Abstract-Survey of terahertz photonics and biophotonics

Kiarash Ahi, Nathan Jessurun, Mohammad-Parsa Hosseini, Navid Asadizanjani

https://www.spiedigitallibrary.org/journals/Optical-Engineering/volume-59/issue-6/061629/Survey-of-terahertz-photonics-and-biophotonics/10.1117/1.OE.59.6.061629.short?SSO=1

We review the advances of terahertz (THz) science and technology in biophotonics, including related challenges and solutions. The main impediment to THz spectroscopy and imaging in this field is the high absorption of the THz beam in water. Hence, transmission imaging and spectroscopy of thick wet tissue using THz radiation has generally been quite difficult. However, the absorption of THz waves by water molecules is so strong that increasing the power of the THz source can lead to structural and functional changes in tissues, so solutions must go beyond a larger power output. In terms of resolution, THz imaging is superior to ultrasound but inferior to visible light microscopy. Owing to its unique material analysis capabilities, promising diagnosis applications have been demonstrated through THz imaging and spectroscopy. Unfortunately, many applications are limited by beam penetration depth and resolution. Hence, researchers from a wide variety of scientific and technical fields have been actively improving these features through the development of electronic devices and materials. In addition, groundbreaking optical architecture and materials to reduce beam absorption in the optics of a system and generate focused beams with smaller diameters have been proposed. On the software side, image processing techniques to computationally enhance the resolution and quality of THz imaging have been proposed. Data science and machine learning to automate the diagnosis of defects and diseases through processing THz images and spectroscopy data have been proposed. We have reviewed the applications of THz radiation in biophotonics and research achievements toward advancing these applications. A conclusion with a roadmap toward increasing the footprint of the THz technology in biophotonics is also proposed.

© 2020 Society of Photo-Optical Instrumentation Engineers (SPIE) 0091-3286/2020/$28.00 © 2020 SPIE

Abstract-A Method and System for Enhancing the Resolution of Terahertz Imaging

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Kiarash Ahi, 

https://www.sciencedirect.com/science/article/pii/S0263224118305700

In this paper, a comprehensive method and process for developing Terahertz (THz) images with enhanced resolution is introduced. This method is implemented as a system comprised of various resolution and quality enhancement techniques. In this system, filters in time and frequency domains are used to filter out the noise, low-frequency spectrum, and diffraction distortions. Point Spread Function (PSF) is modeled and the quality of the output image is enhanced further through deconvolution. This process can be fully implemented on THz Time-Domain Spectroscopy (TDS) systems, and partially on continuous-wave THz imaging systems. By taking advantage of the proposed process, researchers and industrial sectors can achieve a substantial enhancement on the quality and resolution of their THz images.

Abstract-Review of GaN-based devices for terahertz operation

Kiarash Ahi,

http://spie.org/Publications/Journal/10.1117/1.OE.56.9.090901?SSO=1

GaN provides the highest electron saturation velocity, breakdown voltage, operation temperature, and thus the highest combined frequency-power performance among commonly used semiconductors. The industrial need for compact, economical, high-resolution, and high-power terahertz (THz) imaging and spectroscopy systems are promoting the utilization of GaN for implementing the next generation of THz systems. As it is reviewed, the mentioned characteristics of GaN together with its capabilities of providing high two-dimensional election densities and large longitudinal optical phonon of ∼90  meV make it one of the most promising semiconductor materials for the future of the THz emitters, detectors, mixers, and frequency multiplicators. GaN-based devices have shown capabilities of operation in the upper THz frequency band of 5 to 12 THz with relatively high photon densities in room temperature. As a result, THz imaging and spectroscopy systems with high resolution and deep depth of penetration can be realized through utilizing GaN-based devices. A comprehensive review of the history and the state of the art of GaN-based electronic devices, including plasma heterostructure field-effect transistors, negative differential resistances, hetero-dimensional Schottky diodes, impact avalanche transit times, quantum-cascade lasers, high electron mobility transistors, Gunn diodes, and tera field-effect transistors together with their impact on the future of THz imaging and spectroscopy systems is provided.