Unambiguous determination of the THz conductivity in driven YBa$_2$Cu$_3$O$_{6+x}$

Abstract

Mid-infrared pump, THz probe experiments in YBa$_2$Cu$_3$O$_{6+x}$ yield evidence of a high-temperature transient state of the material with the optical properties of a superconductor. However, there are ambiguities in the extracted transient c-axis THz optical properties, primarily resulting from the sublinear growth of the THz response with increasing mid-infrared pump fluence. It was proposed that this effect may be related to saturation of the material's response, which would then perturb its spatial profile and lead one to overestimate the induced change in the transient optical properties. Yet, sublinear growth of the signal could also arise from other effects unrelated to saturation, for example from a change in reflectivity or deposition depth for the pump pulses. Here, we remove this ambiguity by experimentally measuring the fluence-dependent complex optical constants at the 15-20 THz frequencies of the pump. We show that the observed sublinear growth is to be attributed in large part to the enhanced reflection and reduced penetration depth of the pump field at high fluences. When taking into account these findings, we confirm the original interpretation of a light-induced superconducting state both qualitatively and quantitatively.