In this paper, we report on two different approaches that have been explored to realize tunable and reconfigurable THz devices for advanced imaging and adaptive wireless communication. The first approach makes use of electronically tunable varactor diodes. Frequency tunable THz antennas based on this approach have been successfully demonstrated for the first time in G-band, enabling the development of spectroscopic THz detectors and focal-plane imaging arrays. The second approach takes advantages of optical THz spatial modulation based on photo-induced free carriers in semiconductors. Using this approach, high-performance tunable THz modulators/attenuators, reconfigurable masks for THz coded aperture imaging, and photo-induced Fresnel-zone-plate antennas for dynamic THz beam steering and forming have been successfully demonstrated. Our recent study also shows that by employing the so-called mesa array technique, sub-wavelength spatial resolution and higher than 100 dB modulation depth can be achieved, making it possible to develop tunable THz devices (e.g., tunable filters) with performance and versatility far beyond those realized by conventional approaches. On the basis of the above investigation, the prospects of high-speed near-field THz imaging, real-time ultra-sensitive heterodyne imaging and prototype adaptive THz wireless communication links will be discussed.
Continuing advances in scaling of conventional semiconductor devices are enabling mainstream electronics to operate in
the millimeter-wave through THz regime. At the same time, however, novel devices and device concepts are also
emerging to address the key challenges for systems in this frequency range, and may offer performance and functional
advantages for future systems. In addition to new devices, advances in integration technology and novel system
concepts also promise to provide substantial system-level performance and functionality enhancements. Several
emerging devices and device concepts, as well as circuit-level concepts to take advantage of them, are discussed. Based
on unconventional semiconductor device structures and operational principles, these devices offer the potential for
significantly improved system sensitivity and frequency coverage. When combined in arrays, features such as
polarimetric detection and frequency tunability for imaging can be achieved. As examples of emerging devices for
millimeter-wave through THz sensing and imaging, heterostructure backward diodes in the InAs/AlSb/GaSb material
system and GaN-based plasma-wave high electron mobility transistors (HEMTs) will be discussed. Based on interband
tunneling, heterostructure backward diodes offer significantly increased sensitivity and extremely low noise for direct
detection applications, and have been demonstrated with cutoff frequencies exceeding 8 THz. The plasma-wave HEMT
is an emerging device concept that, by leveraging plasma-wave resonances in the two-dimensional electron gas within
the channel of the HEMT, offers the prospect for both tunable narrowband detection as well as low-noise amplification
at frequencies well into the THz. These emerging devices are both amenable to direct integration within compact planar
radiating structures such as annular slot antennas for realization of polarimetric detection and frequency tuning for
spectroscopy and imaging.
We report a technique using photo-induced coded-aperture arrays for potential real-time THz imaging at roomtemperature.
The coded apertures (based on Hadamard coding) were implemented using programmable illumination on
semi-insulating Silicon wafer by a commercial digital-light processing (DLP) projector. Initial imaging experiments
were performed in the 500-750 GHz band using a WR-1.5 vector network analyzer (VNA) as the source and receiver.
Over the entire band, each array pixel can be optically turned on and off with an average modulation depth of ~20 dB
and ~35 dB, for ~4 cm2 and ~0.5 cm2 imaging areas respectively. The modulation speed is ~1.3 kHz using the current
DLP system and data acquisition software. Prototype imaging demonstrations have shown that a 256-pixel image can be
obtained in the order of 10 seconds using compressed sensing (CS), and this speed can be improved greatly for potential
real-time or video-rate THz imaging. This photo-induced coded-aperture imaging (PI-CAI) technique has been
successfully applied to characterize THz beams in quasi-optical systems and THz horn antennas.
Receivers based on superconducting Hot-Electron Bolometers (HEBs) are widely used for terahertz (THz) sensing
owing to their advantages of high sensitivity, low noise, and low LO power requirement. Balanced HEB mixers are
superior to single-element ones since the thermal noise and AM noise from the LO injection can be effectively
suppressed. Although a 1.3 THz balanced waveguide HEB mixer has been reported, waveguide mixer configurations
offer relatively narrow RF bandwidths. We report on the development, fabrication and characterization of a THz quasioptical
balanced superconducting HEB mixer utilizing a dual-polarization sinuous antenna that can potentially achieve
both multiband operation and ultra-high sensitivity. In the balanced mixer configuration, a lens-coupled four-arm
sinuous antenna was designed for operation from 0.2-1.0 THz with a nearly frequency-independent embedding
impedance of ~106 Ω. Two identical superconducting niobium HEB devices have been integrated at the antenna
feedpoints, connecting each opposing pair of antenna arms to form a balanced mixer configuration. An air-bridge was
also fabricated to separate the two mixer branches. The HEB devices were fabricated from 10 nm thick niobium film
sputtered on semi-insulating silicon substrates. Each HEB device has dimensions of 80 nm × 240 nm (3 squares) for
approaching a resistance of 105 Ω for impedance matching. Mixer properties including antenna radiation patterns,
broadband operation and polarization isolation have been characterized. Finally, in order to achieve multiband mixer
operation, electronically reconfigurable THz quasi-optical mesh filters are needed. Frequency-tunable antenna elements
using Schottky varactor diodes suitable for the above applications have been designed, simulated and demonstrated at Gband
(140-220 GHz) showing 50 GHz tuning range.