242 research outputs found
Accurate quantum transport modelling and epitaxial structure design of high-speed and high-power In0.53Ga0.47As/AlAs double-barrier resonant tunnelling diodes for 300-GHz oscillator sources
Terahertz (THz) wave technology is envisioned as an appealing and conceivable solution in the context of several potential high-impact applications, including sixth generation (6G) and beyond consumer-oriented ultra-broadband multi-gigabit wireless data-links, as well as highresolution imaging, radar, and spectroscopy apparatuses employable in biomedicine, industrial processes, security/defence, and material science. Despite the technological challenges posed by the THz gap, recent scientific advancements suggest the practical viability of THz systems. However, the development of transmitters (Tx) and receivers (Rx) based on compact semiconductor devices operating at THz frequencies is urgently demanded to meet the performance requirements calling from emerging THz applications.
Although several are the promising candidates, including high-speed III-V transistors and photo-diodes, resonant tunnelling diode (RTD) technology offers a compact and high performance option in many practical scenarios. However, the main weakness of the technology is currently represented by the low output power capability of RTD THz Tx, which is mainly caused by the underdeveloped and non-optimal device, as well as circuit, design implementation approaches. Indeed, indium phosphide (InP) RTD devices can nowadays deliver only up to around 1 mW of radio-frequency (RF) power at around 300 GHz. In the context of THz wireless data-links, this severely impacts the Tx performance, limiting communication distance and data transfer capabilities which, at the current time, are of the order of few tens of gigabit per second below around 1 m.
However, recent research studies suggest that several milliwatt of output power are required to achieve bit-rate capabilities of several tens of gigabits per second and beyond, and to reach several metres of communication distance in common operating conditions. Currently, the shortterm target is set to 5ā10 mW of output power at around 300 GHz carrier waves, which would allow bit-rates in excess of 100 Gb/s, as well as wireless communications well above 5 m distance, in first-stage short-range scenarios. In order to reach it, maximisation of the RTD highfrequency RF power capability is of utmost importance. Despite that, reliable epitaxial structure design approaches, as well as accurate physical-based numerical simulation tools, aimed at RF power maximisation in the 300 GHz-band are lacking at the current time.
This work aims at proposing practical solutions to address the aforementioned issues. First, a physical-based simulation methodology was developed to accurately and reliably simulate the static current-voltage (IV ) characteristic of indium gallium arsenide/aluminium arsenide (In-GaAs/AlAs) double-barrier RTD devices. The approach relies on the non-equilibrium Greenās function (NEGF) formalism implemented in Silvaco Atlas technology computer-aided design (TCAD) simulation package, requires low computational budget, and allows to correctly model In0.53Ga0.47As/AlAs RTD devices, which are pseudomorphically-grown on lattice-matched to InP substrates, and are commonly employed in oscillators working at around 300 GHz. By selecting the appropriate physical models, and by retrieving the correct materials parameters, together with a suitable discretisation of the associated heterostructure spatial domain through finite-elements, it is shown, by comparing simulation data with experimental results, that the developed numerical approach can reliably compute several quantities of interest that characterise the DC IV curve negative differential resistance (NDR) region, including peak current, peak voltage, and voltage swing, all of which are key parameters in RTD oscillator design.
The demonstrated simulation approach was then used to study the impact of epitaxial structure design parameters, including those characterising the double-barrier quantum well, as well as emitter and collector regions, on the electrical properties of the RTD device. In particular, a comprehensive simulation analysis was conducted, and the retrieved output trends discussed based on the heterostructure band diagram, transmission coefficient energy spectrum, charge distribution, and DC current-density voltage (JV) curve. General design guidelines aimed at enhancing the RTD device maximum RF power gain capability are then deduced and discussed.
To validate the proposed epitaxial design approach, an In0.53Ga0.47As/AlAs double-barrier RTD epitaxial structure providing several milliwatt of RF power was designed by employing the developed simulation methodology, and experimentally-investigated through the microfabrication of RTD devices and subsequent high-frequency characterisation up to 110 GHz. The analysis, which included fabrication optimisation, reveals an expected RF power performance of up to around 5 mW and 10 mW at 300 GHz for 25 Ī¼m2 and 49 Ī¼m2-large RTD devices, respectively, which is up to five times higher compared to the current state-of-the-art. Finally, in order to prove the practical employability of the proposed RTDs in oscillator circuits realised employing low-cost photo-lithography, both coplanar waveguide and microstrip inductive stubs are designed through a full three-dimensional electromagnetic simulation analysis.
In summary, this work makes and important contribution to the rapidly evolving field of THz RTD technology, and demonstrates the practical feasibility of 300-GHz high-power RTD devices realisation, which will underpin the future development of Tx systems capable of the power levels required in the forthcoming THz applications
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Spatiotemporal control of terahertz waves in random media via Nonlinear Ghost Imaging
Harnessing the spatiotemporal control of complex fields is a critical challenge in a plethora of scientific domains, from photonics to ultrasound imaging. The demand for engineering field manipulation techniques is crucial in many disciplines, for instance, the investigation of imaging methodologies in disordered environments. The control of fields through complex media is an established application domain in microwave and ultrasound imaging. In the last two decades, scientists have developed similar approaches for optical waves. There is a great interest in extending this study to the state-of-the-art at Terahertz (THz) frequencies, particularly in the spatiotemporal field manipulation of ultrafast THz pulses, given the significant difference in methodologies and technologies compared to optical frequencies, for instance. Also, this study would be of great interest in telecom applications at THz frequencies, as communications in this band are expected to be more susceptible to scattering when compared to microwaves.
This thesis contains the results obtained in the Emergent Photonics Laboratory, where I have been developing novel field manipulation techniques in random systems. I will illustrate a new route for harnessing the spatiotemporal properties of THz waves by exploiting scattering media as space-time combinatory elements. The state-of-the-art of THz TDS allows approaching wave scattering as a deterministic spatiotemporal event to be used as complex and inexpensive pulse shapers. As a specific case study, I will show the possibility of spatiotemporal superfocusing of ultrafast THz pulse propagating in complex media, corresponding to a simultaneous focusing in space and pulse re-compression in time. It is worth mentioning that the methodology behind the study of field manipulation in complex media is based on the Time-Resolved Nonlinear Ghost Imaging, a novel correlation-based near-field THz imaging that I have contributed to developing throughout my PhD. In this methodology, an electromagnetic image is reconstructed by correlating the known spatial THz patterns projected on the target object with the scattered field measured by a standard time-domain spectroscopy (TDS) detection, a mature approach in the field.
The work is mainly presented in the form of a collection of publications (paper-style) but also includes very recent developments that are still unpublished. In its deployment, this thesis offers a general overview of the topic to introduce the subject field to a general Photonics physicist, presents published materials aggregated per topic, and discusses novel material and results in the final chapter. All the material presented has been generated by myself individually or within teamwork unless otherwise specified. Teamwork outputs are presented in full, with my specific contribution clearly highlighted at the beginning of each chapter. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme Grant agreement nĀ° 725046
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
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Hidden-information extraction from layered structures through terahertz imaging down to ultralow SNR
Supplementary Materials are available online at: https://www.science.org/doi/suppl/10.1126/sciadv.adg8435/suppl_file/sciadv.adg8435_sm.pdf .Copyright Ā© 2023 The Authors. Noninvasive inspection of layered structures has remained a long-standing challenge for time-resolved imaging techniques, where both resolution and contrast are compromised by prominent signal attenuation, interlayer reflections, and dispersion. Our method based on terahertz (THz) time-domain spectroscopy overcomes these limitations by offering fine resolution and a broadband spectrum to efficiently extract hidden structural and content information from layered structures. We exploit local symmetrical characteristics of reflected THz pulses to determine the location of each layer, and apply a statistical process in the spatiotemporal domain to enhance the image contrast. Its superior performance is evidenced by the extraction of alphabetic characters in 26-layer subwavelength papers as well as layer reconstruction and debonding inspection in the conservation of Terra-Cotta Warriors. Our method enables accurate structure reconstruction and high-contrast imaging of layered structures at ultralow signal-to-noise ratio, which holds great potential for internal inspection of cultural artifacts, electronic components, coatings, and composites with dozens of submillimeter layers.National Natural Science Foundation of China (grant. 52175115)
2022 Review of Data-Driven Plasma Science
Data-driven science and technology offer transformative tools and methods to science. This review article highlights the latest development and progress in the interdisciplinary field of data-driven plasma science (DDPS), i.e., plasma science whose progress is driven strongly by data and data analyses. Plasma is considered to be the most ubiquitous form of observable matter in the universe. Data associated with plasmas can, therefore, cover extremely large spatial and temporal scales, and often provide essential information for other scientific disciplines. Thanks to the latest technological developments, plasma experiments, observations, and computation now produce a large amount of data that can no longer be analyzed or interpreted manually. This trend now necessitates a highly sophisticated use of high-performance computers for data analyses, making artificial intelligence and machine learning vital components of DDPS. This article contains seven primary sections, in addition to the introduction and summary. Following an overview of fundamental data-driven science, five other sections cover widely studied topics of plasma science and technologies, i.e., basic plasma physics and laboratory experiments, magnetic confinement fusion, inertial confinement fusion and high-energy-density physics, space and astronomical plasmas, and plasma technologies for industrial and other applications. The final section before the summary discusses plasma-related databases that could significantly contribute to DDPS. Each primary section starts with a brief introduction to the topic, discusses the state-of-the-art developments in the use of data and/or data-scientific approaches, and presents the summary and outlook. Despite the recent impressive signs of progress, the DDPS is still in its infancy. This article attempts to offer a broad perspective on the development of this field and identify where further innovations are required
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
Laser Plasma Study through Simulation and Theory
Department of PhysicsStimulated Raman Scattering (SRS) is a fascinating physical phenomenon that arises from the interaction between a plasma medium and high-energy laser radiation. It involves the transfer of laser energy to plasma waves, resulting in the generation of new waves and the scattering of the incident laser beam. SRS is an important phenomenon in laser plasma interaction, with various applications in fields such as laser fusion, particle acceleration, and high-energy-density physics. The SRS process begins when a high-intensity laser beam interacts with a plasma medium. The laser energy excites plasma waves, which can be longitudinal or transverse depending on the direction of the laser polarization. These waves then undergo a resonance process, where they interact with the plasma ions and transfer energy to them. As a result, new waves are generated, and the incident laser beam is scattered in a different direction. One of the most significant features of SRS is its threshold behavior. SRS only occurs when the laser intensity exceeds a certain threshold value. Below this value, the plasma waves cannot reach the resonance condition and do not transfer energy to the plasma ions. Above the threshold value, however, the plasma waves grow exponentially, leading to a rapid increase in the scattered light intensity. SRS has various applications in laser plasma interaction. In laser fusion, SRS can be a significant obstacle as it leads to the loss of laser energy and can damage the laser system. Researchers have developed various methods to mitigate SRS, such as using frequency conversion techniques, plasma shaping, and polarization smoothing. In particle acceleration, SRS can be used to generate high-energy electron beams. By controlling the laser intensity and plasma conditions, researchers can create plasma wakefields that accelerate charged particles to high energies. Plasma density diagnostics are crucial to understanding the laser plasma interaction process. One diagnostic method involves using a probe laser to measure the plasma density through the interaction with the plasma electrons. Other diagnostic methods include interferometry, Thomson scattering, and Langmuir probes. Raman scattering diagnostics are a powerful tool for measuring the plasma density in a wide range of applications. This technique relies on the inelastic scattering of light from the plasma, which results in a shift in the frequency of the scattered light. By measuring the frequency shift, researchers can determine the plasma density and gain insight into the plasma???s behavior. Overall, Raman scattering diagnostics are a powerful tool for measuring plasma density in a wide range of applications. These techniques can provide valuable insight into the plasma???s behavior and are essential for the development of advanced plasma technologies. Ongoing research continues to improve the sensitivity and accuracy of Raman scattering diagnostics, ensuring that these techniques remain at the forefront of plasma research. Plasma dipole oscillations are a type of collective motion that can occur in a plasma medium when it is excited by an external electromagnetic field. These oscillations result from the motion of the plasma electrons in response to the electromagnetic field, creating a dipole moment that oscillates at a characteristic frequency. Plasma dipole oscillations are an important phenomenon in plasma physics and have various applications, including as a radiation source for plasma diagnostics. Plasma dipole oscillations can also act as a radiation source for plasma diagnostics. When the dipole moment of the plasma oscillates, it generates electromagnetic radiation at the same frequency. This radiation can be in the THzband depending on the plasma parameters. This characteristics create new diagnostic method for plasma density. More easily usage of PDO method, we shot the laser pulse obliquely. The magnetization of the PDO induces the formation of three modes in gyrating electrons: the upper hybrid (H) mode, the right circular mode (R), and the left circular mode (L). The H-mode acts as a resonance point that prevents transmission to the vacuum, whereas X-modes can be transmitted through the plasma. The H-mode diminishes as the magnetic field increases, while X-modes become more prominent. This results in more energy being extracted from the PDO in the form of radiation. This effect is demonstrated by the effective flow, where in the weak field regime, electrons are well organized, resulting in effective longitudinal flow, while in the strong field regime, electron phases are randomized, and circular flows prevail, forming X-modes instead of H-mode.clos
Micro/Nano Structures and Systems
Micro/Nano Structures and Systems: Analysis, Design, Manufacturing, and Reliability is a comprehensive guide that explores the various aspects of micro- and nanostructures and systems. From analysis and design to manufacturing and reliability, this reprint provides a thorough understanding of the latest methods and techniques used in the field. With an emphasis on modern computational and analytical methods and their integration with experimental techniques, this reprint is an invaluable resource for researchers and engineers working in the field of micro- and nanosystems, including micromachines, additive manufacturing at the microscale, micro/nano-electromechanical systems, and more. Written by leading experts in the field, this reprint offers a complete understanding of the physical and mechanical behavior of micro- and nanostructures, making it an essential reference for professionals in this field
Extreme Solar Events: Setting up a Paradigm
The Sun is magnetically active and often produces eruptive events on different energetic and temporal scales. Until recently, the upper limit of such events was unknown and believed to be roughly represented by direct instrumental observations. However, two types of extreme events were discovered recently: extreme solar energetic particle events on the multi-millennial time scale and super-flares on sun-like stars. Both discoveries imply that the Sun might rarely produce events, called extreme solar events (ESE), whose energy could be orders of magnitude greater than anything we have observed during recent decades. During the years following these discoveries, great progress has been achieved in collecting observational evidence, uncovering new events, making statistical analyses, and developing theoretical modelling. The ESE paradigm lives and is being developed. On the other hand, many outstanding questions still remain open and new ones emerge. Here we present an overview of the current state of the art and the forming paradigm of ESE from different points of view: solar physics, stellarāsolar projections, cosmogenic-isotope data, modelling, historical data, as well as terrestrial, technological and societal effects of ESEs. Special focus is paid to open questions and further developments. This review is based on the joint work of the International Space Science Institute (ISSI) team #510 (2020ā2022)
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