In this paper, we present a low-noise, high-gain readout hardware to be used in conjunction with (sub)mm-wave zero bias detectors, to enable high sensitivity (i.e., ∼-50dBm) and fast (i.e., below a second at -50 dBm) power detection.

The developed hardware employs a cascade of two (COTS) programmable gain amplifier (PGA), capable of reaching up to 90 dB gain (volt to volt), each providing an input referred noise level of 16 nV/√Hz. The signal is then digitized on the same board via a 500 kHz 12bit ADC, providing an SNR of 74 dB. The digitized signal is then readout via an ST microcontroller and transferred to the operator PC via a USB interface. The sensitive bias voltages for the PGAs are provided via the microcontroller, or alternatively can be fed by an external lab grade supply to further improve on the (already high) supply noise rejection from the first stage PGA. The proposed hardware is designed to interface with VDI zero bias detectors, high responsivity and low NEP diodes (around 2000 V/W and below 12 pW/√Hz, respectively) operating in monomodal waveguide bands up to 500 GHz. In this contribution we demonstrate the usage of the developed hardware with WR10, WR6.5, WR5 and WR 3 ZBD diodes.@en

Photoconductive antennas (PCAs) are promising candidates for sensing and imaging applications. In recent years, our group has investigated their properties under pulsed laser illumination in transmission using a time-domain (TD) Norton equivalent circuit. Here, we extend this analysis to the link between a photoconductive source and a receiver introducing for the latter a second TD Norton equivalent circuit. We also evaluate the transfer function of a dispersive quasi-optical (QO) link. Specifically, a field correlation approach based on the high-frequency techniques is used to evaluate the spectral transfer function between two bow-tie-based PCAs, including the QO link. The detected currents in the receiving circuit are reconstructed using stroboscopic sampling of the modeled THz pulses, equivalent to what is actually performed by THz TD systems. Both the amplitude and the waveforms of these currents are evaluated. The QO link is then experimentally characterized to validate the proposed methodology. The comparison between the simulations and the measurements is excellent.

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This contribution presents the development of an electrically small lens antenna using an artificially loaded thermoplastic at 140-170GHz. We will present the on-going development of the Fly’s Eye front end antenna concept that was presented in [1]. The antenna is composed on a dual plastic lens, a core lens and a shell lens, fed by a double slot. The core-lens, being presented in this contribution, is a spherical lens made from an artificially loaded plastic of permittivity 9.5. To the best of our knowledge, this thermoplastic material has not been used for lens antennas in this frequency range before. A 4mm lens prototype has been developed using this material, which includes an antireflective layer synthesized by drilling sub-wavelength holes on the lens contour. Full-wave simulations show a negligible degradation of the performance of the anti-reflection layer compared to an ideal homogeneous matching layer. Physical measurements and antenna measurements confirm that the antenna's performance matches the design specifications.@en

Photoconductive antennas (PCAs) are promising candidates for sensing and imaging systems. We have investigated their properties under pulsed laser illumination both in transmission and reception. First, a transmitting PCA has been characterized including a power measurement. Then, a Quasi-Optical (QO) link between a transmitter and a receiver was modelled and analyzed. In this work, we characterize this link with measurement. We use bow-tie based PCAs as examples, and measure the radiated power of the transmitter and the detected current of the receiver. The measurement shows very good agreement with the simulation.@en

State-of-the-art THz pulsed commercial systems operating over large bandwidth suffer from high dispersion or low radiation efficiency due to the poor coupling between the transmitter and receiver photoconductive antennas (PCAs). In this work, we present the fabrication and characterization of a leaky-lens PCA that has the potential to solve this problem. The presented PCA is based on a low-temperature grown gallium arsenide (LT-GaAs) membrane with a 1:15 bandwidth coverage (0.1-1.5 THz), where the frequency response is constant. In order to fabricate the PCA on an LT-GaAs membrane, a novel fabrication process is developed. This process is dramatically faster than previously used processes (∼1.5 h instead of ∼20 h). Furthermore, an experimental validation of the radiated power together with the comparison to a standard bow-tie-based PCA fabricated on the same LT-GaAs wafer is shown in this article. We show that the PCA source on the LT-GaAs membrane is more efficient due to the enhanced leaky wave radiation. The leaky-lens PCA stands out as a great candidate to improve the coupling efficiency in THz pulsed commercial systems, where the maximum laser power that can be used is limited by the dispersion in the optic fiber.

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Drude's description of the response of low-temperature gallium arsenide to optical pulse excitation is used to evaluate the components of a time-domain Norton equivalent circuit of a photoconductive antenna (PCA) source. The saturation of the terahertz (THz) radiated power occurring at large optical excitation levels was previously associated by the scientific community to radiation and charge screening of the bias. With the present circuit, we are able to model accurately the measured saturation as only due to the EM feedback from the antenna to the bias. The predicted THz radiated power is shown to match very accurately the measurements when the circuit is combined with an accurate description of the experimental conditions and the modeling of the THz quasi-optical (QO) channel.

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In this paper, we present an experimental strategy to analyze the harmonic content of mm-wave frequency extenders using the VNA (absolute) power calibration step, without requiring spectrum analyzers and/or separate downconverters. The spectral purity of the upconverted band of the extenders is a key requirement to enable entirely software-based power control required for the accurate analysis of an (active) device under test. The proposed approach is based on the complementary response provided by the calorimeter-based power meter (i.e. VDI PM5) capable of integrating the entire spectral content of the waveguide band, in respect to the extreme frequency selectivity of the narrow-band mixer-based downconverter of the VNA. This complementary integration bandwidth response allows to compare the two results at each input drive level (at the power calibration setup, in-situ) and link the difference to the increased harmonic content contribution, with respect to the spectral content value at the saturation drive level, i.e. nominal manufacturer specified. The paper presents tests carried out in the WR10 (75–110 GHz) and WR6 band (110–170 GHz). The WR10 resulted in a harmonic contribution on the total output power of a maximum of 0.3 dB down to -33 dBc power back off from saturation level, and less than 1 dB down to -38 dBc while the WR6 the same parameter is less than 1 dB over the entire frequency band excluding the lower frequency points.@en

Photoconductive antennas (PCAs) are used for imaging and sensing applications because of their ability to radiate short pulses with large bandwidths in the THz regime. The characterization of PCAs has previously been done using a time-domain Norton equivalent circuit. Thanks to a recent contribution, the size of the excited photoconductive area of PCAs that results in an impedance match with the antenna can be determined analytically using only the available optical power and the material parameters of the photoconductor. Through the impedance matching, the radiated THz power is maximized. These insights are used for the dimensioning of a wide-band photoconductive connected array to be used in the low THz band, excited by a high power laser (∼1W).@en

We present the design, fabrication and characterization of a broadband lossless matching layer for silicon lens arrays. The proposed matching layer is based on silicon frusta (truncated pyramids) on top of the lens array fabricated by means of laser ablation. This matching layer is advantageous over quarter-wavelength dielectric matching layers since it covers more than an octave of bandwidth. We compare the performance of this matching layer with the commonly-used parylene-C matching layer at the center of the targetted band (500 GHz) in a lens-antenna integrated system. We measure a 1.6 dB higher transmission of the proposed silicon frusta matching compared to the parylene-C matching layer.

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Integrated superconducting spectrometer (ISS) technology will enable ultra-wideband, integral-field spectroscopy for (sub)millimeter-wave astronomy, in particular, for uncovering the dust-obscured cosmic star formation and galaxy evolution over cosmic time. Here, we present the development of DESHIMA 2.0, an ISS for ultra-wideband spectroscopy toward high-redshift galaxies. DESHIMA 2.0 is designed to observe the 220–440 GHz band in a single shot, corresponding to a redshift range of z = 3.3–7.6 for the ionized carbon emission ([C II] 158 μ m). The first-light experiment of DESHIMA 1.0, using the 332–377 GHz band, has shown an excellent agreement among the on-sky measurements, the laboratory measurements, and the design. As a successor to DESHIMA 1.0, we plan the commissioning and the scientific observation campaign of DESHIMA 2.0 on the ASTE 10-m telescope in 2023. Ongoing upgrades for the full octave-bandwidth system include the wideband 347-channel chip design and the wideband quasi-optical system. For efficient measurements, we also develop the observation strategy using the mechanical fast sky-position chopper and the sky-noise removal technique based on a novel data-scientific approach. In the paper, we show the recent status of the upgrades and the plans for the scientific observation campaign.

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Advances in far infrared astronomy have been, and will be, defined by instrument capabilities. Especially relevant is the development of imaging spectrometers for the wavelength range of 0.03-3 mm, which are not available at all at this moment. We will discuss recent advances in this field: First, we discuss the development of miniature on-chip spectrometers, that can operate in a 0.09-1 THz by using lossless superconducting circuits to create miniature spectrometers. For higher frequencies this is not possible due to material limitations, moreover instruments have to be operated in space due to the opacity of the atmosphere. Recent proposals for new missions focus on space-based observatories with optics cooled down to 4K, which offer unprecedented spectral imaging speeds, but require large arrays of extremely sensitive detectors. In the second part of this paper we will discuss the development of microwave Kinetic Inductance detectors with a sensitivity of NEP. 3.1.10-20 W/ãHz, sufficient for these applications.@en

A focal plane array of extended-hemispherical silicon lenses coupled to aluminum coplanar-waveguide (CPW) Microwave Kinetic Inductance Detectors (MKIDs) has been designed to operate at 7.8 THz. Low-dispersive leaky-wave radiation has been used to efficiently illuminate the antireflection-coated lenses. To minimize the radiation loss from the antenna feeding lines at these high frequencies, the CPWs have been miniaturized and placed on a dielectric membrane. A test device has been fabricated and its experimental characterization in terms of sensitivity, optical coupling, and beam patterns is ongoing.@en

Photoconductive antennas are devices that provide power up to THz frequencies at a relatively low cost. However, the power radiated by each antenna is typically quite low and arrays have been proposed to increase it. In this paper we present the design of a leaky enhanced array architecture that surpasses the state of the art as it operates efficiently for frequencies up to 1THz, without excessive complications in the manufacturing. This architecture is compared with a ‘standard’ array, showing a broader bandwidth and a higher emitted detected signal.@en

We present the design, fabrication and characterization of a broadband lossless matching layer for shallow lens arrays. The matching layer we propose is based on silicon pyramids fabricated on top of the lens array by means of laser ablation. This matching layer has the advantage that it covers over an octave of bandwidth. We have compared the performance of this matching layer with the commonly used parylene-C matching layer at the center of the targetted band, 500 GHz. The matching layer based on the silicon pyramids has 1.6 dB higher transmission.@en

A novel transmit lens array is proposed to provide broadband quasi-optical power distribution and beam-steering capabilities for array architectures in future submillimeter-wave heterodyne instruments. The transmit array is composed of a double array layer of lens antenna elements with high aperture efficiency. To enable broadband and low loss quasi-optical (QO) power distribution, the transmit lens array is coupled with a high aperture efficiency single lens antenna. The high aperture efficiency is achieved by using a recently introduced multi-mode leaky wave feed. The top lens array can be used to achieve beam-steering capabilities when fed coherently and mechanically translated using a piezo-motor. In this contribution, we present the development of a prototype based on a transmit lens array of 7 elements at 450-650 GHz with measurements showing a good agreement with simulations. This prototype demonstrates a quasi-optical power coupling efficiency of nearly 60%. Moreover it also shows beam-steering of a 36dBi directivity beam to few discrete angles up to +/-25 degrees with less than 2dB scan loss@en

This paper describes the microfabrication and electrical characterization of aluminum-coated superconducting through-silicon vias (TSVs) with sharp superconducting transition above 1 K. The sharp superconducting transition was achieved by means of fully conformal and void-free DC-sputtering of the TSVs with Al, and is here demonstrated in up to 500μ m-deep vias. Full conformality of Al sputtering was made possible by shaping the vias with a tailored hourglass profile, which allowed a metallic layer as thick as 430 nm to be deposited in the center of the vias. Single-via electric resistance as low as 160 mΩ at room temperature and superconductivity at 1.27 K were measured by a three-dimensional (3D) cross-bridge Kelvin resistor structure. This work establishes a CMOS-compatible fabrication process suitable for arrays of superconducting TSVs and 3D integration of superconducting silicon-based devices. [2020-0354].

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The time evolution of voltages and currents in a pulsed photo conductive antenna (PCA) source is evaluated resorting to a rigorous procedure that stems from semiconductor physics first, to define the phenomena involved in the generation of the photocurrent, and then relies on an equivalent circuit in time domain, providing a direct estimation of the power generated by the PCA as well as its spectral distribution. The circuit model is validated via a campaign of measurements of standard PC antenna sources. The saturation phenomena in the THz radiated power occurring at large optical excitation levels, previously observed by the scientific community and associated to different phenomena, are accurately predicted by the present method, which ascribe their main cause to the feedback from the antenna: indeed, the electromagnetic field generated by the device tend to reduce the strength of the forcing field used to accelerate the photo-carriers.

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This paper presents the fabrication and electrical characterization of superconducting high-aspect ratio through-silicon vias DC-sputtered with aluminum. Fully conformal and void-free coating of 300 μm-deep and 50 μmwide vias with Al, a CMOS-compatible and widely available superconductor, was made possible by tailoring a funneled sidewall profile for the axisymmetric vias. Single-via electric resistance as low as 80.44 mΩ at room temperature and superconductivity below 1.28 K were measured by a crossbridge Kelvin resistor structure. This work thus demonstrates the fabrication of functional superconducting interposer layers, suitable for high-density 3D integration of silicon-based quantum computing architectures.@en

We describe a microfabrication process that, thanks to a specifically tailored sidewall profile, enables for the first-time wafer-scale arrays of high-aspect ratio through-silicon vias (TSVs) coated with DC-sputtered Aluminum, achieving at once superconducting and CMOS-compatible 3D interconnects. Void-free conformal coating of up to 500μm-deep and 50μm-wide vias with a mere 2μm-thick layer of Al, a widely available metal in for IC manufacturing, was demonstrated. Single-via electric resistance as low as 468 mΩ at room temperature and superconductivity at 1.25 K were measured by a cross-bridge Kelvin resistor structure. This work establishes the fabrication of functional superconducting interposers suitable for 3D integration of high-density silicon-based quantum computing architectures.

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Recently, we have experimentally demonstrated the generation of mW average power, in the 100-600GHz bandwidth, by photoconductive sources using a connected array of dipoles coherently fed via a free-standing micro-lens array. In this contribution, we extend these sources to a much wider bandwidth reaching 5THz, without compromising the radiated power, by resorting to a tight array periodicity of 15Ϝm between the array elements and the use of an integrated 3D-printed micro-lens array to focus the laser power.

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