Superconducting, transition, and normal state photoresponse in YBCO observed at different temperatures

The photoresponse of YBCO films was measured with a novel technique between 7 K and 200 K. YBCO was exposed to variable-fluence (⩽3 μJ) 300-fs 665-nm 2-kHz laser pulses. Photoinduced impedance changes in the samples biased with a DC current exhibited a transient voltage signal. Typically the voltage signal had a sharp (<50-ps) rise followed by a more gradual decay. Below T_c, the response did not exhibit a simple exponential relaxation time. Relaxation occurred by a serial combination of fast (~200-700 ps) and slow (~ 1-ns) decay time constants. At T_c, the photoresponse was the highest, and the relaxation time constant was typically greater than 1 ns. Above T_c, the relaxation voltage signal was smaller in amplitude and followed a simple exponential dependence. The relaxation time constant for YBCO films on MgO was greater than 2 ns and was about 1 ns for YBCO films on LaAlO₃. The results yield a signal well below T_c (a quantum response); in addition, the negative photoresponse signal corresponds to recombination in the superconducting state. With the Corbino sample geometry, the potential of positive response due to flux flow has been minimized. A response linear with current was observed. The response below 0.9 T_c is temperature independent. At and above T_c, a thermal response due to absorption of the laser energy was observed