Development of the nano-HEB array for low-background far-IR applications

We present an overview of the recent progress made in the development of a far-IR array of ultrasensitive hot-electron nanobolometers (nano-HEB) made from thin titanium (Ti) films. We studied electrical noise, signal and noise bandwidth, single-photon detection, optical noise equivalent power (NEP), and a microwave SQUID (MSQUID) based frequency domain multiplexing (FDM) scheme. The obtained results demonstrate the very low electrical NEP down to 1.5 × 10^(-20) W/Hz^(1/2) at 50 mK determined by the dominating phonon noise. The NEP increases with temperature as ~ T^3 reaching ~10^(-17) W/Hz^(1/2) at the device critical temperature T_C = 330-360 mK. Optical NEP = 8.6 × 10^(-18) W/Hz^(1/2) at 357 mK and 1.4 × 10^(-18) W/Hz^(1/2) at 100 mK respectively, agree with thermal and electrical data. The optical coupling efficiency provided by a planar antenna was greater than 50%. Single 8-μm photons have been detected for the first time using a nano-HEB operating at 50-200 mK thus demonstrating a potential of these detectors for future photon-counting applications in mid-IR and far-IR. In order to accommodate the relatively high detector speed (~μs at 300 mK, ~100 μs at 100 mK), an MSQUID based FDM multiplexed readout with GHz carrier frequencies has been built. Both the readout noise ~2 pA/Hz^(1/2) and the bandwidth > 150 kHz are suitable for nano-HEB detectors.