index film.

9. The optical filter according to claim 8 , wherein the first low-refractive index film has a physical film thickness of 3 nm or more and equal to or lower than half a physical film thickness of the low-refractive index film.

10. The optical filter according to claim 7 , wherein the low-refractive index film becomes higher in density with increasing distance from the interface with the high-refractive index film in a direction vertical to the surface of the transparent substrate.

11. The optical filter according to claim 7 , wherein the transparent substrate is fluorophosphoric acid glass.

12. The optical filter according to claim 1 , wherein the optical multilayer film is formed by vacuum deposition.

13. A near-infrared cut filter, comprising the optical filter of claim 1 .

14. The optical filter according to claim 1 , wherein the transparent substrate is fluorophosphoric acid glass.

15. A method for manufacturing an optical filter, comprising:

forming an optical multilayer film on a surface of a transparent substrate by alternately stacking a low-refractive index film and a high-refractive index film,

wherein the high-refractive index film is higher in refractive index than the low-refractive index film, and the forming comprises forming the low-refractive index film such that the low-refractive index film includes silicon oxide and has a low density portion formed closer to an interface between the low-refractive index film and the high-refractive index film than a middle portion of the low-refractive index film and that a density of the low density portion is lower than a density of the middle portion of the low-refractive index film.

16. The method for manufacturing an optical filter according to claim 15 , wherein the forming comprises stacking a first low-refractive index film forming the low density portion on the high-refractive index film and stacking a second low-refractive index film forming the middle portion higher in density than the first low-refractive index film, on the first low-refractive index film.

17. The method for manufacturing an optical filter according to claim 15 , wherein the forming comprises forming the low-refractive index film such that the low-refractive index film becomes higher in density with increasing distance from the interface with the high-refractive index film in a direction vertical to the surface of the transparent substrate.

18. The method for manufacturing an optical filter according to claim 15 , wherein the forming comprises controlling a degree of vacuum in a film forming chamber such that the density of the low-refractive index film is controlled.

19. The method for manufacturing an optical filter according to claim 18 , wherein the forming comprises controlling a flow rate of an oxygen gas to be introduced into the film forming chamber such that the density of the low-refractive index film is controlled.

20. The method for manufacturing an optical filter according to claim 15 , wherein the forming comprises forming the optical multilayer film by vacuum deposition.