Electric-Field-Induced Infrared Absorption and Raman Scattering in Diamond

We present a detailed discussion of our experimental results on the effect of an externally applied static electric field on the absorption of EM radiation by the $\overrightarrow{\mathrm{q}}\approx 0$ optical modes of diamond. It is shown that the normally infrared-inactive long-wavelength vibrational modes can become infrared active in the presence of the field. From the strength of the field-induced infrared absorption, we determine the absolute value for the Raman tensor of these modes, and its dependence on the field through a higher-order effect. The latter effect also manifests itself as a change in the intensity and/or the polarization selection rules of the first-order Raman scattering.