Silicon
beam splitter for broadband infrared spectroscopy
In Fourier
Transform
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:
* 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.
