Figure 4: Comparison of calculated transverse optical (TO) and transverse acoustic (TA) dispersions at equilibrium and under 0.5% photoexcitation.

(a) Calculated dispersions (TA in red and TO in blue) for PbTe along the Δ direction (marked in reciprocal lattice units, rlu) in the (1 3) zone at both equilibrium (DFT, solid lines) and with 0.5% photoexcitation (CDFT, dashed lines). (b) Relative (percentile) changes in frequency between the DFT and CDFT calculations for both the TO (blue) and TA (red) phonon branches along the Δ. The TA modes experience a softening in frequency across the entire wavevector direction on photoexcitation whereas the TO modes harden dramatically near zone centre and softens near zone edge. (c) Projection of the calculated excited state TA branch eigenvector onto the calculated equilibrium state TO branch eigenvector along the same Δ line. The projection represents the degree of equilibrium TO character that has been translated into the TA modes on photoexcitation and thus the amount of mode-mixing that has occurred. Note that such renormalization of the frequencies and eigenvectors will only result in excitation of two-phonon overtone and combination modes if it is sudden compared with the phonon period.