Superconducting Hot Electron Bolometer Mixer Research Articles

A triple band local oscillator coupling system based on a phase grating for five-meter Dome-A terahertz explorer telescope.

For heterodyne receivers, broadband and integrated local oscillator (LO) coupling schemes are crucial. In this paper, we present a novel terahertz broadband receiver system based on a one-pixel phase grating. We designed a phase grating that can simultaneously couple the LO signals at 690, 850, and 1300GHz to a superconducting hot-electron bolometer mixer at the incident angles of 25.7°, 20.6°, and 13.3°, respectively. At these three frequencies, the measured diffraction efficiencies of the phase grating are as high as 70%, 73%, and 65%, and the total power coupling efficiencies of the LO coupling system are 22%, 23%, and 23%, in good agreement with the simulated results. This efficient and highly integrated multiband receiver system has potential applications in the proposed Five-meter Dome-A Terahertz Explorer telescope.

Superconducting hot-electron bolometer mixers and their applications

Superconducting hot electron bolometer (HEB) mixers are the most sensitive heterodyne detectors at frequencies beyond 1 THz, and are used for many astronomical observations. HEB, consisting of a narrow and short superconducting thin film patch connected to an antenna via contact pads, is brought into a highly sensitive operating regime by applying a DC current and a certain amount of local-oscillator power. Potentially, superconducting HEB mixers are capable of exhibiting high sensitivity for frequencies up to several tens of THz, as their output noise and conversion gain are nearly frequency independent according to the mixing mechanism. Moreover, the understanding of mixing mechanism has evolved over time from lumped-element model to distributed hot-spot model. In this review, we provide an overview of theoretical and experimental studies on HEBs and their applications, especially in astronomical observations on ground, airborne and space telescopes.

A Dual-Band Receiver Based on a Single Superconducting Hot Electron Bolometer Mixer for DATE5 Telescope

Simultaneous multi-frequency observations can be of great advantage when line ratios need to be accurately determined. Five meters Dome A Terahertz Explorer (DATE5) telescope is proposed with two observation bands: 0.85 THz band and 1.35 THz band. In this paper, we report on a dual-band receiver based on a single quasi-optical superconducting hot-electron bolometer mixer. This novel concept allows simultaneous observation of the two bands for DATE5 telescope. Our measurement results show that the dual-band HEB receiver can simultaneously resolve the line spectra in both frequency bands. In particular, an RF tone has been observed with in single and dual frequency mode and the heterodyne performance of the HEB mixer has been compared. Although there is a reduction of mixer conversion gain, the available observation time can be improved by decreasing the time for source pointing and double-beam switching. Whats more, compared with conventional broadband receiver system, this novel technique can achieve compact system and provide a system redundancy for unattended observation.

Linear and nonlinear flux-flow behaviors in superconducting hot-electron bolometer mixers

We study experimentally the resistive behaviors of a niobium nitride superconducting hot-electron bolometer (HEB) device driven by magnetic field and terahertz radiation. We find that the resistance of the superconducting HEB device emerges due to time-dependent changes of the macroscopic quantum phase by flux-flow events. The flux-flow resistance is found to be increased linearly with magnetic field at low vortex velocities and is enhanced by terahertz radiation. At high vortex velocities, there is an obvious non-linear change, which can be well interpreted by viscous flux-flow instability predicted by the Larkin and Ovchinnikov theory. In addition, we demonstrate a technique taking use of the device resistance dependence on magnetic field to stabilize the superconducting HEB device.

Quantum cascade laser based, fiber coupled demultiplexed mid-infrared local oscillator for cryogenic applications

We have developed a fiber coupled demultiplexed local oscillator (LO) source for cryogenic applications based on a distributed feedback quantum cascade laser (QCL) at an operating wavelength of 10.6 μm. The coupling scheme relies on the combination of a phase grating generating multiple LO beams and a polycrystalline fiber delivering the demultiplexed LO signal to a superconducting mixer at 4 K in a closed-cycle cryostat. This scheme eases the use of a beam splitter in front of the mixer. Reflection phase grating generating 2 × 2 beams has been achieved with a total power efficiency of 64% based on a gold covered etched silicon nanostructure. The diffraction beam has been readily coupled into a 2 m long polycrystalline fiber, where a single-lobe circular beam has been delivered at the fiber output. By aligning the fiber core position to the center of a superconducting hot electron bolometer mixer at the rear side, we demonstrate the application of a fiber coupled demultiplexed QCL source for pumping the superconducting mixer with adequate LO power.

10.6 μm heterodyne receiver based on a superconducting hot-electron bolometer mixer and a quantum cascade laser

We report on the development of a heterodyne receiver at mid-infrared wavelength for high-resolution spectroscopy applications. The receiver employs a superconducting NbN hot electron bolometer as a mixer and a room temperature distributed feedback quantum cascade laser operating at 10.6 μm (28.2 THz) as a local oscillator. The stabilization of the heterodyne receiver has been achieved using a feedback loop controlling the output power of the laser. Improved Allan variance times as well as a double sideband receiver noise temperature of 5000 K and a noise bandwidth of 2.8 GHz of the receiver system are demonstrated.

Dielectric Constant and Loss Tangent of Silicon at 700–900 GHz at Cryogenic Temperatures

Quasi-optical superconducting hot-electron bolometer (HEB) mixers have shown a competitive sensitivity at a frequency slightly lower than 1 THz when compared to superconducting SIS mixers. The advantage of low LO power requirement makes superconducting HEB mixers, especially suitable for array application. The far-field beam pattern, as well as coupling efficiency of quasi-optical superconducting HEB mixers, depends strongly upon the lens shape and the dielectric constant of the lens material. Therefore, in this letter we precisely measured the dielectric constant and loss tangent of high resistivity silicon at 4, 77, and 300 K in the frequency range of 700–900 GHz where many important molecular lines located. The measurements are performed by employing a quasi-optical vector network analyzer with a dynamic range as high as 110 dB to ensure high precision measurement. In addition, the dielectric constant of high-density polyethylene (HDPE) and polymethylpentene (TPX/PMP) are also measured since they are widely used in terahertz quasi-optical system.

Engineering Physics of Superconducting Hot-Electron Bolometer Mixers

Superconducting hot-electron bolometers are presently the best performing mixing devices for the frequency range beyond 1.2 THz, where good quality superconductor-insulator-superconductor (SIS) devices do not exist. Their physical appearance is very simple: an antenna consisting of a normal metal, sometimes a normal metal-superconductor bilayer, connected to a thin film of a narrow, short superconductor with a high resistivity in the normal state. The device is brought into an optimal operating regime by applying a dc current and a certain amount of local- oscillator power. Despite this technological simplicity its operation has been found to be controlled by many different aspects of superconductivity, all occurring simultaneously. A core ingredient is the understanding that there are two sources of resistance in a superconductor: a charge conversion resistance occurring at an normal-metal-superconductor interface and a resistance due to time- dependent changes of the superconducting phase. The latter is responsible for the actual mixing process in a non-uniform superconducting environment set up by the bias-conditions and the geometry. The present understanding indicates that further improvement needs to be found in the use of other materials with a faster energy-relaxation rate. Meanwhile several empirical parameters have become physically meaningful indicators of the devices, which will facilitate the technological developments.

Investigation of the Performance of NbN Superconducting HEB Mixers of Different Critical Temperatures

In this paper, we investigate the critical temperature dependence of the performance of niobium nitride superconducting hot electron bolometer (HEB) mixers. The input noise temperature and intermediate frequency (IF) noise bandwidth of superconducting HEB mixers of different critical temperatures $T_{c}$ (between 6 and 12 K) are simulated with a hot spot model. Simulation results show that the mixer input noise temperature is the lowest at $T_{c}$ approximately equal to 7–9.5 K and the IF noise bandwidth increases with $T_{c}$ . We also measure three superconducting HEB mixers of different critical temperatures (i.e., 7.5, 8.8, and 10.3 K) at 0.85 and 1.3 THz. An agreement between measurement and prediction is observed.

Nonequilibrium interpretation of DC properties of NbN superconducting hot electron bolometers

We present a physically consistent interpretation of the dc electrical properties of niobiumnitride (NbN)-based superconducting hot-electron bolometer mixers, using concepts of nonequilibrium superconductivity. Through this, we clarify what physical information can be extracted from the resistive transition and the dc current-voltage characteristics, measured at suitably chosen temperatures, and relevant for device characterization and optimization. We point out that the intrinsic spatial variation of the electronic properties of disordered superconductors, such as NbN, leads to a variation from device to device.

Precise Evaluation of a Phase-Locked THz Quantum Cascade Laser

We have demonstrated the phase-locking of a THz quantum cascade laser (THz-QCL) toward the realization of an accurate and stable local oscillator for a high-resolution receiver. The beat note between the THz-QCL and a THz reference was obtained by heterodyne mixing in a superconducting hot electron bolometer mixer (HEBM) and used for stabilizing the phase of 3.1 THz radiation from the THz-QCL. The phase-locked 3.1 THz radiation was fully evaluated with a superlattice harmonic mixer operating in the THz band, and this revealed that the THz-QCL synchronized with the microwave reference with a fractional frequency instability of 3×10 -15 at an averaging time of 100 s, corresponding to a center frequency deviation within 1 mHz, and the imposed phase noises during the heterodyne mixing were negligibly small.

Terahertz hot electron bolometer waveguide mixers for GREAT

Supplementing the publications based on the first-light observations with the German Receiver for Astronomy at Terahertz frequencies (GREAT) on SOFIA, we present background information on the underlying heterodyne detector technology. We describe the superconducting hot electron bolometer (HEB) detectors that are used as frequency mixers in the L1 (1400 GHz), L2 (1900 GHz), and M (2500 GHz) channels of GREAT. Measured performance of the detectors is presented and background information on their operation in GREAT is given. Our mixer units are waveguide-based and couple to free-space radiation via a feedhorn antenna. The HEB mixers are designed, fabricated, characterized, and flight-qualified in-house. We are able to use the full intermediate frequency bandwidth of the mixers using silicon-germanium multi-octave cryogenic low-noise amplifiers with very low input return loss. Superconducting HEB mixers have proven to be practical and sensitive detectors for high-resolution THz frequency spectroscopy on SOFIA. We show that our niobium-titanium-nitride (NbTiN) material HEBs on silicon nitride (SiN) membrane substrates have an intermediate frequency (IF) noise roll-off frequency above 2.8 GHz, which does not limit the current receiver IF bandwidth. Our mixer technology development efforts culminate in the first successful operation of a waveguide-based HEB mixer at 2.5 THz and deployment for radioastronomy. A significant contribution to the success of GREAT is made by technological development, thorough characterization and performance optimization of the mixer and its IF interface for receiver operation on SOFIA. In particular, the development of an optimized mixer IF interface contributes to the low passband ripple and excellent stability, which GREAT demonstrated during its initial successful astronomical observation runs.

Comparison between hot spot modeling and measurement of a superconducting hot electron bolometer mixer at submillimeter wavelengths

This paper presents the modeling and measurement of a quasioptical niobium nitride superconducting hot electron bolometer mixer at submillimeter wavelengths. The modeling is performed with a distributed hot spot model which is based on solving a heat balance equation for electron temperature along the superconducting microbridge. Particular care has been taken during the modeling concerning the temperature-dependent resistance and the bias current dependence of the critical temperature of the device. The dc and mixing characteristics of this mixer have been computed and we have observed a quite good match between the predicted and the measured results for both dc characteristics and mixing performances at submillimeter wavelengths.

Design and fabrication of superconducting HEB mixer

This paper describes the design and fabrication of superconducting hot electron bolometer (HEB) mixer based on ultra-thin superconducting NbN films. The high quality films were epitaxially grown on high resistance Si substrates. The device was fabricated by magnetron sputtering, electron beam lithography (EBL), reactive ion etching (RIE), lithography, and so on. The device’s resistance-temperature (R-T) curves and current-voltage (I-V) curves were studied. The results of THz response of the device are presented. Y-factor technique was used to measure the device’s noise temperature. When the device was irradiated with a laser radiation of 2.5 THz, the obtained lowest noise temperature of the device was 2213 K.

Characterization of the Performance of a Quasi-Optical NbN Superconducting HEB Mixer

In this paper we focus mainly on the investigation of the performance of a quasi-optical (planar log-spiral antenna) phonon-cooled NbN superconducting hot electron bolometer (HEB) mixer, which is cryogenically cooled by a close-cycled 4-K cryocooler, at 500 and 850 GHz frequency bands. The mixer's noise performance, stability of IF output power, and local oscillator (LO) power requirement are characterized for three NbN superconducting HEB devices of different sizes. The transmission characteristics of Mylar and Zitex films with incidence waves of an elliptical polarization are also examined by measuring the mixer's noise temperature. The lowest receiver noise temperatures (with no corrections) of 750 and 1100 K are measured at 500 and 850 GHz, respectively. Experimental results also demonstrate that the bigger the HEB device is, the higher the stability of IF output power becomes.

Noise Behaviour of a THz Superconducting Hot-ElectronBolometer Mixer

A quasi-optical superconducting NbN hot-electron bolometer (HEB) mixeris measured in the frequency range of 0.5–2.5 THz for understanding ofthe frequency dependence of noise temperature of THz coherentdetectors. It has been found that noise temperature increasing withfrequency is mainly due to the coupling loss between the quasi-opticalplanar antenna and the superconducting HEB bridge when taking accountof non-uniform distribution of high-frequency current. With thecoupling loss corrected, the superconducting HEB mixer demonstrates anoise temperature nearly independent of frequency.

Novel Devices for (Sub)millimeter-Wave Space Applications

Submillimeter-wave space applications require special components for power generation, detection and multiplication. This article presents recent progress in three various solid-state device technologies for submillimeter-wave space instruments: the InP HEMT for ultra-low noise detection at very low power in the IF amplifier, the heterostructure barrier varactor in the frequency multiplier, and the superconducting hot electron bolometer mixer for quantum-limited heterodyne detection above 700 GHz.

Characterization of a quasi-optical NbN superconducting HEB mixer

In this paper, the performance of a quasi-optical NbN superconducting hot-electron bolometer (HEB) mixer, cryogenically cooled by a close-cycled 4-K refrigerator, is thoroughly investigated at 300, 500, and 850 GHz. The lowest receiver noise temperatures measured at the respective three frequencies are 1400, 900, and 1350 K, which can go down to 659, 413, and 529 K, respectively, after correcting the loss and associated noise contribution of the quasi-optical system before the measured superconducting HEB mixer. The stability of the quasi-optical superconducting HEB mixer is also investigated here. The Allan variance time measured with a local oscillator pumping at 500 GHz and an IF bandwidth of 110 MHz is 1.5 s at the dc-bias voltage exhibiting the lowest noise temperature and increases to 2.5 s at a dc bias twice that voltage.

Characterization of a Quasi-optical NbN Superconducting Hot-electron Bolometer Mixer

In this paper, we report the performance of a quasi-optical NbN superconducting HEB (hot electron bolometer) mixer measured at 500 GHz. The quasi-optical NbN superconducting HEB mixer is cryogenically cooled by a 4-K close-cycled refrigerator. Its receiver noise temperature and conversion gain are thoroughly investigated for different LO pumping levels and dc biases. The lowest receiver noise temperature is found to be approximately 1200 K, and reduced to about 445 K after correcting the loss of the measurement system. The stability of the mixer’s IF output power is also demonstrated.

Low noise NbN superconducting hot electron bolometer mixers at 1.9 and 2.5 THz

NbN phonon-cooled hot electron bolometer mixers (HEBs) have been realized withnegligible contact resistance between the bolometer itself and the contact structure. Usinga combination of in situ cleaning of the NbN film and the use of an additionalsuperconducting interlayer of a 10 nm NbTiN layer between the Au of the contactstructure and the NbN film superior noise temperatures have been obtained aslow as 950 K at 2.5 THz and 750 K at 1.9 THz. Here we address in detail the DCcharacterization of these devices, the interface transparencies between the bolometers andthe contacts and the consequences of these factors on the mixer performance.