Coherent Phonon Generation from Prethermal Carrier Populations in a Semiconductor
Ultrafast laser excitation of carriers in semiconductors often results in the creation of fully symmetric coherent optical phonons, typically understood in terms of a thermalized transient carrier population. We test this understanding with systematic measurements and first-principles calculations of the photon-energy dependence of ultrafast-laser-driven coherent phonon generation in tellurium, using excitation wavelengths around the electronic band gap. We observe that, contrary to the expectations of models assuming a thermalized carrier population, the coherent amplitude of the fully symmetric $A_1$ mode changes only weakly when tuning wavelength across the band gap. The measured spectral dependence is instead reproduced by a transition-resolved prethermal force weighted by optical matrix elements and band- and momentum-dependent $A_1$ deformation potentials. Fitting this model to the data yields an excited transition state lifetime of a few femtoseconds. These results identify transition selectivity, rather than a thermalized total carrier density, as a key factor controlling coherent-phonon generation in semiconductors excited near a band gap.