Giant second-harmonic generation (SHG) in the terahertz (THz) frequency range is observed in a thin film of an s-wave superconductor NbN, where the time-reversal ($𝒯$-) and space-inversion ($𝒫$-) symmetries are simultaneously broken by supercurrent injection. We demonstrate that the phase of the second-harmonic (SH) signal flips when the direction of supercurrent is inverted, i.e., the signal is ascribed to the nonreciprocal response that occurs under broken $𝒫$- and $𝒯$-symmetries. The temperature dependence of the SH signal exhibits a sharp resonance, which is accounted for by the vortex motion driven by the THz electric field in an anharmonic pinning potential. The maximum conversion ratio ${\eta }_{SHG}$ reaches $\approx {10}^{-2}$ in a thin film NbN with the thickness of 25,nm after the field cooling with a very small magnetic field of $\approx 1$,Oe, for a relatively weak incident THz electric field of 2.8,kV/cm at 0.48,THz.

中文翻译：

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超电流注入下超导NbN的不可逆太赫兹二次谐波生成

在s波超导体NbN的薄膜中观察到了太赫兹（THz）频率范围内的巨型二次谐波（SHG），其中时间反转（$𝒯$-）和空间反转（$𝒫$-）通过超电流注入同时破坏对称性。我们证明了当超电流的方向反转时，第二谐波（SH）信号的相位会翻转，即，该信号归因于在断路情况下发生的不可逆响应$𝒫$-和 $𝒯$-对称。SH信号的温度依赖性表现出尖锐的共振，这是由非谐波钉扎电势中THz电场驱动的涡旋运动引起的。最大转换率${\eta }_{\mathrm{小号}HG}$ 到达 $\approx {10}^{-2}$ 磁场冷却后，在厚度为25nm的NbN薄膜中 $\approx \mathrm{1个}$，Oe，对于0.48，THz的2.8，kV / cm的相对较弱的入射THz电场。