Epitaxial YBa2Cu3O7−δ thin films, grown with a controllable misalignment between c-axis and surface normal, exhibit a sensitive photoresponse to far infrared laser radiation at temperatures below Tc. While no photoresponse was observed in c-axis normal films, a sensitivity of 0.2 V/W was measured at a wavelength of 432 μm for a film with a 20° c-axis tilt. Difference frequency mixing of two laser modes has also been demonstrated at intermediate frequencies between 200 kHz and 6 MHz. Both mixing and video detection are believed to originate in the ac Josephson effect: the effect of the tilted growth is to allow a component of the radiation field to be applied along the c-axis, thus modulating the relatively weak Josephson coupling in this direction.
Pulsed-laser heating of current-biased percolative YBa2Cu3O7−δ films can be used to generate and release self-field-induced vortices. A laser pulse yields a voltage signal due to penetration of bias-current-induced magnetic flux in absence of an external magnetic field. Upon recooling, strongly pinned vortices remain in the film. These remanent vortices have been detected after disconnecting the bias source in the currentless film. Applying a subsequent second laser pulse again yields a signal voltage but of opposite sign due to magnetic flux redistribution. This phenomenon could be applied as write-read memory.
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