Precise Evaluation of a Phase-Locked THz Quantum Cascade Laser

Superconducting hot electron bolometer mixers are now a competitive alternative to Schottky diode mixers in the terahertz frequency range because of their ultra wideband (from millimeter waves to visible light), high conversion gain, and low intrinsic noise level. High Tc superconductor materials can be used to make hot electron bolometers and present some advantage in term of operating temperature and cooling. In this paper, we present first a model for the study of superconducting hot electron bolometers responsivity in direct detection mode, in order to establish a firm basis for the design of future THz mixers. Secondly, an original process to realize YBaCuO hot electron bolometer mixers will be described. Submicron YBaCuO superconducting structures are expitaxially sputter deposited on MgO substrates and patterned by using electron beam lithography in combination with optical lithography. Metal masks achieved by electron beam lithography are insuring a good bridge definition and protection during ion etching. Finally, detection experiments are being performed with a laser at 850 nm wavelength, in homodyne mode in order to prove the feasibility and potential performances of these devices.

We present microwave (30 GHz) measurements on aluminum superconducting hot-electron bolometer (HEB) mixers. Aluminum HEB mixers have a lower superconducting transition temperature than niobium and niobium nitride devices, and are predicted to have improved sensitivity and require less local oscillator power. The devices studied consist of a narrow superconducting aluminum microbridge with contacts comprised of thick aluminum, titanium, and gold. A perpendicular magnetic field is used to suppress superconductivity in the contacts. The device length is much shorter than the electron–phonon inelastic length [L≪(Dτe–ph)1/2] and thus the intermediate frequency (IF) bandwidth is set by the diffusion time of hot electrons from the microbridge into the contacts. We discuss limitations on the IF bandwidth in very short HEB mixers with fully normal contacts. Overall, the microwave heterodyne mixing results show good performance with a mixer noise temperature inferred from the conversion efficiency and output noise of TM⩾4 K, double side band (DSB). Measurements at 618 GHz yield a conversion efficiency approximately 10 dB lower than observed at 30 GHz. Output saturation by background radiation is postulated to contribute significantly to this difference. A similar decrease in conversion efficiency in the microwave mixing measurements is observed when broadband noise is added to the rf input signal. We discuss the tradeoff between increased sensitivity and the occurrence of saturation effects in low transition temperature HEBs.

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.

In this paper, we present our studies on a technology demonstrator for a balanced waveguide hot-electron bolometer (HEB) mixer operating in the 1.6–2.0 THz band. The design employs a novel layout for the HEB mixer combining several key technologies: all-metal THz waveguide micromachining, ultra-thin NbN film deposition and a micromachining of a silicon-on-insulator (SOI) substrate to manufacture the HEB mixer. In this paper, we present a novel mixer layout that greatly facilitates handling and mounting of the mixer chip via self-aligning as well as provides easy electrical interfacing. In our opinion, this opens up a real prospective for building multi-pixel waveguide THz receivers. Such receivers could be of interest for SOFIA, possible follow up of the Herschel HIFI, and even for ground based telescopes yet over limited periods of time with extremely dry weather (PWV less than 0.1 mm).

We have studied the phase locking and spectral linewidth of an ∼2.7THz quantum cascade laser by mixing its two lateral lasing modes. The beat signal at about 8GHz is compared with a microwave reference by applying conventional phase lock loop circuitry with feedback to the laser bias current. Phase locking has been demonstrated, resulting in a narrow beat linewidth of less than 10Hz. Under frequency stabilization we find that the terahertz line profile is essentially Lorentzian with a minimum linewidth of ∼6.3kHz. Power dependent measurements suggest that this linewidth does not approach the Schawlow-Townes limit.

Superconducting hot electron bolometers (HEBs) are widely used as terahertz (THz) heterodyne detectors (mixers) and becoming the most advantageous candidate for low noise receiver at frequencies above 1.2 THz. However, there is still lack of accurate theoretical modeling which explains the complex mechanism of different aspects of superconducting HEBs. These difficulties do not prevent researchers around the world from making experimental progress. In the last few years, superconducting HEBs are also used as THz direct detectors to develop the array for the applications in biology, medicine and security systems. Recently, a noise equivalent power (NEP) of 0.2 pW/√Hz at 2.5 THz is reported for the superconducting HEBs with thermal biasing. Instead of thermal one, microwave (MW) which was first used to stabilize the HEB mixer is also applied to bias the device as a direct detector. Furthermore, THz can be used to bias the HEBs, if THz sources can be provided easy. Although MW biasing can improve sensitivity of the superconducting HEB direct detectors, little work has been done to compare the responsivity and NEP among the THz direct detectors with thermal, THz and MW biasing. To find a best biasing method, we have studied the similarities and differences of the direct detectors based on the superconducting HEBs with three different biasing methods. The fabricated NbN HEB detectors consist of a complementary logarithmic-spiral antenna made of gold and an NbN film (bridge) connecting across the antenna's inner terminals. We have fabricated the niobium nitride (NbN) superconducting HEB mixers with the system double sideband (DSB) noise temperature better than 8 times of the quantum limit: hf/kB (where h is the Planck constant, kB is the Boltzmann constant and f is the operating frequency) at the frequencies higher than 1 THz. Here, the mixer chips with the system DSB noise temperature of about 400 K and the intermediate frequency (IF) gain bandwidth (GBW) of larger than 5 GHz at 4.2 K and 0.65 THz have been used. As the direct detectors, the current responsivity of 290 A/W and 105 A/W are obtained for MW and THz biasing, which about one order higher than that of thermal biasing. The NEP of 2.7 pW/√Hz is obtained with MW biasing, which can be expected to be improved in the future.

Hot electron bolometric mixers: new terahertz technology

The development of artificially structured electromagnetic materials, termed metamaterials, has led to the realization of phenomena that cannot be obtained with natural materials. This is especially important for the technologically relevant terahertz (1 THz = 10(12) Hz) frequency regime; many materials inherently do not respond to THz radiation, and the tools that are necessary to construct devices operating within this range-sources, lenses, switches, modulators and detectors-largely do not exist. Considerable efforts are underway to fill this 'THz gap' in view of the useful potential applications of THz radiation. Moderate progress has been made in THz generation and detection; THz quantum cascade lasers are a recent example. However, techniques to control and manipulate THz waves are lagging behind. Here we demonstrate an active metamaterial device capable of efficient real-time control and manipulation of THz radiation. The device consists of an array of gold electric resonator elements (the metamaterial) fabricated on a semiconductor substrate. The metamaterial array and substrate together effectively form a Schottky diode, which enables modulation of THz transmission by 50 per cent, an order of magnitude improvement over existing devices.

A tunable continuous-wave source of a far-infrared and THz radiation based on a semiconductor nanoheterostructure with asymmetric quantum wells is suggested. It utilizes Rabi oscillations at a transition between quantum well subbands excited by external femtosecond pulses of a mid-infrared electromagnetic field. Due to quantum well broken inversion symmetry the subbands possess different average dipole moments, which enables the creation of polarization at the Rabi frequency as the subband populations change. It is shown that if this polarization is excited so that it is periodic in space, then, though being pulsed, it can produce continuous-wave output radiation. Changing the polarization space period and the time intervals between the exciting pulses, one can tune the frequency of this radiation throughout the far-infrared and THz range. In the present work a concrete multiple quantum well heterostructure design and a scheme of its space-periodic polarization are suggested. It is shown that for existing sources of mid-infrared femtosecond pulses the proposed scheme can provide a continuous-wave output power of order the power of far-infrared and THz quantum cascade lasers. Being added to the possibility of its output frequency tuning, this can make the suggested device attractive for fundamental research and various applications.

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.

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.

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.

In this paper we study the effect of NbN-Au contact pads on the performance of a superconducting NbN hot electron bolometer (HEB) mixer. The receiver noise temperature (Trec) and IF noise bandwidth (BN) at a LO frequency of 1.3 THz are measured at different bath temperatures between 3.3 K and 8.5 K. The measured Trec and BN of the superconducting HEB mixer are nearly constant for Tbath/Tc less than 0.7, and increase quickly with the further increase of Tbath. Measurement results demonstrate that the NbN-Au contact pads in superconducting state nearly have no influence on Trec and Bn, but the contact pads in resistive state make Trec and BN increase quickly with Tbath, indicated by the slope change of current-voltage curves and IF output power peak.

The Oxygen Spectrometer for Atmospheric Science on a Balloon (OSAS-B) is a heterodyne receiver for the thermally excited ground state transition of neutral atomic oxygen at 4.75 THz [1]. It has been shown that this transition is favorable for the determination of atomic oxygen in the mesosphere and lower thermosphere (MLT) region of Earth [2]. Due to water absorption, it cannot be observed from ground. Atomic oxygen is the dominant species in the MLT, and thus plays an important role for the chemistry and energy balance in the MLT region as well as for the deceleration of low-earth orbit satellites. OSAS-B uses a combined helium/nitrogen cryostat for the detector of the instrument, a superconducting hot-electron bolometer mixer, as well as for cooling the quantum-cascade laser, which serves as the local oscillator for heterodyne detection. A turning mirror allows for measurements at different vertical inclinations and for radiometric calibration against two blackbody sources. The first flight took place as a one-day flight in September 2022 from Esrange, Sweden in the framework of the EU-funded Hemera 2020 program. During the course of the flight, several hundred spectra for different elevation angles and azimuth directions were recorded. Preliminary results show a reasonable agreement with predictions from the current MSIS model (NRL MSIS 2.0/2.1). Besides, we are able to observe the spectral signatures of shear winds in the MLT as they are predicted by the horizontal-wind model (HWM 2014).[1] Wienold, M. et al. 48th IRMMW-THz, Montreal, Canada (2023), doi: 10.1109/IRMMW-THz57677.2023.10299165[2] Richter, H. et al. Commun Earth Environ 2,19 (2021), doi: 10.1038/s43247-020-00084-5

THz wave spectroscopy and imaging technologies are promising in security inspection applications. However, the following hurdles prevent THz technologies to be used in in situ applications, especially when standoff detection is required. First of all, the attenuation of THz waves in the atmosphere is higher than 100 dB/km, so it was previously thought impossible to perform long distance broadband THz wave sensing and spectroscopy, due to severe water vapor attenuation. Secondly, pulsed THz wave emitters using either real or virtual photocurrents, saturate when high excitation intensities are used. Further increase of the excitation power may even cause damage to the emitter. The saturation and damage of THz wave emitter limits the strength of the THz fields that can be generated from such emitters. Additionally, although pulsed THz wave generation and detection systems provide broadband spectral coverage, the spectrum does not generally cover the entire terahertz band continuously. Semiconductors or nonlinear crystals usually have phonon modes in the THz band. Absorption and dispersion due to photons result in dark areas in the measured THz spectrum. Finally, the reflection of THz waves, by both surfaces of the emitter or sensor, generates interference patterns in the THz spectrum. Confronted by those hurdles, using ambient air as the THz wave emitter and sensor becomes more and more interesting. By using ambient air as THz wave emitter and sensor, one can generate and detect THz waves close to the sample. Sending an optical beam instead a of THz wave, benefits long-distance standoff detection due to the relatively low attenuation experienced in the atmosphere. Since air or other gases are easily replaceable, damage is not a concern even if a strong laser field is used to generate the THz pulses. As a result, it is preferable in the generation of high intensity THz pulses. Finally, dry air has neither phonon bands nor boundary reflection surfaces, and thus provides continuous coverage along the entire bandwidth.