Digital Frequency-Domain Multiplexing (DfMux) is a transition edge sensor multiplexing technique that has been used in mm-wave receivers with multiplexing factors as high as 68. It is the baseline readout technology for LiteBIRD and a potential upscope option for PICO. Recent efforts have been directed toward simplifying packaging, reducing parasitic impedance, and improving readout noise performance by integrating all cryogenic readout components onto a single cryogenic stage. Here we present recent progress including further improved performance and an increase in the scale of operation. This work marks an important step toward the development of DfMux for space-based mm-wave receivers.
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/Hz1/2 at 50 mK determined by the dominating phonon noise. The NEP increases with temperature as ~ T3
reaching ~ 10-17 W/Hz1/2 at the device critical temperature TC = 330-360 mK. Optical NEP = 8.6×10-18 W/Hz1/2 at 357
mK and 1.4×10-18 W/Hz1/2 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/Hz1/2 and the bandwidth > 150 kHz are suitable for nano-HEB detectors.
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