Picosecond Infrared Detection and Optical Mixing in YBa2Cu3O7-δ Thin Films
Doctoral thesis, 1994
Studies of infrared photoresponse in high Tc superconducting YBa2Cu3O7-.delta. (YBCO) thin films are presented in this dissertation. Detectors fabricated from YBCO films of different thicknesses have been investigated, in terms of responsitivity, speed, and detection mechanisms, by the use of semiconductor lasers at near infrared wavelengths and a CO2 laser at 10.6 µm wavelength.
Granular films, thicker than the optical penetration depth, showed only bolometric response and response times in the nanosecond range. Detectors patterned in films from different laboratories displayed large variations in responsivity, which could not be explained by material characterization. When epitaxial films of the same thickness as the optical penetration depth were used, major improvements were observed. Both the speed and the responsivity were considerably higher. The response was found to be mainly bolometric but, for the shortest laser pulses, indications of a vortex depairing response mechanism was found.
By the use of even thinner films and a special detector structure, a high speed detection mechanism attributed to nonequilibrium phenomena was revealed. An electron heating mechanism, with the quasiparticles at an elevated temperature compared with the phonons of the superconductor, fitted well with our measured data. Furthermore, the detector was operated as a mixer for infrared radiation at intermediate frequencies up to 18 GHz. Based on the electron heating model, the bandwidth of the electron heating mechanism was calculated to be 130 GHz.
The photoresponse from a single YBCO grain boundary Josephson junction was also studied. A bolometric response was found, in which the critical current of the junction was reduced as the impinging radiation raised the temperature. The flexibility of a Josephson junction type bolometer was also discussed.