Theory of the Elasto-Optic Effect in Nonmetallic Crystals

General expressions are derived for the magnitude and dispersion of elasto-optic coefficients in terms of strain-induced modifications of both the electronic energy-band structure, and, if present, the excitonic structure. This analysis uses the deformation potential concept in conjunction with oscillator models for the important optical transitions to describe strain-induced energy shifts, and also emphasizes the importance of strain-dependent oscillator strengths. Results are compared with existing elasto-optic dispersion data in materials having no excitonic contribution (e.g., LiNb${\mathrm{O}}_3$ and Si) and in materials with important excitonic contributions (e.g., alkali halides and CdS). In the case of ferroelectric crystals, two important ferroelectricity-related contributions to the elasto-optic effect are identified. The first relates to a strain-dependent Curie temperature, and the second to the enhancing effect of polarization fluctuations near the Curie point in the paraelectric phase.