Transient renormalization of the Josephson plasma frequency

Abstract

Layered superconductors are emerging as compact sources of intense, continuous and coherent terahertz (THz) waves [Welp et al., Nat. Photonics 7, 702 (2013)]. Apart from that, they also represent nonlinear media that can exhibit a host of nonlinear optical effects such as odd harmonic generation, slowing down of THz waves, self-focusing and self-induced transparency [Savel'ev et al., Nat. Phys. 2, 521 (2006)] that further widen their range of applications. In the linear regime, electromagnetic waves can penetrate them only if their frequency is larger than a gap-frequency, the so-called Josephson plasma frequency, whose signature is characterized by an edge in the THz reflectivity spectrum. In this work, transient renormalization of the Josephson plasma frequency in THz-perturbed La1.84Sr0.16CuO4 has been experimentally observed using the time-resolved THz time-domain spectroscopy. As the perturbing THz field evolves in time, the reflectivity edge measured from the raw-reflectivity data is found to exhibit a red shift, thus implying a reduction in the Josephson plasma frequency. This is a nonlinear effect that renders the layered superconductor transparent over a narrow spectral range close to the Josephson plasma frequency—THz waves with frequencies smaller than the unperturbed Josephson plasma frequency can penetrate the sample. The results presented in this paper should stimulate the development of nonlinear physics in layered superconductors and contribute in developing new concepts for well-controllable THz devices.