Anomalous Terahertz Emission in Striped Cuprates

Terahertz emission is observed after impulsive optical excitation only in media in which inversion or time-reversal symmetry are broken. For this reason, in centrosymmetric superconductors this phenomenon is generally not seen, unless a current bias or a magnetic field are applied. Here, we report evidence for anomalous terahertz emission in unbiased cuprates in which charge stripes coexist with superconductivity. Emission is only observed when stripes are either incommensurate with the lattice or fluctuating, such as in La$_{1.905}$Ba$_{0.095}$CuO$_4$ ($x=9.5$%) and in La$_{1.845}$Ba$_{0.155}$CuO$_4$ ($x=15.5$%). A sharp response at frequencies immediately below the bulk Josephson plasma resonance suggests that this radiation originates from surface Josephson plasmons, which are generally dark modes but appear to be coupled to the electromagnetic continuum in these materials. We attribute this activated anomalous emission to the fact that incommensurate stripes break inversion symmetry in the out-of-plane direction and fold the plasmon dispersion curve onto the light cone.