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.
Two heterostructure barrier varactor (HBV) frequency multipliers, a 300 GHz tripler and a 210 GHz quintupler, are designed, fabricated, and tested. The frequency tripler is fabricated with integrated technology, and the quintupler uses flip-chip mounted HBV diodes. The 210 GHz frequency quintupler shows record output power and efficiency. Moreover, the agreement between the simulation and measurement results validates our design methodology. The frequency tripler exhibits a measured output power of 4 mW and efficiency of 5% at 300 GHz. The 210 GHz frequency quintupler also achieves 5% conversion efficiency with 100 mW of input power. With an input E-H tuner, it can provide over 2 mW output power with over 10% bandwidth.
Design, modeling and testing of these frequency multipliers are described and presented in this paper. Some possible methods to improve these frequency multipliers are addressed.
THz frequency sources have a variety of applications ranging from
molecular spectroscopy, atmospheric remote sensing, scaled radar
range systems, sensing and monitoring of chemical and biological
molecules to wireless communications. However, there is a lack of
frequency tunable sources at these wavelengths. A frequency
upconverter can be used to generate frequency tunable sidebands as
a tunable high frequency source from a fixed source, such as Far
Infrared (FIR) Laser. The development of 1.6 THz frequency
upconverters with integrated diode circuit are described in this
paper. The integration of the diode with the embedding circuit
enhances mechanical robustness and makes the circuits easy to
handle compared with a whisker-contacted diode structure. A
nonlinear analysis is used to determine the optimum varactor diode
parameters. Through the optimization, the circuit quartz substrate
thickness is chosen to be 10 um and the anode diameter is
determined to be 1 um. With the non-ohmic cathode contact
technique and air bridge process (eliminating the surface channel
etch process), the 1.6 THz integrated circuits were fabricated in
University of Virginia with high yield. Furthermore, the
conversion loss is measured and presented. The test setup consists
of an FIR Laser, beam splitter, polarizer, parabolic mirror,
silicon etalon and other optical components. The average
conversion loss was measured to be approximatly 25 dB over 8 GHz
microwave pump. Equivalent circuit models and simulations are
presented to corroborate these results.
Through a US Army sponsored SBIR project we are developing terahertz components based on integrated GaAs Schottky diodes for the frequency range from 200 - 700 GHz. These new components are inherently
broadband and therefore require no mechanical tuners. Rather, they can be electronically swept across significant frequency bands and are therefore useful for chemical and biological spectroscopy. This talk will focus on our demonstration of a terahertz frequency Transmit / Receive capability which may be of use for CB detection and secure communications.
Sideband generation is a method for producing tunable sources in the far IR frequency range by mixing a tunable microwave source with a fixed laser source to produce tunable sidebands. A 36 element array of planar Schottky diodes was used to mix the output of a CO2 pumped laser at 1.6 THz with a 1-20 GHz microwave source to generate 5.9 (mu) W of DSB power for a conversion efficiency of 28 dB. The array produces sidebands by modulating the amplitude of the laser with a low duty cycle and no matching network which is not the optimal condition. For unmatched conditions at 180 degree phase modulation by a square wave with a 50 percent duty cycle will provide 4 dB SB conversion efficiency. This can be implemented by resonating an inductor with a varactor to obtain a short circuit and then modulating away form resonance for an open circuit. A proof of principle demonstration was implemented in waveguide at 80 GHz which resulted in 9 dB conversion efficiency for sinusoidal phase modulation and about 3 GHz bandwidth. This technique will be attempted at 1.6 THz in waveguide.
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.
The performance of receivers incorporating subharmonically pumped mixers is presented over the signal frequency (RF) range of 585 GHz to 2.1 Thz. The mixers employ whisker contacted Schottky barrier diodes in corner cubes. Free standing resonant metal mesh bandpass filters are used to suppress fundamental mode mixing by blocking signals at fLO +/- fIF while allowing second harmonic mixing by passing radiation at 2fLO +/- fIF. The spectral performance of the mesh filters is presented and fabrication techniques are briefly discussed. Comparisons are made to similar fundamental mode mixers.
NASA's Mission to Planet Earth attempts to address issues related to environmental change through extensive scientific investigation and global monitoring. As part of this effort, the Earth Observing System Microwave Limb Sounder (EOS-MLS), Joe W. Waters principal investigator, was proposed and is currently in development. The Submillimeter-Wave Radiometer Development group at JPL along with collaborators at the Rutherford Appleton Laboratory in the United Kingdom and a small number of US laboratories are developing space-borne radiometer components to satisfy the detection requirements for EOS-MLS from 200 to 650 GHz with possible extension up to 2.5 THz (119 micrometers ). This conference paper summarizes the development that has been ongoing, with emphasis on the millimeter- and submillimeter-wave mixers. Detailed design and performance data for a subharmonically pumped antiparallel-pair planar-diode mixer are presented including computational simulations and measured mixer noise and conversion loss at 215 and 640 GHz. Results from a modest test program comparing the performance at 215 GHz of planar GaAs antiparallel-pair mixer diodes, planar In53Ga47As devices, GaAs planar-doped-barrier diodes and a GaAs millimeter-wave integrated circuit (MMIC) mixer are also presented. Finally, current and future development efforts in the areas of submillimeter-wave local oscillators, integrated planar-diode mixers, IF amplifiers, and THz radiometers are outlined.
GaAs Schottky barrier mixer diodes have been successfully used in heterodyne receivers throughout the millimeter and submillimeter wavelength ranges [1,2]. These receivers combine excellent spectral resolution (∆ν/ν=10-6) and exceptional sensitivity, and have given scientists an important tool for use in such fields as radio astronomy , atmospheric studies , chemical spectroscopy  and plasma diagnostics . Although superconducting devices (SIS junctions) have recently achieved better performance at millimeter wavelengths , it is not certain when, or if, SIS technology will be extended to terahertz frequencies. Since there are a great many scientific programs which rely on submillimeter wavelength heterodyne technology, including studies of ozone depletion  and space based astronomy , it is critical that the Schottky technology be pushed to its fundamental limits.