We report on the first room-temperature modular multi-pixel Schottky diode-based, tunable, frequency-multiplied local
oscillator sub-system at 1.9 THz. This source has been developed to enable efficient high-resolution mapping of the C+
line using suborbital platforms such as the SOFIA aircraft and balloons, as well as space instruments. This compact LO
source features four multipliers (X3X2X3X3) to up-convert Ka-band power to 1.9 THz. Preliminary results at 300 K
demonstrate more than 5 μW per pixel at 1.9 THz. The source is designed to provide a large output power dynamic
range and can be expanded to larger array receivers.
Terahertz high-resolution spectroscopy of interstellar molecular clouds greatly relies on hot-electron superconducting bolometric (HEB) mixers. Current state-of-the-art receivers use mixer devices made from ultrathin (~ 3-5 nm) films of NbN with critical temperature ~ 9-11 K. Such mixers have been deployed on a number of groundbased, suborbital, and orbital platforms including the HIFI instrument on the Hershel Space Observatory. Despite its good sensitivity and well-established fabrication process, the NbN HEB mixer suffers from the narrow intermediate frequency (IF) bandwidth ~ 2-3 GHz and is limited to operation at liquid Helium temperature. As the heterodyne receivers are now trending towards “high THz” frequencies, the need in a larger IF bandwidth becomes more pressing since the same velocity resolution for a Doppler shifted line at 5 THz requires a 5-times greater IF bandwidth than at 1 THz. Our work is focusing on the realization of practical HEB mixers using ultrathin (10-20 nm) MgB2 films. They are prepared using a Hybrid Physical-Chemical Vapor Deposition (HPCVD) process yielding ultrathin films with critical temperature ~ 37-39 K. The expectation is that the combination of small thickness, high acoustic phonon transparency at the interface with the substrate, and very short electron-phonon relaxation time may lead to IF bandwidth ~ 10 GHz or even higher. SiC continues to be the most favorable substrate for MgB2 growth and as a result, a study has been conducted on the transparency of SiC at THz frequencies. FTIR measurements show that semi-insulating SiC substrates are at least as transparent as Si up to 2.5 THz. Currently films are passivated using a thin (10 nm) SiO2 layer which is deposited ex-situ via RF magnetron sputtering. Micron-sized spiral antenna-coupled HEB mixers have been fabricated using MgB2 films as thin as 10 nm. Fabrication was done using contact UV lithography and Ar Ion milling, with E-beam evaporated Au films deposited for the antenna. Measurements have been carried out on these devices in the DC, Microwave, and THz regimes. The devices are capable of mixing signals above 20 K indicating that operation may be possible using a cryogen-free cooling system. We will report the results of all measurements taken to indicate the local oscillator power requirements and the IF bandwidth of MgB2 HEB mixers.
We report on the development of waveguide-based mixers for operation beyond 2 THz. The mixer element is a
superconducting hot-electron bolometer (HEB) fabricated on a silicon-on-insulator (SOI) substrate. Because it is beyond
the capability of conventional machining techniques to produce the fine structures required for the waveguide embedding
circuit for use at such high frequencies, we employ two lithography-based approaches to produce the waveguide circuit:
a metallic micro-plating process akin to 3-D printing and deep reactive ion etching (DRIE) silicon micromachining.
Various mixer configurations have been successfully produced using these approaches. A single-ended mixer produced
by the metal plating technique has been demonstrated with a receiver noise temperature of 970 K (DSB) at a localoscillator
frequency of 2.74 THz. A similar mixer, produced using a silicon-based micro-machining technique, has a
noise temperature of 2000 K (DSB) at 2.56 THz. In another example, we have successfully produced a waveguide RF
hybrid for operation at 2.74 THz. This is a key component in a balanced mixer, a configuration that efficiently utilizes
local oscillator power, which is scarce at these frequencies. In addition to allowing us to extend the frequency of
operation of waveguide-based receivers beyond 2 THz, these technologies we employ here are amenable to the
production of large array receivers, where numerous copies of the same circuit, precisely the same and aligned to each
other, are required.
The Stratospheric TeraHertz Observatory (STO) is a NASA funded, Long Duration Balloon (LDB) experiment designed to
address a key problem in modern astrophysics: understanding the Life Cycle of the Interstellar Medium (ISM). STO will
survey a section of the Galactic plane in the dominant interstellar cooling line [C II] (1.9 THz) and the important star
formation tracer [N II] (1.46 THz) at ~1 arc minute angular resolution, sufficient to spatially resolve atomic, ionic and
molecular clouds at 10 kpc. STO itself has three main components; 1) an 80 cm optical telescope, 2) a THz instrument
package, and 3) a gondola . Both the telescope and gondola have flown on previous experiments [2,3]. They have been reoptimized
for the current mission. The science flight receiver package will contain four [CII] and four [NII] HEB mixers,
coupled to a digital spectrometer. The first engineering test flight of STO was from Ft. Sumner, NM on October 15, 2009.
The ~30 day science flight is scheduled for December 2011.
In the wavelength regime between 60 and 300 microns there are a number of atomic and molecular emission lines that
are key diagnostic probes of the interstellar medium. These include transitions of [CII], [NII], [OI], HD, H2D+, OH, CO,
and H2O, some of which are among the brightest global and local far-infrared lines in the Galaxy. In Giant Molecular
Clouds (GMCs), evolved star envelopes, and planetary nebulae, these emission lines can be extended over many arc
minutes and possess complicated, often self absorbed, line profiles. High spectral resolution (R> 105) observations of
these lines at sub-arcminute angular resolution are crucial to understanding the complicated interplay between the
interstellar medium and the stars that form from it. This feedback is central to all theories of galactic evolution. Large
format heterodyne array receivers can provide the spectral resolution and spatial coverage to probe these lines over
The advent of large format (~100 pixel) spectroscopic imaging cameras in the far-infrared (FIR) will fundamentally
change the way astronomy is performed in this important wavelength regime. While the possibility of such instruments
has been discussed for more than two decades, only recently have advances in mixer and local oscillator technology,
device fabrication, micromachining, and digital signal processing made the construction of such instruments tractable.
These technologies can be implemented to construct a sensitive, flexible, heterodyne array facility instrument for
SOFIA. The instrument concept for StratoSTAR: Stratospheric Submm/THz Array Receiver includes a common user
mounting, control system, IF processor, spectrometer, and cryogenic system. The cryogenic system will be designed to
accept a frontend insert. The frontend insert and associated local oscillator system/relay optics would be provided by
individual user groups and reflect their scientific interests. Rapid technology development in this field makes SOFIA the
ideal platform to operate such a modular, continuously evolving instrument.
We present an overview of the recent progress made in the development of a far-IR array of ultrasensitive hot-electron
nanobolometers (nano-HEB) made from thin titanium (Ti) films. We studied electrical noise, signal and noise
bandwidth, single-photon detection, optical noise equivalent power (NEP), and a microwave SQUID (MSQUID) based
frequency domain multiplexing (FDM) scheme. The obtained results demonstrate the very low electrical NEP down to
1.5×10-20 W/Hz1/2 at 50 mK determined by the dominating phonon noise. The NEP increases with temperature as ~ T3
reaching ~ 10-17 W/Hz1/2 at the device critical temperature TC = 330-360 mK. Optical NEP = 8.6×10-18 W/Hz1/2 at 357
mK and 1.4×10-18 W/Hz1/2 at 100 mK respectively, agree with thermal and electrical data. The optical coupling
efficiency provided by a planar antenna was greater than 50%. Single 8-μm photons have been detected for the first time
using a nano-HEB operating at 50-200 mK thus demonstrating a potential of these detectors for future photon-counting
applications in mid-IR and far-IR. In order to accommodate the relatively high detector speed (~ μs at 300 mK, ~ 100 μs
at 100 mK), an MSQUID based FDM multiplexed readout with GHz carrier frequencies has been built. Both the readout
noise ~ 2 pA/Hz1/2 and the bandwidth > 150 kHz are suitable for nano-HEB detectors.
This paper describes the Heterodyne Instrument for the Far-Infrared (HIFI), to be launched onboard of ESA's Herschel Space Observatory, by 2008. It includes the first results from the instrument level tests. The instrument is designed to be electronically tuneable over a wide and continuous frequency range in the Far Infrared, with velocity resolutions better than 0.1 km/s with a high sensitivity. This will enable detailed investigations of a wide variety of astronomical sources, ranging from solar system objects, star formation regions to nuclei of galaxies.
The instrument comprises 5 frequency bands covering 480-1150 GHz with SIS mixers and a sixth dual frequency band, for the 1410-1910 GHz range, with Hot Electron Bolometer Mixers (HEB). The Local Oscillator (LO) subsystem consists of a dedicated Ka-band synthesizer followed by 7 times 2 chains of frequency multipliers, 2 chains for each frequency band. A pair of Auto-Correlators and a pair of Acousto-Optic spectrometers process the two IF signals from the dual-polarization front-ends to provide instantaneous frequency coverage of 4 GHz, with a set of resolutions (140 kHz to 1 MHz), better than < 0.1 km/s. After a successful qualification program, the flight instrument was delivered and entered the testing phase at satellite level. We will also report on the pre-flight test and calibration results together with the expected in-flight performance.
A 1.5 THz superconducting receiver has been in operation at the Receiver Lab Telescope of the Smithsonian
Astrophysical Observatory in Northern Chile since December 2004. This receiver incorporates a Hot Electron Bolometer
(HEB) mixer chip made from a thin film of Niobium Titanium Nitride (NbTiN), which is mounted in a precisionmachined
waveguide mixer block attached to a corrugated waveguide horn assembly. With a noise temperature of
around 1500 K, this receiver is sensitive enough for use in the pioneering field of ground-based terahertz spectral-line
astronomy. A number of innovative techniques have been employed in the construction and deployment of this receiver.
These include near-field vector beam mapping to enable accurate coupling to the telescope optics, the use of tunerless
planar-diode based local oscillator unit capable of generating a few μW at 1.5 THz, and special calibration techniques
required for terahertz astronomy. In this paper, we will report on the design, set-up and operation of this state-of-the-art
We are developing a hot-electron superconducting transition-edge sensor (TES) that is capable of counting THz photons
and operates at T = 0.3K. The main driver for this work is moderate resolution spectroscopy (R ~ 1000) on the future
space telescopes with cryogenically cooled (~ 4 K) mirrors. The detectors for these telescopes must be background-limited
with a noise equivalent power (NEP) ~ 10-19-10-20 W/Hz1/2 over the range ν=0.3-10 THz. Above about 1 THz,
the background photon arrival rate is expected to be ~ 10-100 s-1, and photon counting detectors may be preferable to an
integrating type. We fabricated superconducting Ti nanosensors with a volume of ~ 3×10-3 μm3 on planar Si substrate
and have measured the thermal conductance G to the thermal bath. A very low G=4×10-14 W/K, measured at 0.3 K, is
due to the weak electron-phonon coupling in the material and the thermal isolation provided by superconducting Nb
contacts. This low G corresponds to NEP(0.3K) = 3×10-19 W/Hz1/2. This Hot-Electron Direct Detector (HEDD) is
expected to have a sufficient energy resolution for detecting individual photons with ν > 0.3 THz at 0.3 K. With the
sensor time constant of a few microseconds, the dynamic range is ~ 50 dB.
The Heterodyne Instrument for Far Infrared (HIFI) on ESA's Herschel Space Observatory utilizes a variety of novel RF components in its five SIS receiver channels covering 480- 1250 GHz and two HEB receiver channels covering 1410-1910 GHz. The local oscillator unit will be passively cooled while the focal plane unit is cooled by superfluid helium and cold helium vapors. HIFI employs W-band GaAs amplifiers, InP HEMT low noise IF amplifiers, fixed tuned broadband planar diode multipliers, high power W-band Isolators, and novel material systems in the SIS mixers. The National Aeronautics and Space Administration through the Jet Propulsion Laboratory is managing the development of the highest frequency (1119-1250 GHz) SIS mixers, the local oscillators for the three highest frequency receivers as well as W-band power amplifiers, high power W-band isolators, varactor diode devices for all high frequency multipliers and InP HEMT components for all the receiver channels intermediate frequency amplifiers. The NASA developed components represent a significant advancement in the available performance. This paper presents an update of the performance and the current state of development.
The Heterodyne Instrument for Far Infrared (HIFI) on ESA's Herschel Space Observatory is comprised of five SIS receiver channels covering 480-1250 GHz and two HEB receiver channels covering 1410-1910 GHz. Two fixed tuned local oscillator sub-bands are derived from a common synthesizer to provide the front-end frequency coverage for each channel. The local oscillator unti will be passively cooled while the focal plane unit is cooled by superfluid helium and cold helium vapors. HIFI employs W-band GaAs amplifiers, InP HEMT low noise IF amplifiers, fixed tuned broadband planar diode multipliers, and novel material systems in the SIS mixtures. The National Aeronautics and Space Administration's Jet Propulsion Laboratory is managing the development of the highest frequency (1119-1250 GHz) SIS mixers, the highest frequency (1650-1910 GHz) HEB mixers, local oscillators for the three highest frequency receivers as well as W-band power amplifiers, varactor diode devices for all high frequency multipliers and InP HEMT components for all the receiver channels intermediate frequency amplifiers. The NASA developed components represent a significant advancement in the available performance. The current state of the art for each of these devices is presented along with a programmatic view of the development effort.
We are developing terahertz mixers to cover the highest frequency band ("6H") for the heterodyne instrument (HIFI) aboard the Herschel Space Observatory. The mixer will be optimized for operation at 1.8 THz, with an input bandwidth of at least 0.2 THz. Some of the key spectroscopic lines in this frequency band are the fine-structure transition of ionized carbon at 1.9 THz, and numerous rotational transitions of water vapor and other hydrides. The mixers will employ a superconductive hot-electron bolometer as the mixing element, for which we will use a diffusion-cooled niobium microbridge. This variant allows an IF bandwidth that meets the range required for HIFI's 4-8 GHz IF. The mixer will be operated at ~2 K bath temperature. The sensitivity requirement is a double sideband mixer noise temperature of Tmix / ν ~ 1,000 K / THz , which has been previously demonstrated with this type of mixer. The mixer is a quasioptical design, employing a twin-slot planar antenna mounted on the backside of an elliptical silicon lens. Initial measurements indicate that that these mixers can be adequately pumped with a solid-state 1.5 THz local-oscillator source. HEB mixers are extremely delicate and susceptible to environmental damage; we have therefore focused a good deal of attention to engineering a rugged, flyable mixer.
SIS heterodyne mixer technology based on niobium tunnel junctions has now been pushed to frequencies over 1 THz, clearly demonstrating that the SIS junctions are capable of mixing at frequencies up to twice the energy gap frequency (4(Delta) /h). However, the performance degrades rapidly above the gap frequency of niobium (2(Delta) /h approximately equals 700 GHz) due to substantial ohmic losses in the on-chip tuning circuit. To solve this problem, the tuning circuit should be fabricated using a superconducting film with a larger energy gap, such as NbN; unfortunately, NbN films often have a substantial excess surface resistance in the submillimeter band. In contrast, the SIS mixer measurements we present in this paper indicate that the losses for NbTiN thin films can be quite low.