Plasmons in Graphene for Amplification of THz Radiation
More than 40 years ago, a new direction in physics opened up with the arrival of plasma-wave electronics. The possibility that the plasma waves could propagate faster than electrons fascinated all. Therefore, it was initially expected that plasmonic devices, including detectors and generators of electromagnetic radiation, would be able to work effectively in the very high frequencies – terahertz (THz) range, inaccessible to standard electronic devices. However, numerous experimental attempts to realize such amplifiers or emitters failed: the intensity of radiation turned out to be too small, plasma resonances too broad, or devices operated only at cryogenic temperatures. Thus, the creation of compact, tunable, room temperature operating THz amplifiers and sources is still a challenging task.
In the lecture we will demonstrate graphene nanostructures can be used for THz amplification. We will show that gate voltage tunable resonant plasmon absorption, increases with increas of the current and then turns to THz radiation amplification with a gain going up to 9%. These very recent results were interpreted using a dissipative plasmonics crystal model, which captures the main trends and basic physics of the amplification phenomena. Specifically, the model predicts that increasing current drives the system into an amplification regime, wherein the plasma waves may transfer energy to the incoming electromagnetic waves.
All results were obtained at room temperature. Therefore, they pave the way towards a future THz plasmonic technology with a new generation of all-electronic, resonant, voltage-controlled THz amplifiers. [10.1103/PhysRevX.10.031004].
Millimeter-wave dielectric waveguides and their application in sensing and imaging
Dielectric rod waveguides (DRW) are promising transmission lines when low loss dielectric materials are used and can be combined with semiconductor devices (oscillators, detectors, mixers, etc.) in the hybrid and/or monolithic integrated circuits. These developments offer a new opportunity for passive and active component perfThe distribution of electric field of the guided wave in a rectangular dielectric waveguide as a function of its width and height is considered. From this analysis, preferable dimensions of the dielectric waveguide for the near-field applications are determined. Two types of metal-to-dielectric waveguide transitions needed to build measurement setups are proposed. Methods of thin-sheet material characterization in the transmission mode in the frequency band of 50 to 75 GHz and a point-by-point scanning example with subwavelength resolution are presented.
THz Schottky Detectors: Design and application for Particle Accelerators
The main obstacle for many scientific and commercial applications is the lack of suitable THz sources and detectors with enough power and sensitivity with small footprints and portability. Currently, available photonic based THz systems have already demonstrated great potential in terms of high tunability, standard room temperature operation, and signal quality, however, they are still suffering from many drawbacks, such as big size equipment (needs an optical table), mechanical disturbance (additional to noise an
Wideband THz direct detectors are powerful instruments for the diagnostic of coherent THz radiation in various particle accelerators. The investigation of the beam properties allows for bunch-by-bunch (Linac) as well as turn-by-turn (Synchrotron) diagnostics of electron bunches in the accelerator.
Zero-bias Schottky diode detectors operated at room-temperature are the choice for applications, where the ultimate sensitivity of a cryogenic detector is not required. Furthermore, Schottky detectors are intrinsically much faster than the latter ones.
In this talk, first an overview about THz generating accelerators is given, followed by current State of the Art techniques to detect THz radiation. Finally, recent results of compact quasi-optically coupled zero-bias planar Schottky detectors for monitoring picosecond pulses of intense, coherent far-infrared radiation in Free Electron Lasers are investigated
Record-breaking performance of low-dimensional solid photodetectors
In the last decade, papers on low-dimensional solid photodetectors have appeared in the available literature reporting the achievement of record-breaking photodetectors, better than those available on the commercial market. Several previously published papers have pointed to unrealistic claims about detector performance. The purpose of this paper is to present these record-breaking data and to try to draw attention to the obvious physical limitations of photodetectors that are sometimes ignored in the estimation of photodetector performance.
Enhanced Terahertz Devices Toward Systems-On-Chip Enabled By Photonic Integrated Circuits
The main obstacle for many scientific and commercial applications is the lack of suitable THz sources and detectors with enough power and sensitivity with small footprints and portability. Currently, available photonic based THz systems have already demonstrated great potential in terms of high tunability, standard room temperature operation, and signal quality, however, they are still suffering from many drawbacks, such as big size equipment (needs an optical table), mechanical disturbance (additional to noise and alignment), high power consumption (electrical and optical), and low flexibility system (each application needs a new setup). Therefore, we propose a new THz system platform, aimed to overcome all the above drawbacks, based on photonic integrated circuits (PICs) and nanotechnology. This can not happen just by resize the big systems and devices to be fits for intergradation, but it required manger changing in power requirements and enhanced performance to be suitable and compatible with PICs technology. The target system-on-chip incorporates a fully integrated THz source and detector with increased emission power and sensitivities by using nano-contacts based photomixers with cointegrated electronic and air-interface. The THz system-on-chip will find applications in various areas such as sensing, communications, food safety, and biomedicine among others.
The use of THz waves for communication applications has been pushed forward over the last years, with numerous demonstrations of laboratory links up to 300 GHz, as well as the first outdoor field trials.
However, the key limiting factor is still the available output power of the THz transmitters. Leveraging recent year developments, amplifiers now reach the 300 GHz band, using III-V semi-conductors so far, and looking towards future applications characterization of these active circuits using modulated THz signals to complete continuous-wave measurements is of utmost importance. Using a photonics-driven IQ generator in the 300 GHz range, combined with a GaAs reference receiver, we analyze the error-vector magnitude degradation induced by the III-V amplifier, the building blocks (Tx and Rx) of a superheterodyne THz transmission system developed during the European Project THOR. Laboratory validation of the THz link will be shown at the conference
Dmitri V. Lioubtchenko
Passive And Active Devices Based On Dielectric Rod Waveguides Form Future THz Applications
Dielectric rod waveguides (DRW) are promising transmission lines when low loss dielectric materials are used and can be combined with semiconductor devices (oscillators, detectors, mixers, etc.) in the hybrid and/or monolithic integrated circuits. These developments offer a new opportunity for passive and active component performance, as it allows to decrease the insertion losses. Besides, DRWs have no cut-off frequency enabling broadband operation. A big variety of passive and active components can be integrated into the DRW to obtain a compact transceiver module for THz applications.
Terahertz Imaging and Spectroscopy. Terahertz Free Electron Laser – Polfel Project
The last twenty years have been characterized by strong development of the technique using radiation from the border of far infrared and microwaves in the range of about 0.1–10 THz, which due to its unique properties has been called the terahertz band (THz). THz radiation penetrates through most non-metallic and non-polar substances such as paper, cardboard, plastic, clothes, etc. and reflects perfectly on metal, which in connection with its non-ionizing and therefore safe nature is the basis for the operation of body scanners detecting suspicious objects (e.g., weapons, bombs) under human clothing or non-destructive testing of materials, e.g., composites. In this spectral range, many substances, including some materials, including explosives, have absorptive characteristics (spectral signatures) resulting from vibrational vibrations and interactions between molecules, which can be used to identify covered materials in packaging.
In the 1980s, the first Time-Domain Spectroscopy (TDS) setup was demonstrated, which developed rapidly and found many applications in science and technology. The TDS is a synchronized system for generating and detecting a pulse of electromagnetic radiation lasting approximately 0.5 ps using a femtosecond laser and photoconductive antennas. This pulse has a broad spectrum in the range of about 0.1-3 THz. In the first part of the presentation, the characteristics of terahertz radiation and the measurement systems used will be presented.
The presented research results are divided into two groups – spectroscopy and imaging. The presentation will show the results related to general studies of THz wave interactions with various materials, including explosives, pharmaceuticals, liquid crystals, and metamaterials. Then, the results of imaging the internal structure of composite materials will be presented together with signal processing techniques. THz imaging allows for the detection of defects (such as delamination, cracks, air bubbles) or moisture, e.g., in glass fiber reinforced polymers.
Currently, the project “PolFEL – Polish free-electron laser” is being implemented in Poland. The aim of the PolFEL project is to build a new research infrastructure at NCBJ in Świerk, Poland, in the form of a source of coherent electromagnetic radiation – a free-electron laser (FEL). Radiation in FEL lasers is generated because of the interaction of a beam of relativistic electrons with a system of strong magnetic fields. The generated radiation is characterized by unique properties that make FEL lasers extremely useful in research and development. The PolFEL laser will generate ultra-short, about 100 fs duration, pulses of high-power radiation in the spectral range of vacuum ultraviolet, infrared and THz. In the third part of the presentation, the progress of work in the PolFEL project will be presented, with particular emphasis on terahertz systems – a diagnostic station and two measuring stations intended for the end-user.
Linear And Nonlinear Stability Analysis Of Power Amplifiers
In this lecture, the stability issues of high frequency circuits will be reviewed, both in the linear and in the nonlinear regime. The general stability of linear circuits is first introduced from a general point of view. The stability criteria in linear regime available in standard CAD tools are then reviewed, including in particular the stability factors K/B1, µ1/µ2, etc. The special cases of even/odd mode stability, inner loops analysis, and generalised linear stability analysis are given a detailed description.
The stability analysis is then extended to nonlinear regime. The conversion matrix approach is introduced, and the derived methods available in commercial CAD tools for the detection of spurious modes are described. In particular, the approaches by Rizzoli, Collantes/STAN, Di Paolo, etc. are introduced, including the analysis of even/odd modes.
Terahertz Photonic Spectrum Analyzer Systems
The presentation will introduce photonic concepts for generation and detection of terahertz radiation. Two laser beams that are spaced in frequency by the THz frequency to generate are mixed in a photoconductor where it serves as a local oscillator. If a THz signal is coupled to the photoconductor as well, it will be mixed with the optical LO down to an intermediate frequency. The first part of the presentation demonstrates the implementation of these devices as active mixing element in photonic spectrum analyzer concepts will be illustrated with several application examples. The second part will focus on optical systems driving the photoconductors in order to generate Hz-Level linewidth spectrum analyzers.
Mm-wave and THz Spectroscopy for Biomedical and Clinical Applications
During the last years, we have seen an increasing interest on the use of the mm-wave and THz spectral range for biomedical applications. In this talk, we will review the challenges and practical issues associated to the use of such frequency ranges for biomedical and clinical applications. As an example, we will discuss the use of a sub-THz spectroscopy system for the study of the evolution of hyperglycemia in animal models and humans.
Microwave Vector Measurements
III-V Terahertz MMIC Design and Applications for 6G Communications and Radar
This talk will summarize the challenges for III-V Terahertz MMIC design and realization emphasizing on future applications in 6G communications and radar. The talk will go into detail on routes for MIMO systems and 2D arrays and potential pitfalls and difficulties. In addition, the talk will briefly discuss the packaging and interconnect technologies for the frequencies beyond 100 GHz.
Yahya M. Meziani
2D material based devices for terahertz detection
Nils G. Weimann
InP-based electronic THz components
Two-Dimensional Materials For Gas Sensing Applications
Gas sensing is a rapidly developing technology that enhances living standards, security, and medical diagnosis. Commercial sensors are popular resistive gas sensors made of metal oxides (e.g., SnO2, WO3, TiO2, NiO) and were proposed a few decades ago. These sensors operate at elevated temperatures and require at least tenmWfor continuous operation. Two-dimensional (2D) materials (e.g., graphene oxide, phosphorene, MoS2, WS2) can reach better results of gas sensing than metal oxides. It is because of theirtremendoussurface-to-volume ratio and gas sensitivity at room temperature. These sensors require power for bias only, evenup to a few mW.Their sensitivity can reach a level of single moleculesand can be enhanced by modulation techniques (e.g., UV light irradiation, surface functionalization, bias conditions) or flicker noise measurements.
In this talk, recent results of gas sensing by selected 2D materials are presented. We discuss the supposed development of gas sensors with 2D materials (e.g., graphene back-gated FET, sensing layer of graphene flakes decorated with TiO2 nanoparticles)and applied detection algorithm. We present the possibility of detecting gas mixture components by using a single resistive gas sensor. Moreover, we show the results of gas sensing under UV irradiation by a fluctuation enhanced gas sensing method utilizing low-frequency noise as an additional source of information about the ambientof the gas sensor.
Thz Resonant-Tunnelling Diodes, Oscillators, Detectors, Applications
The presentation will give an overview on the present status of the resonant-tunnelling diodes (RTDs) in the area of THz electronics. The contemporarystatus of RTD oscillators will be discussed, in particular: the level of the output power and operating frequencies of RTD oscillators;overview on the different types of RTD oscillators, their advantages and disadvantages; tunability and frequency stability of the RTD oscillators. Use of RTDs as THz detectors will be also discussed, in particular: operating principles, limitations, application examples. Further on, an overview on the application examples of RTDs will be given: RTDs oscillators in high-data-rate wireless transmission systems, imaging applications, spectroscopy, etc.
Deep learning in microwave design
In the talk, we use an artificial intelligence for the design and the analysis of selected planar microwave components (filters, antennas). When analyzing, edges of a layout are identified and depending on the shape of the structure, the microwave component is classified. A database of classified elements is assumed to be created. When designing, a proper prototype component can be selected from the database to meet required parameters roughly. A better match can be reached consequently using a proper optimization. That way, simple microwave components can be designed automatically.
Syed Umer Abbas Shah
Micromachining for Terahertz Frequencies
Micromachined devices, also known as microelectromechanical systems (MEMS), are manufactured in billions of devices per year and at an extremely low cost. Such large-scale usage is enabled by the inherent advantages of micromachining as a fabrication technology. The advantages include very low-cost, excellent product uniformity, very high level of integration and miniaturization, batch fabrication and volume manufacturing capability. In contrast, device fabrication at terahertz frequencies still relies on CNC-milling as a packaging and integration technology, which is sequential and not scalable either. This talk summarizes the state of the art in silicon micromachining, discusses advantages and disadvantages and describes several millimeter-wave and submillimeter-wave devices implemented in silicon micromachined waveguide technology.
Principles of detection in the THz frequency range
Terahertz frequency range is still referred as being the one of the least exploited spectrum range of electromagnetic radiation. It is located between the millimeter waves and infrared with a loosely defined span from about 300 GHz to 10 THz. During the last two decades, there was a noticeable scientific interest devoted for the development of THz devices and techniques which resulted in the invention of novel sources and detectors.
The lecture will give an overview of different, novel as well as well-established detection schemes which are used to detect THz radiation and will address to the underlying physical principles, their potentials, fields of applications and limitations. It will start from the introduction to a thermal detection principle and how this principle can be applied to implement detectors operating at room temperature as well as at cryogenic temperatures and to be “trimmed” to be able to detect just few THz photons. Special attention will be given to detectors based on electric nonlinear characteristics such as Schottky diodes and field-effect-transistor based detectors (TeraFETs). Furthermore, there will be addressed the main differences between the power and amplitude detection and the state-of-the-art devices used for detection of single THz photons.
Graphene In Field Effect Transistors And Thz Detectors
Graphene is an excellent material to implement in field effect transistors (FETs) and FET THz detectors. The wafer scale fabrication of graphene on up to 8” wafers allows to create large arrays of GFETs with mobilities up to 3500 cm2/Vs. The on-wafer DC characterization allows to extract the mobility and contact resistances using the designated test structures. The fabrication process of GFETs is tailored so that the graphene is encapsulated under the layer of gate dielectric through the fabrication process. The design of graphene-based THz detectors includes the antenna integration with the transistor and tuning the response to combine the plasmonic mixing with thermoelectric rectification. The best GFET THz detector voltage responsivity is 74 V/W.
Time Modulated Antenna Array as Effective Smart Beamforming in THz
Intelligent (smart) or adaptive antennas are the most suitable for wireless communication, especially for fifth generation and higher communication systems. The key property of intelligent technology is the ability to respond automatically by changing an appropriate radiation pattern. Phase-array based smart antennas are used as the main beamforming structure. The development and application of the phase-array in THz frequency range is very problematic. A TIME–MODULATED antenna array (TMAA) can be used as a cheaper alternative.
TMAA is based on periodical ON/OFF switching of signals received/transmitted from/to each antenna array element; hence, continuous wave signals are modulated to pulsed RF signals. The spectrum of a signal after time–modulation is composed of a carrier component and harmonic components (sidebands). When a TMAA is used to receive a signal at the carrier frequency f0, and the switching frequency fp ≪ f0, sideband components will appear in the receiver. The carrier component can be used for sidelobe reduction, while siedebands are suitable for beam–scanning. The advantage of TMAAs lies in beamforming, which is achieved with switches instead of phase–shifters. RF switches based on semiconductors can be low–cost and high power handling components operating in high frequency range. This advantage might be a key factor enabling TMAAs to be a low–cost solution applicable to future intelligent antenna systems for mm–wave communication. RF switches, which use a combination of graphene and two-dimensional high-density electron gas (2DEG) in the AlGaN/GaN system, were proposed and studied. The switches were integrated into the coplanar waveguide, which allows them to be used in any system without the use of, e.g., bonding, flip-chip and other technologies and avoiding the matching problems. The use of such a switch can provide up to 20 MHz of bandwidth in time-modulated systems, which is an outstanding result for such systems.
Teodora – Nicoleta Cremene
Publishing with MDPI Journal— Micromachines
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Hartmut G. Roskos
Strong coupling of metamaterials in a photonic crystal cavity: A classical plasmonic playground to emulate strong coupling in quantum optical systems
We investigate (ultra)strong coupling phenomena of planar metallic metamaterials placed into an external dielectric cavity, which consist of silicon slabs separated by air spacers. The presentation first introduces basic one-photon–one-plasmon and two-photon–one-plasmon coupling scenarios with simple metamaterials and their electrodynamically complementary structures [1, 2]. Then, we extend the studies in two directions. First, we will discuss metamaterials, which exhibit electromagnetically induced transparency (EIT) . The unit cell of these metamaterials consists of interacting pairs of split-ring resonators (SRRs), which are rotated by 90° relative to each other (see top of Fig. 1a). If electromagnetic radiation, polarized as shown in Fig. 1a, is tuned to the resonance of the left SRR, then this SRR is excited, while the right one is not (dark mode). If one now places the metamaterial into a 1D dielectric cavity consisting of several silicon slabs as shown in Fig. 1a, with the slabs and the spacers chosen for the cavity resonance frequency matching the resonance of the bright mode, then one observes in transmission measurements the occurrence of four polariton modes. This is corroborated by simulations, as shown in the bottom part of Fig. 1a. Here, the size of the right SRR is varied, and with it the resonance frequency of the dark mode, which yields four polariton branches (two upper polariton branches, UP1 and UP2, and two lower ones, LP1 and LP2) which exhibit three anti-crossing features. Interestingly, the same study with a metamaterial, which has the right SRR in the unit cell back-rotated to the orientation of the left SRR, produces only three polariton modes. We will show that this difference is the result of different coupling among the photons of the cavity and the plasmons of the SRRs which in the first case is hierarchical, and in the second case is not. The second topic of discussion is devoted to dynamical aspects of cavity-coupled systems .
Here, we investigate a complementary Swiss-cross metamaterial in a two-slab dielectric cavity (see metamaterial unit cell and cavity arrangement in the top part of Fig. 1b). Visible light pulses from an amplifier laser  excite charge carriers in the silicon slab which carries the metamaterial, rendering the silicon in the free space of the Swiss cross elements electrically conductive, and thus abruptly – on a time scale of 100 fs – switching off the Swiss-cross plasmon resonance. The bottom part of Fig. 1b displays the measured decay of the two polariton modes of the vacuum ground-state. Although the coupling is switched off with a subcycle speed, it requires about one oscillation cycle for the polariton modes to collapse to the pure cavity mode. Closer inspection reveals that the dynamics of the upper and lower polariton branches differ. While the upper polariton decreases in frequency and merges into the cavity mode, the second polariton shows a rather abrupt collapse.
Upon variation of the arrival time of the visible pump pulse relative to the THz probe pulse, one identifies additional intriguing differences (data not shown). The probe pulse exhibits a beat-note with a period of 8 ps, which is the temporal fingerprint of the coherent superposition of the oscillations of the two polariton modes. A node (zero-crossing) of the beat-note corresponds to the energy of the coupled modes residing for that moment in the plasmons of the complementary metamaterial, while an anti-node (peak amplitude of the field envelope) sees the energy mainly in the radiation field. If the pump pulse arrives at an antinode, the beat-note disappears and the frequency shifts to that of the cavity mode, and this without much loss of amplitude and hence little attenuation. In contrast, when the optical pulse arrives at a node, one observes an instantaneous and strong loss of amplitude, and the antinode, which should follow a half-cycle later, cannot rebuild. The loss of amplitude is a consequence of the coupled-mode energy just then being mostly concentrated in the metamaterial plasmons. As the optical excitation destroys the constituent plasmon mode, its energy content is mostly dissipated by electrical shock currents in the metamaterial and the mobile carriers generated in the silicon.
Beyond the emulation of quantum-optical systems, such double-resonator (metamaterial plus external resonator) approaches as introduced here offer application options to be explored further in their own right. One of them, which we will briefly address in the presentation, is the modification and potential improvement of metamaterialbased chemical sensors , another one the enforcement of coherence between semiconductor-based oscillators and lasers.
Acknowledgements: This research is funded by the Deutsche Forschungsgemeinschaft, project RO 770/46-1.
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