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Si is a high refractive index material (3.4 @ 8 microns) with very broad spectral transmission in the mid-IR and far-IR regions of the spectrum. However, this broad spectral transmission is not contiguous and Si has very strong absorption in the mid-IR due to impurities; interstitial oxygen at 1107 cm-1, and substitutional carbon at 605 cm-1. These impurity bands can be reduced by using Si produced by the “float zone” technique but another absorption mechanism is also present in Si, namely the multi-phonon lattice bands, the “two phonon band” centered at 630 cm-1 and the “three phonon band” centered at 1118 cm-1. All of these absorption bands are fairly broad and thus overlap. As a result Si optical elements have never been popular in general mid-IR spectroscopic applications. Alternatives are ZnSe and Ge.
Advantages for all Spectral Regions and Application
Advantages as a Far-IR Beamsplitter
Disadvantages as a Far-IR Beamsplitter
Advantages for General Spectroscopy Sampling, Window or ATR
Disadvantages for General Spectroscopy Sampling, Window or ATR
Advantages for Use in Si Semiconductor Process Chemical Research
Melting Point: 1420 °C
Density: 2.3 g/cm3
Solubility in H2O: Insoluble
Hardness: 1150 kg/mm2
Short Wavelength Limit: 8900 cm-1 (1 mm)
Long Wavelength Limit (mid-IR): 624 cm-1 (1 mm), 969 cm-1 (4 mm)
The infrared throughput of Si can be significantly increased with application of specialized BBAR coatings to levels exceeding 95% transmission. It may also be coated to control depth of penetration in ATR applications.
Short and Long Wavelength Limits defined for which transmissivity is greater than 50% of stated crystal thickness.
1. Reference for above RI values: The Infrared Handbook, The Infrared Information Analysis Center, Environmental Institute of Michigan, 1985.