The temperature dependence of the optical constants of materials (refractive index, absorption and extinction coefficients, and dielectric function) can be determined with spectroscopic ellipsometry over a broad range of temperatures and photon energies or wavelengths. Such results have practical value, for example for applications of optical materials at cryogenic or elevated temperatures. The temperature dependence of optical gaps and their broadenings also provides insight into the scattering of electrons and holes with other quasiparticles, such as phonons or magnons. This review presents a detailed discussion of the experimental considerations for temperature-dependent ellipsometry and selected results for insulators, semiconductors, and metals in the infrared to ultraviolet spectral regions.

Herein, the complex pseudodielectric function of Ge and Si from femtosecond pump–probe spectroscopic ellipsometry with 267, 400, and 800 nm pump–pulse wavelengths is analyzed by fitting analytical lineshapes to the second derivatives of the pseudodielectric function with respect to energy. This yields the critical point parameters (threshold energy, lifetime broadening, amplitude, and excitonic phase angle) of E 1 and E 1 + Δ 1 in Ge and E 1 in Si as functions of delay time. Coherent longitudinal acoustic phonon oscillations with a period of about 11 ps are observed in the transient critical point parameters of Ge. From the amplitude of these oscillations, the laser‐induced strain is found to be on the order of 0.03% for Ge measured with the 800 nm pump pulse, which is in reasonable agreement with the strain calculated from theory.

The dielectric function of bulk Ge is determined between 0.5 and 6.3 eV in a temperature range of 10–738 K using spectroscopic ellipsometry. The authors provide the data in a tabulated format that can be interpolated as a function of photon energy and temperature using commercial software. Another focus of this paper lies on the analysis of critical points, in particular, on the investigation of the temperature dependence of the direct bandgap E0 and the critical point E0+Δ0, where Δ0 is the spin–orbit splitting. To explore the temperature dependence of critical points, the parameters that characterize their line shapes are calculated using three different techniques. First, the common method of numerically calculating and analyzing the second derivatives of the dielectric function works well for critical points at higher energies. Second, an analysis in reciprocal space by performing a discrete Fourier transform and analyzing the resulting Fourier coefficients yields values for the energies of E0 and E0+Δ0. Third, the energy determined from a parametric semiconductor model is shown as a function of temperature. The authors observe a temperature dependent redshift of the E0 and E0+Δ0 critical point energies as well as an increase in the broadening of E0 with temperature.