I am preparing an optical system in infrared, however I will need to stick two optical components. They are made from CaF2.
Can you recommend index matching-liquid? I am interested in the range 1um-15um. I see that other researchers use paraffin oil, but still i would like to make sure of all possibilities.
Or maybe there are few liquids, that would work eg. in 1um-5um, 5um-10um and so on?
I can't solve your problem, but I can give you some hints and things to keep in mind. First of all, CaF2 is transparent in the range you are interested in, but has a very strong band below about 30 microns. Accordingly, it has normal dispersion, but a comparably strong one: The index of refraction is about 1.4 at 5 microns which decreases to 1.3 at 10 microns (see e.g. https://refractiveindex.info/?shelf=main&book=CaF2&page=Malitson ). On the other hand, I am not aware of one liquid that is completely transparent in the infrared range and this means that their index of refraction is increasing with increasing wavelength. Maybe some Haloalkanes (no hydrogens left!) might work for you.
A few came to mind, but none is perfect. You will need to choose based on your application. Each of these below has certain low absorption window in the MIR and FIR, and the optical dispersion (or refractive indices) for some may need to be further characterized.
(1) Acetonitrile
See "Schafer SA et al., Mechanism of Biliary Stone Fragmentation Using the Ho:YAG Laser, IEEE Trans Biomed Eng, vol 41, no 3, 1994."
(2) Glycerol, 1,3-Butylene glycol, trimethylolpropane, Topicare(TM)
See "Viator JA, et al., Spectra from 2.5-15 um of tissue phantom materials, optical clearing agents and ex vivo human skin: implications for depth profiling of human skin, Phys Med Biol, 48 (2003) N15-N24."
Best of luck,
Kin
Dear Martin and Kin, Regarding Acetonitrile: Acetonitrile, 99%, exhibits its two strongest infra red absorption bands at wavelengths of 7.0 and 7.3 micrometers. https://pubchem.ncbi.nlm.nih.gov/compound/acetonitrile#section=Spectral-Properties Regards, Shigeo
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