Stress-induced optical dichroism of[Li]0defects in MgO

Abstract

Stress-induced optical-dichroism and optical-absorption spectra are reported for MgO ${\mathrm{}\left[\mathrm{Li}\right]\mathrm{}}^0$ centers that were produced by quenching MgO:Li from 1200°C. An analysis of the spectra by the method of moments is presented. A strong temperature-dependent dichroism is attributed to the paraelastic alignment of inherently anisotropic ${\mathrm{}\left[\mathrm{Li}\right]\mathrm{}}^0$ defects. The anisotropy of the defects can be described by a ratio of the transition-dipole strengths, $\frac{D_{\parallel }}{D_{\perp }}\approx 2.2$, and an energy splitting, $\frac{E_{\parallel }}{E_{\perp }}\approx 0.37$ eV, for light polarized parallel ($\parallel$) and perpendicular ($\perp$) to the defect symmetry axis. These results are well explained by the polaron model of the defect, and no evidence is found for an ion-model contribution to the optical band, although the numerical uncertainties preclude a definite conclusion. At liquid-helium temperature the stress-induced alignment of the defects is hindered by random crystal fields, which can be described by an effective temperature, $T_c\approx 11$ K. These fields can be attributed to a random distribution of charged point defects, but they cannot be attributed to $F^{+}$ centers.