Md. Itrat Bin Shams

Coded-Aperture Imaging Using Photo-Induced Reconfigurable Aperture Arrays for Mapping Terahertz Beams

We report terahertz coded-aperture imaging using photo-induced reconfigurable aperture arrays on a silicon wafer. The coded aperture was implemented using programmable illumination from a commercially available digital light processing projector. At 590 GHz, each of the array element apertures can be optically turned on and off with a modulation depth of 20 dB and a modulation rate of ~ 1.3 kHz. Prototype demonstrations of 4 ×4 coded-aperture imaging using Hadamard coding have been performed. Continuous THz imaging with 8 ×8 pixels has also been demonstrated, using a slowly moving metal strip as the target. In addition, this technique has been successfully applied to mapping THz beams by using a 6 ×6 aperture array at 590 GHz. The imaging results agree closely with theoretical calculations based on Gaussian beam propagation, demonstrating that this technique is promising for realizing real-time and low-cost terahertz cameras for many applications. The reported approach provides a simple but powerful means to visualize THz beams, which is highly desired in quasi-optical system alignment, quantum-cascade laser design and characterization, and THz antenna characterization.

Photo-induced spatial modulation of THz waves: opportunities and limitations

Programmable conductive patterns created by photoexcitation of semiconductor substrates using digital light processing (DLP) provides a versatile approach for spatial and temporal modulation of THz waves. The reconfigurable nature of the technology has great potential in implementing several promising THz applications, such as THz beam steering, THz imaging or THz remote sensing, in a simple, cost-effective manner. In this paper, we provide physical insight about how the semiconducting materials, substrate dimension, optical illumination wavelength and illumination size impact the performance of THz modulation, including modulation depth, modulation speed and spatial resolution. The analysis establishes design guidelines for the development of photo-induced THz modulation technology. Evolved from the theoretical analysis, a new mesa array technology composed by a matrix of sub-THz wavelength structures is introduced to maximize both spatial resolution and modulation depth for THz modulation with low-power photoexcitation by prohibiting the lateral diffusion of photogenerated carriers.

A terahertz reconfigurable photo-induced fresnel-zone-plate antenna for dynamic two-dimensional beam steering and forming

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.

Quasi-Optical Terahertz Microfluidic Devices for Chemical Sensing and Imaging

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.

Investigation and Demonstration of a WR-4.3 Optically Controlled Waveguide Attenuator

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 split waveguide 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.

Tunable and reconfigurable THz devices for advanced imaging and adaptive wireless communication

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.

Approaching real-time terahertz imaging using photo-induced reconfigurable aperture arrays

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.