Epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/(7-x) thin films on CeO/sub 2/ buffered sapphire for optical mixers

Optical mixers based on YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films deposited on CeO/sub 2/ thin film buffered sapphire substrates were developed and used for photoresponse measurements. Firstly, a 50 nm CeO/sub 2/ thin film was laser deposited on sapphire at a substrate temperature (T/sub s/) of 760/spl deg/C and an oxygen pressure (p/sub O2/) of 1.0/spl times/10/sup -2/ mbar, followed by an annealing for 15 min at 850/spl deg/C. Subsequently, a /spl sim/50 nm thick YBCO layer was laser deposited at T/sub s/=760/spl deg/C and p/sub O2/=1.0 mbar. Finally, a 150 nm thick Au film for contacts was laser deposited in situ. X-ray diffraction and /spl phi/-scan showed a high degree of c-axis and in-plane orientation. The critical temperature was 89 K and the critical current, j/sub c//spl sim/2.10/sup 6/ A/cm/sup 2/ at 77 K. Two diode lasers (/spl lambda/=1.56 /spl mu/m) were coupled into a single mode fiber, which was centered at a patterned mixer structure in the YBCO film. Optical mixing was detected up to 7 GHz at a temperature of 77 K.