David W. Porterfield

Resonant metal-mesh bandpass filters for the far infrared

The spectral performance of freestanding resonant metal-mesh bandpass filters operating with center frequencies ranging from 585 GHz to 2.1 THz is presented. These filters are made up of a 12-µm-thick copper film with an array of cross-shaped apertures that fill a circular area with a 50-mm diameter. The filters exhibit power transmission in the range 97-100% at their respective center frequencies and stop-band rejection in excess of 18 dB. The theoretically predicted nondiffracting properties of the meshes are experimentally verified through high-resolution beam mapping. Scalability of the filter spectra with mesh dimensions is demonstrated over a wide spectral range. Several modeling methods are considered, and results from the models are shown.

A high-power fixed-tuned millimeter-wave balanced frequency doubler

We report on the design and evaluation of a 40-80-GHz (40/80-GHz) high-power wide-band fixed-tuned balanced doubler. The active device is a single GaAs chip comprising a linear array of six planar Schottky varactors. The varactors and a quartz microstrip circuit are embedded in a split waveguide block. We have achieved a measured 3-dB fixed-tuned bandwidth of 17% and measured flange-to-flange peak efficiency of 48% at an input-power level of 200 mW. The doubler operates at near-peak efficiency (45%) at an input power of 250 mW. We have cooled the block to 14 K and achieved an efficiency of 61% at an input-power level of 175 mW and an efficiency of 48% at an input-power level of 365 mW. Emphasis has been placed on making the design easy to fabricate and scalable to higher frequencies.

High-Efficiency Terahertz Frequency Triplers

Design and experimental analysis of high-power and high-efficiency frequency triplers to the 220 GHz and 440 GHz bands are presented. Test data for the 220 GHz tripler show 23 mW output power with 16% efficiency. Test data for the 440 GHz tripler show 13 mW output power with 12% efficiency. The 3 dB bandwidth for both triplers is about 7%. This performance is comparable to the best reported in the literature at these frequencies. There are no mechanical tuners and thus the triplers may be electronically swept to any frequency in the band. The triplers comprise a waveguide housing, a pair of quartz microstrip circuits and a Virginia Diodes (VDI) GaAs Schottky varactor chip. The simple circuit topology makes it easy to assemble the multipliers and bias the varactors. A version to 800 GHz has been designed and should be available for testing in 2007. The design is scalable to frequencies above 1 THz.

Terahertz sources and detectors

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.

Integrated terahertz transmit/receive modules

We report on research and development of millimeter-wave components used in prototype 600 GHz transmit and receive systems. These systems offer state-of-the-art performance in terms of noise, power, ease of use and mechanical and electrical robustness. The transmitters comprise high-power frequency multiplier chains driven by commercial power amplifiers in the 18-45 GHz range and lower frequency fundamental oscillators. The receivers comprise a subharmonic mixer and an LO multiplier chain. All of the components are based on planar GaAs Schottky diodes in fixed-tuned broadband embedding structures. These designs are scalable to any frequency in the band from 100 GHz through 1 THz and we are exploring the challenges of scaling to the 1-5 THz band.

Quarter‐micrometer GaAs Schottky barrier diode with high video responsivity at 118 μm

A quarter‐micrometer diameter Schottky barrier mixer diode has been fabricated on n+ GaAs using electron beam lithography and reactive ion etching (RIE). The anodes were formed using a Pt/Au electroplate technique. The diode zero‐bias capacitance of 0.25 fF and series resistance of about 25 Ω, measured at dc, correspond to a ‘‘figure‐of‐merit’’ cuttoff frequency of about 25 THz. The video responsivity at 118 μm (2540 GHz) was as high as 200 V/W, over three times higher than the best previously reported. The design, fabrication, and evaluation of this diode is described.

Development of multiplier based sources for up to 2 THz

Nonlinear diodes are used to extend the functionality of microwave electronics into the terahertz frequency band. Systems using this technology achieve useful transmitter power and receiver sensitivity throughout the frequency range from about 100 GHz through several terahertz. This talk reviews this nonlinear diode technology, with emphasis on the ongoing research and development that will enable this terahertz technology to transition from a tool for basic science into commercial systems suitable for broader applications. Emphasis is placed on terahertz sources. Three recent VDI sources are described.

An 80/160 GHz broadband, fixed-tuned, balanced frequency doubler

We report on the development of a high-power, broadband, fixed-tuned 80/160 GHz frequency doubler. The design is based on a similar 40/80 GHz doubler which exhibited a measured 3 dB bandwidth of 17% and peak efficiency of 48% at an output power of 100 mW. Simulations for the new 80/160 GHz doubler indicate similar bandwidth and efficiency. The focus of this paper is the design and simulation of the 80/160 GHz doubler. Test results will be presented at the conference.

A high pulsed power frequency doubler to 190 GHz

A high-pulsed-power varactor doubler has been developed to efficiently transfer the power from a pulsed 95 GHz IMPATT oscillator to the 190 GHz band. The frequency doubler uses waveguide based embedding structures employing high-thermal conductivity circuits and Virginia Diodes, Inc. (VDI) proprietary GaAs Schottky varactor diode technology. The embedding circuitry is based on a balanced doubler topology that delivers state-of-the-art power and fixed-tuned bandwidth at millimeter-wave frequencies. The waveguide structure is modified to provide ample room for the large diode arrays while simultaneously blocking propagation of the unwanted TM modes. Special attention was given to maximizing the heat conduction pathways in the embedding structure to minimize heating of the varactor devices.

Submillimeter-wave and Terahertz Diodes, Components and Subsystems

Nonlinear diodes are used to extend the functionality of microwave electronics into the terahertz frequency band. Systems using this technology achieve useful transmitter power and receiver sensitivity throughout the frequency range from about 100 GHz through several terahertz. This talk will review this nonlinear diode technology, with emphasis on the ongoing research and development that will enable this terahertz technology to transition from a tool for basic science into broader applications will also be discussed.