A very promising application of high temperature superconductors is the use of these materials as broadband optical and infrared detectors. In previous investigations on thin films operated in a biased mode like a photoconductor slow bolometric signals and high-speed non-thermal response has been observed 1,2 covering an extremely wide spectral range from uv to far-infrared (FIR). Additionally photovoltaic voltage signals due to a Nernst effect were found in samples subjected to a moderate external magnetic field) ,4

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INFRARED DETECTION BY TI-Ba-Ca-Cu-O SUPERCONDUCTING FILMS

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.

Square-law Josephson detection of far-infrared radiation with current-biased granular Tl2Ba2CaCu2O8 thin films

The origin of temperature and radiation power response dependencies to millimeter wave radiation is analyzed for Y–Ba–Cu–O thin films. These dependencies were both experimentally measured and described by an empirical formula. The results which were obtained indicate that the peak shape of temperature dependence for the nonbolometric response is determined by the temperature of the thermostat and is less inertial than for τ=1/f (f=35 GHz).

Response of superconducting Y–Ba–Cu–O films to millimeter wave radiation

Nonequilibrium processes resulting from the interaction of high-temperature superconductors with electromagnetic radiation are considered from microwave to optical range. Emphasis is laid on the dependence of surface or dc resistance on external parameters (temperature, bias current, modulation frequency, magnetic field, radiation power, and frequency), which is characteristic of every nonbolometric response mechanism considered by us. The most frequently used methods for monitoring the response of HTSC to electromagnetic radiation are described.

Response of high-temperature superconductors to electromagnetic radiation: (A Review)

Nonequilibrium photoresponse behavior has been investigated for YBa2Cu3O7−x (YBCO) granular films to 8 mm microwave radiation under various bias currents and magnetic fields. The measurements reveal that the nonequilibrium photoresponse mode occurs only in the tail region of the resistance transition curve R(T) from the normal to the superconducting state, where transportation behavior of the granular superconducting film is found to be characterized by the Kosterlitz–Thouless (KT) phase transition model. Based on the KT model, the photoresponse mechanism has been interpreted in terms of the depinning process of the unbinding vortices, which are generated from the decoupling process of the vortex–antivortex pairs by current, and are held at the intrinsic pinning sites of the granular high-Tc superconducting films at low temperature. Under the co-action of the bias current and the incident microwave photons, these unbinding vortices will be driven out of the pinning center, creating viscous motion in the Josephson junction array system. An analytical result of the unbinding vortices density n(T,I) induced by applied current has been worked out based on the model of two-dimensional Josephson junction arrays that is employed as a model system for the YBCO granular films. The distribution of the n(T,I) is found to be analogous to that of the photoresponse measured in the temperature region of 2/3TKT<T<TKT. Additionally, the measurements reveal that the magnitude of the photoresponse is linearly increased with an increase of the incident microwave power. These results imply that the nonequilibrium photoresponse induced by microwave irradiation may be intrinsically related to the decoupling process of the vortex–antivortex pairs, as well as to the depinning dynamics of the unbinding vortices in the granular high-Tc superconducting films.

Nonequilibrium photoresponse of YBa2Cu3O7−x granular films to 8 mm microwave radiation

The photoresponses of Pb doped 2223 phase BiSrCaCuO films to low power 5 mm cw radiation were measured at temperatures near Tc. To identify the involved detection mechanisms, dependence of responsivity on temperature, supplied current, radiation power, and film granularity were investigated. At temperatures in the vicinity of the transition to the normal state, good agreement was obtained with the responses derived from a bolometric model. At temperatures below the transition, a nonbolometric response mode was found to occur when the film was biased into a partially resistive regime. Transient measurements were carried out showing a response time for this mode of at least 5 orders of magnitude smaller than that of the bolometric response. Further characteristics inherent to the nonbolometric mode include square law detection, nonlinear current dependence, and short wavelength cutoff. In addition, a correlation between responsivity and film granularity was established. For strongly granular films with an a...

I. Shih

Papers

Photoresponses of granular (Pb)BiSrCaCuO films to millimeter wavelength radiation