Terahertz nonlinear optics is a viable method to interrogate collective phenomena in quantum materials spanning ferroelectrics, charge-density waves, and superconductivity. In superconductors this includes the Higgs amplitude and Josephson phase modes. We have investigated the nonlinear c-axis response of optimally doped La1.85Sr0.15CuO4 using high-field THz time domain spectroscopy (THz-TDS) at field strengths up to ∼80 kV/cm. With increasing field, we observe a distinct red-shift of the Josephson plasma edge and enhanced reflectivity (above the plasma edge) arising from third harmonic generation. The non-monotonic temperature dependent response is consistent with nonlinear drive of the Josephson Plasma Mode (JPM) as verified with comparison to theoretical expectations. Our results add to the understanding that, using THz light, the JPM (in addition to the Higgs mode) provides a route to interrogate and control superconducting properties.
@article{arxiv.2211.17184,
title = {Terahertz third harmonic generation in c-axis La$_{1.85}$Sr$_{0.15}$CuO$_4$},
author = {Kelson Kaj and Kevin A. Cremin and Ian Hammock and Jacob Schalch and Dmitri N. Basov and Richard D. Averitt},
journal= {arXiv preprint arXiv:2211.17184},
year = {2024}
}