Future Instruments Corporation

Silicon beamsplitter for broadband infrared spectroscopy
In Fourier Transform spectroscopy at long wavelengths, the far infrared or terahertz spectral range, the standard beamsplitter is the pelical mylar film. The pelical 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:
* Different thicknesses are required for different spectral ranges,
* The maximum efficiency is not close to ideal,
* At low frequencies the efficiency falls off as the square of the wavenumber,
* At high frequencies absorption in the mylar further reduces the beamsplitter
   efficiency.
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.



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