We measured the double sideband (DSB) receiver noise temperature (TrecDSB) of an NbN hot electron bolometer (HEB) mixer at three local oscillator frequencies of 1.6, 2.5, and 5.3 THz. The HEB has cleaned contact interfaces with a 200 nm thick Au layer. The measuredTrecDSB values are 530 ± 11 K, 640 ±18 K, and 2190 ±150 K at 1.6, 2.5, and 5.3 THz, respectively, using an air setup with total optical losses of 2.60 ± 0.04, 2.63 ± 0.16, and 4.70 ± 0.24 dB, respectively. We derived low mixer noise temperatures (TmixerDSB) of 240 ± 6 K at 1.6 THz and 290 ± 13 K at 2.5 THz, achieving over 30% improvement compared to published NbN HEB mixers. This enhancement can reduce the integration time of a heterodyne instrument by roughly a factor of 2. At 5.3 THz,TmixerDSB is 620 ± 55 K, showing limited improvement due to non-optimized antenna geometry. These results also contribute to understanding the device physics of a wide HEB (4 μm) at high frequencies.

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We have demonstrated three 4×2 hot electron bolometer (HEB) mixer arrays for operation at local oscillator (LO) frequencies of 1.4, 1.9 and 4.7 THz, respectively. These arrays consist of spiral antenna coupled NbN HEB mixers combined with elliptical lenses. These are to date the highest pixel count arrays using a quasi-optical coupling scheme at supra-THz frequencies. At 1.4 THz, we obtained an average double sideband mixer noise temperature of 330 K, a mixer conversion loss of 5.2 dB, and an optimum LO power of 210 nW. The array at 1.9 THz has an average mixer noise temperature of 425K, a mixer conversion loss of 6.4 dB, and an optimum LO power of 190 nW. For the array at 4.7 THz we obtained an average mixer noise temperature of 715 K, a mixer conversion loss of 8.9 dB, and an optimum LO power of 240 nW. We found the arrays to be uniform regarding the mixer noise temperature with a standard deviation of 3-4%, the conversion loss with a standard deviation of 8-11%, and optimum LO power with a standard deviation of 5-6%. The noise bandwidth was also measured, being 3.5 GHz for the three arrays. These performances are comparable to previously reported values in the literature for single pixels and also other detector arrays at similar frequencies. Our arrays met the requirements and were employed in the Galactic/Extra-Galactic ULDB Spectroscopic Terahertz Observatory (GUSTO), a NASA balloon borne observatory. GUSTO launched from Antarctica on the 31st December 2023 having a successful flight of 57 days, the longest ever recorded by NASA for such mission profile.@en

We have studied THz heterodyne detection in sub-micrometer MgB2 hot electron bolometer (HEB) mixers based on superconducting MgB2 films of ∼ 5 nm (HEB-A), corresponding to a critical temperature (Tc) of 33.9 K, and ∼ 7 nm (HEB-B), corresponding to a T c of 38.4 K. We have measured a double sideband (DSB) receiver noise temperature of 2590 K for HEB-A and 2160 K for HEB-B at 1.6 THz and 5 K. By correcting for optical losses, both HEBs show receiver noise temperatures of ∼1600 K referenced to the front of anti-reflection (AR)-coated Si lenses. An intermediate frequency (IF) noise bandwidth of 11 GHz has been measured for both devices. The required local oscillator (LO) power is about 13 μW for both HEBs. We have also measured a DSB receiver noise temperature of 3290 K at 2.5 THz and 5 K but with an AR-coated lens optimized for 1.6 THz. Besides, we have observed a step-like structure in current voltage (IV) curves, which becomes weaker when the LO power increases and observable only in their differential resistance. Such a correlated structure appears also in the receiver output power as a function of voltage, which is likely due to electronic inhomogeneities intrinsic to the variations in the thickness of the MgB2 films. Different behavior in the IV curves around the low bias voltages, pumped with the same LO power at 1.6 and 5.3 THz, was observed for HEB-B, suggesting the presence of a high-energy σ-gap in the MgB2 film.

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Heterodyne receivers combining a NbN HEB mixer with a local oscillator (LO) are the work horse for high resolution (≥106) spectroscopic observations at supra-terahertz frequencies. We report an MgB2 HEB mixer working at 5.3 THz with
20 K operation temperature based on a previously published paper [Y. Gan et al, Appl. Phys. Lett., 119, 202601 (2021)]. The HEB consists of a 7 nm thick MgB2 submicron-bridge contacted with a spiral antenna. It has a Tc of 38.4 K. By using hot/cold blackbody loads and a Mylar beam splitter all in vacuum, and applying a 5.25 THz FIR gas laser as the LO, we measured a minimal DSB receiver noise temperature of 3960 K. The latter gives a DSB mixer noise temperature of 1470 K. This sensitivity is 28 times better than a room temperature Schottky mixer at 4.7 THz, but about 2.5 times less sensitive than an NbN HEB mixer. The latter must be operated around 4 K. The IF noise bandwidth is about 10 GHz, which is 2.5-3 times larger than an NbN HEB. With further optimization, such MgB2 HEBs are expected to reach a better sensitivity. That the low noise, wide IF bandwidth MgB2 HEB mixers can be operated in a compact, low dissipation 20 K Stirling cooler can significantly reduce the cost and complexity of heterodyne instruments and therefore facilitate new space missions.@en

Gal/Xgal U/LDB Spectroscopic/ Stratospheric THz Observatory (GUSTO) is a NASA Explorers Mission of Opportunity that will make large scale maps of the Milky Way and Large Magellanic Cloud in three important interstellar lines: [CII], [OI], and [NII] at 158, 63, and 205 μm, respectively. During its ~75 day stratospheric (~36 km) flight, GUSTO’s 0.9-meter balloon-borne telescope and THz heterodyne array receivers will provide the spectral and spatial resolution needed to untangle the complexities of the interstellar medium by probing all phases of its Life Cycle. The GUSTO payload consists of (1) a telescope; (2) three 8-pixel heterodyne array receivers; (3) autocorrelator spectrometers; (4) instrument control electronics; and (5) a cryostat. The GUSTO gondola is derived from successful APL designs. Much of the GUSTO instrument architecture and hardware is based on the experience gained in developing and flying the Stratospheric Terahertz Observatory (STO). GUSTO is currently undergoing integration and test and will launch from the NASA Long Duration Balloon (LDB) Facility near McMurdo, Antarctica in December 2023.@en

We present an analysis of the bandwidth of an asymmetric 8-beam Fourier grating as the beam multiplexer for a 4.7 THz local oscillator used in a heterodyne receiver. We take the grating designed for NASA GUSTO balloon observatory as an example to address the bandwidth question although it does not need to operate over a wide frequency range. By illuminating the grating at different frequencies from 4.445 to 5.045 THz, we simulated the changes of its performance in three aspects using COMSOL Multiphysics: diffraction efficiency, power uniformity, and the angular distribution of the output beams. These parameters can affect the coupling efficiency between the output beams of the grating and the beams of a mixer array. The bandwidth of the grating is found to be 230 GHz, corresponding to 4.9% of the operating frequency, which is sufficient for many applications.

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We have demonstrated a low noise superconducting MgB2 hot electron bolometer (HEB) mixer working at the frequency of 5.3 terahertz (THz) with 20 K operation temperature. The bolometer consists of a 7 nm thick MgB2 submicrometer bridge contacted with a spiral antenna to couple THz radiation through a high resistive Si lens, and it has a superconducting critical temperature of 38 K. By using hot/cold blackbody loads and a Mylar beam splitter all in vacuum and applying a 5.25 THz far-infrared gas laser as a local oscillator, we measured a minimal double sideband receiver noise temperature of 3960 K at the LO power of 9.5 μW. This can be further reduced to 2920 K if a Si lens with an antireflection coating optimized at this frequency and a 3 μm beam splitter are used. The measured intermediate frequency (IF) noise bandwidth is 9.5 GHz. The low noise, wide IF bandwidth mixers, which can be operated in a compact, low dissipation Stirling cooler, are more suitable for space applications than the existing HEB mixers. Furthermore, we likely observed a signature of the double-gap in MgB2 by comparing current-voltage curves pumped at 5.3 and 1.6 THz.

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Large heterodyne receiver arrays (~100 pixel) allow astronomical instrumentations mapping more area within limited space mission lifetime. One challenge is to generate multiple local oscillator (LO) beams. Here, We succeeded in generating 81 beams at 3.86 THz by combining a reflective, metallic Fourier grating with an unidirectional antenna coupled 3rd-order distributed feedback (DFB) quantum cascade laser (QCL). We have measured the diffracted 81 beams by scanning a single pyroelectric detector at a plane, which is in the far field for the diffraction beams. The measured output beam pattern agrees well with a simulated result from COMSOL Multiphysics with respect to the angular distribution and power distribution among the 81 beams. We also derived the diffraction efficiency to be 94\pm 3\%, which is very close to what was simulated for a manufactured Fourier grating (97%). For an array of equal superconducting hot electron bolometer mixers, 64 out of 81 beams can pump the HEB mixers with similar power, resulting in receiver sensitivities within 10%. Such a combination of a Fourier grating and a QCL can create an LO with 100 beams or more, enabling a new generation of large heterodyne arrays for astronomical instrumentation. This paper is essentially a copy of our paper in Optics Express.

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We present a terahertz spatial filter consisting of two back-to-back (B2B) mounted elliptical silicon lenses and an opening aperture defined on a thin gold layer between the lenses. The beam filtering efficiency of the B2B lens system is investigated by simulation and experiment. Using a unidirectional antenna coupled 3rd-order distributed feedback (DFB) quantum cascade laser (QCL) at 3.86 THz as the source, the B2B lens system shows 72% transmissivity experimentally with a fundamental Gaussian mode as the input, in reasonably good agreement with the simulated value of 80%. With a proper aperture size, the B2B lens system is capable of filtering the non-Gaussian beam from the QCL to a nearly fundamental Gaussian beam, where Gaussicity increases from 74% to 99%, and achieves a transmissivity larger than 30%. Thus, this approach is proven to be an effective beam shaping technique for QCLs, making them to be suitable local oscillators in the terahertz range with a Gaussian beam. Besides, the B2B lens system is applicable to a wide frequency range if the wavelength dependent part is properly scaled.

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The grating design for the 4.7 THz channel of GUSTO (Galactic/ Extragalactic ULDB Spectroscopic Terahertz Observatory) has been reported in this paper, which acts as a beam multiplexer for coupling a single QCL (Quantum Cascade Laser) beam to an 8-pixel mixer array. The design and analysis are based on modeling and simulations showing a successful accommodation of the requirements from the designed optical system. The asymmetric feature is used and to be applied for the first time at such a frequency range.@en

We present an analysis of the bandwidth of a preliminary designed asymmetric 8-pixel Fourier grating as the beam multiplexer for the 4.7 THz local oscillator of the GUSTO mission. We take the GUSTO grating as an example to address the bandwidth question although GUSTO itself does not need to operate over a wide frequency range. By illuminating single beams with different frequencies from 4.445 THz to 5.045 THz to the grating, we simulated the changes in the grating’s performance in three aspects using COMSOL Multiphysics: diffraction efficiency, power distribution, and the angular distribution of the output beams. These parameters can reduce the coupling efficiency between the output beams of the grating and the beams of the mixer array of GUSTO. The grating’s bandwidth is calculated to be 250 GHz, which is sufficient for many applications.

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Large heterodyne receiver arrays (~100 pixel) allow astronomical instrumentations to map more area within limited space mission lifetime. One challenge is to generate multiple local oscillator (LO) beams. Here, we succeeded in generating 81 beams at 3.86 THz by combining a reflective, metallic Fourier grating with an unidirectional antenna coupled 3rd-order distributed feedback (DFB) quantum cascade laser (QCL). We have measured the diffracted 81 beams by scanning a single pyroelectric detector at a plane, which is in the far field for the diffraction beams. The measured output beam pattern agrees well with a simulated result from COMSOL Multiphysics, with respect to the angular distribution and power distribution among the 81 beams. We also derived the diffraction efficiency to be 94 ± 3%, which is very close to what was simulated for a manufactured Fourier grating (97%). For an array of equal superconducting hot electron bolometer mixers, 64 out of 81 beams can pump the HEB mixers with similar power, resulting in receiver sensitivities within 10%. Such a combination of a Fourier grating and a QCL can create an LO with 100 beams or more, enabling a new generation of large heterodyne arrays for astronomical instrumentation.

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GUSTO will be a NASA balloon borne terahertz observatory to be launched from Antarctica in late 2021 for a flight duration of 100-170 days. It aims at reviewing the life cycle of interstellar medium of our galaxy by simultaneously mapping the three brightest interstellar cooling lines: [OI] at 4.7 THz, [CII] at 1.9 THz, and [NII] at 1.4 THz; along the 124 degrees of the galactic plane and through a part of the Large Magellanic Cloud. It will use three arrays of 4x2 mixers based on NbN hot electron bolometers (HEBs), which are currently the most sensitive mixers for high resolution spectroscopic astronomy at these frequencies. Here we report on the design of a novel 4.7 THz receiver for GUSTO. The receiver consists mainly of two subsystems: a 4×2 HEB quasi-optical mixer array and a 4.7 THz multi-beam LO. We describe the mixer array, which is designed as a compact monolithic unit. We show, for example, 10 potential HEB detectors with the state of the art sensitivity of 720 K measured at 2.5 THz. They have a small variation in sensitivity, being less than 3%, while also meet the LO uniformity requirements. For the multi-beam LO we demonstrate the combination of a phase grating and a single QCL at 4.7 THz, which generates 8 sub-LO beams, where the phase grating shows an efficiency of 75%. A preliminary concept for the integrated LO unit, including QCL, phase grating and beam matching optics is presented.

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We compare the results of simulated and measured power efficiency and far-field beam pattern, for two reflective Fourier phase gratings, designed to generate 2 x 2 and 2 x 4 beams respectively from a single-beam, coherent source at 1.4 THz. The designed surface structures were manufactured on aluminum plates by a computer numerical control (CNC) micro-milling machine. Despite small differences between the designed and fabricated gratings, we measured power efficiencies of both gratings to be around 70%, which is in a good agreement with the simulated values. We also find a good agreement between the simulated and measured diffracted beam size and spatial distribution. We demonstrate the application of both gratings as multiple beam local oscillators to simultaneously pump (or operate) a 4-pixel array of superconducting heterodyne mixers.

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We present an 8-beam local oscillator (LO) for the astronomically significant [OI] line at 4.7 THz. The beams are generated using a quantum cascade laser (QCL) in combination with a Fourier phase grating. The grating is fully characterized using a third order distributed feedback (DFB) QCL with a single mode emission at 4.7 THz as the input. The measured diffraction efficiency of 74.3% is in an excellent agreement with the calculated result of 75.4% using a 3D simulation. We show that the power distribution among the diffracted beams is uniform enough for pumping an array receiver. To validate the grating bandwidth, we apply a far-infrared (FIR) gas laser emission at 5.3 THz as the input and find a very similar performance in terms of efficiency, power distribution, and spatial configuration of the diffracted beams. Both results represent the highest operating frequencies of THz phase gratings reported in the literature. By injecting one of the eight diffracted 4.7 THz beams into a superconducting hot electron bolometer (HEB) mixer, we find that the coupled power, taking the optical loss into account, is in consistency with the QCL power value.

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Modelling, manufacturing and characterization of two 4 and 8-pixel Fourier phase gratings operated at 1.4 THz are reported, mainly applicable as local oscillator multiplexers for heterodyne receivers. Comparing the measurements with full 3D simulations shows good agreements and provides good understanding. Power efficiency of around 70% is experimentally derived for both gratings. We demonstrate the application of both, as multiple beam local oscillators to simultaneously pump (or operate) an array of 4-pixel superconducting mixers.

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