Scanning the issue: T-ray imaging, sensing, and retection

Copyright © 2007 IEEEDerek Abbott, Xi-Cheng Zhan

The 2017 Terahertz Science and Technology Roadmap

Science and technologies based on terahertz frequency electromagnetic radiation (100GHz-30THz) have developed rapidly over the last 30 years. For most of the 20th century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to “real world” applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2016, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 17 sections that cover most of the key areas of THz Science and Technology. We hope that The 2016 Roadmap on THz Science and Technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies

Molecular Screening for Terahertz Detection with Machine-Learning-Based Methods

The molecular requirements are explored for achieving efficient signal up-conversion in a recently developed technique for terahertz (THz) detection based on molecular optomechanics. We discuss which molecular and spectroscopic properties are most important for predicting efficient THz detection and outline a computational approach based on quantum-chemistry and machine-learning methods for calculating these properties. We validate this approach by bulk and surface-enhanced Raman scattering and infrared absorption measurements. We develop a virtual screening methodology performed on databases of millions of commercially available compounds. Quantum-chemistry calculations for about 3000 compounds are complemented by machine-learning methods to predict applicability of 93 000 organic molecules for detection. Training is performed on vibrational spectroscopic properties based on absorption and Raman scattering intensities. Our top molecules have conversion intensity two orders of magnitude higher than an average molecule from the database. We also discuss how other properties like molecular shape and self-assembling properties influence the detection efficiency. We identify molecular moieties whose presence in the molecules indicates high activity for THz detection and show an example where a simple modification of a frequently used self-assembling compound can enhance activity 85-fold. The capabilities of our screening method are demonstrated on narrow-band and broadband detection examples, and its possible applications in surface-enhanced spectroscopy are also discussed

Ultra-high-resolution software-defined photonic terahertz spectroscopy

A novel technique for high-resolution 1.5 µm photonics-enabled terahertz (THz) spectroscopy using software control of the illumination spectral line shape (SLS) is presented. The technique enhances the performance of a continuous-wave THz spectrometer to reveal previously inaccessible details of closely spaced spectral peaks. We demonstrate the technique by performing spectroscopy on LiYF4:Ho3+, a material of interest for quantum science and technology, where we discriminate between inhomogeneous Gaussian and homogeneous Lorentzian contributions to absorption lines near 0.2 THz. Ultra-high-resolution (<100 Hz full-width at half maximum) frequency-domain spectroscopy with quality factor Q > 2 × 109 is achieved using an exact frequency spacing comb source in the optical communications band, with a custom uni-traveling-carrier photodiode mixer and coherent down-conversion detection. Software-defined time-domain modulation of one of the comb lines is demonstrated and used to resolve the sample SLS and to obtain a magnetic field-free readout of the electronuclear spectrum for the Ho3+ ions in LiYF4:Ho3+. In particular, homogeneous and inhomogeneous contributions to the spectrum are readily separated. The experiment reveals previously unmeasured information regarding the hyperfine structure of the first excited state in the 5 I8 manifold complementing the results reported in Phys. Rev. B 94, 205132 (2016)

Study of propagation and detection methods of terahertz radiation for spectroscopy and imaging

The applications of terahertz (THz, 1 THz is 1012 cycles per second or 300 pm in wavelength) radiation are rapidly expanding. In particular, THz imaging is emerging as a powerful technique to spatially map a wide variety of objects with spectral features which are present for many materials in THz region. Objects buried within dielectric structures can also be imaged due to the transparency of most dielectrics in this regime. Unfortunately, the image quality in such applications is inherently influenced by the scattering introduced by the sample inhomogeneities and by the presence of barriers that reduces both the transmitted power and the spatial resolution in particular frequency components. For continued development in THz radiation imaging, a comprehensive understanding of the role of these factors on THz radiation propagation and detection is vital. This dissertation focuses on the various aspects like scattering, attenuation, frequency filtering and waveguide propagation of THz radiation and its subsequent application to a stand-off THz interferometric imager under development. Using THz Time Domain spectroscopic set-up, the effect of scattering, guided THz propagation with loss and dispersion profile of hollow-core waveguides and various filtering structures are investigated. Interferometric detection scheme and subsequent agent identification is studied in detail using extensive simulation and modeling of various imaging system parameters

The 2023 terahertz science and technology roadmap

Terahertz (THz) radiation encompasses a wide spectral range within the electromagnetic spectrum that extends from microwaves to the far infrared (100 GHz–∼30 THz). Within its frequency boundaries exist a broad variety of scientific disciplines that have presented, and continue to present, technical challenges to researchers. During the past 50 years, for instance, the demands of the scientific community have substantially evolved and with a need for advanced instrumentation to support radio astronomy, Earth observation, weather forecasting, security imaging, telecommunications, non-destructive device testing and much more. Furthermore, applications have required an emergence of technology from the laboratory environment to production-scale supply and in-the-field deployments ranging from harsh ground-based locations to deep space. In addressing these requirements, the research and development community has advanced related technology and bridged the transition between electronics and photonics that high frequency operation demands. The multidisciplinary nature of THz work was our stimulus for creating the 2017 THz Science and Technology Roadmap (Dhillon et al 2017 J. Phys. D: Appl. Phys. 50 043001). As one might envisage, though, there remains much to explore both scientifically and technically and the field has continued to develop and expand rapidly. It is timely, therefore, to revise our previous roadmap and in this 2023 version we both provide an update on key developments in established technical areas that have important scientific and public benefit, and highlight new and emerging areas that show particular promise. The developments that we describe thus span from fundamental scientific research, such as THz astronomy and the emergent area of THz quantum optics, to highly applied and commercially and societally impactful subjects that include 6G THz communications, medical imaging, and climate monitoring and prediction. Our Roadmap vision draws upon the expertise and perspective of multiple international specialists that together provide an overview of past developments and the likely challenges facing the field of THz science and technology in future decades. The document is written in a form that is accessible to policy makers who wish to gain an overview of the current state of the THz art, and for the non-specialist and curious who wish to understand available technology and challenges. A such, our experts deliver a 'snapshot' introduction to the current status of the field and provide suggestions for exciting future technical development directions. Ultimately, we intend the Roadmap to portray the advantages and benefits of the THz domain and to stimulate further exploration of the field in support of scientific research and commercial realisation

The Boston University Photonics Center annual report 2007-2008

This repository item contains an annual report that summarizes activities of the Boston University Photonics Center in the 2007-2008 academic year. The report provides quantitative and descriptive information regarding photonics programs in education, interdisciplinary research, business innovation, and technology development. The Boston University Photonics Center (BUPC) is an interdisciplinary hub for education, research, scholarship, innovation, and technology development associated with practical uses of light.The following report summarizes activities of the Boston University Photonics Center (BUPC) during the period July 2007 through June 2008. These activities span the Center’s complementary missions in education, research, technology development, and commercialization. Faculty research activity reached an all time high when evaluated by the accepted metrics of external funding, scholarly publications, honors and awards. The Center’s educational programs were bolstered by two summer programs hosting more than 40 undergraduate interns, and by the renewal of a competitive graduate fellowship program sponsoring ten BUPC graduate fellowships. In technology development, the prototype RedOwl sniper detection system pioneered by Center faculty, staff, and industry partners was field-tested by the US Department of Defense. RedOwl has been officially transitioned to industry partners for further commercial development along with the soldier wearable Enhanced Acoustic Gear for Locating Enemies (EAGLE) system. Three defense/security prototypes were developed by BUPC to address critical national defense needs in the past year. Four faculty development projects were supported in collaboration with the Army Research Laboratory’s Sensors and Electron Devices Directorate (ARL-SEDD) to fill the technology pipeline for our future defense-related prototyping efforts. The Center’s business incubator is at capacity with various technology companies ranging from photonics to life sciences. The incubator provides a stimulating intellectual environment, outstanding facilities, and professional business expertise to participating companies

Scattering of the near field of an electric dipole by a single-wall carbon nanotube

The use of carbon nanotubes as optical probes for scanning near-field optical microscopy requires an understanding of their near-field response. As a first step in this direction, we investigated the lateral resolution of a carbon nanotube tip with respect to an ideal electric dipole representing an elementary detected object. A Fredholm integral equation of the first kind was formulated for the surface electric current density induced on a single-wall carbon nanotube (SWNT) by the electromagnetic field due to an arbitrarily oriented electric dipole located outside the SWNT. The response of the SWNT to the near field of a source electric dipole can be classified into two types, because surface-wave propagation occurs with (i) low damping at frequencies less than ~ 200-250 THz and (ii) high damping at higher frequencies. The interaction between the source electric dipole and the SWNT depends critically on their relative location and relative orientation, and shows evidence of the geometrical resonances of the SWNT in the low-frequency regime. These resonances disappear when the relaxation time of the SWNT is sufficiently low. The far-field radiation intensity is much higher when the source electric dipole is placed near an edge of SWNT than at the centroid of the SWNT. The use of an SWNT tip in scattering-type scanning near-field optical microscopy can deliver a resolution less than ~ 20 nm. Moreover, our study shows that the relative orientation and distance between the SWNT and the nanoscale dipole source can be detected.Comment: 23 pages, 16 figure

Fiber-based Terahertz Time-Domain Spectroscopy Systems Operated in the Telecom Band

The aim of the doctoral thesis is the study of Terahertz time domain spectrometers relying on telecommunication fiber technology. Optical fiber offers low losses, high stability and compactness, features that ease the deployment of this kind of sensing instruments in industrial scenarios. The development of terahertz signal sources working at telecom wavelengths has enabled the employment of mature, telecom-related photonic components that allowed a transition within THz research from being mainly object of scientific interest to an application-oriented technology. In this thesis, fiber terahertz systems utilizing ultrafast photoconductors with integrated antenna structures have been investigated at different levels, including the control of the photoconductor structure, as well as at instrument and system levels. The carrier transport in InGaAs-InAlAs multilayer hetero-structures, present in the employed photoconductive antennas, has been investigated under the additional injection of a continuous optical wave. By varying the amplitude level of the respective optical signal injected into either the emitter or the receiver, it has been shown that the amplitude of the detected photocurrent could be controlled without affecting its bandwidth. Unlike increasing the optical power of the pulsed signal, raising the continuous optical power results in a reduction of the measured photocurrent. This lowering of the conductivity is related to changes in the instantaneous carrier momentum relaxation time in the photoactive material, rather than to variations of the free carrier density level. This behavior affects systems including continuous-wave optical components, as, for instance, optical amplifiers. The effect has been further exploited to modulate the operation conditions of photoconductive antennas, enabling an all-optical control of the THz amplitude. This represents a method to implement a signal modulation, necessary, for instance, for lock-in signal detection. Different industrial applications and THz imaging systems require fast data acquisition. Slow, stepwise working mechanical optical delay lines are about to be replaced by faster schemes. A fast THz-time-domain spectroscopy system using a coil-based rapid mechanic delay line has been set up and analyzed. A convenience of usage of optical fibers is the simplicity of signal multiplication and distribution. It can be exploited to allow centralized operation of a set of parallel terahertz sensing units. A centralized architecture with optical source sharing simplifies the implementation as well as the cost of nondestructive inspection platforms, where several sensing units would have to work in the same facility, for example at quality control in factories or security checkpoints. The cost of such a distribution system is evaluated, its feasibility experimentally demonstrated, and key features relevant to the system performance are discussed. The present document is formally structured in a brief introduction, Chapter 2, which review common terahertz technology as a whole, with the focus on optoelectronic schemes and respective technology in the telecom band. Chapter 3 includes work carried out dealing with the carrier dynamics under continuous optical wave irradiation of the photoconductive antenna modules and the application of the effect as modulation method. Chapter 4 deals with the implementation of the fast delay in the system and Chapter 5 describes and analyses architecture for parallel, remotely controlled sensing. Finally, Chapter 6 provides conclusion and future work perspectives.El objetivo de la presente Tesis Doctoral es el estudio de espectroscopios temporales de Terahercios basados en tecnología de fibra óptica para telecomunicaciones. La fibra óptica ofrece bajas pérdidas de propagación, alta estabilidad y la capacidad de implementar sistemas robustos y compactos, características que facilitan el despliegue de este tipo de instrumentos de sensado en escenarios industriales. El desarrollo de fuentes de THz que operan en la banda infrarroja empleada en telecomunicaciones permite el uso de componentes maduros de la industria de las comunicaciones ópticas, lo que a su vez se ha traducido en una transición desde el uso de la banda de THz básicamente para intereses científicos al desarrollo de sistemas para aplicaciones industriales. En la presente tesis se investigan sistemas de THz basados en antenas fotoconductivas y fibra óptica a distintos niveles: control de la estructura fotoconductiva, instrumento y sistema. El transporte de portadores en heteroestructuras multicapa InGaAs-InAlAs, empleadas actualmente en antenas fotoconductivas, se ha investigado bajo la inyección de una onda óptica continua. Se ha observado que variando el nivel de amplitud de esta onda continua tanto en el emisor como en el receptor es posible controlar la fotocorriente detectada sin afectar a su ancho de banda. A diferencia de un incremento en la potencia óptica de la señal pulsada, elevar el nivel de continua resulta en una reducción de la fotocorriente medida. Esta reducción de la conductividad se relaciona con cambios en el tiempo de relajación del momento de los portadores en el material fotoactivo en lugar de variaciones de la densidad de portadores libres. Este comportamiento puede tener un efecto en sistemas que introduzcan componentes ópticos continuos como por ejemplo sistemas de sensado que empleen amplificadores ópticos. Este efecto puede ser usado para modular las condiciones de operación de las antenas fotoconductivas permitiendo el control todo-óptico del sistema. Este método permite modular la señal, lo que resulta necesario por ejemplo para realizar detección lock-in. Tanto diferentes aplicaciones industriales como los sistemas de imagen en THz requieren sistemas rápidos de captura. Para ello es necesario sustituir las líneas de retardo ópticas tradicionales basadas en motores paso-a-paso por otros sistemas de mayor velocidad. Se ha implementado y caracterizado un sistema THz-TDS usando una línea de retardo rápida basada en bobinas de voz. Una característica fundamental de la fibra óptica es su extraordinaria simplicidad para realizar la distribución de señales ópticas. Esta característica puede ser explotada para permitir la operación centralizada de un conjunto paralelo de sensores de THz. Una arquitectura centralizada en la que la fuente óptica se comparte entre muchos sensores simplifica la implementación y reduce el coste de sistemas de inspección no destructiva que requieran de múltiples sensores en paralelo, como, por ejemplo, en control de calidad industrial o en controles de seguridad. Se ha evaluado el coste de estos sistemas distribuidos, se ha validado experimentalmente su viabilidad y se han identificado y estudiado sus prestaciones. El documento de la tesis doctoral se estructura formalmente en una breve introducción, el capítulo 2, en el que se revisa la tecnología de THz en su conjunto, los esquemas optoelectrónicos y el uso de tecnologías ópticas basadas en la banda de las telecomunicaciones. El capítulo 3 incluye el estudio realizado sobre la dinámica de los portadores bajo la irradiación dela antena fotoconductiva con una onda óptica continua y su uso como técnica de modulación. El capítulo 4 trata con la implementación de un sistema THz-TDS rápido mientras que el capítulo 5 describe y analiza una arquitectura de sensado paralela para reducir costes. Finalmente el capítulo 6 recoge las conclusiones y futuras líneas de actuación.L'objectiu de la present Tesi Doctoral és l'estudi d'espectroscopis temporals de terahertzs basats en tecnologia de fibra òptica per a telecomunicacions. La fibra òptica ofereix baixes pèrdues de propagació, alta estabilitat i la capacitat d'implementar sistemes robustos i compactes, característiques que faciliten el desplegament d'aquest tipus d'instruments de sensat en escenaris industrials. El desenvolupament de fonts de THz que operen a la banda infraroja emprada en telecomunicacions permet l'ús de components madurs de la indústria de les comunicacions òptiques, el que al seu torn s'ha traduït en una transició des de l'ús de la banda de THz bàsicament per interessos científics al desenvolupament de sistemes per a aplicacions industrials. En la present tesi s'investiguen sistemes de THz basats en antenes fotoconductivas i fibra òptica a diferents nivells: control de l'estructura fotoconductiva, instrument i sistema. El transport de portadors en heteroestructures multicapa InGaAs-InAlAs, emprades actualment en antenes fotoconductivas, s'ha investigat sota la injecció d'una ona òptica contínua. S'ha observat que variant el nivell d'amplitud d'aquesta ona contínua tant en l'emissor com en el receptor és possible controlar la fotocorriente detectada sense afectar el seu ample de banda. A diferència d'un increment en la potència òptica del senyal polsada, elevar el nivell de contínua resulta en una reducció de la fotocorrent mesurada. Aquesta reducció de la conductivitat es relaciona amb canvis en el temps de relaxació del moment dels portadors en el material fotoactiu en lloc de variacions de la densitat de portadors lliures. Aquest comportament pot tenir un efecte en sistemes que introdueixin components òptics continus com ara sistemes de sensat que utilitzen amplificadors òptics. Aquest efecte pot ser usat per modular les condicions d'operació de les antenes fotoconductivas permetent el control tot-òptic del sistema. Aquest mètode permet modular el senyal, el que resulta necessari per exemple per realitzar detecció lock-in. Tant diferents aplicacions industrials com els sistemes d'imatge en THz requereixen sistemes ràpids de captura. Per a això és necessari substituir les línies de retard òptiques tradicionals basades en motors pas-a-pas per altres sistemes de major velocitat. S'ha implementat i caracteritzat un sistema THz-TDS usant una línia de retard ràpida basada en bobines de veu. Una característica fonamental de la fibra òptica és la seua extraordinària simplicitat per realitzar la distribució de senyals òptiques. Aquesta característica pot ser explotada per a permetre l'operació centralitzada d'un conjunt paral·lel de sensors de THz. Una arquitectura centralitzada en la qual la font òptica es comparteix entre molts sensors simplifica la implementació i redueix el cost de sistemes d'inspecció no destructiva que requereixin de múltiples sensors en paral·lel, com, per exemple, en control de qualitat industrial o en controls de seguretat . S'ha avaluat el cost d'aquests sistemes distribuïts, s'ha validat experimentalment la seua viabilitat i s'han identificat i estudiat les seues prestacions. El document de la tesi doctoral s'estructura formalment en una breu introducció, capítol 2, en el qual es revisa la tecnologia de THz en el seu conjunt, els esquemes optoelectrònics i l'ús de tecnologies òptiques basades en la banda de les telecomunicacions. El capítol 4 inclou l'estudi realitzat sobre la dinàmica dels portadors sota la irradiació de la antena fotoconductiva amb una ona òptica contínua i el seu ús com a tècnica de modulació. El capítol 5 tracta la implementació d'un sistema THz-TDS ràpid mentre que el capítol 6 descriu i analitza una arquitectura de sensat paral·lela per reduir costos. Finalment, el capítol 7 recull les conclusions i futures línies d'actuació.Bockelt, AS. (2017). Fiber-based Terahertz Time-Domain Spectroscopy Systems Operated in the Telecom Band [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/86148TESI