Electrical NEP in hot-electron titanium superconducting bolometers

摘要

We are presenting the current progress on the titanium (Ti) hot-electron transition-edge devices. The ultimate goal of thiswork is to develop a submillimeter Hot-Electron Direct Detector (HEDD) with the noise equivalent power NEP = 10~(-18)-10~(-20) w/Hz~(1/2)for the moderate resolution spectroscopy and Cosmic Microwave Background (CMB) studies on futurespace telescope (e.g., SPICA, SAFIR, SPECS, CMBPol) with cryogenically cooled (~ 4-5 K) mirrors. Recently', wehave achieved the extremely low thermal conductance (~ 20 fW/K at 300 mK and ~ 0.1 fW/K at 40 mK) due to theelectron-phonon decoupling in Ti nanodevices with niobium (Nb) Andreev contacts. This thermal conductance translatesinto the "phonon-noise" NEP≈ 3x10~(-21) W/Hz~(1/2)at 40 mK and NEP ≈ 3x10~(-19)W/Hz~(1/2)300 mK. These record dataindicate the great potential of the hot-electron detector for meeting many application needs. Beside the extremely lowphonon-noise NEP, the nanobolometers have a very low electron heat capacitance that makes them promising asdetectors of single THz photons". As the next step towards the practical demonstration of the HEDD, we fabricated andtested somewhat larger than in Ref.1 devices (~ 6 μm x 0.35 μm x 40 nm) whose critical temperature is well reproducedin the range 300-350 mK. The output electrical noise measured in these devices with a low-noise dc SQUID isdominated by the thermal energy fluctuations (ETF) aka "phonon noise". This indicates the high electrothermal loopgain that effectively suppresses the contributions of the Johnson noise and the amplifier (SQUID) noise. The electricalNEP = 6.7x10~(-18)W/Hz~(1/2)derived from these measurements is in good agreement with the predictions based on thethermal conductance data. The very low NEP and the high speed (~ μs) are a unique combination not found in otherdetectors.