Emerging Approaches for THz Array Imaging: A Tutorial Review and Software Tool

Accelerated by the increasing attention drawn by 5G, 6G, and Internet of Things applications, communication and sensing technologies have rapidly evolved from millimeter-wave (mmWave) to terahertz (THz) in recent years. Enabled by significant advancements in electromagnetic (EM) hardware, mmWave and THz frequency regimes spanning 30 GHz to 300 GHz and 300 GHz to 3000 GHz, respectively, can be employed for a host of applications. The main feature of THz systems is high-bandwidth transmission, enabling ultra-high-resolution imaging and high-throughput communications; however, challenges in both the hardware and algorithmic arenas remain for the ubiquitous adoption of THz technology. Spectra comprising mmWave and THz frequencies are well-suited for synthetic aperture radar (SAR) imaging at sub-millimeter resolutions for a wide spectrum of tasks like material characterization and nondestructive testing (NDT). This article provides a tutorial review of systems and algorithms for THz SAR in the near-field with an emphasis on emerging algorithms that combine signal processing and machine learning techniques. As part of this study, an overview of classical and data-driven THz SAR algorithms is provided, focusing on object detection for security applications and SAR image super-resolution. We also discuss relevant issues, challenges, and future research directions for emerging algorithms and THz SAR, including standardization of system and algorithm benchmarking, adoption of state-of-the-art deep learning techniques, signal processing-optimized machine learning, and hybrid data-driven signal processing algorithms...Comment: Submitted to Proceedings of IEE

Novel Approaches for Nondestructive Testing and Evaluation

Nondestructive testing and evaluation (NDT&E) is one of the most important techniques for determining the quality and safety of materials, components, devices, and structures. NDT&E technologies include ultrasonic testing (UT), magnetic particle testing (MT), magnetic flux leakage testing (MFLT), eddy current testing (ECT), radiation testing (RT), penetrant testing (PT), and visual testing (VT), and these are widely used throughout the modern industry. However, some NDT processes, such as those for cleaning specimens and removing paint, cause environmental pollution and must only be considered in limited environments (time, space, and sensor selection). Thus, NDT&E is classified as a typical 3D (dirty, dangerous, and difficult) job. In addition, NDT operators judge the presence of damage based on experience and subjective judgment, so in some cases, a flaw may not be detected during the test. Therefore, to obtain clearer test results, a means for the operator to determine flaws more easily should be provided. In addition, the test results should be organized systemically in order to identify the cause of the abnormality in the test specimen and to identify the progress of the damage quantitatively

OCM 2023 - Optical Characterization of Materials : Conference Proceedings

The state of the art in the optical characterization of materials is advancing rapidly. New insights have been gained into the theoretical foundations of this research and exciting developments have been made in practice, driven by new applications and innovative sensor technologies that are constantly evolving. The great success of past conferences proves the necessity of a platform for presentation, discussion and evaluation of the latest research results in this interdisciplinary field

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance

Beam scanning by liquid-crystal biasing in a modified SIW structure

A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium

Synthetic Aperture Radar (SAR) Meets Deep Learning

This reprint focuses on the application of the combination of synthetic aperture radars and depth learning technology. It aims to further promote the development of SAR image intelligent interpretation technology. A synthetic aperture radar (SAR) is an important active microwave imaging sensor, whose all-day and all-weather working capacity give it an important place in the remote sensing community. Since the United States launched the first SAR satellite, SAR has received much attention in the remote sensing community, e.g., in geological exploration, topographic mapping, disaster forecast, and traffic monitoring. It is valuable and meaningful, therefore, to study SAR-based remote sensing applications. In recent years, deep learning represented by convolution neural networks has promoted significant progress in the computer vision community, e.g., in face recognition, the driverless field and Internet of things (IoT). Deep learning can enable computational models with multiple processing layers to learn data representations with multiple-level abstractions. This can greatly improve the performance of various applications. This reprint provides a platform for researchers to handle the above significant challenges and present their innovative and cutting-edge research results when applying deep learning to SAR in various manuscript types, e.g., articles, letters, reviews and technical reports

Photonic Technology for Precision Metrology

Photonics has had a decisive influence on recent scientific and technological achievements. It includes aspects of photon generation and photon–matter interaction. Although it finds many applications in the whole optical range of the wavelengths, most solutions operate in the visible and infrared range. Since the invention of the laser, a source of highly coherent optical radiation, optical measurements have become the perfect tool for highly precise and accurate measurements. Such measurements have the additional advantages of requiring no contact and a fast rate suitable for in-process metrology. However, their extreme precision is ultimately limited by, e.g., the noise of both lasers and photodetectors. The Special Issue of the Applied Science is devoted to the cutting-edge uses of optical sources, detectors, and optoelectronics systems in numerous fields of science and technology (e.g., industry, environment, healthcare, telecommunication, security, and space). The aim is to provide detail on state-of-the-art photonic technology for precision metrology and identify future developmental directions. This issue focuses on metrology principles and measurement instrumentation in optical technology to solve challenging engineering problems

Roadmap on holography

From its inception holography has proven an extremely productive and attractive area of research. While specific technical applications give rise to 'hot topics', and three-dimensional (3D) visualisation comes in and out of fashion, the core principals involved continue to lead to exciting innovations in a wide range of areas. We humbly submit that it is impossible, in any journal document of this type, to fully reflect current and potential activity; however, our valiant contributors have produced a series of documents that go no small way to neatly capture progress across a wide range of core activities. As editors we have attempted to spread our net wide in order to illustrate the breadth of international activity. In relation to this we believe we have been at least partially successful.This work was supported by Ministerio de Economía, Industria y Competitividad (Spain) under projects FIS2017-82919-R (MINECO/AEI/FEDER, UE) and FIS2015-66570-P (MINECO/FEDER), and by Generalitat Valenciana (Spain) under project PROMETEO II/2015/015

Waveplates based on metasurfaces in the THz range

Los platos de onda basados en metasuperficies son componentes clave en electromagnetismo, ya que permiten un control total de la polarización de las ondas electromagnéticas con la ventaja de presentar estructuras más compactas que los platos de onda convencionales. Además, la aplicación del principio de Pancharatnam Berry (PB) a los platos de media onda (HWP) basados en metasuperficies, permite la manipulación de frentes de onda junto con la conversión de la polarización de las ondas incidentes circularmente polarizadas, simplemente girando los meta-átomos que componen la metasuperficie. Para lograr altos niveles de eficiencia de transmisión con platos de onda basados en metasuperficies, generalmente se requieren diseños multicapa. Esto implica estructuras voluminosas y complica el proceso de fabricación, restando importancia a la ventaja de utilizar metasuperficies. El propósito de esta tesis realizada en la Universidad Pública de Navarra y en L’École Polytechnique Fédérale de Lausanne, Suiza (EPFL) es ofrecer una ventaja tecnológica tanto para el control de polarización como para la manipulación del frente de onda y contribuir al desarrollo de dispositivos basados en metamasuperficies, incluyendo su fabricación y verificación experimental. Los resultados incluyen lo siguiente: Un plato de media onda operando en transmisión, ultradelgado y basado en una metasuperficie en zigzag de dos capas que opera en la parte baja del espectro del THz con un 90% de eficiencia de transmisión, que se demuestra numérica y experimentalmente. Se lleva a cabo un análisis detallado de la robustez del dispositivo con respecto a los desalineamientos de las capas mediante el diseño y la fabricación de dos dispositivos adicionales con el máximo desalineamiento entre capas en ambas direcciones transversales. Una metalente ultradelgada y compacta basada en el principio Pancharatnam Berry con solo dos capas alcanzando un 90% de eficiencia de transmisión, enfocando el frente de onda de una onda incidente polarizada circularmente y convirtiendo su polarización. La estructura es estudiada semi-analítica y numéricamente y medida experimentalmente, comprobándose un excelente comportamiento como HWP PB metalente a 87 GHz. Una aplicación de ingeniería de frentes de onda para la manipulación de los mismos se demuestra numéricamente en el rango de ondas milimétricas mediante la integración de la metalente en un sistema de antena-metalente, que se estudia semi-analíticamente y se corrobora experimentalmente. El sistema convierte la polarización de las ondas polarizadas circularmente, logrando un incremento de la directividad de antena de 17 dB a ⁓35 dB a 87 GHz con un AR inferior a 0.5 dB. Finalmente, se presentan dos configuraciones extra del sistema para trabajar entre los extremos del rango de frecuencia comprendido entre 75 GHz y 105 GHz, con directividades ⁓32 dB y AR < 3 dB.Transmissive waveplates based on metasurfaces are key components in electromagnetism, as they allow for a full control of the electromagnetic wave polarization with the advantage of presenting structures more compact than conventional waveplates. Moreover, applying the Pancharatnam Berry (PB) principle to half-wave plate (HWP) metasurfaces allows the manipulation of wavefronts along with the conversion of the handedness of circularly polarized incident waves by simply rotating the meta-atoms that compose the metasurface. For achieving high levels of transmission efficiency with transmissive waveplates based on metasurfaces, multiple layer designs are usually required. It implies bulky structures and complicates the fabrication process, downplaying the aim of the use of metasurfaces. The purpose of this thesis performed at the Public University of Navarre and at École Polytechnique Fédérale de Lausanne (EPFL) is to offer a technological advantage both for polarization controlling as wavefront manipulation and contribute to the development of metasurface-based devices, including their fabrication and experimental verification. The results include the following: An ultrathin transmissive half-wave plate based on a bi-layered zigzag metasurface operating at the lower-frequency edge of the THz spectrum with a 90% of transmission efficiency, which is numerically and experimentally demonstrated. A detailed analysis of the device robustness with respect to layer misalignments is carried out by designing and fabricating two additional devices with the maximum possible shift between layers along both transverse directions. A compact ultrathin metalens based on the Pancharatnam Berry principle with only two layers with a 90% of transmission efficiency, focusing the wavefront of a circularly polarized incident wave and converting its handedness. The structure is semi-analytically and numerically studied and experimentally measured, verifying an excellent behavior as HWP PB metalens at 87 GHz. A wavefront engineering application for wavefront manipulation is numerically demonstrated in the millimeter-wave range by the integration of the metalens in an antenna-metalens system, which is semi-analytically studied and experimentally corroborated. The system converts the handedness of circular polarized waves, achieving an increment of the antenna directivity from17 dB to ⁓35 dB at 87 GHz with an AR lesser than 0.5 dB. Finally, two extra system configurations are presented to work around the frequency range extremes comprised between 75 GHz and 105 GHz, with directivities ⁓32 dB and AR < 3 dB.Programa de Doctorado en Tecnologías de las Comunicaciones, Bioingeniería y de las Energías Renovables (RD 99/2011)Bioingeniaritzako eta Komunikazioen eta Energia Berriztagarrien Teknologietako Doktoretza Programa (ED 99/2011

Metamaterials and Metasurfaces

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