spectroscopy at long wavelengths, the far infrared or terahertz
spectral range, the standard beamsplitter is the pellicle mylar film.
The pellicle thickness is chosen to give a peak maximum beamsplitter
efficiency at a desired frequency because of interference in the thin
film. The disadvantages of the mylar beamsplitters are:
These problems are eliminated by using a beamsplitter comprising of a thick Si element. This beamsplitter can be used for the entire spectral range normally covered by mylar (25 - 1000 inverse cm). The average efficiency is better than the maximum efficiency of the mylar beamsplitters. The maximum spectral resolution is set by the thickness of the Si element (0.7 inverse cm for a 2 mm thickness). The high purity Si is ideally transparent except for a narrow (10 inverse cm) phonon absorption band at 600 inverse cm. Therefore a single Si beamsplitter provides a uniform average efficiency that replaces the mylar for spectroscopy with a resolution set by the thickness of the Si element.