The electro-optic effect in crystals can be separated into two types of microscopic interaction: an electron-lattice contribution in which the applied field produces a lattice displacement, which in turn modifies the electronic polarizability (or refractive index), and a direct electron-field contribution in which the applied field modifies the electronic polarizability in the absence of lattice displacements. The latter contribution in LiNb and LiTa can be estimated from second-harmonic-generation experiments by Miller and Savage, and accounts for less than 10% of the refractive-index change. Each polar-lattice optic mode in LiNb and LiTa () contributes separately to the electro-optic effect an amount proportional to the product of its Raman-scattering efficiency and infrared oscillator strength. We have measured the absolute scattering efficiencies for LiNb and LiTa. The oscillator strengths for LiNb have been measured by Barker and Loudon. We find that the dominant contribution to the electro-optic coefficients and comes from the lowest-frequency mode; and to and from the next lowest mode. These same modes dominate the low-frequency dielectric constant. The absolute values of , , , and calculated from the combined Raman, infrared, and second-harmonic-generation data are in excellent agreement with the electro-optic coefficients measured directly by Turner. In addition to the absolute scattering efficiencies for all the transverse and longitudinal modes in LiTa and LiNb, we have also determined the mode frequencies and linewidths, which are important in calculating Raman gain.
- Received 27 March 1967
- Published in the issue dated August 1967
© 1967 The American Physical Society