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Advances in modern semiconductor integrated circuits have always demanded faster and more sensitive analytical methods on a large-scale wafer. The surface of wafers is fundamentally essential to start building circuits, and quantitative measures of the surface potential, defects, contamination, passivation quality, and uniformity are subject to inspection. The present study provides a new approach to access those by means of terahertz (THz) emission spectroscopy. Upon femtosecond laser illumination, THz radiation, which is sensitive to the surface electric fields of the wafer, is generated. Here, we systematically research the THz emission properties of silicon surfaces under different surface conditions, such as the initial surface with a native oxide layer, a fluorine-terminated surface, and a hydrogen-terminated surface. Meanwhile, a strong doping concentration dependence of the THz emission amplitude from the silicon surface has been revealed in different surface conditions, which implies a semiquantitative connection between the THz emission and the surface band bending with the surface dipoles. Laser-induced THz emission spectroscopy is a promising method for evaluating local surface properties on a wafer scale.
Long-term stable excitons, bound pairs of electrons and holes, in van der Waals materials were found to be handled at room temperature, paving the way to develop optoelectronic/photonic devices for future high-speed communication technology. To miniaturize and integrate such functions to achieve highly efficient excitonic devices, understanding and manipulation of exciton dynamics in the nanoscale structures is absolutely essential. Since the spatial resolution of the optical techniques, which have been mainly used in the research of this field, is limited to μm to several tens of nm, the introduction of new technology is strongly desired. Here, we demonstrate a method to probe and visualize neutral excitons using the laser-combined multiprobe scanning tunneling microscopy (STM) system. Ultrafast dynamics of excitons in the nanostructures produced in an in-plane WS 2 /WSe 2 heterostructure, such as dynamics in the ps region of many-body effects under high density and spatial variation in the effect of local defects on lifetime, was successfully revealed with a spatial resolution of 1 nm order. This method is expected to accelerate research on exciton dynamics and the development of applications directly based on the experimental results of nanoscale.
In our study, we determine the alignment of magnetic domains in a CoFeB layer using THz radiation. We generate THz pulses by fs laser pulses in magnetized CoFeB/Pt heterostructures based on spin currents. An LT-GaAs Auston switch detects the radiation phase sensitively and allows us to determine the magnetization alignment. Our scanning technique with motorized stages, with step sizes in the sub-micrometer range, allows us to image two dimensional magnetic structures. Theoretically, the resolution is restricted to half of the wavelength if focusing optics in the far-field limit are used. By applying near-field imaging, the spatial resolution is enhanced to the single digit micrometer range. For this purpose, spintronic emitters in diverse geometric shapes, e.g., circles, triangles, squares, and sizes are prepared to observe the formation of magnetization patterns. The alignment of the emitted THz radiation can be influenced by applying unidirectional external magnetic fields. We demonstrate how magnetic domains with opposite alignment and different shapes divided by domain walls are created by demagnetizing the patterns using minor loops and imaged using phase sensitive THz radiation detection. For analysis, the data are compared to Kerr microscope images. The possibility of combining this method with THz range spectroscopic information of magnetic texture or antiferromagnets in direct vicinity to the spintronic emitter makes this detection method interesting for a much wider range of applications probing THz excitation in spin systems with high resolution beyond the Abbe diffraction limit, limited solely by the laser excitation area.
According to quantum field theory, empty space—the ground state with all real excitations removed—is not empty, but filled with quantum-vacuum fluctuations. Their presence can manifest itself through phenomena such as the Casimir force, spontaneous emission, or dispersion forces. These fluctuating fields possess correlations between space-time points outside the light cone, i.e. points causally disconnected according to special relativity. As a consequence, two initially uncorrelated quantum objects in empty space which are located in causally disconnected space-time regions, and therefore unable to exchange information, can become correlated. Here, we have experimentally demonstrated the existence of correlations of the vacuum fields for non-causally connected space-time points by using electro-optic sampling. This result is obtained by detecting vacuum-induced correlations between two 195 fs laser pulses separated by a time of flight of 470 fs. This work marks a first step in analyzing the space-time structure of vacuum correlations in quantum field theory. There is interest in measuring the vacuum fluctuations of electromagnetic radiation. Here the authors demonstrate quantum-vacuum electric field correlations between two regions which are outside the light-cone.
Spin transport is crucial for future spintronic devices operating at bandwidths up to the terahertz range. In F|N thin-film stacks made of a ferromagnetic/ferrimagnetic layer F and a normal-metal layer N, spin transport is mediated by (1) spin-polarized conduction electrons and/or (2) torque between electron spins. To identify a crossover from (1) to (2), we study laser-driven spin currents in F|Pt stacks where F consists of model materials with different degrees of electrical conductivity. For the magnetic insulators yttrium iron garnet, gadolinium iron garnet (GIG) and γ-Fe2O3, identical dynamics is observed. It arises from the terahertz interfacial spin Seebeck effect (SSE), is fully determined by the relaxation of the electrons in the metal layer, and provides a rough estimate of the spin-mixing conductance of the GIG/Pt and γ-Fe2O3/Pt interfaces. Remarkably, in the half-metallic ferrimagnet Fe3O4 (magnetite), our measurements reveal two spin-current components with opposite direction. The slower, positive component exhibits SSE dynamics and is assigned to torque-type magnon excitation of the A- and B-spin sublattices of Fe3O4. The faster, negative component arises from the pyrospintronic effect and can consistently be assigned to ultrafast demagnetization of minority-spin hopping electrons. This observation supports the magneto-electronic model of Fe3O4. In general, our results provide a route to the contact-free separation of torque- and conduction-electron-mediated spin currents.
We report on a time-domain polarimetry (TDP) system for generating and detecting broadband terahertz (THz) waves of different polarization angles. We generate THz waves from two-color laser filaments and determine their polarization states with a detection bandwidth of up to 8 THz using a spinning gallium phosphide crystal. The polarization of THz emission can be controlled by adjusting the position and tilt angle of the β-barium borate crystal. We characterize the precision of this system for polarimetric measurements at fixed time delay to be [Formula: see text] and [Formula: see text] for complete time-domain waveforms. We also demonstrate the feasibility of our TDP system by measuring broadband optical properties of anisotropic samples in both transmission and reflection geometries. The THz-TDP technique can be easily integrated in conventional THz time-domain spectroscopy setups using nonlinear crystal detectors.
Spintronic terahertz emitters are broadband and efficient sources of terahertz radiation, which emerged at the intersection of ultrafast spintronics and terahertz photonics. They are based on efficient spin-current generation, spin-to-charge-current conversion, and current-to-field conversion at terahertz rates. In this Editorial, we review the recent developments and applications, the current understanding of the physical processes, and the future challenges and perspectives of broadband spintronic terahertz emitters.
Diabetic foot syndrome, a long term consequence of Diabetes Mellitus, is the most common cause of non-traumatic amputations. Around 8% of the world population suffers from diabetes, 15% of diabetic patients present a diabetic foot ulcer which leads to amputation in 2.5% of the cases. There is no objective method for the early diagnosis and prevention of the syndrome and its consequences. We test terahertz imaging, which is capable of mapping the cutaneous hydration, for the evaluation of the diabetic foot deterioration as an early diagnostic test as well as ulcers prevention and tracking tool. Furthermore, the analysis of our terahertz measurements combined with neurological and vascular assessment of the patients indicates that the dehydration is mainly related to the peripheral neuropathy without a significant vascular cause.
Terahertz (THz) communications have emerged as a promising candidate to support the heavy data traffic and exploding network capacity in the future 6G wireless networks. However, THz communications face many challenges for practical implementation, such as propagation loss, signal blockage, and hardware cost. In this article, an emerging paradigm of intelligent reflecting surface (IRS)-assisted THz communications is analyzed to address the above issues by leveraging joint active and passive beamforming to enhance the communication quality and reduce overhead. Aimed at practical implementation, an overview of the currently available approaches of realizing THz active/passive beam steering at the transmitter and IRS is presented. Based on these approaches, a beam training strategy for establishing joint beamforming is then investigated in THz communications. Moreover, various emerging and appealing 6G scenarios that integrate IRS into THz communications are envisioned. Open challenges and future research directions for this new paradigm are finally highlighted.
Significance
The Pt(111)–electrolyte interface is one of the most important model interfaces in physical electrochemistry. Here, we present detailed capacitance measurements of Pt(111) in contact with nonspecifically adsorbed electrolytes in combination with modeling to show that this interface does not follow classic Gouy–Chapman–Stern behavior. Our combined experimental-theoretical work suggests a picture of the double layer of Pt(111) with an increased ion concentration close to the interface as a consequence of a weak attractive ion–surface interaction and a changing polarizability of the Pt(111)–water interface due to the potential-dependent water adsorption and orientation.
The accuracy of clinical assessment techniques in diagnosing partial-thickness burn injuries has remained as low as 50–76%. Depending on the burn depth and environmental factors in the wound, such as reactive oxygen species, inflammation, and autophagy, partial-thickness burns can heal spontaneously or require surgical intervention. Herein, it is demonstrated that terahertz time-domain spectral imaging (THz-TDSI) is a promising tool for in vivo quantitative assessment and monitoring of partial-thickness burn injuries in large animals. We used a novel handheld THz-TDSI scanner to characterize burn injuries in a porcine scald model with histopathological controls. Statistical analysis (n = 40) indicates that the THz-TDSI modality can accurately differentiate between partial-thickness and full-thickness burn injuries (1-way ANOVA, p < 0.05). THz-TDSI has the potential to improve burn care outcomes by helping surgeons in making objective decisions for early excision of the wound.
In the field of optical science, it is becoming increasingly important to observe and manipulate matter at the atomic scale using ultrashort pulsed light. For the first time, we have performed the ab initio simulation solving the Maxwell equation for light electromagnetic fields, the time-dependent Kohn-Sham equation for electrons, and the Newton equation for ions in extended systems. In the simulation, the most time-consuming parts were stencil and nonlocal pseudopotential operations on the electron orbitals as well as fast Fourier transforms for the electron density. Code optimization was thoroughly performed on the Fujitsu A64FX processor to achieve the highest performance. A simulation of amorphous SiO 2 thin film composed of more than 10,000 atoms was performed using 27,648 nodes of the Fugaku supercomputer. The simulation achieved excellent time-to-solution with the performance close to the maximum possible value in view of the memory bandwidth bound, as well as excellent weak scalability.
Featured Application
Compact lasers integrated onto SOI for optical communications.
Abstract
The tremendous demand for low-cost, low-consumption and high-capacity optical transmitters in data centers challenges the current InP-photonics platform. The use of silicon (Si) photonics platform to fabricate photonic integrated circuits (PICs) is a promising approach for low-cost large-scale fabrication considering the CMOS-technology maturity and scalability. However, Si itself cannot provide an efficient emitting light source due to its indirect bandgap. Therefore, the integration of III-V semiconductors on Si wafers allows us to benefit from the III-V emitting properties combined with benefits offered by the Si photonics platform. Direct epitaxy of InP-based materials on 300 mm Si wafers is the most promising approach to reduce the costs. However, the differences between InP and Si in terms of lattice mismatch, thermal coefficients and polarity inducing defects are challenging issues to overcome. III-V/Si hetero-integration platform by wafer-bonding is the most mature integration scheme. However, no additional epitaxial regrowth steps are implemented after the bonding step. Considering the much larger epitaxial toolkit available in the conventional monolithic InP platform, where several epitaxial steps are often implemented, this represents a significant limitation. In this paper, we review an advanced integration scheme of AlGaInAs-based laser sources on Si wafers by bonding a thin InP seed on which further regrowth steps are implemented. A 3 µm-thick AlGaInAs-based MutiQuantum Wells (MQW) laser structure was grown onto on InP-SiO2/Si (InPoSi) wafer and compared to the same structure grown on InP wafer as a reference. The 400 ppm thermal strain on the structure grown on InPoSi, induced by the difference of coefficient of thermal expansion between InP and Si, was assessed at growth temperature. We also showed that this structure demonstrates laser performance similar to the ones obtained for the same structure grown on InP. Therefore, no material degradation was observed in spite of the thermal strain. Then, we developed the Selective Area Growth (SAG) technique to grow multi-wavelength laser sources from a single growth step on InPoSi. A 155 nm-wide spectral range from 1515 nm to 1670 nm was achieved. Furthermore, an AlGaInAs MQW-based laser source was successfully grown on InP-SOI wafers and efficiently coupled to Si-photonic DBR cavities. Altogether, the regrowth on InP-SOI wafers holds great promises to combine the best from the III-V monolithic platform combined with the possibilities offered by the Si photonics circuitry via efficient light-coupling.
Beam forming and beam steering are inevitable technologies for the practical application of high-frequency electromagnetic waves. Specifically, beam control technology using a phased array for terahertz waves above 100 GHz is necessary to realize the future of high-speed wireless communication. By photomixing, which is a promising method for generating terahertz waves, the phase of the generated waves can be tuned in the optical domain, so that the beam from the phased array can be controlled by photonics technologies. Directing the beam of a terahertz wave enables wireless communication to be improved not only via an increase in power efficiency but also in security in the physical layer of the wireless transmission. By utilizing this advantage and using coherent detection at the receiver, a secured wireless communication system is proposed, and the fundamental mechanism is demonstrated in a feasibility experiment.
The effect of cavities or plates upon the electromagnetic quantum vacuum are considered in the context of electro-optic sampling, revealing how they can be directly studied. These modifications are at the heart of e.g. the Casimir force or the Purcell effect such that a link between electro-optic sampling of the quantum vacuum and environment-induced vacuum effects is forged. Furthermore, we discuss the microscopic processes underlying electro-optic sampling of quantum-vacuum fluctuations, leading to an interpretation of these experiments in terms of exchange of virtual photons. With this in mind it is shown how one can reveal the dynamics of vacuum fluctuations by resolving them in the frequency and time domains using electro-optic sampling experiments.
Metamaterials enable precise tailoring of light–matter interactions at the subwavelength scale, permitting access to the full range of electromagnetic responses encoded in Maxwell’s equations and operating across a huge swath of the electromagnetic spectrum. These salient features have, during the past two decades, fuelled fundamental metamaterials research which, in turn, has unveiled an impressive assemblage of potential applications. Imaging is at the forefront of these developments, leveraging the ability of metamaterials to achieve arbitrary, specific and tunable scattering responses that are poised for integration with a host of materials and devices. We review the impact of metamaterials and metasurface on imaging science and technology from microwave to optical frequencies. Our focus is on metamaterial-based imaging components and the associated imaging modalities that benefit from these advances.
Atomically precise electronics operating at optical frequencies require tools that can characterize them on their intrinsic length and time scales to guide device design. Lightwave-driven scanning tunnelling microscopy is a promising technique towards this purpose. It achieves simultaneous sub-ångström and sub-picosecond spatio-temporal resolution through ultrafast coherent control by single-cycle field transients that are coupled to the scanning probe tip from free space. Here, we utilize lightwave-driven terahertz scanning tunnelling microscopy and spectroscopy to investigate atomically precise seven-atom-wide armchair graphene nanoribbons on a gold surface at ultralow tip heights, unveiling highly localized wavefunctions that are inaccessible by conventional scanning tunnelling microscopy. Tomographic imaging of their electron densities reveals vertical decays that depend sensitively on wavefunction and lateral position. Lightwave-driven scanning tunnelling spectroscopy on the ångström scale paves the way for ultrafast measurements of wavefunction dynamics in atomically precise nanostructures and future optoelectronic devices based on locally tailored electronic properties. Here, the authors perform lightwave-driven terahertz scanning tunnelling microscopy and spectroscopy of graphene nanoribbons with atomic resolution in three dimensions, revealing localized wavefunctions that are inaccessible by conventional scanning tunnelling microscopy.
Near-field microscopy discloses a peculiar potential to explore novel quantum state of matter at the nanoscale, providing an intriguing playground to investigate, locally, carrier dynamics or propagation of photoexcited modes as plasmons, phonons, plasmon-polaritons or phonon-polaritons. Here, we exploit a combination of hyperspectral time domain spectroscopy nano-imaging and detectorless scattering near-field optical microscopy, at multiple terahertz frequencies, to explore the rich physics of layered topological insulators as Bi 2 Se 3 and Bi 2 Te 2.2 Se 0.8 , hyperbolic materials with topologically protected surface states. By mapping the near-field scattering signal from a set of thin flakes of Bi 2 Se 3 and Bi 2 Te 2.2 Se 0.8 of various thicknesses, we shed light on the nature of the collective modes dominating their optical response in the 2-3 THz range. We capture snapshots of the activation of transverse and longitudinal optical phonons and reveal the propagation of sub-diffractional hyperbolic phonon-polariton modes influenced by the Dirac plasmons arising from the topological surface states and of bulk plasmons, prospecting new research directions in plasmonics, tailored nanophotonics, spintronics and quantum technologies.
The exploitation of ultrafast electron dynamics in quantum cascade lasers (QCLs) holds enormous potential for intense, compact mode-locked terahertz (THz) sources, squeezed THz light, frequency mixers, and comb-based metrology systems. Yet the important sub-cycle dynamics have been notoriously difficult to access in operational THz QCLs. Here, we employ high-field THz pulses to perform the first ultrafast two-dimensional spectroscopy of a free-running THz QCL. Strong incoherent and coherent nonlinearities up to eight-wave mixing are detected below and above the laser threshold. These data not only reveal extremely short gain recovery times of 2 ps at the laser threshold, they also reflect the nonlinear polarization dynamics of the QCL laser transition for the first time, where we quantify the corresponding dephasing times between 0.9 and 1.5 ps with increasing bias currents. A density-matrix approach reproducing the emergence of all nonlinearities and their ultrafast evolution, simultaneously, allows us to map the coherently induced trajectory of the Bloch vector. The observed high-order multi-wave mixing nonlinearities benefit from resonant enhancement in the absence of absorption losses and bear potential for a number of future applications, ranging from efficient intracavity frequency conversion, mode proliferation to passive mode locking.
Thanks to the abrupt advances in semiconductor technologies, particularly in terms of the operating frequency, the last decade has seen various efforts and trials in attempts to achieve high-throughput wireless communications at THz frequencies. Based on several potential device technologies, not only III-V heterojunction bipolar and high-electron-mobility transistors but also with silicon CMOSs and photonic technologies, high data rates of 100 Gbps or more have been successfully demonstrated so far. Meanwhile, the next generation of wireless communications systems, so called 6G, is already being discussed in research communities worldwide. A terabit per second peak data rate is envisioned, which essentially requires huge spectral resources. This recent great attention on 6G communications is implicitly going to THz communications but is more than a technical curiosity unlike the past. In this article, we briefly review the progress of THz communications made in the last decade and discuss some of the most challenging issues that need to be overcome for future wireless systems at THz frequencies, including not only the 6G network but also short-range connectivity and fixed wireless links
Passive microwave satellite observations provide critical information for global forecast models, particularly in cloudy and/or precipitating conditions. The limited temporal sampling provided by current operational polar orbiters cannot capture rapidly changing conditions such as the development of convective storms. This is a significant issue for open-ocean weather systems such as tropical cyclones and hurricanes that can only be effectively monitored from satellites. The recent development and demonstration of miniaturized microwave radiometers on-board low-cost CubeSat satellites has the potential to dramatically improve the temporal and spatial sampling of all-sky microwave observations by deploying a substantial constellation of satellites in low Earth orbit. Two constellations of 60 CubeSats in 550 km orbits are compared to the current operational microwave sensors. One approach employs all polar orbiters, while the other approach uses multiple inclination orbits for increased sampling over convective storm regions. Both approaches reduce average revisit times to approximately ~20-30 minutes globally, and the multi-inclination approach also provides irregular 5-10 minute sampling over selected latitudes. Improved global temporal sampling would provide all-sky observations to global forecast models over rapidly-changing environments, while millimeter-wave observations over convective storm regions would be valuable for both forecasting and studying the development of convective storms. This study demonstrated that a constellation of low-cost CubeSats with microwave radiometers has the potential to provide equivalent temporal resolution to that observed from sensors on geostationary orbit.
Significance
For centuries the double layer at the solid/electrolyte interface has been a central concept in electrochemistry. Today, it is still crucial for virtually all renewable energy storage and conversion technologies. Here, the double-layer formation is probed by THz spectroscopy with ultrabright synchrotron light as a source. Our results capture the molecular details of double-layer formation at positively/negatively charged Au electrodes for an NaCl electrolyte. We reveal a contrasting response applying positive versus negative bias, which is dictated by the interfacial water network and rationalized by accompanying molecular dynamics simulations and electronic-structure calculations. While Na ⁺ is directly attracted toward the negatively charged electrode, stripping of the Cl ⁻ hydration shell is observed only at larger potential values.
A linear scanning system utilizing constant wave 280 GHz radiation has been developed and characterized. The system comprises a linear array of detectors based on a unique plasma wave approach in terahertz sensing, an impact ionization avalanche transit-time-diode signal generator coupled to a frequency multiplier and an optical system. The performed tests allowed us to estimate the resolution of the system reaching the value of 2.3 mm and to determine the dynamic range of the system to be around 200. The imaging capabilities of the scanner were tested in realistic cases of non-destructive testing and security screening.
The New Space Era has increased communication traffic in space by new space missions led by public space agencies and private companies. Mars colonization is also targeted by crewed missions in the near future. Due to increasing space traffic near Earth and Mars, the bandwidth is getting congested. Moreover, the downlink performance of the current missions is not satisfactory in terms of delay and data rate. Therefore, to meet the increasing demand in space links, Terahertz band (0.1-10 THz) wireless communications are proposed in this study. In line with this, we discuss the major challenges that the realization of THz band space links pose and possible solutions. Moreover, we simulate Mars-space THz links for the case of a clear Mars atmosphere, and a heavy dust storm to show that even in the worst conditions, a large bandwidth is available for Mars communication traffic.
Because of the ultrafast and photon-driven nature of the transport in their active region, we demonstrate that quantum cascade lasers can be operated as resonantly amplified terahertz detectors with wide RF bandwidth. Tunable responsivities up to 50 V/W and noise equivalent powers down to 100 pW/Hz1/2 are demonstrated at 4.7 THz. Constant peak responsivities with respect to the detector temperature are observed up to 80 K. Thanks to the ≈ps intersub-band lifetime, electrical bandwidths larger than 20 GHz can be obtained, allowing the detection of optical beatnotes from quantum cascade THz frequency combs.
The scattering-type scanning near-field optical microscope (s-SNOM) has emerged as a powerful tool for resolving nanoscale inhomogeneities in laterally heterogeneous samples. However, most analytical models used to predict the scattering near-field signals are assuming homogenous landscapes (bulk materials), resulting in inconsistencies when applied to samples with more complex configurations. In this work, we combine the point-dipole model (PDM) to the finite-element method (FEM) to account for the lateral and vertical heterogeneities while keeping the computation time manageable. Full images, spectra, or hyperspectral line profiles can be simulated by calculating the self-consistent dipole radiation demodulated at higher harmonics of the tip oscillation, mimicking real experimental procedures. Using this formalism, we clarify several important yet puzzling experimental observations in near-field images on samples with rich typography and complex material compositions, heterostructures of two-dimensional material flakes, and plasmonic antennas. The developed method serves as a basis for future investigations of nano-systems with nontrivial topography.
We present a high-power source of broadband terahertz (THz) radiation covering the whole THz spectral region (0.1–30 THz). The two-color gas plasma generation process is driven by a state-of-the-art ytterbium fiber chirped pulse amplification system based on coherent combination of 16 rod-type amplifiers. Prior to the THz generation, the pulses are spectrally broadened in a multipass cell and compressed to 37 fs with a pulse energy of 1.3 mJ at a repetition rate of 500 kHz. A gas-jet scheme has been employed for the THz generation, increasing the efficiency of the process to 0.1%. The air-biased coherent detection scheme is implemented to characterize the full bandwidth of the generated radiation. A THz average power of 640 mW is generated, which is the highest THz average power achieved to date. This makes this source suitable for a variety of applications, e.g., spectroscopy of strongly absorbing samples or driving nonlinear effects for the studies of material properties.
Topologically protected surface states present rich physics and promising spintronic, optoelectronic, and photonic applications that require a proper understanding of their ultrafast carrier dynamics. Here, we investigate these dynamics in topological insulators (TIs) of the bismuth and antimony chalcogenide family, where we isolate the response of Dirac fermions at the surface from the response of bulk carriers by combining photoexcitation with below-bandgap terahertz (THz) photons and TI samples with varying Fermi level, including one sample with the Fermi level located within the bandgap. We identify distinctly faster relaxation of charge carriers in the topologically protected Dirac surface states (few hundred femtoseconds), compared to bulk carriers (few picoseconds). In agreement with such fast cooling dynamics, we observe THz harmonic generation without any saturation effects for increasing incident fields, unlike graphene which exhibits strong saturation. This opens up promising avenues for increased THz nonlinear conversion efficiencies, and high-bandwidth optoelectronic and spintronic information and communication applications.
Gallium nitride (GaN) is one of the most technologically important semiconductors and a fundamental component in many optoelectronic and power devices. Low-resistivity GaN wafers are in demand and actively being developed to improve the performance of vertical GaN power devices necessary for high-voltage and high-frequency applications. For the development of GaN devices, nondestructive characterization of electrical properties particularly for carrier densities in the order of 1019 cm−3 or higher is highly favorable. In this study, we investigated GaN single crystals with different carrier densities of up to 1020 cm−3 using THz time-domain ellipsometry in reflection configuration. The p- and s-polarized THz waves reflected off the GaN samples are measured and then corrected based on the analysis of multiple waveforms measured with a rotating analyzer. We show that performing such analysis leads to a ten times higher precision than by merely measuring the polarization components. As a result, the carrier density and mobility parameters can be unambiguously determined even at high conductivities.
The properties of the water network in concentrated HCl acid pools in nanometer‐sized reverse nonionic micelles were probed with TeraHertz absorption, dielectric relaxation spectroscopy, and reactive force field simulations capable of describing proton hopping mechanisms. We identify that only at a critical micelle size of W0=9 do solvated proton complexes form in the water pool, accompanied by a change in mechanism from Grotthuss forward shuttling to one that favors local oscillatory hopping. This is due to a preference for H⁺ and Cl⁻ ions to adsorb to the micelle interface, together with an acid concentration effect that causes a “traffic jam” in which the short‐circuiting of the hydrogen‐bonding motif of the hydronium ion decreases the forward hopping rate throughout the water interior even as the micelle size increases. These findings have implications for atmospheric chemistry, biochemical and biophysical environments, and energy materials, as transport of protons vital to these processes can be suppressed due to confinement, aggregation, and/or concentration.
Tin-iodide perovskites are an important group of semiconductors for photovoltaic applications, promising higher intrinsic charge-carrier mobilities and lower toxicity than their lead-based counterparts. Controllable tin vacancy formation and the ensuing hole doping provide interesting opportunities to investigate dynamic intraband transitions of charge carriers in these materials. Here, we present for the first time an experimental implementation of a novel Optical-Pump–IR-Push–THz-Probe spectroscopic technique and demonstrate its suitability to investigate the intraband relaxation dynamics of charge carriers brought into nonequilibrium by an infrared “push” pulse. We observe a push-induced decrease of terahertz conductivity for both chemically- and photodoped FA0.83Cs0.17SnI3 thin films and show that these effects derive from stimulated THz emission. We use this technique to reveal that newly photogenerated charge carriers relax within the bands of FA0.83Cs0.17SnI3 on a subpicosecond time scale when a large, already fully thermalized (cold) population of charge-carriers is present. Such rapid dissipation of the initial charge-carrier energy suggests that the propensity of tin halide perovskites toward unintentional self-doping resulting from tin vacancy formation makes these materials less suited to implementation in hot-carrier solar cells than their lead-based counterparts.
Spin–charge conversion is a spin–orbit coupling phenomena where electrical currents can generate transverse spin currents and vice versa. It is one of the central topics in spintronics and widely applied to manipulate the spin and charge degrees of freedom in materials. Previous research on spin–charge conversion was mainly carried out by transport measurements, which lies in the (quasi)equilibrium and DC/low-frequency limit. The recent development of THz emission spectroscopy applied to this field provides additional insights into the dynamics of the spin–charge conversion process, i.e., its ultrafast timescales. Here, the underlying physics and the latest progress of THz studies on spintronics are introduced. The technical details and some features of this technique are summarized, including spin current generation, signal detection, and data analysis. Finally, some possible developments are discussed as well as future research and applications.
By sampling terahertz waveforms emitted from InAs surfaces, we reveal how the entire, realistic geometry of typical near-field probes drastically impacts the broadband electromagnetic fields. In the time domain, these modifications manifest as a shift in the carrier-envelope phase and emergence of a replica pulse with a time delay dictated by the length of the cantilever. This interpretation is fully corroborated by quantitative simulations of terahertz emission nanoscopy based on the finite element method. Our approach provides a solid theoretical framework for quantitative nanospectroscopy and sets the stage for a reliable description of subcycle, near-field microscopy at terahertz frequencies.
In this roadmap article, we have focused on the most recent advances in terahertz (THz) imaging with particular attention paid to the optimization and miniaturization of the THz imaging systems. Such systems entail enhanced functionality, reduced power consumption, and increased convenience, thus being geared toward the implementation of THz imaging systems in real operational conditions. The article will touch upon the advanced solid-state-based THz imaging systems, including room temperature THz sensors and arrays, as well as their on-chip integration with diffractive THz optical components. We will cover the current-state of compact room temperature THz emission sources, both optolectronic and electrically driven; particular emphasis is attributed to the beam-forming role in THz imaging, THz holography and spatial filtering, THz nano-imaging, and computational imaging. A number of advanced THz techniques, such as light-field THz imaging, homodyne spectroscopy, and phase sensitive spectrometry, THz modulated continuous wave imaging, room temperature THz frequency combs, and passive THz imaging, as well as the use of artificial intelligence in THz data processing and optics development, will be reviewed. This roadmap presents a structured snapshot of current advances in THz imaging as of 2021 and provides an opinion on contemporary scientific and technological challenges in this field, as well as extrapolations of possible further evolution in THz imaging.
THz communications is envisaged for wide bandwidth mobile communications eventually reaching data capacities exceeding 100 Gbit/s. The technology enabling compact chip-integrated transceivers with highly directive, steerable antennas is the key challenge at THz frequencies to overcome the very high free-space path losses and to support user mobility. In this article, we report on mobile and multi-user THz communications using a photonic THz transmitter chip featuring 1D beam steering for the first time. In the proposed approach, 1D THz beam steering is achieved by using a photodiode excited leaky-wave antenna (LWA) in the transmitter chip. The on-chip LWA allows to steer the directive THz beam from 6° to 39° within the upper WR3-band (0.28-0.33 THz). The antenna’s directivity is 14 dBi which is further increased to 23 dBi using an additional hemicylindrical Teflon lens. The 3-dB beam width and coherence bandwidth of the fabricated THz transmitter chips with lens are 9° and 12 GHz, respectively. The proposed approach allows steering the THz beam via the beat frequency of an optical heterodyne system at a speed up to 28°/s. Without using a THz amplifier in the transmitter chip, a data rate of 24 Gbit/s is achieved for a single user for all beam directions and at short wireless distances up to 6 cm. The wireless distance is successfully increased to 32 cm for a lower data rate of 4 Gbit/s, still without using a transmitter amplifier. Also, multi-user THz communications and the overall capacity of the developed THz transmitter chip is studied revealing that up to 12 users could be supported together with a total wireless data capacity of 48 Gbit/s. Fully integrated 2D transmitter chips are expected to reach wireless distances of several meters without additional amplifiers.
Deep neural networks (DNNs) are empirically derived systems that have transformed traditional research methods, and are driving scientific discovery. Artificial electromagnetic materials (AEMs)—including electromagnetic metamaterials, photonic crystals, and plasmonics—are research fields where DNN results valorize the data driven approach; especially in cases where conventional methods have failed. In view of the great potential of deep learning for the future of artificial electromagnetic materials research, the status of the field with a focus on recent advances, key limitations, and future directions is reviewed. Strategies, guidance, evaluation, and limits of using deep networks for both forward and inverse AEM problems are presented. Deep learning is rapidly transforming traditional research methods and driving scientific discovery. Artificial electromagnetic materials (AEMs)—including electromagnetic metamaterials, photonic crystals, and plasmonics—are fields where deep learning has tremendous potential. This comprehensive review article presents deep learning techniques for forward and inverse design of AEMs, with a focus on recent advances, key limitations, and future directions.
We are standing at the crossroads of powerful new facilities emerging in the next decade on the ground and in space like ELT, SKA, JWST, and Athena. Turning the narrative of the star formation potential of galaxies into a quantitative theory will provide answers to many outstanding questions in astrophysics, from the formation of planets to the evolution of galaxies and the origin of heavy elements. To achieve this goal, there is an urgent need for a dedicated space-borne, far-infrared spectroscopic facility capable of delivering, for the first time, large scale, high spectral resolution (velocity resolved) multiwavelength studies of the chemistry and dynamics of the ISM of our own Milky Way and nearby galaxies. The Far Infrared Spectroscopic Surveyor (FIRSS) fulfills these requirements and by exploiting the legacy of recent photometric surveys it seizes the opportunity to shed light on the fundamental building processes of our Universe.
Exotic quantum vacuum phenomena are predicted in cavity quantum electrodynamics systems with ultrastrong light-matter interactions. Their ground states are predicted to be vacuum squeezed states with suppressed quantum fluctuations owing to antiresonant terms in the Hamiltonian. However, such predictions have not been realized because antiresonant interactions are typically negligible compared to resonant interactions in light-matter systems. Here we report an unusual, ultrastrongly coupled matter-matter system of magnons that is analytically described by a unique Hamiltonian in which the relative importance of resonant and antiresonant interactions can be easily tuned and the latter can be made vastly dominant. We found a regime where vacuum Bloch-Siegert shifts, the hallmark of antiresonant interactions, greatly exceed analogous frequency shifts from resonant interactions. Further, we theoretically explored the system’s ground state and calculated up to 5.9 dB of quantum fluctuation suppression. These observations demonstrate that magnonic systems provide an ideal platform for exploring exotic quantum vacuum phenomena predicted in ultrastrongly coupled light-matter systems. Ultrastrong light-matter interactions with dominant antiresonant terms are expected to give rise to interesting phenomena such as quantum fluctuation suppression. Here, the authors propose a system of ultrastrongly coupled magnon modes in a rare earth orthoferrite as a platform for exploring such phenomena.
We have developed a continuous wave sub-wavelength terahertz (THz) imaging system that combines two prominent classical optical techniques: solid immersion microscopy and interferometric detection. This combination allows for resolution beyond the diffraction limit at 703 GHz. We experimentally demonstrate sub-wavelength spatial resolution working with a relatively low-cost pyroelectric detector and with both high and low contrast samples.
Tunnelling is one of the most fundamental manifestations of quantum mechanics. The recent advent of lightwave-driven scanning tunnelling microscopy has revolutionized ultrafast nanoscience by directly resolving electron tunnelling in electrically conducting samples on the relevant ultrashort length- and timescales. Here, we introduce a complementary approach based on terahertz near-field microscopy to perform ultrafast nano-videography of tunnelling processes even in insulators. The central idea is to probe the evolution of the local polarizability of electron–hole pairs with evanescent terahertz fields, which we detect with subcycle temporal resolution. In a proof of concept, we resolve femtosecond interlayer transport in van der Waals heterobilayers and reveal pronounced variations of the local formation and annihilation of interlayer excitons on deeply subwavelength, nanometre scales. Such contact-free nanoscopy of tunnelling-induced dynamics should be universally applicable to conducting and non-conducting samples and reveal how ultrafast transport processes shape functionalities in a wide range of condensed matter systems. Subcycle nano-videography of charge-transfer dynamics in WSe2/WS2 heterostructures is obtained by using a terahertz near-field microscopy. The central idea is to probe the local polarizability of electron–hole pairs with evanescent terahertz fields.
Anisotropic magnetoresistance (AMR) is a ubiquitous and versatile probe of magnetic order in contemporary spintronics research. Its origins are usually ascribed to extrinsic effects (i.e., spin-dependent electron scattering), whereas intrinsic (i.e., scattering-independent) contributions are neglected. Here, we measure AMR of polycrystalline thin films of the standard ferromagnets Co, Ni, Ni81Fe19, and Ni50Fe50 over the frequency range from dc to 28 THz. The large bandwidth covers the regimes of both diffusive and ballistic intraband electron transport and, thus, allows us to separate extrinsic and intrinsic AMR components. Analysis of the THz response based on Boltzmann transport theory reveals that the AMR of the Ni, Ni81Fe19, and Ni50Fe50 samples is of predominantly extrinsic nature. However, the Co thin film exhibits a sizable intrinsic AMR contribution, which is constant up to 28 THz and amounts to more than 2/3 of the dc AMR contrast of 1%. These features are attributed to the hexagonal structure of the Co crystallites. They are interesting for applications in terahertz spintronics and terahertz photonics. Our results show that broadband terahertz electromagnetic pulses provide new and contact-free insights into magnetotransport phenomena of standard magnetic thin films on ultrafast timescales.
We investigate photoconductive terahertz (THz) emitters compatible with 1550 nm excitation for THz time-domain spectroscopy (TDS). The emitters are based on rhodium (Rh) doped InGaAs grown by molecular beam epitaxy. InGaAs:Rh exhibits a unique combination of ultrashort trapping time, high electron mobility, and high resistivity. THz emitters made of InGaAs:Rh feature an emitted THz power of 637 μW at 28 mW optical power and 60 kV/cm electrical bias field. In particular for a fiber coupled photoconductive emitter, this is an outstanding result. When these emitters are combined with InGaAs:Rh based receivers in a THz TDS system, 6.5 THz bandwidth and a record peak dynamic range of 111 dB can be achieved for a measurement time of 120 s.
Employing electro-optic sampling (EOS) with ultrashort probe pulses, recent experiments showed direct measurements of quantum vacuum fields and their correlations on subcycle timescales. Here, we propose a quantum-enhanced EOS where bright photon-number entangled twin beams are used to derive conditioned nonclassical probes. In the case of the quantum vacuum, this leads to a sixfold improvement in the signal-to-noise ratio over the classically probed EOS. In addition, engineering of the conditioning protocol yields a reliable way to extract higher-order moments of the quantum noise distribution and robust discrimination of the input quantum states, for instance, a vacuum and a few-photon cat state. These improvements open a viable route toward robust tomography of quantum fields in space-time, an equivalent of homodyne detection in energy-momentum space, and the possibility of precise experiments in real-space quantum electrodynamics.
We have demonstrated a low noise superconducting MgB2 hot electron bolometer (HEB) mixer working at the frequency of 5.3 terahertz (THz) with 20 K operation temperature. The bolometer consists of a 7 nm thick MgB2 submicrometer bridge contacted with a spiral antenna to couple THz radiation through a high resistive Si lens, and it has a superconducting critical temperature of 38 K. By using hot/cold blackbody loads and a Mylar beam splitter all in vacuum and applying a 5.25 THz far-infrared gas laser as a local oscillator, we measured a minimal double sideband receiver noise temperature of 3960 K at the LO power of 9.5 μW. This can be further reduced to 2920 K if a Si lens with an antireflection coating optimized at this frequency and a 3 μm beam splitter are used. The measured intermediate frequency (IF) noise bandwidth is 9.5 GHz. The low noise, wide IF bandwidth mixers, which can be operated in a compact, low dissipation Stirling cooler, are more suitable for space applications than the existing HEB mixers. Furthermore, we likely observed a signature of the double-gap in MgB2 by comparing current–voltage curves pumped at 5.3 and 1.6 THz.
The Galaxy Evolution Probe (GEP) is a concept for a mid- and far-infrared space observatory to measure key properties of large samples of galaxies with large and unbiased surveys. GEP will attempt to achieve zodiacal light and Galactic dust emission photon background-limited observations by utilizing a 6-K, 2.0-m primary mirror and sensitive arrays of kinetic inductance detectors (KIDs). It will have two instrument modules: a 10 to 400 μm hyperspectral imager with spectral resolution R = λ / Δλ ≥ 8 (GEP-I) and a 24 to 193 μm, R = 200 grating spectrometer (GEP-S). GEP-I surveys will identify star-forming galaxies via their thermal dust emission and simultaneously measure redshifts using polycyclic aromatic hydrocarbon emission lines. Galaxy luminosities derived from star formation and nuclear supermassive black hole accretion will be measured for each source, enabling the cosmic star formation history to be measured to much greater precision than previously possible. Using optically thin far-infrared fine-structure lines, surveys with GEP-S will measure the growth of metallicity in the hearts of galaxies over cosmic time and extraplanar gas will be mapped in spiral galaxies in the local universe to investigate feedback processes. The science case and mission architecture designed to meet the science requirements is described, and the KID and readout electronics state of the art and needed developments are described. This paper supersedes the GEP concept study report cited in it by providing new content, including: a summary of recent mid-infrared KID development, a discussion of microlens array fabrication for mid-infrared KIDs, and additional context for galaxy surveys. The reader interested in more technical details may want to consult the concept study report.
Measuring terahertz (THz) conductivity on an ultrafast timescale is an excellent way to observe charge-carrier dynamics in semiconductors as a function of time after photoexcitation. However, a conductivity measurement alone cannot separate the effects of charge-carrier recombination from effective mass changes as charges cool and experience different regions of the electronic band structure. Here we present a form of time-resolved magneto-THz spectroscopy that allows us to measure cyclotron effective mass on a picosecond timescale. We demonstrate this technique by observing electron cooling in the technologically significant narrow-bandgap semiconductor indium antimonide. A significant reduction of electron effective mass from 0.032 to 0.017 me is observed in the first 200 ps after injecting hot electrons. The measured electron effective mass in InSb as a function of photoinjected electron density agrees well with conduction band nonparabolicity predictions from ab initio calculations of the quasiparticle band structure.