M. Justen

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.

NbTiN Hot Electron Bolometer Waveguide Mixers on

We report on NbTiN hot electron bolometer (HEB) mixer design and fabrication for the 1.4, 1.9 and 2.5 THz frequency bands. The mixers under discussion are our contribution to the multi-band single-pixel receivers of the German Receiver for Astronomy at Terahertz Frequencies (GREAT), which is a first light instrument for the airborne Stratospheric Observatory for Infrared Astronomy (SOFIA), and the focal plane array receiver on the balloon-borne Stratospheric Terahertz Observatory (STO). We measure device noise vs. intermediate frequency (IF) and analyse the receiver system output power stability and IF band ripple with newly developed SiGe low-noise amplifiers from the S. Weinreb group (Caltech). The mixers use waveguide technology with the device coupled to the fundamental waveguide mode via an integrated probe antenna. The device is electrically connected through beam leads, which reliably suspend the 2 μm thin Si3N4 membrane with micrometer mounting precision. Electron beam lithography defines the 400 nm long and 4 nm thick NbTiN microbridges and a novel deep reactive-ion etch is used for shaping of the substrates.

{\rm Si}_{3}{\rm N}_{4}

We present a 380?520?GHz balanced superconductor?insulator?superconductor (SIS) mixer on a single silicon substrate. All radio-frequency (RF) circuit components are fabricated on a 9??m thick membrane. The intermediate frequency (IF) is separately amplified and combined. The balanced mixer chip, using Nb/Al/Al2O3/Nb SIS junctions, is mounted in a tellurium copper waveguide block at 4.2?K using Au beam lead contacts. We find uncorrected minimum receiver double-sideband noise temperatures of 70?K and a noise suppression of up to 18?dB, measured within a 440?495?GHz RF and a 4?8?GHz IF bandwidth, representing state-of-the-art device performance.

Membranes at THz Frequencies

This article presents a heterodyne experiment which uses a 380–520 GHz planar circuit balanced Nb-Al2O3-Nb superconductor-insulator-superconductor (SIS) quasiparticle mixer with 4–8 GHz instantaneous intermediate frequency (IF) bandwidth to quantitatively determine local oscillator (LO) noise. A balanced mixer is a unique tool to separate noise at the mixers LO port from other noise sources. This is not possible in single-ended mixers. The antisymmetric IV characteristic of a SIS mixer further helps to simplify the measurements. The double-sideband receiver sensitivity of the balanced mixer is 2–4 times the quantum noise limit hν/kB over the measured frequencies with a maximum LO noise rejection of 15 dB. This work presents independent measurements with three different LO sources that produce the reference frequency but also an amount of near-carrier noise power which is quantified in the experiment as a function of the LO and IF frequency in terms of an equivalent noise temperature TLO. Two types of LO ...

Balanced superconductor–insulator–superconductor mixer on a 9 µm silicon membrane

2×2 parallel fed and 3×3 serial fed patch antenna arrays on a benzocyclobutene (BCB) polymer layer are integrated with a 70 μm wide, dry etched, double metal waveguide quantum cascade laser, operating at about 1.9 THz. The BCB surrounds the quantum cascade laser ridge and is planarized to fit precisely its height. The patch antenna arrays emit a linearly polarized, highly symmetric beam perpendicular to the antenna plane. The beams have a FWHM angle of 49° (2×2) and 35° (3×3). Both measurements and simulations indicate coupling factors to a Gaussian beam of over 90%. The antenna design is strongly governed by the high thickness (h=13.6  μm) and the low dielectric constant (ϵr=2.45) of the BCB substrate. Because the patch array has a very low input reflectivity of -13 to -20  dB over the 1.7-2.1 THz operation band, the laser needs a partially transmitting reflector to maintain the Q-factor of the active medium resonator to assure lasing in the antennas operation band. By changing the dimensions of the reflector, the facet transparency can be designed in a wide range from fully transmissive to highly reflective.

A 490 GHz planar circuit balanced Nb-Al2O3-Nb quasiparticle mixer for radio astronomy: Application to quantitative local oscillator noise determination

We present the recently assembled flight models of the SIS mixer units (MU) for band 2 of the HIFI instrument for the Herschel satellite observatory. Design, device fabrication, most of the mixer hardware fabrication, space-qualification, and final assembly are all realized at KOSMA. Thanks to the modular design RF- and IF-performance of different parts of the MU are tested separately before final assembly. Latest test results show a noise temperature between 80 K in the band centre towards 220 K at the upper band edge at 2.8 K operating temperature. The mixer IF-ripple is below 2 dB over a 4-8 GHz bandwidth including the bias-T which provides a very high EMI and ESD protection. The mixer performance and design are discussed under the aspect of the careful trade off between performance, reliability, timely delivery, and resources.

2D patch antenna array on a double metal quantum cascade laser with >90% coupling to a Gaussian beam and selectable facet transparency at 1.9 THz.

We introduce a double metal terahertz quantum cascade laser meant for astrophysical heterodyne measurements. The laser ridge is embedded in benzocyclobutene, and the device exhibits single mode, continuous wave operation around 4.745 THz with a peak power of almost 1.8 mW at 10 K and a power consumption of ≈1.6 W. Moreover, thanks to the integration of a top metal contact with a patch array antenna for light out-coupling the beam of the emitted light has a low-divergence single-lobe profile and an FWHM of ≈30°.

SIS flight mixers for band 2 of the HIFI instrument of the Herschel Space Observatory

GaAs/AlGaAs quantum cascade lasers based on four quantum well structures operating at 4.7 THz are reported. A large current density dynamic range is observed, leading to a maximum operation temperature of 150 K for the double metal waveguide device and a high peak output power more than 200 mW for the single surface plasmon waveguide device. A continuous wave, single mode, third order distributed feedback laser with a low electrical power dissipation and a narrow far-field beam pattern, which is required for a local oscillator in astronomy heterodyne spectrometers, is also demonstrated.

A patch-array antenna single-mode low electrical dissipation continuous wave terahertz quantum cascade laser

We present the details of a coupling structure to embed a 1.9-THz double-metal quantum cascade lasers (QCLs) into a 120-

High performance 4.7 THz GaAs quantum cascade lasers based on four quantum wells

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