Revealing the frequency-dependent oscillations in the nonlinear terahertz response induced by the Josephson current

TL;DR

This paper investigates the nonlinear terahertz response of high-temperature superconductors, revealing frequency-dependent oscillations explained by Josephson nonlinear electrodynamics, advancing understanding in strong-field terahertz spectroscopy.

Contribution

It demonstrates that Josephson-type nonlinear electrodynamics explains observed oscillations, providing new insights into superconductor responses to intense terahertz radiation.

Findings

Oscillatory reflectivity depends linearly on probe frequency.

The physical emission frequency is near the Josephson plasma edge.

Theoretical model matches experimental observations.

Abstract

Nonlinear responses of superconductors to intense terahertz radiation has been an active research frontier. Using terahertz pump-terahertz probe spectroscopy, we investigate the c-axis nonlinear optical response of a high-temperature superconducting cuprate. After excitation by a single-cycle terahertz pump pulse, the reflectivity of the probe pulse oscillates as the pump-probe delay is varied. Interestingly, the oscillatory central frequency scales linearly with the probe frequency, a fact widely overlooked in pump-probe experiments. By theoretically solving the nonlinear optical reflection problem on the interface, we show that our observation is well explained by the Josephson-type third-order nonlinear electrodynamics, together with the emission coefficient from inside the material into free space. The latter results in a strong enhancement of the emitted signal whose physical frequency is around the Josephson plasma edge. Our result offers a benchmark for and new insights into strong-field terahertz spectroscopy of related quantum materials.