G. Schneider

Fast nonlinear photoresponse of current biased thin‐film Bi2Sr2CaCu2O8 to pulsed far‐infrared radiation

The response of a polycrystalline thin‐film Bi2Sr2CaCu2O8 superconducting stripe to short‐pulse far‐infrared (λ=447 μm) radiation is reported. Under constant current bias, a photosignal is generated when the maximum zero voltage current is exceeded. Measurements of the sensitivity as a function of temperature, bias current, and intensity reveal the signal source to be nonbolometric. The response is found to obey a (power)1/2 law over more than 2 orders of magnitude. We believe the detection mechanism arises from the interaction of grain boundary Josephson junctions with radiation induced screening currents.

Superconducting Energy Gap from Infrared Response Measurements in Granular Tl2Ba2CaCu2O8 Films

The resistance response of polycrystalline Tl2Ba2CaCu2O8 films to short laser pulses has been investigated for a wide range of far-infrared frequencies. For temperatures T ll Tc a drastic change in response time is observed at a critical frequency νc simeq 4 THz. The time constant increases from about 1 ns at ν νc. The critical frequency is attributed to a superconducting energy gap with 2Δ(0)/kB Tc simeq 2.

Infrared Detection by Tl-Ba-Ca-Cu-O superconducting Films

The response of polycrystalline Tl−Ba−Ca−Cu−O superconducting thin films on short laser pulses has been investigated for radiation between 10 μm and 500 μm wavelength. Fast signals with time constants less than 1 ns were observed for wavelengths longer than about 100 μm whereas for shorter wavelengths only a bolometric signal could be detected.

Spectral dependence of nonbolometric far‐infrared detection with thin‐film Bi2Sr2CaCu2O8

The photoresponse of granular current biased thin‐film Bi2Sr2CaCu2O8 superconductor to short pulse far infrared radiation has been measured for 16 laser lines, with frequencies between 10 and 1000 cm−1. A strong dependence of the signal on the frequency ω, approximately proportional to ω−2.3, is observed. It is shown, by calculations based on the resistively shunted junction model, that this photosignal can be understood to arise from the ac Josephson effect at grain boundary weak links.

Far infrared response of thin film Bi2Sr2CaCu2O8 using a free electron laser

The far infrared response of granular thin-film Bi2Sr2CaCu2O8 superconductor has been investigated using long (≈5 μs) but sharply truncated free electron laser pulses in the frequency range between 50 cm−1 and 125 cm−1. Under constant current bias, a fast response and a slow bolometric signal component could be identified in this energy range, which is below the BCS energy gap (≈ 200 cm−1). Measurements of the power dependences of the signal voltages showed that both the fast and the thermal responses are consistent with the predictions of the resistively shunted Josephson junction model.

Magnetic field dependence of Josephson photoresponse in high-Tc superconductor thin films

The Josephson photoresponse of granular high-Tc superconductor films to pulsed far infrared laser radiation has been investigated in magnetic fields of up to 3T. Its value is strongly influenced by fields less than 50mT and shows a pronounced hysteresis here. At low bias current densities, jb 100A/cm2 the photoresponse is depressed. All changes have occured by a field of B < 50mT: for higher fields the photoresponse remains constant. This dependence is interpreted to arise from micrometre sized grain boundary junctions with a strongly inhomogenous critical current distribution on a sub-nanometre scale.

Enhancement of Josephson photoresponse of granular high‐Tc superconductor thin films by deoxygenation

The dependence of the far infrared Josephson photoresponse of current biased granular high‐Tc superconductor thin films on deoxygenation is presented. Bi2Sr2CaCu2O8 and TlBa2Ca2Cu3O9 thin films were heated in vacuum to temperatures of between 200 and 500 °C. The resulting deoxygenation weakens the intergrain coupling, thereby reducing the critical current and enhancing the photoresponse. In this way the optimum temperature for the fast Josephson response may be tuned to lie outside the temperature region of the slow bolometric signal.

A simple method of producing far-infrared high-pass filters

A method of making high-pass waveguide filters for the far infrared is described. The filters are formed from arrays of tightly packed narrow-bore metal tubes. Such filters feature sharp cut-off transitions in the region between 20 cm-1 and 40 cm-1. Transmission above cut-off is comparable to that in similar filters produced by drilling holes in thin metal sheets. Typically the transmissions just above cut-off are about 5% while that just below was measured as less than 0.1%.

Sensitivity Improvement of Granular Thin Film High Tc Superconductor FIR Detection by a post Annealing procedure

The use of high Tc superconducting thin films as detectors for UV to microwave radiation is currently under investigation. Bolometric effects, nonequilibrium electron heating and Josephson video detection have been reported [1]. In the far infrared (FIR), the photon energies are smaller than the BCS energy gap and so Josephson detection in granular films seems the most promising. Granular films represent distributed antennas with the detection occuring at all intergrain weak links. They are therefore especially suitable for multimode detection. The sensitivity of detectors reported varies considerably and a reliable production technique leading to reproducible results has not been reported. We have established that Tl and Bi based thin films can be heat treated under vacuum to increase their sensitivities by several orders of magnitude. Sensitivities exceeding 1V/W are obtained. Presented below are results relating different heat treatments to the photoresponse.

High speed nonlinear detection of short pulses of mind and far infrared radiation with current biased granular fhin film of Bi₂Sr₂CaCu₂O₈

The work introduced below has extended previous reports of far infrared (FIR: 10 cm-1 < ω < 100 cm-1) responses from granular Y-Ba-Cu-O [1] and Tl-Ba-Sr-Cu-O [2] films to the Bi2Sr2CaCu2O8 system for the first time. More importantly, the photosignal was found to depend on √Power over a two orders of magnitude range of incident powers. The detection mechanism is therefore believed to arise from the interaction of current biased intergranular Josephson junctions with radiation induced screening currents. A non bolometric response is also observed at low temperatures at a mid infrared frequency of 903 cm-1.