Si, Ge, or InAs wafers with an AR coating. IR semiconductor filters function as long-pass filters with very good blocking.
Strictly speaking, semiconductor filters are not thin film filters, but absorption filters that rely on the electronic band structure of semiconductors for their action.
In order to use a semiconductor as an optical filter, two problems must be solved. Firstly, there is always some residual free carrier absorption in the pass region and, secondly, the high refractive index of semiconductors makes the reflection loss at the surface rather large.
The first difficulty is overcome by using a thin slice of very pure material, the second by depositing an anti-reflection coating on the surface.
Semiconductor filters have long-wave pass characteristics and consist of coated, optically polished discs of semiconductors that are mounted in holders for protection. They feature a very high rate of absorption in the suppression region.
The following IR semiconductor filters are available ring mounted with a diameter of approximately 25 mm as standard products.
Depending on the material used, they can be used for the following different wavelengths:
