SUPERCONDUCTING FAST MICROBOLOMETERS OPERATING BELOW THEIR CRITICAL-TEMPERATURE

The bolometric nature of the optical sensitivity of YBCO (YBa2Cu3O7-delta )hin-film microstrips in the superconducting state is demonstrated. Below the critical temperature T-c the critical current temperature dependence of the device provides a suitable temperature sensor. A theoretical thermal model that gives reliable forecasts for the sensor time constant and sensitivity over the 10-90 K temperature range has been developed. Sensitivity measurements on high-quality YBCO films deposited on MgO are in quite go;od agreement with the model, showing that the observed signals are entirely bolometric. The effective heat capacity of the sensor is that of the film in the irradiated area. Owing to the high thermal conductivity of MgO, the substrate stays at the heat sink temperature and the main resistance to heat flow is the film/substrate interface. The heat capacity of the irradiated area of the film, strongly coupled to the heat sink through the thermal boundary resistance gives high-sped bolometers behaving as first-order systems. For SrTiO3 or ZrO2 substrates, their lower thermal conductivity would lead to higher time constants and more complicated behaviors. The precise origin of the measured sensor noise has still to be established. On a 0.2X10x15 mu m(3) microbridge at 85 K (below the critical temperature), a noise equivalent power of 120 pW/root Hz and a time constant tau of 20 ns were measured; this corresponds to a noise equivalent temperature of 60 nK/root Hz and a specific detectivity D* of 1.8X10(6) cm root Hz/W. A more relevant criterion for comparing optical sensors is the D*/root tau ratio. In the case presented, this ratio is equal to 1.3x10(11) cm/J which is one of the best values published at this time. Such performances should give competitive composite br antenna coupled bolometers compared to existing sensors for wavelengths greater than 20 mu m.