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.
The Heterodyne Instrument for Far Infrared (HIFI) on ESA's Herschel Space Observatory is comprised of five SIS receiver channels covering 480-1250 GHz and two HEB receiver channels covering 1410-1910 GHz. Two fixed tuned local oscillator sub-bands are derived from a common synthesizer to provide the front-end frequency coverage for each channel. The local oscillator unti will be passively cooled while the focal plane unit is cooled by superfluid helium and cold helium vapors. HIFI employs W-band GaAs amplifiers, InP HEMT low noise IF amplifiers, fixed tuned broadband planar diode multipliers, and novel material systems in the SIS mixtures. The National Aeronautics and Space Administration's Jet Propulsion Laboratory is managing the development of the highest frequency (1119-1250 GHz) SIS mixers, the highest frequency (1650-1910 GHz) HEB mixers, local oscillators for the three highest frequency receivers as well as W-band power amplifiers, varactor diode devices for all high frequency multipliers and InP HEMT components for all the receiver channels intermediate frequency amplifiers. The NASA developed components represent a significant advancement in the available performance. The current state of the art for each of these devices is presented along with a programmatic view of the development effort.
We are developing terahertz mixers to cover the highest frequency band ("6H") for the heterodyne instrument (HIFI) aboard the Herschel Space Observatory. The mixer will be optimized for operation at 1.8 THz, with an input bandwidth of at least 0.2 THz. Some of the key spectroscopic lines in this frequency band are the fine-structure transition of ionized carbon at 1.9 THz, and numerous rotational transitions of water vapor and other hydrides. The mixers will employ a superconductive hot-electron bolometer as the mixing element, for which we will use a diffusion-cooled niobium microbridge. This variant allows an IF bandwidth that meets the range required for HIFI's 4-8 GHz IF. The mixer will be operated at ~2 K bath temperature. The sensitivity requirement is a double sideband mixer noise temperature of Tmix / ν ~ 1,000 K / THz , which has been previously demonstrated with this type of mixer. The mixer is a quasioptical design, employing a twin-slot planar antenna mounted on the backside of an elliptical silicon lens. Initial measurements indicate that that these mixers can be adequately pumped with a solid-state 1.5 THz local-oscillator source. HEB mixers are extremely delicate and susceptible to environmental damage; we have therefore focused a good deal of attention to engineering a rugged, flyable mixer.