Pair-breaking of the superconducting thin films induced by the intense terahertz pulses

High-power terahertz time-domain spectroscopy (THz-TDS) was used to examine YBCO and NbN thin films when transmitted by intense single-cycle THz pulses. This allowed for an investigation of the nonlinear, time-resolved behavior of superconducting thin films in the presence of strong THz electric fields with the field strengths of tens of kV cm$^{\mathrm{-1}}$. In the case of low field strengths, the behavior of the thin films agrees with previous examinations by means of conventional, low-power THz-TDS. However, for strong THz electric fields, it was found by analysis with the two-fluid model that the superfluid population decreases dramatically, possibly due to Cooper pair breakup. This was accompanied by a drop in the imaginary part of the conductivity in the THz frequency range. Moreover, a high-intense THz-punp - THz-probe measurement was conducted with the both YBCO and NbN thin films and estimated the recombination time of quasiparticles excited by intense THz electric field in superconductos. As a results, It was found that the recombination time of YBCO was several picosecond and much shorter than that of NbN.