SUBMILLIMETER OPTICAL-PROPERTIES OF HEXAGONAL BORON-NITRIDE

The submillimeter optical properties of hot-pressed polycrystalline boron nitride with a predominantly hexagonal crystal structure were studied at room temperature from approximately 20 to 120 cm-1 (lambda = 500-84 mum) with a Fourier-transform spectrometer. Several grades were studied and probed both parallel and perpendicular to the material's optic axis. The material was found to behave as a negatively uniaxial birefringent crystal. The birefringence, defined as the difference between the real part of the extraordinary and ordinary indices of refraction (n(e)-n(o)), was quite large in one case with a value of -0. 152. The material's absorption properties were also studied. For certain grades, a modest dichroism was observed. The low absorption (alpha < 1 cm-1 ) for grade A at frequencies below 38 cm-1 suggests the possibility for millimeter/submillimeter wave applications. Results are compared with data obtained by other researchers on related materials.