Strong-Field Pulsed THz Study of Superconductivity Breakdown in NbN

We report the ultra-fast breakdown of the superconducting state in a NbN thin film (${T}_C\approx\mathrm{14K}$) when exposed to an intense single-cycle THz pulse. The THz pulse's transform-limited spectral content was kept below the NbN pair-breaking energy threshold near 2$\Delta/hc=$ 35 cm$^{-1}$ (i.e., $<$1 THz). Thus, the initial electronic response was dominated by the inductive behavior of the pair condensate. At low THz E-field strength, the NbN film transmitted less for the superconducting state than for the normal state, as expected. As a function of increasing THz E-field strength, the film transmittance remained constant until a threshold range was reached, after which the transmittance changed over to its normal state value. Through this threshold range we also observed a significant non-linear response in the form of THz upconversion to frequencies approaching 3 times the optical gap, corresponding to time scales well below 1 picosecond.