The terahertz region of the electromagnetic spectrum has unique properties that make it especially useful for imaging and spectroscopic detection of concealed weapons, explosives and chemical and biological materials. However, terahertz energy is difficult to generate and detect, and this has led to a technology gap in this frequency band. Nonlinear diodes can be used to bridge this gap by translating the functionality achieved at microwave frequencies to the terahertz band. Basic building blocks include low-noise mixers, frequency multipliers, sideband generators and direct detectors. These terahertz components rely on planar Schottky diodes and recently developed integrated diode circuits make them easier to assemble and more robust. The new generation of terahertz sources and receivers requires no mechanical tuning, yet achieves high efficiency and broad bandwidth. This paper reviews the basic design of terahertz transmitters and receivers, with special emphasis on the recent development of systems that are compact, easy to use and have excellent performance.

Through the support of the US Army Research Office we are developing terahertz sources and detectors suitable for use in the spectroscopy of chemical and biological materials as well as for use in imaging systems to detect concealed weapons. Our technology relies on nonlinear diodes to translate the functionality achieved at microwave frequencies to the terahertz band. Basic building blocks that have been developed for this application include low-noise mixers, frequency multipliers, sideband generators and direct detectors. These components rely on planar Schottky diodes and integrated diode circuits and are therefore easy to assemble and robust. They require no mechanical tuners to achieve high efficiency and broad bandwidth. This paper will review the range of performance that has been achieved with these terahertz components and briefly discuss preliminary results achieved with a spectroscopy system and the development of sources for imaging systems.

An analysis is presented of a new horn antenna, fabricated by a novel micromachining technique, that uses crystallographic etching of silicon and ultraviolet lithography of an ultra-thick photoresist (SU-8). The horn was found to have low cross-polarized field levels and a predicted Gaussian coupling efficiency of 92.5%. The horn shape is governed by the crystal planes of the silicon substrate and the thickness of the photoresist and has up to four independent design parameters that allow a wide range of antenna patterns. A design for the horn that yields symmetric beam patterns was investigated by computer analysis, microwave scale modeling, and measurements of a micromachined horn at 585 GHz. The major features of the 585-GHz beam patterns agree well with the computer-generated and scaled beam patterns. We have thus demonstrated a new micromachinable horn that has great potential for integration into array structures

A novel wafer bonding process has been used to integrate high quality GaAs devices on quartz substrates. The method of adhesion by spin-on-dielectric temperature enhanced reflow (MASTER) uses a spin-on-dielectric as a bonding agent to achieve a robust bond that in no way degrades either high frequency performance or reliability. A 585 GHz integrated mixer fabricated using this process has achieved record double-sideband mixer noise temperatures of 1,150 K at room temperature and 880 K at 77 K. Furthermore, the integrated mixers require no mechanical tuning, are easy to assemble, and repeatable. Precise control of the circuit geometry, coupled with the reduction of parasitic elements, allows greater accuracy of computer simulations and will therefore lead to better high frequency performance and bandwidth. This new technology is easily extended to other circuit designs and will allow the development of a new generation of submillimeter-wave integrated circuits.

Standard semiconductor fabrication processes have been used to form waveguide components for the submillimeter wavelength;range. A 585 GHz fundamentally pumped Schottky mixer with record performance demonstrates this technology. It consists of an etched silicon horn, a diced waveguide, and a lithographically formed microstrip channel for the diode circuit. The block dimensions are precisely controlled and extremely sharp. The measured mixer noise temperature is 1200K (DSB), which is equivalent to the best result obtained with standard metal machining.

The high cost of fabricating waveguide components is one of the primary factors limiting the development of terahertz technology. This paper reviews the development of an inexpensive micromachining technology that is suitable for the frequency range from 500 GHz through 5 THz. Our first effort was a 585 GHz direct detector that allowed us to measure the beam patterns of our new micromachined horn antenna. The results were quite good and matched both theoretical predictions and the patterns of a low frequency scaled model of the horn. More recently, a high quality 585 GHz mixer was assembled and tested. The performance was equivalent to that obtained from a traditionally machined block, Tmix,dsb = 1,200K. We are now extending this technology to 1.6 THz. A sideband generator and a mixer circuit are being fabricated and the first circuits demonstrate excellent control of the critical features. This paper overviews the new micromachined block fabrication process, summarizes our measurements at 585 GHz and shows the first fabrication results at 1.6 THz.

Sensitive and robust heterodyne mixers are needed for future atmospheric remote sensing missions. This data from satellites such as NASA's Earth Observing System (EOS) lends great insight into molecular interactions in our environment. The Microwave Limb Sounder (MLS) on EOS will detect radiation emitted from 03, ClO, and OH molecules which are critical to our understanding of ozone depletion and greenhouse warming. The heterodyne mixers on MLS must exhibit sufficient spectral sensitivity, wide bandwidth, low noise, and minimal LO power requirements. Planar GaAs Schottky diodes currently are the most promising technology for space-borne radiometers where cryogenic cooling is not desirable. In this work we present progress on a novel wafer bonding technology, MASTER, used to integrate submillimeter wavelength planar GaAs Schottky mixer diodes with quartz microstrip circuitry. Problems associated with wafer expansion after bonding, open- circuited devices, and Ti/Pt/Au metallization removal have been solved and device yield is significantly improved. FTIR measurements of the bonding adhesive's properties at submillimeter wavelengths are discussed. We have fabricated 640 GHz subharmonic mixers for EOS-MLS which nearly match state-of-the-art performance at this frequency with DSB Tmix equals 2396 K and Lmix equals 10.98 dB using 4.67 mW of LO power. RF testing of a new higher yield batch of MASTER mixers is in progress.

We have designed and tested broadband fixed-tuned waveguide to microstrip transitions in which the probe is extended across the waveguide. We have performed scale-model testing of several configurations of waveguide to microstrip transitions, and have measured a fixed-Wiled bandwidth of 32%. Simulations of these structures were performed using Hewlett Packard's High Frequency Structure Simulator (HFSS). The sensitivity of the transition bandwidth to changes in geometry and microstrip circuit layout are discussed.