Quench-drive spectroscopy and high-harmonic generation in BCS superconductors

TL;DR

This paper introduces a novel spectroscopy method combining quench and drive pulses to analyze superconductors, revealing transient harmonic modulations that probe quasiparticle and collective excitations.

Contribution

It develops a two-dimensional spectroscopy technique with a quench-drive setup and characterizes the superconductor's response using an Anderson-pseudospin model.

Findings

Transient modulation of higher harmonics observed

Wave mixing probes quasiparticle excitations

Fourier analysis reveals collective modes

Abstract

In pump-probe spectroscopies, THz pulses are used to quench a system, which is subsequently probed by either a THz or optical pulse. In contrast, third-harmonic generation experiments employ a single multicycle driving pulse and measure the induced third harmonic. In this work, we analyze a spectroscopy setup where both a quench and a drive are applied and two-dimensional spectra as a function of time and quench-drive delay are recorded. We calculate the time evolution of the nonlinear current generated in the superconductor within an Anderson-pseudospin framework and characterize all experimental signatures using a quasiequilibrium approach. We analyze the superconducting response in Fourier space with respect to both the frequencies corresponding to the real time and the quench-drive delay time. In particular, we show the presence of a transient modulation of higher harmonics, induced by a wave mixing process of the drive with the quench pulse, which probes both quasiparticle and collective excitations of the superconducting condensate.