SUBMILLIMETER OPTICAL-PROPERTIES OF HEXAGONAL BORON-NITRIDE

The submillimeter optical properties of hot-pressed polycrystalline bo ron nitride with a predominantly hexagonal crystal structure were stud ied at room temperature from approximately 20 to 120 cm-1 (lambda = 50 0-84 mum) with a Fourier-transform spectrometer. Several grades were s tudied and probed both parallel and perpendicular to the material's op tic axis. The material was found to behave as a negatively uniaxial bi refringent crystal. The birefringence, defined as the difference betwe en the real part of the extraordinary and ordinary indices of refracti on (n(e)-n(o)), was quite large in one case with a value of -0. 152. T he material's absorption properties were also studied. For certain gra des, 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.