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 effects. Typically, the patterned samples had critical temperatures around 86 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 wavelength (17 ps full width at half maximum) showed a approximately 30 ps response, 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 relaxations in the photoresponse. Phonon escape from the film (approximately 3 ns) is the limiting process, followed by heat diffusion in the substrate. Similar relaxations were also seen for lambda = 10.6 mum. The photoresponse measurements were made with the film in the resistive state and extended into the normal state. These states were created by supercritical bias currents. Measurements between 75 and 95 K (i.e., from below to above T(c)) showed that the photoresponse was proportional to dR/dT for f(mod) = 1 MHz and 4 GHz. The fast response is limited by the electron-phonon scattering time, estimated to 1.8 ps from experimental data. The responsivity both at 0.8 and 10.6 mum wavelength was approximately 1.2 V/W at f(mod) = 1 GHz and the noise equivalent power was calculated to 1.5 X 10(-9) WHz-1/2 for the fast response.