Significant blue-shift in photoluminescence excitation spectra of Nd³⁺:LaF₃ potential laser medium at low-temperature

Temperature-dependent optical properties of bulk Nd³⁺:LaF₃ crystals are reported. A blue-shift in the photoluminescence excitation (PLE) spectrum is observed at 30 K. The 173.2-nm emission peak wavelength at 300 K shifted to 172.8 nm at 30 K, consistent with the 6-nm blue-shift in transmission edge and 2437-cm^-1 increase in the lowest energy level of the 4f²5d configuration. Thermal broadening of the 5d-4f emission bands with increasing temperature is also observed as the dip at around 178.5 nm present at 30 K disappears at 300 K. A smaller spectral overlap between the PLE and emission spectra is observed as temperature is decreased. Our results suggest that absorption cross-section at the peak fluorescence wavelength is expected to decrease at 30 K.