dc responsivity of proximity effect bridges to high frequency radiation

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Abstract
The effect of microwave radiation on the dc current-voltage characteristic of proximity effect bridges has been studied at frequencies of 10, 35 and 90 GHz. We report the properties of bridges fabricated in layered films of Nb and Ta. The fraction ε of the incident radiant power absorbed by the bridge is measured calorimetrically, permitting determination of the intrinsic responsivity (volts per watt of power absorbed when operated in a current-biased mode). In our test geometry ε is typically ∼ 10-4 for a 0.1 Ω Nb-Ta bridge, and increases roughly linearly with dc junction resistance R. Broad-band responsivitv is generally independent of incident power. increases with increasing critical current Ic (decreasing temperature), and decreases with increasing frequency, varying approximately as Ω-3 where Ω is the normalized frequency ħω/2 eRIc. Peak responsivities achieved to date are ∼ 105 V/W (absorbed) at 10 GHz and ∼ 102 V/W (absorbed) at 90 GHz. By mounting junctions in vacuum and exposing them to near-infrared radiation, we have found that their purely bolometric (thermal) response is much weaker than the microwave response at 90 GHz