Abstract
The far-infrared photoresponse of granular thin film Tl2Ba2CaCu2O8 has been investigated at relatively low optical powers obtained from a c.w. laser source. Both bolometric and Josephson detection has been observed at wavelengths between 214 mu m and 1011 mu m. In contrast to measurements made with higher-power pulsed radiation, a Josephson photosignal proportional to incident power is obtained in this spectral range. The responsivity is found to increase strongly with wavelength, approximately proportionally to lambda 2.5, with a measured sensitivity of 12 V W-1 at 1011 mu m and a corresponding NEP of 2*10-8 W Hz- 12 /. The photoresponse dependences on wavelength and power are compared with recent numerical predictions of the behaviour of two-dimensional Josephson junction arrays with random defects (Cai et al. 1994 Phys. Rev. B 49 4015). The observed square-law response and strongly increased responsivity at longer wavelengths indicate the potential of utilizing such films as rugged, fast, large-area video detectors for the sub-mm/microwave spectral range.
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