Terahertz third harmonic generation in c-axis La$_{1.85}$Sr$_{0.15}$CuO$_4$

TL;DR

This study demonstrates third harmonic generation in the c-axis of optimally doped La$_{1.85}$Sr$_{0.15}$CuO$_4$ using high-field THz spectroscopy, revealing nonlinear Josephson plasma mode behavior related to superconductivity.

Contribution

It provides new experimental evidence of third harmonic generation linked to Josephson plasma modes in cuprate superconductors under high-field THz excitation.

Findings

Red-shift of Josephson plasma edge with increasing field

Enhanced reflectivity due to third harmonic generation

Non-monotonic temperature dependence consistent with theoretical models

Abstract

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 La$_{1.85}$Sr$_{0.15}$CuO$_4$ using high-field THz time domain spectroscopy (THz-TDS) at field strengths up to $\sim$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.