Abstract

Since the discovery of high-temperature superconductors (HTS), there have been intensive investigations of their transient response under ultrashort laser-pulse excitation. Recently, using the femtosecond pump-probe technique, Han et al.[1] observed a very fast (several picoseconds) optical response on YBa₂Cu₃O_x (YBCO) in the superconducting state under weak excitation and attributed it to the generation and recombination of quasi- particles. As the ambient temperature drops below the superconductor’s critical temperature (T_c), the transient reflectivity signal abruptly changes both its sign and temporal behavior—this striking behavior appears to be correlated to the onset of superconductivity. However, the physical significance of such an ultrafast response is still not clear, since the position of the Fermi level (E_F) and the density of states near E_F are not well known. Independently, a series of transient photovoltage measurements have also been performed on YBCO. Recent experimental work [2] demonstrated that for ultra-thin films the relative magnitude of the nonthermal component was enhanced and could be as fast as tens of picoseconds. The fact that the electrical response is at least one order of magnitude slower than the optical response suggests that more in-depth studies are necessary in order to find the ultimate speed of optoelectronic devices made of HTS.

© 1993 Optical Society of America