Thin-Film Absorption Filters
A thin-film absorption filter is a material layer with an absorption edge at the desired wavelength, typically designed as a longwave-pass filter. These filters utilize semiconductors that exhibit a sharp transition from opacity to transparency at the intrinsic edge, making them ideal for simple, effective longwave-pass applications. While the edge transitions are not as steep as those achieved by interference filters, absorption filters are easier to fabricate.
Reflection Loss in the Pass Region
The main challenge is managing reflection loss in the pass region due to the high refractive index of the film. For example:
- Germanium, with an absorption edge at 1.65 μm and a refractive index of 4.0, requires a thickness of at least several quarter-waves to achieve useful rejection.
- Without additional coatings, interference fringes cause reflectance variations from substrate levels at half-wave positions to as high as 68% at quarter-wave positions.
This issue can be addressed by applying antireflection coatings:
- Between the substrate and the germanium layer.
- Between the germanium layer and the air.
Antireflection Coating Design
Single quarter-wave antireflection coatings are typically sufficient. Optimal refractive indices for these coatings are:
- 2.46 between glass and germanium,
- 2.0 between germanium and air.
Zinc sulfide, with an index of 2.35, is a practical choice for both interfaces. It reduces reflectance near the peak of the quarter-wave coatings to about 2.1% for wavelengths where the germanium layer is an odd multiple of quarter-waves. At half-wave multiples, the germanium layer acts as an absentee layer, and the two zinc sulfide layers combine to form a half-wave, maintaining a reflectance of 4%.
Materials for Absorption Filters
Other materials commonly used for single-layer absorption filters include:
- Cerium dioxide: For ultraviolet rejection with visible transmission.
- Silicon: For longwave-pass filters with an edge at 1 μm.
- Lead telluride: For longwave-pass filters with an edge at 3.4 μm.
Practical Example
A practical lead telluride filter is illustrated in Figure 7.1, along with its design. The two zinc sulfide layers are quarter-waves at 3.0 μm. Improved performance might be achieved by increasing their thicknesses to quarter-waves at 4.5 μm.
