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

TL;DR

This paper demonstrates terahertz second harmonic generation in superconductors with symmetric states, driven by lightwave-induced symmetry breaking and supercurrents, revealing new nonlinear optical phenomena.

Contribution

It introduces a microscopic quantum kinetic model explaining THz SHG in superconductors via dynamical symmetry breaking and supercurrents driven by lightwave acceleration.

Findings

Observation of THz SHG in superconductors below critical temperature.

SHG signal depends on the THz field and disappears above Tc.

Nonlinear supercurrents are driven by sub-cycle Cooper pair acceleration.

Abstract

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.