Lattice dynamics of photoexcited insulators from constrained density-functional perturbation theory

We present a constrained density-functional perturbation theory scheme for the calculation of structural and harmonic vibrational properties of insulators in the presence of an excited and thermalized electron-hole plasma. The method is ideal to tame ultrafast light-induced structural transitions in the regime where the photocarriers thermalize faster than the lattice, the electron-hole recombination time is longer than the phonon period, and the photocarrier concentration is large enough to be approximated by an electron-hole plasma. The complete derivation presented here includes total energy, forces and stress tensor, variable cell structural optimization, harmonic vibrational properties, and the electron-phonon interaction. We discuss in detail the case of zone-center optical phonons not conserving the number of electrons and inducing a Fermi shift in the photoelectron and hole distributions. We validate our implementation by comparing with finite differences in Te and VSe2. By calculating the evolution of the phonon spectrum of Te, Si, and GaAs as a function of the fluence of the incoming laser light, we demonstrate that even at low fluences, corresponding to approximately 0.05 photocarriers per atom, the phonon spectrum is substantially modified with respect to the ground-state one with new Kohn anomalies appearing and a substantial softening of zone-center optical phonons. Our implementation can be efficiently used to detect reversible transient phases and irreversible structural transitions induced by ultrafast light absorption.