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

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