Terahertz spectroscopy systems use far-infrared radiation to extract molecular spectral information in an otherwise inaccessible portion of the electromagnetic spectrum. Materials research is an essential component of modern terahertz systems: novel, higher-power terahertz sources rely heavily on new materials such as quantum cascade structures. At the same time, terahertz spectroscopy and imaging provide a powerful tool for the characterization of a broad range of materials, including semiconductors and biomolecules.

Materials for terahertz science and technology

The Earth Observing System Microwave Limb Sounder measures several atmospheric chemical species (OH, HO/sub 2/, H/sub 2/O, O/sub 3/, HCl, ClO, HOCl, BrO, HNO/sub 3/, N/sub 2/O, CO, HCN, CH/sub 3/CN, volcanic SO/sub 2/), cloud ice, temperature, and geopotential height to improve our understanding of stratospheric ozone chemistry, the interaction of composition and climate, and pollution in the upper troposphere. All measurements are made simultaneously and continuously, during both day and night. The instrument uses heterodyne radiometers that observe thermal emission from the atmospheric limb in broad spectral regions centered near 118, 190, 240, and 640 GHz, and 2.5 THz. It was launched July 15, 2004 on the National Aeronautics and Space Administration's Aura satellite and started full-up science operations on August 13, 2004. An atmospheric limb scan and radiometric calibration for all bands are performed routinely every 25 s. Vertical profiles are retrieved every 165 km along the suborbital track, covering 82/spl deg/S to 82/spl deg/N latitudes on each orbit. Instrument performance to date has been excellent; data have been made publicly available; and initial science results have been obtained.

https://aura.gsfc.nasa.gov/science/publications/Waters_Froidevaux_Harwood_IEEE2006.pdf

The Earth observing system microwave limb sounder (EOS MLS) on the aura Satellite

Terahertz irradiation and sensing is being applied for the first time to a wide range of fields outside the traditional niches of space science, molecular line spectroscopy, and plasma diagnostics. This paper surveys some of the terahertz measurements and applications of interest in the biological and medical sciences.

Terahertz technology in biology and medicine

A summary of the NASA Jet Propulsion Laboratory's 675 GHz imaging radar is presented, with an emphasis on several key design aspects that enable fast, reliable through-clothes imaging of person-borne concealed objects. Using the frequency-modulated continuous-wave (FMCW) radar technique with a nearly 30 GHz bandwidth, sub-centimeter range resolution is achieved. To optimize the radar's range resolution, a reliable software calibration procedure compensates for signal distortion from radar waveform nonlinearities. Low-noise, high dynamic range detection comes from the radar's heterodyne RF architecture, low-noise chirp source, and high-performance 675 GHz transceiver. The radar's optical design permits low-distortion fast beam scanning for single-pixel imaging, and a real-time radar image frame rate of 1 Hz is now possible. Still faster speeds are on the horizon as multi-beam THz transceivers are developed.

THz Imaging Radar for Standoff Personnel Screening

Terahertz (THz) spectroscopy, and especially THz imaging, holds large potential in the field of nondestructive, contact-free testing. The ongoing advances in the development of THz systems, as well as the appearance of the first related commercial products, indicate that large-scale market introduction of THz systems is rapidly approaching. We review selected industrial applications for THz systems, comprising inline monitoring of compounding processes, plastic weld joint inspection, birefringence analysis of fiber-reinforced components, water distribution monitoring in polymers and plants, as well as quality inspection of food products employing both continuous wave and pulsed THz systems.

Terahertz imaging: applications and perspectives

A novel GaAs monolithic membrane-diode (MOMED) structure has been developed and implemented as a 2.5-THz Schottky diode mixer. The mixer blends conventional machined metallic waveguide with micromachined monolithic GaAs circuitry to form, for the first time, a robust, easily fabricated, and assembled room-temperature planar diode receiver at frequencies above 2 THz. Measurements of receiver performance, in air, yield at T/sub receiver/ of 16500-K double sideband (DSB) at 8.4-GHz intermediate frequency (IF) using a 150-K commercial Miteq amplifier. The receiver conversion loss (diplexer through IF amplifier input) measures 16.9 dB in air, yielding a derived "front-end" noise temperature below 9000-K DSB at 2514 GHz. Using a CO/sub 2/-pumped methanol far-infrared laser as a local oscillator at 2522 GHz, injected via a Martin-Puplett diplexer, the required power is /spl ap/5 mW for optimum pumping and can be reduced to less than 3 mW with a 15% increase in receiver noise. Although demonstrated as a simple submillimeter-wave mixer, the all-GaAs membrane structure that has been developed is suited to a wide variety of low-loss high-frequency radio-frequency circuits.

/pdf/2-5-thz-gaas-monolithic-membrane-diode-mixer-39yudyz7ub.pdf

2.5-THz GaAs monolithic membrane-diode mixer

Multiplier device fabrication techniques have been developed to enable robust implementation of monolithic circuits well into the THz frequency range. To minimize the dielectric loading of the waveguides, some circuits are realized entirely on a 3 /spl mu/m thick GaAs membrane with metal beamleads acting as RF probes and DC contact points. Other designs retain some thicker GaAs as a support and handling structure, allowing a membrane of bare metal or thin GaAs to be suspended across an input or output waveguide. Extensive use is made of selective etches, both reactive ion (RIE) and wet chemical, to maintain critical dimensions. Electron beam (e-beam) lithography provides the small contact areas required at the highest frequencies. Planar multiplier circuits for 200 GHz to 2700 GHz have been demonstrated using a variety of metal and GaAs membrane configurations made available by these fabrication techniques.

/pdf/fabrication-of-200-to-2700-ghz-multiplier-devices-using-gaas-s8ba7hi4gp.pdf

Fabrication of 200 to 2700 GHz multiplier devices using GaAs and metal membranes

The OH radical is an important player in known ozone depletion cycles; however, due to its location in the atmosphere, it must be studied from either a balloon or spaceborne platform. For long-term mapping over large portions of the earth, a spaceborne platform is the most desirable. NASA's Earth Observing System Microwave Limb Sounder instrument is slated to house a 2.5-THz Schottky-diode receiver for such measurements. In this paper, we describe the design, fabrication, and testing of the receiver front end. Measured double-sideband (DSB) receiver noise temperatures of better than 9000 K are reproducibly achieved with all devices of our best design. Estimated mixer noise is 3500-K DSB for optimal bias conditions and at room temperature. Selected components will be used in the first terahertz heterodyne receiver to be flown in space.

A 2.5-THz receiver front end for spaceborne applications

High bandwidths are obtained with heterojunction bipolar transistors by thinning the base and collector layers, increasing emitter current density, decreasing emitter contact resistivity, and reducing the emitter and collector junction widths In mesa HBTs, minimum dimensions required for the base contact impose a minimum width for the collector junction, frustrating device scaling Narrow collector junctions can be obtained by using substrate transfer processes, or -if contact resistivity is greatly reduced -by reducing the width of the base Ohmic contacts in a mesa structure HBTs with submicron collector junctions exhibit extremely high fmax and high gains in mm-wave ICs Logic gate delays are primarily set by depletion-layer charging times, and neither fτ nor fmax is indicative of logic speed For high speed logic, epitaxial layers must be thinned, emitter and collector junction widths reduced, current density increased, and emitter parasitic resistance decreased Transferred-substrate HBTs have obtained 21 dB unilateral power gain at 100 GHz If extrapolated at -20 dB/decade, the power gain cutoff frequency fmax is 11 THz Transferred-substrate HBTs have obtained 295 GHz fτ Demonstrated ICs include lumped and distributed amplifiers with bandwidths to 85 GHz, 66 GHz master-slave flip-flops, and 18 GHz clock rate Δ-Σ ADCs

SCALING OF InGaAs/InAlAsHBTs FOR HIGH SPEED MIXED-SIGNAL AND mm-WAVE ICs

A broadband planar Schottky balanced doubler at 800 GHz has been designed and built. The design utilizes two Schottky diodes in a balanced configuration on a 12 /spl mu/m thick gallium arsenide (GaAs) substrate as a supporting frame. This broadband doubler (designed for 735 GHz to 850 GHz) uses a split waveguide block and has a relatively simple, fast, and robust assembly procedure. The doubler achieved /spl ap/10% efficiency at 765 GHz, giving 1.1 mW of peak output power when pumped with about 9 mW of input power at room temperature.

/pdf/a-broadband-800-ghz-schottky-balanced-doubler-5ytpn1y33o.pdf

S.C. Martin

Papers

2.5-THz GaAs monolithic membrane-diode mixer

Fabrication of 200 to 2700 GHz multiplier devices using GaAs and metal membranes

A 2.5-THz receiver front end for spaceborne applications

SCALING OF InGaAs/InAlAsHBTs FOR HIGH SPEED MIXED-SIGNAL AND mm-WAVE ICs

A broadband 800 GHz Schottky balanced doubler