Disentangling Temperature-Induced Variations in Absorption and Refractive Index in Ultrafast Transient Reflection Spectroscopy.

Accurately interpreting ultrafast carrier dynamics in solid-state materials requires understanding the distinct contributions of absorption and refractive index variations to transient optical responses. In this study, we systematically disentangle temperature-induced changes in the absorption coefficient and refractive index of silicon crystals as a model system. Our approach utilizes transient reflection spectroscopy, complemented by in situ temperature-dependent spectroscopic ellipsometry and density functional theory (DFT) calculations. We examine and decipher the observed spectral shifts and profile modifications in correlation with lattice expansion from 298 to 403 K. Our findings reveal that thermal-induced variations in the refractive index account for approximately 85% of the transient reflection signal at 0.2 ps in the visible range, rising to about 93% at 10 ps. This behavior is attributed to temperature-driven modifications in the band structure at the Γ and X points, inducing significant changes in the dielectric function. These changes distinctly impact absorption and refractive index, each exhibiting unique spectral signatures and temporal behaviors. This work establishes a framework for decoupling optical parameters in transient measurements, providing valuable insights for the study of solid-state optoelectronic materials.