Optical Properties of Single-Crystal Paratellurite (TeO2)

Absorption at the fundamental band edge, the dispersion characteristics of the refractive indices, the optical rotatory power, associated temperature coefficients, and the photoelastic constants have been investigated. Absorption at the band edge has been found to obey the exponential Urbach rule up to 5 × ${10}^2$ ${\mathrm{cm}}^{-1\mathrm{}}$. Refractive-index data between 0.4 and 1.0 μm are excellently fitted to the two-term Sellmeier dispersion formula with oscillators located at 9.24 and 4.70 eV. The contribution of the former oscillator to the refractive index in the visible to infrared region is larger than that of the latter. The dispersion energy ${\mathcal{E}}_{\mathrm{di}}=\frac{{\mathcal{F}}_i}{{\mathcal{E}}_i}$ (where ${\mathcal{F}}_i$ is the oscillator strength factor and ${\mathcal{E}}_i$ is the oscillator position) defined by Wemple and DiDomenico has been determined as ∼ 25 eV for the transition at 9.24 eV, and is in agreement with the value derived for the average single oscillator. Dispersion of the rotatory power has been also explained by the two-term Chandrasekhar's formula with oscillator positions nearly equal to those found from the refractive indices. Dispersion measurements of the photoelastic constants reveal that a large anisotropy exists between the changes of ${\mathcal{F}}_i$ and ${\mathcal{E}}_i$ induced by the strain $S_1$ and those induced by $S_3$. The main contribution to the positive value of the temperature coefficient of the refractive indices comes from an intrinsic temperature effect, and the contribution of the photoelastic effect associated with the thermal expansion is negative. Anomalous behavior has been observed in $\frac{\mathrm{dn}}{n}\mathrm{dT}$ and $\frac{d\rho }{\rho }\mathrm{dT}$ between 0.5 and 0.6 μm, and is probably attributable to extremely weak absorption peaks located in this wavelength region.