Terahertz time-domain spectroscopy of transient metallic and superconducting states

The use of terahertz time-resolved studies in conducting and superconducting systems is gaining considerable interest, especially in the light of recent studies demonstrating induction of superconductivity in various materials with optical impulsive excitation. The authors carry out a theoretical analysis of the response functions typically measured by terahertz time-domain spectroscopy (THz-TDS) in metallic and superconducting samples. Within the Drude model, the authors found that THz-TDS does not simply measure the instantaneous conductivity change, in agreement with previous reports. They identify limits wherein the time-resolved THz-TDS spectrum does and does not approach the instantaneous conductivity change. They further show that the difference between the THz-TDS response and the instantaneous conductivity is largest when the photo-induced state changes from a normal metal to a superconductor.