  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.
