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

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.

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• Observation of terahertz second harmonic generation from Dirac surface states in the topological insulator Bi${}_2$Se${}_3$
• Revealing the frequency-dependent oscillations in the nonlinear terahertz response induced by the Josephson current
• Investigating Josephson plasmons in layered cuprates via nonlinear terahertz spectroscopy
• Ghost Josephson plasmon in bilayer superconductors
• Optical absorption in tilted geometries as an indirect measurement of longitudinal plasma waves in layered cuprates
• Dynamical signatures and control of time-reversal breaking in twisted nodal superconductors
• Two-dimensional THz spectroscopy in electronic systems: a many-body diagrammatic approach
• Raman response in superconducting multiorbital systems with application to nickelates