Spectral filter for green and shorter wavelengths

• US 20040004779A1
• Filed: 06/04/2003
• Published: 01/08/2004
• Est. Priority Date: 06/04/2002
• Status: Active Grant

• Current Assignee: Lake Shore Cryotronics, Inc.
• Original Assignee: Lake Shore Cryotronics, Inc.
• Inventors: Kochergin, Vladimir, Swinehart, Philip
• Granted Patent: US 7,031,566 B2
• Time in Patent Office: Days
• US Class Current: 359/885
• CPC Class Codes: B82Y 20/00 Nanooptics, e.g. quantum optics or photonic crystals
• CPC Class Codes: G02B 5/201 in the form of arrays
• CPC Class Codes: G02B 5/204 in which spectral selection is performed by means of a conductive grid or array, e.g. frequency selective surfaces for use with wavelengths longer than the infrared light H01Q15/0006
• CPC Class Codes: G02B 5/207 comprising semiconducting materials
• CPC Class Codes: G02B 5/208 for use with infrared or ultraviolet radiation, e.g. for separating visible light from infrared and/or ultraviolet radiation
• CPC Class Codes: G02B 5/22 Absorbing filters G02B5/201 - G02B5/208 take precedence
• CPC Class Codes: G02B 6/1225 comprising photonic band-gap structures or photonic lattices
• CPC Class Codes: G03F 7/70575 Wavelength control, e.g. control of bandwidth, multiple wavelength, selection of wavelength or matching of optical components to wavelength
• CPC Class Codes: G03F 7/70941 Stray fields and charges, e.g. stray light, scattered light, flare, transmission loss
• CPC Class Codes: G03F 7/70958 Optical materials or coatings, e.g. with particular transmittance, reflectance or anti-reflection properties

Abstract

The UV, deep UV and/or far UV (ultraviolet) filter transmission spectrum of an MPSi spectral filter is optimized by introducing at least one layer of substantially transparent dielectric material on the pore walls. Such a layer will modify strongly the spectral dependences of the leaky waveguide loss coefficients through constructive and/or destructive interference of the leaky waveguide mode inside the layer. Increased blocking of unwanted wavelengths is obtained by applying a metal layer to one or both of the principal surfaces of the filter normal to the pore directions. The resulting filters are stable, do not degrade over time and exposure to UV irradiation, and offer superior transmittance for use as bandpass filters. Such filters are useful for a wide variety of applications including but not limited to spectroscopy and biomedical analysis systems.

Claims

1. An optical spectral filter comprising

a substrate or host wafer having an array of substantially uniform parallel hollow pores there through, the pores having a characteristic lateral dimensions in the plane of the host wafer within the range of from about 0.1 μ

m to about 20 μ

m, each pore supporting at least one waveguide mode in the transparency wavelength range of said filter, said wafer having first and second surfaces substantially perpendicular to the axis of the pores, and wherein the walls of each pore are coated with at least one layer of substantially transparent material at the transparency wavelength range of said spectral filter and wherein the thickness of each of said layers of transparent material is at least 10 nm.

2. A spectral filter of claim 1 wherein the wafer is comprised at least partially of porous semiconductor material.

3. A spectral filter of claim 2 wherein said porous semiconductor material is macroporous silicon.

4. A spectral filter of claim 2 said porous semiconductor material is mesoporous silicon.

5. A spectral filter of claim 2 wherein said porous semiconductor material is porous indium phosphide.

6. A spectral filter of claim 2 wherein said porous semiconductor material is porous gallium arsenide.

7. A spectral filter of claim 2 wherein said porous semiconductor material is chosen from the full possible range of alloys and compounds of zinc, cadmium, mercury, silicon, germanium, tin, lead, aluminum, gallium, indium, bismuth, nitrogen, oxygen, phosphorus, arsenic, antimony, sulfur, selenium and tellurium.

8. A spectral filter of claim 1 wherein the wafer is comprised at least partially of porous aluminum oxide.

9. A spectral filter of claim 1, wherein the wafer has a thickness of from about 1 to about 5000 times the characteristic lateral dimension of the pores.

10. A spectral filter of claim 1, wherein each layer of the transparent pore coating in said filter is material selected from the group consisting of oxides, nitrides, oxynitrides and fluorides

11. A spectral filter of claim 1, wherein each layer of said optically transparent pore coating material has a thickness of about 10 nm to about 1000 nm.

12. A spectral filter of claim 1, wherein said layer of optically transparent material comprises in turn a multilayer composed of different optically transparent materials having a thickness not exceeding half the diameter of the pores.

13. A spectral filter of claim 1, wherein the filter is a long-pass filter.

14. A spectral filter of claim 1, wherein the filter is a short-pass filter.

15. A spectral filter of claim 1, wherein the filter is a band-pass filter.

16. A spectral filter of claim 1, wherein the filter is a band-blocking filter.

17. A spectral filter of claim 1, wherein centers of said pores are placed apart by a distance in the range of 0.1 micrometers to 20 micrometers, said distance being more than the smallest lateral dimension of said pores.

18. A spectral filter of claim 1, wherein said pores are spatially ordered in the plane of said wafer into a predetermined pattern having predetermined symmetry.

19. A spectral filter of claim 17, wherein said symmetry is hexagonal symmetry.

20. A spectral filter of claim 18, wherein said symmetry is cubic symmetry.

21. A spectral filter of claim 1, wherein said pores are spatially disordered in the plane of said wafer.

22. A spectral filter of claim 1, wherein said pores are disposed such that the pore pattern has a complex order having complex symmetry

23. A spectral filter of claim 1, wherein said pores are displaced such as the pore pattern has a complex order that does not have any simple symmetry.

24. A spectral filter of claim 1, wherein said pores have substantially circular cross sections.

25. A spectral filter of claim 1, wherein said pores have approximately square cross sections.

26. A spectral filter of claim 1, wherein said pores have elliptical, oval or rectangular cross sections with the length of one axis of said shape being different than that of another orthogonal axis of said shape.

27. A spectral filter of claim 26, wherein said length of one axis of the ellipsoid being different than that of another axis of ellipsoid by a multiple of from 1 to 100.

28. A spectral filter of claim 26, wherein said filter exhibits transmission in the transparency wavelength range that is substantially different for light having polarization aligned along the two orthogonal axes of said ellipsoid.

29. A spectral filter of claim 26, wherein the filter functions in an optical assembly as an optical polarizer within the transparency wavelength range of said spectral filter.

30. A spectral filter of claim 1, wherein said pores exhibit approximately constant lateral cross-section over the length of said pores.

31. A spectral filter of claim 1, wherein said pores are made to exhibit a modulated lateral cross section over at least some part of the length of said pores.

32. A spectral filter of claim 31, wherein said modulation is periodical with the period from about 50 nm to about 20 μ

m.

33. A spectral filter of claim 31, wherein said modulation is the superposition of two or more periodical modulations with periods from about 50 nm to about 20 μ

m each.

34. A spectral filter of claim 31, wherein said modulation is quasi-periodical with the period slowly changing along the depth of said pores in a predetermined fashion.

35. A spectral filter of claim 31, wherein said pores have more than one length segment of modulation along their depth.

36. A spectral filter of claim 35, wherein said length segments of modulation are of the same modulation and are spaced such that 180 degree optical phase shifts are formed between them, thus creating at least one narrow band of transmission through the filter.

37. A spectral filter of claim 35, wherein said length segments of modulation are of different periods and/or structures of modulation.

38. A spectral filter of claim 1, wherein said pores have at least one end tapered.

39. A spectral filter of claim 38 wherein said tapering is created such that the pore cross section is gradually increased when approaching said pore end with the rate of increase being in the range of 1 to 55 degrees with respect to the pore axis.

40. A spectral filter of claim 1 wherein said waveguide mode is a leaky waveguide mode.

41. A spectral filter of claim 1, wherein the structure of said least one layer of substantially transparent pore coating material is chosen to minimize losses of the leaky waveguide modes supported by each of said pores within at least part of the transparency wavelength range of said spectral filter.

42. A spectral filter of claim 1, wherein the structure of said at least one layer of substantially transparent pore coating material is chosen to maximize the losses of leaky waveguide modes supported by each of said pores in predetermined wavelength ranges outside the transparency wavelength range of said spectral filter.

43. A spectral filter of claim 1, wherein said pores are dimensioned and coated such that substantially only a fundamental leaky waveguide mode is supported within most of the transparency wavelength range of said spectral filter.

44. A spectral filter of claim 1, wherein said pores are dimensioned and coated such that two or more leaky waveguide modes are supported within most of the transparency wavelength range of said spectral filter.

45. A spectral filter of claim 1 wherein at least one layer of coating material that absorbs in at least some wavelength ranges outside the transparency wavelength range of said spectral filter material is disposed on at least one of the first or second surfaces of the filter wafer such as at least some portion of said each pore length is left uncoated by said at least one layer of absorbing material.

46. A spectral filter of claim 45 wherein said at least one layer of absorbing material comprises at least one layer of metal.

47. A spectral filter of claim 45 wherein said metal is chosen from the group consisting of Ag, Al, Cu, Ni, Fe, Au, In, Sn, Pt, Pd, Rh, Ru, and conducting oxides, nitrides and oxynitrides of metals.

48. A spectral filter of claim 45 wherein said at least one layer of absorbing material comprises at least one layer of a semiconductor.

49. A spectral filter of claim 1 wherein at least one layer of coating material that is reflecting at least at some wavelength ranges outside the transparency wavelength range of said spectral filter material is disposed on at least one of the first or second surfaces of the filter wafer such that at least some portion of each pore length is left uncoated by said at least one layer of reflective coating material.

50. A spectral filter of claim 1 wherein a dielectric multilayer coating that is highly reflective in at least some wavelength ranges outside the transparency wavelength range of said spectral filter is disposed on at least one of the first or second surfaces of the filter wafer such that at least some portion of each pore length is left uncoated by said at least one layer of reflective multilayer coating material.

51. A spectral filter of claim 1 wherein said wafer is disposed between two plates of material that is transparent in a predetermined spectral range.

52. A spectral filter of claim 51 wherein said plates are made of the material selected from the group consisting of silicon dioxide, UV-enhanced silicon dioxide, quartz, fused quartz, magnesium-, calcium-, barium-, lead- and lithium fluorides, cryolite (Na3AlF6), zinc sulfide and sapphire.

53. A spectral filter of claim 51 wherein said plates have both surfaces substantially flat and parallel.

54. A spectral filter of claim 51 wherein at least one surface of at least one of said plates is of a lens-like shape.

55. A spectral filter of claim 1 wherein the filter surface is plastically deformed to a predetermined non-planar shape.

56. A spectral filter of claim 1 wherein the last from the pore wall of said at least one layer of substantially transparent material completely fills said pores.

57. A spectral filter of claim 55 wherein said layer of substantially transparent material completely filling said pores comprises the core of said waveguide.

58. A spectral filter of claim 1 wherein said spectral filter is disposed contiguous to an optical detection means.

59. A spectral filter of claim 1 wherein said spectral filter is disposed in the optical system at some distance from the optical detection means.

60. An optical spectral filter for green and shorter wavelengths comprising

a substrate having an array of substantially uniform parallel hollow pores therethrough, the pores having a characteristic lateral dimensions in the plane of the substrate, each pore supporting at least one waveguide mode in a transparency wavelength range of said filter, said substrate having first and second surfaces substantially perpendicular to the axis of the pores, and wherein the walls of each pore are coated with at least one layer of substantially transparent material at the transparency wavelength range of said spectral filter.