state dynamics in pump-probe conductivity'
---

Introduction
============

The nonequilibrium response of superconductors has been the subject of considerable interest for a long time [@kopninBook; @Orenstein12]. In the past few years, fueled by recent advances in femtosecond terahertz (THz) laser technology [@Fulop12], many experimental [@matsunagaPRL13; @matsunagaPRL12; @mansartPNAS13; @Beck2013; @DalScience2013; @beckDemsarPRL11; @Rett10; @leitensdorferPRL10; @Mans09; @rubhausenPRL09; @PerfettiPRL07; @Kain05; @Ave01; @kaindlScience00] and theoretical [@volkov1974; @amin2004; @barankov2004; @Yuz05; @Yuz06; @nicolePRB03; @kabanovPRL05; @Unter08; @Papen07; @Papen08; @Papen09; @Schny11; @Akba13; @zachmannNJP13] studies have focused on the ultrafast time response of the BCS state subjected to nonadiabatic perturbations. For example, Matsunaga *et al.*[@matsunagaPRL12; @matsunagaPRL13] have employed THz pump-THz probe spectroscopy to investigate the nonadiabatic dynamics of superconducting thin films after the injection of Bogoliubov quasiparticles with energies just above the gap edge. Transient oscillations with the frequency of the amplitude mode of the superconducting order parameter have been observed in these measurements. Besides pump-probe experiments, also tunneling spectroscopy has recently been used to study the ultrafast response of superconducting tunnel junctions [@ponomarev2013].

On the theory side, intensive efforts have been devoted to the analysis of persistent and damped oscillations in the Bogoliubov quasiparticle occupations, which are induced by either an interaction quench [@volkov1974; @amin2004; @barankov2004; @Yuz05; @Yuz06] or 