characterization of a zero-bias quasi-optical terahertz detector based on monolithically integrated heterostructure backward diodes (HBDs) for operation at G-band. The reported detectors consist of HBDs with 0.7 x 0.7 mu m(2) active device area and submicrometer-scale airbridges, integrated with lens-coupled high-impedance planar folded dipole antennas. Measurements of the HBD detector show that a peak-measured detector sensitivity of approximately 2400 V/W and a minimum noise equivalent power (NEPmin) of 2.14 pW/root Hz have been obtained at 170 GHz. If an antireflection coating was used on the lens, a sensitivity of approximately 3500 V/W and NEPmin of 1.48 pW/root Hz is projected. The radiation patterns of the quasi-optical detector in both E-and H-planes have been measured, and good agreement has been achieved between simulation and measurement. The performance of this detector can be further improved by scaling the HBD device active area. The reported approach using monolithically integrated heterostructure backward tunneling diodes and submicrometer airbridges is promising for developing high performance and compact detectors and focal-plane arrays for millimeter-wave and terahertz sensing and imaging applications.

Electromagnetic band gap (EBG) structures offer unique solutions for effectively manipulating electromagnetic waves over a broad range of frequencies for a wide range of applications. However, most EBG designs reported so far either require sophisticated fabrication processes or have limited tunability and reconfigurability. In this paper, we investigate the potential to implement high performance tunable and reconfigurable EBG components using a novel optical control approach. This technology allows the generation of EBG structures through spatially-resolved photogeneration of free carriers in a semiconductor, without any complex fabrication processes. As a prototype demonstration, a reconfigurable microwave frequency tunable band-stop filter (BSF) based on photo-induced uniplanar EBG structures has been investigated through simulation. In this approach, the required EBG patterns are directly illuminated onto a Ge ground plane mounted to the bottom of a Duroid substrate for tunability using a digital light processing (DLP) projector. On the basis of HFSS simulations, the bandwidth of the BSF can be tuned by modifying the EBG pattern filling factor. The center frequency of the BSF could also be tuned from 8-12 GHz by adjusting the period of the EBG structure. In addition, two limiting factors, i.e., localized heating effects and finite lateral spatial resolution (due to carrier diffusion), that may affect the circuit performance in this technology have been investigated and discussed. By using a mesa-array structured ground plane, this approach is promising for developing tunable and reconfigurable circuits such as filters from the microwave to terahertz regimes.

We report the design, fabrication, and characterization of a high-performance optically controlled WR-4.3 variable attenuator. The attenuation is based on the interaction between the incident waves and photogenerated free carriers inside an E-plane micromachined silicon absorber. Tuning is realized by illuminating the silicon with different light intensities using three 808-nm infrared laser diodes. Measurement results show that an average attenuation range of 60 dB, a 0.7-dB insertion loss, and a greater than 15-dB return loss have been achieved over the entire WR-4.3 band. In addition, a 3-dB modulation bandwidth of 350 kHz has been obtained experimentally. Initial thermal stability test of the device has also been performed, with less than 0.4-dB amplitude drift and 2° drift in phase over 30-min demonstration.

In this paper, we report the design and simulation of two optically controlled tunable and reconfigurable G-band (140220 GHz) waveguide devices based on electromagnetic band gap (EBG) structures. The first device based on a pre-patterned EBG structure can be reconfigured between a band-stop filter (BSF) and a transmission line. The second device based on photoinduced EBG structures using a mesa-array shows increased level of tunability and reconfigurability with a BSF center frequency tunable from 175-200 GHz. The proposed tunable and reconfigurable THz devices are promising for a wide range of applications including multiple-frequency-band THz wireless communications.

We report a 0.74-THz beam-steering and forming antenna based on dynamically reconfigurable photo-induced Fresnel zone plates (PI-FZPs) formed by patterned illumination of a high-resistivity silicon wafer using a digital light processing (DLP) projector. This approach eliminates the need for fabrication of circuits or devices on the semiconductor wafer. Using computer-generated PI-FZPs, the beam from a diagonal horn antenna at 740 GHz has been steered from -8. to + 8., consistent with expectations from design and simulations. In addition, the antenna beam width can be dynamically tuned using PI-FZPs with different focal lengths. Both the beam-steering and forming functionalities can be performed simultaneously, enabling applications in THz communications, sensing, and imaging. The PI-FZP antenna performance and design considerations including gain, efficiency, frequency scaling and operation speed are discussed on the basis of the experimental results.

Owing to the unprecedented development in terahertz (THz) sources and detectors in the last decade, technologists and researchers have intensified their efforts to develop advanced THz sensing and imaging systems with superior performance and unique functionalities. One of the key elements to realize such systems is the ability to monolithically integrate high-performance semiconductor devices with THz antennas and other passive structures. In recent years, devices based on interband tunneling in III-V heterostructures have emerged as promising candidates for THz detection that offer extremely high nonlinearity, high sensitivity, low noise, fast response, and room temperature operation. In this paper, we first review the development of heterostructure backward tunnel diodes (HBDs) in the InAs/AlSb/AlGaSb material system that have been demonstrated with detection sensitivity that outperforms the current state of the art (e.g., the fundamental limit of Schottky diodes) and with noise-equivalent power (NEP) below 0.2 pW/Hz½. We then present the monolithic integration of HBDs with planar folded dipole antennas (FDAs) using submicrometer-scale airbridges to achieve optimized impedance matching for high-performance and compact THz detectors and focal-plane array (FPA) imaging systems. In addition, the potential of using HBDs for realizing THz systems with advanced functionalities such as spectroscopic FPAs (using frequency-tunable THz antennas) and polariametric detection/imaging systems will be discussed. Finally, the integration of HBDs into waveguides for more advanced THz sensing and imaging (e.g., a six-port reflectometer for near-field imaging) will be discussed.

Tunable and reconfigurable terahertz (THz) devices such as modulators/variable attenuators, tunable filters, coded apertures, phase shifters and high-level switches (e.g., DPDT) that are required for advanced imaging and adaptive wireless communication applications are challenging to realize. We report a promising approach to develop the above THz devices based on spatially-resolved optical modulation (SROM) using photo-induced (PI) free carriers in semiconductors. The fundamental mechanism for this approach will first be introduced followed by prototype demonstrations for reconfigurable coded-aperture imaging masks, beam steering/forming antennas and waveguide-based tunable attenuators. The potential to develop more advanced tunable/reconfigurable THz devices (e.g., tunable delay lines, SPDT, DPDT switches) using optically-controlled waveguide architectures such as PI electromagnetic band gap (EBG) structures and dynamically-reconfigurable PI substrate-integrated waveguides (SIWs) will also be discussed on the basis of performance-improved SROM using the so-called mesa-array technique.

We report the design and demonstration of a compact WR-4.3 (170-260 GHz, equivalent to WR-4 band in Electronics Industries Alliance band designation) optically controlled waveguide attenuator using an E-plane tapered high-resistivity micromachined silicon absorber. Variable attenuation is realized by illuminating the silicon absorber with different light intensities from a fiber-guided infrared laser diode. Finite element method simulation has shown that high attenuator performance can be potentially achieved. For a prototype demonstration, a WR-4.3 optically controlled attenuator has been designed and implemented using an E-plane splitwaveguide configuration. The attenuator has been characterized in the WR-4.3 waveguide band using a vector network analyzer and the results show that a 0.6-dB insertion loss, greater than 10-dB return loss, and an average of approximately 25-dB tuning range have been achieved over most of the WR-4.3 band (i.e., 170-230 GHz). This approach is promising for developing high-performance variable waveguide attenuators into the millimeter-wave and terahertz regime.

We report the development of a compact WR-4 (170260 GHz) optically-tunable waveguide attenuator based on the interaction between electromagnetic waves and photo-induced free carriers in semiconductors. This approach is promising for achieving superior performance, including a ~50 dB tuning range, low insertion loss, lowreturn loss (VSWR), and high tuning speed. Based on full-wave simulation results, an average attenuation level of ~50 dB can be achieved using a 1 mm long Ge absorber at a light intensity of 1 W/cm. By employing an E-plane taper design and energy absorption mechanism, return loss lower than -13 dB have been achieved. A prototype attenuator with Si absorber has been implemented using an E-plane split waveguide design. Initial measurement results show that ~14 dB attenuation can be achieved using a single illumination spot. The attenuation level can be improved by increasing the illumination region length employing multiple fibers.

We first review the development of a frequency domain quasi-optical terahertz (THz) chemical sensing and imaging platform consisting of a quartz-based microfluidic subsystem in our previous work. We then report the application of this platform to sensing and characterizing of several selected liquid chemical samples from 570–630 GHz. THz sensing of chemical mixtures including isopropylalcohol-water (IPA-H2O) mixtures and acetonitrile-water (ACN-H2O) mixtures have been successfully demonstrated and the results have shown completely different hydrogen bond dynamics detected in different mixture systems. In addition, the developed platform has been applied to study molecule diffusion at the interface between adjacent liquids in the multi-stream laminar flow inside the microfluidic subsystem. The reported THz microfluidic platform promises real-time and label-free chemical/biological sensing and imaging with extremely broad bandwidth, high spectral resolution, and high spatial resolution.

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.

Sensing and imaging at millimeter-wave and THz frequencies is promising for a wide range of applications, including security, industrial control, healthcare, and scientific metrology. The development of high-sensitivity, low noise detectors based on interband tunneling in III-V heterostructure devices, and their integration into subsystems is promising for realizing the potential of these applications. This paper describes recent work on both heterostructure backward diodes and tunneling field-effect transistors as sensitive detectors in the microwave through THz frequency ranges, as well as their integration into tunable pixel elements and focal plane arrays for imaging and sensing applications.

In this work, the authors report integration and fabrication of high-performance Sb-based heterostructure backward diodes (HBDs) with planar folded dipole antennas (FDAs) using submicron-scale airbridges for terahertz (THz) detection. By integrating HBDs into FDAs, high detector responsivity of 20 000 V/W at 200 GHz and 9500 V/W at 585 GHz could be potentially achieved due to the optimized impedance matching between the antenna and HBD detector. In order to minimize interconnect parasitics, the HBD integration is accomplished using submicron-scale airbridges. Electromagnetic simulations coupled to device models show that by introducing submicron-scale airbridges and optimizing the device layout, parasitic capacitance and spreading resistance can be significantly reduced. This allows performance nearly equal to the intrinsic device performance to be obtained. To achieve this level of performance, a novel fabrication and integration process has been developed. The process includes mix-and-match electron beam and optical lithography to span the size scales required for both the FDA and small-area HBDs, and offers high accuracy and reproducibility while requiring fewer critical fabrication steps compared to conventional hybrid integration techniques. Devices fabricated using this process have obtained a record-high device curvature coefficient of −58 V−1, indicating the quality of the devices that can be achieved. The process is scalable—in terms of device size, frequency range, and array size—enabling the development of THz focal plane arrays in a wide frequency range (100 GHz to beyond 1 THz).

A versatile flexible biocomposite metal-like conductor (approximate to 10 Omega/upsilon) that can function under severe folding and abrasion events and can potentially replace metal conductors in a wide range of electronic applications ranging from detection of small stochastic biomedical signals (electromyography) to terahertz shielding for high speed electronics.

We report a novel approach for characterization of terahertz (THz) antennas using the photoinduced coded-aperture imaging (PI-CAI) technique. For a prototype demonstration, a WR-1.5 (500-750 GHz) diagonal horn antenna has been fully characterized. The THz beam radiated from the antenna was imaged using the PI-CAI technique at different distances from the antenna aperture. The measured beam profiles show near-Gaussian shapes, with the expected propagation properties. The far-field antenna radiation patterns were then extracted from the far-field beam image and compared with conventional measurement results, theoretical calculation and full-wave simulation. Finally, the antenna Gaussian coupling efficiency was calculated to be similar to 83% based on the two-dimensional (2-D) beam mapping technique. Antenna radiation parameters obtained using the above approach show good agreement with published literature and manufacturer datasheets, demonstrating the potential of the PI-CAI approach for fast and accurate characterization of THz antennas. (C) 2015 Wiley Periodicals, Inc.

An acrylic emulsion artists’ paint containing chlorinated copper phthalocyanine pigment was modified with variable-size multilayer graphene (exfoliated graphite) to induce low electrical resistance; composite films were spray-cast on common printing paper, heat-cured, and subsequently polished under mild compression, to produce highly conductive paper. The mechanically robust conductive paint showed excellent adhesion to the underlying paper, as determined by Taber abrasion and tape peel tests, which displayed no adhesive failure under the test conditions studied. The conductivity of the paper substrates were tuned by changing the concentration and the size of the multilayer graphene particles. Detailed conductivity measurements showed stable Ohmic current–voltage behavior. The optimum graphene-in-paint formulations resulted in sheet resistances of the order of 10Ω/sq. Standard electrostatic force microscopy measurements showed uniform surface electric field gradient distribution strongly correlating with the surface topography. Similarly, scanning Kelvin probe microscopy measurements indicated stable work functions close to 5eV, comparable to highly-ordered pyrolytic graphite. Furthermore, Kelvin probe measurements were more sensitive to surface charges related to copper phthalocyanine domains, which are known to have semiconducting properties. Finally, the conductive papers were also tested in the 0.50–0.75 terahertz frequency range for electromagnetic interference shielding (EMI) characterization and displayed quasi-metallic shielding performance.

We report a novel and simple approach to realize terahertz (THz) dynamic two-dimensional (2D) beam steering and forming antennas, based on reconfigurable photo-induced Fresnel zone plates (PI-FZPs). The FZPs are formed by directly illuminating a high-resistivity silicon wafer with the desired patterns using a digital light processing (DLP) projector, without any circuit or device fabrication. At 750 GHz, the THz beam from a diagonal horn antenna has been steered two dimensionally over a range from approximately -12° to +12° from the antenna boresight, by projecting different PI-FZP patterns. In addition, using PI-FZPs with different focal lengths, the THz beam size can be dynamically tuned. Both the beam steering and forming can be performed simultaneously without affecting the antenna performance, making this an enabling technology for emerging THz applications such as sensing, imaging, tracking, adaptive wireless communications and short-range high-speed interconnections.

The authors report the design, simulation and characterisation of lens-coupled folded-dipole antennas (LC-FDAs) for terahertz (THz) detection and focal-plane imaging arrays. LC-FDAs operating at 200 GHz have been designed on semi-insulating silicon wafers with a resistivity larger than 20 000 Ω·cm. Even–odd mode analysis (EOA) has been performed to extract the currents of the two modes. The embedding impedance of the designed LC-FDAs has been simulated using conventional numerical electromagnetics, and the results show that a wide range of impedance values (both real and imaginary parts) can be achieved by changing the antenna geometry. This property makes LC-FDAs suitable for high-performance THz detectors in which impedance matching between antennas and devices is desired. From the currents calculated using EOA, the LC-FDA far-field radiation patterns, antenna directivity and Gaussian coupling efficiency for different lens structures have been examined using ray-tracing techniques. Good agreement between calculation results and measurement has been observed demonstrating the effectiveness of the above analysis approach. The single element FDA design has also been explored for use in high-resolution two-dimensional THz focal plane arrays by optimising the lens structure and evaluating the off-axis radiation patterns.

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.