标题： 太赫兹二次谐波生成的发现：从对称性破缺非线性超电流的光波加速中 显示英文标题

标题： Discovery of Terahertz Second Harmonic Generation from Lightwave Acceleration of Symmetry--Breaking Nonlinear Supercurrents

摘要： 我们报告在具有反演对称平衡态的超导体(SC)中观察到太赫兹(THz)二次谐波生成(SHG)，这种反演对称性禁止偶数阶非线性。这种SHG信号通过用低于SC能隙的多周期THz电场进行周期驱动，在单脉冲发射中被观测到，并在SC临界温度以上消失。我们通过使用强THz光波非线性和脉冲传播相互作用的量子动力学模型，解释了亚太赫兹周期下的动态对称性破缺原理。由此产生的SC内部非零积分脉冲面积由于亚周期 Cooper 对加速而驱动光波非线性超电流，这与直流偏置超导体不同，其可通过带结构和THz泵浦场进行控制。 显示英文摘要

摘要： We report terahertz (THz) second harmonic generation (SHG) in superconductors (SC) with inversion symmetric equilibrium states that forbid even-order nonlinearities. Such SHG signal is observed in single-pulse emission by periodic driving with a multi-cycle THz electric field tuned below the SC energy gap and vanishes above the SC critical temperature. We explain the microscopic physics by a dynamical symmetry breaking principle at sub-THz-cycle by using quantum kinetic modeling of the interplay between strong THz-lightwave nonlinearity and pulse propagation. The resulting non-zero integrated pulse area inside the SC drives lightwave nonlinear supercurrents due to sub--cycle Cooper pair acceleration, in contrast to d.c.-biased superconductors, which can be controlled by the bandstructure and the THz pump field.