Long Wave Pass Filters

Abstract

Random arrays of macropores in Si scatter light at wavelengths smaller than the average pore geometry, e.g., the pore-to-pore distance. This chapter describes how this property can be used for making an infrared long-wave pass filter (LWPF). Such a filter would scatter light with wavelengths below an edge wavelength that is defined by the porous layer geometry and morphology, but transmit the light effectively and uniformly for wavelengths above this edge. Infrared LWPFs have a number of important applications. They are used both in combination with narrowband pass filters (in order to achieve a narrow pass band with wide and deep rejection bands) and by themselves in applications where the radiation from shorter wavelengths should be suppressed in order to improve the signal-to-noise performance of detectors (as in astronomy, Fourier transform spectroscopy, etc).