NONEQUILIBRIUM AND BOLOMETRIC PHOTORESPONSE IN PATTERNED YBA2CU3O7-DELTA THIN-FILMS

Epitaxial laser deposited YBa2Cu3O7-delta films of approximately 50 nm thickness were patterned into detectors consisting of ten parallel 1 mum wide strips in order to study nonequilibrium and bolometric effect s. Typically, the patterned samples had critical temperatures around 8 6 K, transition widths around 2 K and critical current densities above 1 X 10(6) A/cm2 at 77 K Pulsed laser measurements at 0.8 mum waveleng th (17 ps full width at half maximum) showed a approximately 30 ps res ponse, attributed to electron heating, followed by a slower bolometric decay. Amplitude modulation in the band f(mod) = 100 kHz-10 GHz of a laser with wavelength lambda = 0.8 mum showed two different thermal re laxations in the photoresponse. Phonon escape from the film (approxima tely 3 ns) is the limiting process, followed by heat diffusion in the substrate. Similar relaxations were also seen for lambda = 10.6 mum. T he photoresponse measurements were made with the film in the resistive state and extended into the normal state. These states were created b y supercritical bias currents. Measurements between 75 and 95 K (i.e., from below to above T(c)) showed that the photoresponse was proportio nal to dR/dT for f(mod) = 1 MHz and 4 GHz. The fast response is limite d by the electron-phonon scattering time, estimated to 1.8 ps from exp erimental data. The responsivity both at 0.8 and 10.6 mum wavelength w as approximately 1.2 V/W at f(mod) = 1 GHz and the noise equivalent po wer was calculated to 1.5 X 10(-9) WHz-1/2 for the fast response.