Abstract-Terahertz Coded Aperture Mask using a Vanadium Dioxide Bowtie Antenna Array

Souheil Nadri, Rebecca Percy, Lin Kittiwatanakul, Alex Arsenovic, Jiwei Lu, Stu Wolf, Robert M. Weikle II

http://arxiv.org/abs/1512.02697

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 to 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 to 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.

Probes Promise Precise On-Wafer Measurements At THz Frequencies

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Terahertz-frequency integrated circuits (ICs) offer tremendous promise in terms of available bandwidth for short-range communications. Although such devices have been fabricated for use at frequencies through 3000 GHz (3 THz), the on-wafer commercial probes for characterizing these high-frequency ICs are limited to about 340 GHz. As a solution, Theodore J. Reck, Lihan Chen, Chunhu Zhang, Alex Arsenovic, Christopher Groppi, Arthur W. Lichtenberger, Robert M. Weikle II, and N. Scott Barker—who combined their talents from the University of Virginia and Arizona State University—have presented a scalable approach to the fabrication of high-frequency wafer probes that integrates a rectangular-waveguide probe and coplanar-waveguide (CPW) wafer probe onto a single silicon chip. The wafer probes feature a ground-signal-ground (GSG) configuration on a 15-micron-thick silicon substrate.
The experimental probe consists of an E-plane split waveguide block that houses the silicon chip. Tabs of silicon electroplated with gold are clamped between the two halves of the block, both for mechanical support and alignment of the housing. The probe chip couples from rectangular waveguide through a radial E-plane waveguide probe into a transmission-line channel. This channel emerges from the block where the signal mode is converted to the GSG wafer probe.
To verify that the design was capable of providing sufficient force to achieve a low-resistance contact with an IC under test, a probe with a single tip was fabricated and evaluated with different contact forces. A contact resistance of 0.07 Ω was achieved for a contact force of 1 mN. See “Micromachined Probes for Submillimeter-Wave On-Wafer Measurements—Part 1: Mechanical Design and Characterization” and “Part 2: RF Design and Characterization,” IEEE Transactions on Terahertz Science and Technology, November 2011, pp. 349 and 357.