Refraction in plane-parallel plate—Reconsideration of method of measurement of refractive indices

Abstract

The prism (minimum deviation) method of measurement of refractive indices in solids has a couple of disadvantages including problems associated with manufacturing of a good prism itself. Besides this method is unsuitable to be used in piezo- and electro-optical experiments when one needs to achieve so-called homogeneous distribution of an interaction (electric field, stress, strain) within the sample. Besides, worked out with a large expenditure of energy and cost, the prism is usually useless in other applications (destructive method). Therefore, an “elder” method of measurement of refractive indices by measuring shift of the refracted beam of light in a plane-parallel plate has been reconsidered in this paper. The sample itself may be a cuboid (if one wishes to measure all of three principal refractive indices in optically biaxial crystal), or just a crystalline boule in which one is cutting off the conical and tail parts and by suitable mechanical treatment (lapping and polishing) reveals a pair of plane parallel surfaces properly oriented to crystallographic (or principal axes of the optical indicatrix) directions. In such case, however, one can only measure one (isotropic crystals) or two (optically uniaxial crystals) refractive indices. However, in case of this method as opposed to the prism method one can repeat the measurement a couple of times, e.g. after a series of consecutive annealing, when this is either impossible or undesirable to cut the boule into smaller pieces prior to the final annealing (nondestructive method). The error depends on the angle of incidence and on sample (boule) thickness and is usually of the order of 0.001. However, in the future it is expected that the error can be reduced by implementing an electronic readout of the beam shift below 1μm. Working of the method has been illustrated with a couple of examples in GdCOB (biaxial), LiNbO3 (uniaxial) and YAG:Cr,Mg (isotropic) crystals.