In this paper we give a detailed analysis of the expected sensitivity and operating conditions in the power detection mode of a hot-electron bolometer (HEB) made from a few μm2 of monolayer graphene (MLG) flake which can be embedded into either a planar antenna or waveguide circuit via NbN (or NbTiN) superconducting contacts with critical temperature ~ 14 K. Recent data on the strength of the electron-phonon coupling are used in the present analysis and the contribution of the readout noise to the Noise Equivalent Power (NEP) is explicitly computed. The readout scheme utilizes Johnson Noise Thermometry (JNT) allowing for Frequency-Domain Multiplexing (FDM) using narrowband filter coupling of the HEBs. In general, the filter bandwidth and the summing amplifier noise have a significant effect on the overall system sensitivity. The analysis shows that the readout contribution can be reduced to that of the bolometer phonon noise if the detector device is operated at 0.05 K and the JNT signal is read at about 10 GHz where the Johnson noise emitted in equilibrium is substantially reduced. Beside the high sensitivity (NEP < 10-20 W/Hz1/2), this bolometer does not have any hard saturation limit and thus can be used for far-IR sky imaging with arbitrary contrast. By changing the operating temperature of the bolometer the sensitivity can be fine tuned to accommodate the background photon flux in a particular application. By using a broadband low-noise kinetic inductance parametric amplifier, ~100s of graphene HEBs can be read simultaneously without saturation of the system output.
Terahertz high-resolution spectroscopy of interstellar molecular clouds greatly relies on hot-electron superconducting bolometric (HEB) mixers. Current state-of-the-art receivers use mixer devices made from ultrathin (~ 3-5 nm) films of NbN with critical temperature ~ 9-11 K. Such mixers have been deployed on a number of groundbased, suborbital, and orbital platforms including the HIFI instrument on the Hershel Space Observatory. Despite its good sensitivity and well-established fabrication process, the NbN HEB mixer suffers from the narrow intermediate frequency (IF) bandwidth ~ 2-3 GHz and is limited to operation at liquid Helium temperature. As the heterodyne receivers are now trending towards “high THz” frequencies, the need in a larger IF bandwidth becomes more pressing since the same velocity resolution for a Doppler shifted line at 5 THz requires a 5-times greater IF bandwidth than at 1 THz. Our work is focusing on the realization of practical HEB mixers using ultrathin (10-20 nm) MgB2 films. They are prepared using a Hybrid Physical-Chemical Vapor Deposition (HPCVD) process yielding ultrathin films with critical temperature ~ 37-39 K. The expectation is that the combination of small thickness, high acoustic phonon transparency at the interface with the substrate, and very short electron-phonon relaxation time may lead to IF bandwidth ~ 10 GHz or even higher. SiC continues to be the most favorable substrate for MgB2 growth and as a result, a study has been conducted on the transparency of SiC at THz frequencies. FTIR measurements show that semi-insulating SiC substrates are at least as transparent as Si up to 2.5 THz. Currently films are passivated using a thin (10 nm) SiO2 layer which is deposited ex-situ via RF magnetron sputtering. Micron-sized spiral antenna-coupled HEB mixers have been fabricated using MgB2 films as thin as 10 nm. Fabrication was done using contact UV lithography and Ar Ion milling, with E-beam evaporated Au films deposited for the antenna. Measurements have been carried out on these devices in the DC, Microwave, and THz regimes. The devices are capable of mixing signals above 20 K indicating that operation may be possible using a cryogen-free cooling system. We will report the results of all measurements taken to indicate the local oscillator power requirements and the IF bandwidth of MgB2 HEB mixers.
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.
We are presenting the current progress on the titanium (Ti)
hot-electron transition-edge devices. The ultimate goal of this work is to develop a submillimeter Hot-Electron Direct Detector (HEDD) with the noise equivalent power NEP = 10-18-10-20 W/Hz1/2 for the moderate resolution spectroscopy and Cosmic Microwave Background (CMB) studies on future space telescope (e.g., SPICA, SAFIR, SPECS, CMBPol) with cryogenically cooled (~ 4-5 K) mirrors. Recentlyi, we have achieved the extremely low thermal conductance (~ 20 fW/K at 300 mK and ~ 0.1 fW/K at 40 mK) due to the electron-phonon decoupling in Ti nanodevices with niobium (Nb) Andreev contacts. This thermal conductance translates into the "phonon-noise" NEP ≈ 3×10-21 W/Hz1/2 at 40 mK and NEP ≈ 3×10-19 W/Hz1/2 at 300 mK. These record data indicate the great potential of the hot-electron detector for meeting many application needs. Beside the extremely low phonon-noise NEP, the nanobolometers have a very low electron heat capacitance that makes them promising as
detectors of single THz photonsii. As the next step towards the practical demonstration of the HEDD, we fabricated and
tested somewhat larger than in Ref.1 devices (~ 6 μm × 0.35 μm × 40 nm) whose critical temperature is well reproduced in the range 300-350 mK. The output electrical noise measured in these devices with a low-noise dc SQUID is dominated by the thermal energy fluctuations (ETF) aka "phonon noise". This indicates the high electrothermal loop gain that effectively suppresses the contributions of the Johnson noise and the amplifier (SQUID) noise. The electrical NEP = 6.7×10-18 W/Hz1/2 derived from these measurements is in good agreement with the predictions based on the thermal conductance data. The very low NEP and the high speed (~ μs) are a unique combination not found in other detectors.
We are developing a hot-electron superconducting transition-edge sensor (TES) that is capable of counting THz photons
and operates at T = 0.3K. The main driver for this work is moderate resolution spectroscopy (R ~ 1000) on the future
space telescopes with cryogenically cooled (~ 4 K) mirrors. The detectors for these telescopes must be background-limited
with a noise equivalent power (NEP) ~ 10-19-10-20 W/Hz1/2 over the range ν=0.3-10 THz. Above about 1 THz,
the background photon arrival rate is expected to be ~ 10-100 s-1, and photon counting detectors may be preferable to an
integrating type. We fabricated superconducting Ti nanosensors with a volume of ~ 3×10-3 μm3 on planar Si substrate
and have measured the thermal conductance G to the thermal bath. A very low G=4×10-14 W/K, measured at 0.3 K, is
due to the weak electron-phonon coupling in the material and the thermal isolation provided by superconducting Nb
contacts. This low G corresponds to NEP(0.3K) = 3×10-19 W/Hz1/2. This Hot-Electron Direct Detector (HEDD) is
expected to have a sufficient energy resolution for detecting individual photons with ν > 0.3 THz at 0.3 K. With the
sensor time constant of a few microseconds, the dynamic range is ~ 50 dB.
We have developed a concept design for a large (~10k × 10k) CMOS imaging array whose elements are grouped in small subarrays with N pixels in each. The subarrays are code-division multiplexed using the Hadamard Transform (HT) based encoding. The Hadamard code improves the signal-to-noise (SNR) ratio to the reference of the read-out amplifier noise by a factor of N1/2. This way of grouping pixels reduces the number of hybridization bumps by N. A single chip layout has been designed and the architecture of the imager has been developed to accommodate the HT based multiplexing into the existing CMOS technology. The imager architecture allows for a trade-off between the speed and the sensitivity. The envisioned imager would operate at a speed >100 fps with the pixel noise < 20 e-. The power dissipation would be ~ 100 pW/pixel. The combination of the large format, high speed, high sensitivity and low power dissipation are very attractive for space applications.
Superconductive hot-electron bolometer (HEB) mixers have been built and tested in the frequency range from 1.1 THz to 2.5 THz. The mixer device employs diffusion as a cooling mechanism for hot electrons. The double sideband receiver noise temperature was measured to be approximately equals 2750 K at 2.5 K at 2.5 THz; and mixer IF bandwidths as high as 9 GHz are achieved for 0.1 micrometers long devices. The local oscillator power dissipated in the HEB microbridge was in the range 20- 100 nW. Further reductions in LO power and mixer noise can be potentially achieved by using Al microbridges. The advantages and parameters of such devices are evaluated. A distributed-temperature model has been developed to properly describe the operation of the diffusion-cooled HEB mixer. The HEB mixer is a primary candidate for ground based, airborne and spaceborne heterodyne instruments at THz frequencies.
Superconductive hot-electron bolometer (HEB) mixers have been built and tested in the frequency range from 1.1 THz to 2.5 THz. The mixer device is a 0.15 - 0.3 micrometer microbridge made from a 10 nm thick Nb film. This device employs diffusion as a cooling mechanism for hot electrons. The double sideband noise temperature was measured to be less than or equal to 3000 K at 2.5 THz and the mixer IF bandwidth is expected to be at least 10 GHz for a 0.1 micrometer long device. The local oscillator (LO) power dissipated in the HEB microbridge was 20 - 100 nW. Further improvement of the mixer characteristics can be potentially achieved by using Al microbridges. The advantages and parameters of such devices are evaluated. The HEB mixer is a primary candidate for ground based, airborne and spaceborne heterodyne instruments at THz frequencies. HEB receivers are planned for use on the NASA Stratospheric Observatory for Infrared Astronomy (SOFIA) and the ESA Far Infrared and Submillimeter Space Telescope (FIRST). The prospects of a submicron-size YBa2Cu3O7-(delta ) (YBCO) HEB are discussed. The expected LO power of 1 - 10 (mu) W and SSB noise temperature of approximately equals 2000 K may make this mixer attractive for various remote sensing applications.
We report on the development of quasioptical Nb hot-electron bolometer mixers for heterodyne receivers operating at 1 THz 3 THz. The devices have submicron in-plane sizes, thus exploiting diffusion as the electron cooling mechanism. Quasioptical mixer circuits have been developed with planar double-dipole or twin-slot antennas. The measured (DSB) receiver noise temperatures are 1670 K at 1.1 THz, with an estimated mixer noise temperature of approximately equals 1060 K, and 2750 K at 2.5 THz, with an estimated mixer noise temperature of approximately equals 900 K. The IF bandwidth is found to scale as the length-squared, and bandwidths as high as 8 GHz have been measured. These results demonstrate the low-noise, broadband operation of the diffusion-cooled bolometer mixer over a wide range of far-infrared wavelengths.
We report on a study of S-N and N-S current switching in high quality YBaCuO films deposited onto ZrO2 and NdGaO3 substrates. The films 60-120 nm thick prepared by laser ablation were structured into single strips and were provided with gold contacts. We monitored the time dependence of the resistance upon application of the voltage step on the film. Experiment performed within certain ranges of voltage amplitudes and temperatures showed the occurrence of the fast stage both in S-N (shorter than 300 ps) and N-S transition. We discuss the mechanism of switching taking into account the hot electron phenomena in YBaCuO. The contributions of various thermal processes in the subsequent stage of the resistance dynamic are also discussed. The basic limiting characteristics (average dissipated power, minimum work done for switching, maximum repetition rate) of a picosecond switch which is proposed to be developed are estimated.
The transient voltage response in both epitaxial and granular YBaCuO thin films to 20 ps pulses of YAG:Nd laser radiation with 0.63 micrometers and 1.54 micrometers was studied. In normal and resistive states both types of films demonstrate two components: nonequilibrium picosecond component and following bolometric nanosecond. The normalized amplitudes are almost the same for all films. In superconducting state we observed a kinetic inductive response and two-component shape after integration. The normalized amplitude of the response in granular films is up to several orders of magnitude larger than in epitaxial films. We interpret the nonequilibrium response in terms of a suppression of order parameter by the excess of quasiparticles followed by the change of resistance in normal and resistive states or kinetic inductance in superconducting state. The sharp rise of inductive response in granular films is explained both by a diminishing of the crossection for current percolation through the disordered network os Josephson weak links and by a decrease of condensate density in neighboring regions.
Nonequilibrium picosecond and bolometric responses of YBCO films 500 angstroms thick patterned into 20 X 20 micrometers 2 size structure to 17 ps laser pulses and modulated radiation of GaAs and CO2 lasers have been studied. The modulation frequencies up to 10 GHz for GaAs laser and up to 1 GHz for CO2 were attained. The use of small radiation power (1 - 10 mW/cm2 for cw radiation and 10 - 100 nJ/cm2 for pulse radiation) in combination with high sensitive read-out system made possible to avoid any non-linear transient processes caused by an overheating of sample above a critical temperature or S-N switching enhanced by an intense radiation. Responses due to the change of kinetic inductance were believed to be negligible. The only signals observed were caused by a small change of the film resistance either in the resistive state created by a bias current or in the normal state. The data obtained by means of pulse and modulation techniques are in agreement. The responsivity about 1 V/W was measured at 1 GHz modulation frequency both for 0.85 micrometers and 10.6 micrometers wavelengths. The sensitivity of high-Tc fast wideband infrared detector is discussed.