Thermo-optic behaviour of solids

Summary

The refractive index of zinc-blende increases appreciably on heating for all wavelengths in the visible region the increase being 0·0212 and 0.0107 respectively for wavelengths 4358 Å.U. and 7320 Å.U. on heating from 0° to 205° C. It is shown that by applying the general theory of Part I one can satisfactorily explain both the variation ofdn/dt with wavelength and the courses of the temperature—refractive index curves from −80° to 700° C. for different wavelengths. The calculations show that the ultra-violet dispersion frequency at 2532 Å.U. alters appreciably with temperature, the proportional rate of change χ=−d (log ν)/dt being 70×10⁻⁶. This is nearly ten times the corresponding rate for diamond. The rate of change becomes less and less on lowering the temperature. It is pointed out that, for diamond, χ actually vanishes at very low temperatures and this is also probably the case for zinc-blende.