We explored the effect of Cr dopant on the transport behaviors of polycrystalline VO2 thin films in order to suppress the sharp metal-insulator transition, and tune the temperature coefficient of resistivity (TCR) value. A reactive bias target ion beam deposition was used for combinational sputtering to Cr doped VO2 thin films (~100 nm). The addition of Cr led a structural change in the semiconducting phase of VO2. With the Cr content >7 at. %, the sharp metal-semiconductor transition and the hysteresis loop was suppressed in thin film VO2. A further increase of Cr content reduced the TCR. Separately the effect of the oxygen flow rate was investigated to modulate the TCR and the resistivity value of Cr doped VO2. We demonstrated the resistivity of Cr doped VO2 was modulated by 2 orders of magnitude with a very small change in the oxygen flow rate. We obtained TCR of ~ 4.5 %/K in Cr doped VO2 grown on single crystal sapphire substrate near room temperature.
Terahertz imaging systems have received substantial attention from the scientific community for their use in astronomy, spectroscopy, plasma diagnostics and security. One approach to designing such systems is to use focal plane arrays. Although the principle of these systems is straightforward, realizing practical architectures has proven deceptively difficult. A different approach to imaging consists of spatially encoding the incoming flux of electromagnetic energy prior to detection using a reconfigurable mask. This technique is referred to as coded aperture"or Hadamard"imaging. This paper details the design, fabrication and testing of a prototype coded aperture mask operating at WR-1.5 (500-750 GHz) that uses the switching properties of vanadium dioxide(VO2). The reconfigurable mask consists of bowtie antennas with vanadium dioxide VO2 elements at the feed points. From the symmetry, a unit cell of the array can be represented by an equivalent waveguide whose dimensions limit the maximum operating frequency. In this design, the cutoff frequency of the unit cell is 640 GHz. The VO2 devices are grown using reactive-biased target ion beam deposition. A reflection coefficient (S11) measurement of the mask in the WR-1.5 (500-750 GHz) band is conducted. The results are compared with circuit models and found to be in good agreement. A simulation of the transmission response of the mask is conducted and shows a transmission modulation of up to 28 dB. This project is a first step towards the development of a full coded aperture imaging system operating at WR-1.5 with VO2 as the mask switching element.
Terahertz components and devices are typically interfaced with measurement instrumentation and characterized using fixtures equipped with waveguide flanges or antennas. Such fixtures are known to introduce significant uncertainty and error in measurements. It is preferable to characterize such devices in-situ,
where the device under test can be measured on-wafer, prior to dicing and separately from the circuit housing to which it is ultimately affixed. This is commonly done in the RF and millimeter-wave region with a probe station equipped with coplanar launchers. Commercial coplanar waveguide probes have generally been available to the WR-2.2 band (325—500 GHz) but few options currently exist for on-wafer measurements
above these frequencies. This paper describes recent work at the University of Virginia and Dominion Microprobes, Inc. to extend on-wafer measurement capabilities to terahertz frequencies through the design and implementation of coplanar probes based on silicon micromachining. At present micromachined on-wafer probes operating to WR1.2 (600 to 900 GHz) have been demonstrated and exhibit typical insertion losses lower than 7 dB with return loss of 15 dB or greater over a full waveguide band.
The design, construction, and investigation of a compact reflectometer suitable for measuring return loss magnitude and phase of biological materials and chemical agents at submillimeter wavelengths is presented. The instrument, which consists of a section of rectangular waveguide and an ensemble of Schottky diode power detectors is designed as a proof-of-principle demonstration and is a relatively simple implementation of the six-port analyzer originally
investigated by Engen and coworkers. Design considerations for the reflectometer are presented and measurements in the 270 GHz to 320 GHz range are discussed. In addition, preliminary return loss measurements on sample biological materials (salmon and herring DNA) in the millimeter-wave region are presented and described.
The design, construction, and investigation of a reflectometer for measuring return loss magnitude and phase at submillimeter wavelengths is presented. The instrument, which consists of a section of rectangular waveguide and an ensemble of Schottky diode power detectors is designed as a proof-of-principle demonstration and is a relatively simple implementation of the six-port analyzer originally investigated by Engen and coworkers. Design considerations for the reflectometer are presented and measurements in the 270 GHz to 320 GHz range are discussed.
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.
Quasi-optical power-combining offers the most promising method for extracting large amounts of power from solid-state devices in the microwave and millimeter-wave range. This technique can be applied to a variety of devices. The difficulties associated with traditional waveguides power-combiners such as skin-effect losses are eliminated because the signals are combined in free-space. The solid-state devices are embedded in a two-dimensional grid configuration and placed in a Fabry-Perot cavity. In this respect, the quasi-optical power-combiner is analogous to a laser oscillator in which the active medium of the laser is replaced with an array of active devices. The grid presents a reflection coefficient to an incident plane wave which is larger than unity and the resonator provides feedback to couple the devices together. The two-dimensional structure of the grid is amenable to modern photolithographic processing and potentially allows thousands of devices to be integrated monolithically.
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