Transient Increase of the Energy Gap of Superconducting NbN Thin Films Excited by Resonant Narrow-Band Terahertz Pulses

Observations of radiation-enhanced superconductivity have thus far been limited to a few type-I superconductors (Al, Sn) excited at frequencies between the inelastic scattering rate and the superconducting gap frequency $2\Delta /h$. Utilizing intense, narrow-band, picosecond, terahertz pulses, tuned to just below and above $2\Delta /h$ of a BCS superconductor NbN, we demonstrate that the superconducting gap can be transiently increased also in a type-II dirty-limit superconductor. The effect is particularly pronounced at higher temperatures and is attributed to radiation induced nonthermal electron distribution persisting on a 100 ps time scale.

Figure 1

(a) Transmission intensity Tr as a function of $T_0$ as measured at 1.26 THz (circles) and 2.08 THz (diamonds), respectively. Dotted lines are the calculated values obtained by transfer matrix method. Inset to (a) presents the expected $\delta \mathrm{Tr}/\mathrm{Tr}$ for the data recorded at $\sim 10\mathrm{K}$ and $\nu =1.26\mathrm{THz}$, whereby the gap is reduced on the resolution limited time scale (solid line) and recovers on the 100 ps time scale (dashed)—see also the corresponding arrows in the main panel. (b) $T$ dependence of Tr from the broadband measurements, with the extracted $T$ dependence of the SC gap shown in the inset.

Figure 2

(a) Dynamics of the transmission change $\delta \mathrm{Tr}$ (normalized to ${\mathrm{Tr}}_{4.3\mathrm{K}}-{\mathrm{Tr}}_{16\mathrm{K}}$) when pumping with 2.08 THz pulses of different absorbed energy density $A$ (in $\mathrm{mJ}/{\mathrm{cm}}^3$). The shaded region presents the pump-probe intensity autocorrelation. (b) The maximum gap reduction $\delta \Delta /\Delta$ as a function of $A$ for different temperatures. Solid lines are simple saturation fits (see text). (c) The extracted SC depletion energy density $A_{\mathrm{dep}}$ as a function of $T$, measured with NIR (1.55 eV) [9] and THz (2.08 THz) pumping. The dashed line represents $E_{\mathrm{cond}}(T)\propto {\Delta }^2(T)$.

Figure 3

Dynamics of the relative transmission change $\delta \mathrm{Tr}/\mathrm{Tr}$ at ${\nu }_{<}=1.26\mathrm{THz}$ recorded at several temperatures. The shaded region presents the pump-probe intensity autocorrelation. For each temperature the maximum relative change in the gap $\delta \Delta /\Delta$ is calculated from the measured $\delta \mathrm{Tr}/\mathrm{Tr}$ assuming the $T^{*}$ model. The dotted (blue) lines and dashed (red) lines show the predicted $\delta \mathrm{Tr}/\mathrm{Tr}(t_d)$ based on the $T^{*}$ model (see text).