Several types of boron nitride (BN) were studied via terahertz time-domain spectroscopy by Mira Naftaly of the National Physical Laboratory (Teddington, England) and Jon Leist of Momentive Performance Materials (Strongsville, OH), with an eye toward terahertz optical materials. The terahertz time-domain spectrometer contained a femtosecond laser and gallium arsenide photoconductive emitter, off-axis parabolic mirrors, and light detection via a zinc telluride crystal and photodiodes. Types of BN tested included HBN, HBR, HBC, and HBT (all versions of BN manufactured using different binders) as well as pBN (pyrolytic BN, fully dense and manufactured by chemical vapor deposition with no binder).
All five types of BN had refractive indices on the order of 2 and were birefringent, with ne - no equaling -0.392 for pBN and ranging between -0.110 and -0.187 for the rest. The one type of BN having a reasonably good optical transmission was pBN, which has measured values of 0.62 cm-1 for the extraordinary ray and 4.1 cm-1 for the ordinary ray. Low-cost pBN is hard and durable. It also has low optical dispersion in the terahertz region. Contact Naftaly atmira.naftaly@npl.co.uk.
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