Resonant generation of coherent phonons in a superconductor by ultrafast optical pump pulses

TL;DR

This paper investigates how ultrafast optical pulses can resonantly generate coherent phonons in a superconductor by inducing nonadiabatic oscillations of the order parameter, with the phonon generation enhanced when their energies match.

Contribution

It introduces a quantum kinetic model to analyze the resonant generation of coherent phonons driven by ultrafast pulses in superconductors, highlighting the nonadiabatic regime.

Findings

Resonant enhancement of phonon generation occurs when order-parameter oscillation frequency matches phonon energy.

Superconducting order parameter oscillates nonadiabatically after ultrafast excitation.

Pump pulse intensity can tune the resonance condition in experiments.

Abstract

We study the generation of coherent phonons in a superconductor by ultrafast optical pump pulses. The nonequilibrium dynamics of the coupled Bogoliubov quasiparticle-phonon system after excitation with the pump pulse is analyzed by means of the density-matrix formalism with the phonons treated at a full quantum kinetic level. For ultrashort excitation pulses, the superconductor exhibits a nonadiabatic behavior in which the superconducting order parameter oscillates. We find that in this nonadiabatic regime the generation of coherent phonons is resonantly enhanced when the frequency of the order-parameter oscillation is tuned to the phonon energy, a condition that can be achieved in experiments by varying the integrated pump pulse intensity.