Terahertz time-domain spectroscopy of transient metallic and superconducting states

TL;DR

This paper analyzes how terahertz time-domain spectroscopy measures transient metallic and superconducting states, revealing that the measured response function may not directly reflect the instantaneous optical conductivity, especially in superconducting conditions.

Contribution

It clarifies the relationship between the measured response function and the actual optical conductivity in time-resolved THz spectroscopy, highlighting limitations and signatures of photoinduced superconductivity.

Findings

$oldsymbol{ ext{Sigma}(oldsymbol{ extomega};oldsymbol{t})}$ is not always equal to $oldsymbol{ ext{ extsigma}(oldsymbol{ extomega};oldsymbol{t})}$.

Signatures of photoinduced superconductivity can be identified in the response function.

Differences are most pronounced with long momentum relaxation times.

Abstract

Time-resolved terahertz time-domain spectroscopy (THz-TDS) is an ideal tool for probing photoinduced nonequilibrium metallic and superconducting states. Here, we focus on the interpretation of the two-dimensional response function $\Sigma(\omega;t)$ that it measures, examining whether it provides an accurate snapshot of the instantaneous optical conductivity, $\sigma(\omega;t)$. For the Drude model with a time-dependent carrier density, we show that $\Sigma(\omega;t)$ is not simply related to $\sigma(\omega;t)$. The difference in the two response functions is most pronounced when the momentum relaxation rate of photocarriers is long, as would be the case in a system that becomes superconducting following pulsed photoexcitation. From the analysis of our model, we identify signatures of photoinduced superconductivity that could be seen by time-resolved THz-TDS.