SUPERCONDUCTING, TRANSITION, AND NORMAL STATE PHOTORESPONSE IN YBCO OBSERVED AT DIFFERENT TEMPERATURES

Photoresponse of YBCO films was measured with a new technique between 7 K and 200 K. YBCO was exposed to variable fluence (less-than-or-equal-to 3 mu-J) 300 fsec 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 psec) 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 (approximately 200 - 700 psec) and slow (approximately 1 nsec) decay time constants. At T(c) the photoresponse was the highest and the relaxation time constant was typically greater than 1 nsec. 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 nsec and about 1 nsec for YBCO films on LaAlO3.