Nonlinear Response of the High-Temperature Superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ to the Transmission of Intense Terahertz Pulses

High-Power Terahertz Time-Domain Spectroscopy (THz-TDS) was used to examine YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ thin films when transmitted by intense single-cycle THz pulses. This allowed for an investigation of the nonlinear, time-resolved behavior of YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ in the presence of strong THz electric fields for the first time. High field strengths of tens of kV cm$^{-1}$ were achieved by improving the efficiency of optical rectification in LiNbO$_{3}$ through the tilted-pulse-front method and by ensuring a tight focusing of the THz beam. In the case of low field strengths, the behavior of the thin films agrees with previous examinations of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ 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 investigated frequency range of 0.2 to 0.8 THz. The results further suggest a decrease of the effective mass of the carriers for strong THz fields.