Frequency Conversion Research Articles

Controlling surface acoustic waves (SAWs) via temporally graded metasurfaces

In this manuscript, the temporal rainbow effect for surface acoustic waves (SAWs) is illustrated through a temporal analog of space metagradings. We show that a time-modulated array of mechanical resonators induces a wavenumber-preserving frequency transformation which, in turn, dictates Rayleigh-to-Shear wave conversion. The process is unfolded through the adiabatic theorem, which allows us to delineate the transition between a solely frequency-converted wave packet and a temporally-driven mode conversion. In other words, our paper explores the role of time modulation in the context of elastic metasurfaces, and we envision our implementation to be suitable for designing a new family of SAW devices with frequency conversion, mode conversion, and unusual transport capabilities.

Molecularization of Metasurfaces for Multifunctional Ultrafast All‐Optical Terahertz Waves

Hybrid metasurfaces incorporated by active materials hold great promise for state‐of‐the‐art terahertz functional devices. However, it is still a major challenge to achieve ultrafast, dynamic, and multifunctional effective control of THz waves via hybrid metasurfaces. Herein, a modulator consisting of split rings and cut‐wires is first demonstrated, with an amplitude of −35.6 dB at 0.524 THz. By embedding semiconductor silicon into specified locations to form a hybrid metasurface, the ultrastrong connectivity of the silicon bridges leads to rapid optical molecularization. Under photoexcitation, the frequency tuning range is 26.7%, the phase shifting reaches 357.5°, and the maximal modulation depth is 94.54%. Taking advantage of the rapid relaxation of photocarriers in the silicon bridges, the ultrafast frequency switching is within 1400 ps. More interestingly, by changing the positions of the silicon bridges, the frequency tuning range is further promoted to 60%, the phase shifting is 353.5°, the modulation depth of 100% is achieved, and the full recovery time is 1600 ps. Furthermore, the underlying mechanism of the ultrafast tuning process is elucidated. This work demonstrates the feasibility of all‐optical‐controlled hybrid metasurface to achieve multifunctional dynamic modulation of THz waves, which has tremendous potential for applications in optical switching, signal processing, and frequency conversion.

Generation of squeezed high-order harmonics

For decades, most research of high harmonic generation (HHG) considered matter as quantum but light as classical. Recently, HHG driven by quantum states of light such as bright squeezed vacuum was predicted to reach beyond the classical HHG cutoff. Moreover, in squeezed coherent illumination, it was shown that the underlying dynamics are significantly modified by the photon statistics effective force. Here we show that HHG driven by quantum light results in quantum high harmonics. We derive a formula for the quantum state of the high harmonics, when driven by arbitrary quantum light states, and then explore specific cases of experimental relevance. Specifically, for a moderately squeezed pump, HHG driven by squeezed coherent light results in squeezed high harmonics. Harmonic squeezing is optimized by syncing ionization times with the pump's squeezing phase. Beyond this regime, as pump squeezing is increased, the harmonics initially acquire squeezed thermal photon statistics, and then occupy an intricate quantum state which strongly depends on the semiclassical nonlinear response function of the interacting system. Our results pave the way for generation of squeezed extreme-ultraviolet ultrashort pulses, and more generally, quantum frequency conversion into previously inaccessible spectral ranges, which may enable ultrasensitive attosecond metrology. Published by the American Physical Society 2024

An improved wireless power transmission system for micro unmanned aerial vehicles

AbstractDue to its limited load capacity, the piggybacking of wireless charging systems for micro unmanned aerial vehicles has become a challenging task in the design of micro unmanned aerial vehicles. Therefore, this paper proposes an improved lightweight design method for micro unmanned aerial vehicles to carry a wireless charging system. The improved system uses an inductor–capacitor–capacitor‐parallel (LCC‐P) topology compensation network instead of a bilateral inductor–capacitor–capacitor (LCC) topology compensation network to achieve constant current charging (CC) and constant voltage charging (CV). Then the adaptive frequency conversion control is used to realize the automatic conversion from CC charging to CV charging during the charging process. Compared with the traditional wireless charging system based on a direct current‐direct current converter (DC‐DC converter) to achieve CC charging and CV charging, this improved system reduces the total weight of the receiving end of the wireless charging system based on ensuring the charging efficiency, which meets the demand of wireless charging systems carried by micro drone aircraft. The test results of the constructed physical system coincide with the simulation design results, and the total weight of the receiving end is only 8 g, which verifies the correctness and effectiveness of the improved design scheme.

Electrically tunable third-harmonic generation using intersubband polaritonic metasurfaces

Nonlinear intersubband polaritonic metasurfaces, which integrate giant nonlinear responses derived from intersubband transitions of multiple quantum wells (MQWs) with plasmonic nanoresonators, not only facilitate efficient frequency conversion at pump intensities on the order of few tens of kW cm-2 but also enable electrical modulation of nonlinear responses at the individual meta-atom level and dynamic beam manipulation. The electrical modulation characteristics of the magnitude and phase of the nonlinear optical response are realized through Stark tuning of the resonant intersubband nonlinearity. In this study, we report, for the first time, experimental implementations of electrical modulation characteristics of mid-infrared third-harmonic generation (THG) using an intersubband polaritonic metasurface based on MQW with electrically tunable third-order nonlinear response. Experimentally, we achieved a 450% modulation depth of the THG signal, 86% suppression of zero-order THG diffraction tuning based on local phase tuning exceeding 180 degrees, and THG beam steering using phase gradients. Our work proposes a new route for electrically tunable flat nonlinear optical elements with versatile functionalities.

A3Sc(SO4)3 (A = K, Rb, Cs): Three Deep‐Ultraviolet Transparent Alkali‐Scandium Sulfates with Strong Second‐Harmonic Generation Response and High Thermal Stability

AbstractNonlinear optical (NLO) crystals that have the capability of achieving tunable lasers through frequency conversion are of great importance in solid‐state lasers. Herein, three new deep‐UV transparent alkali‐scandium sulfates with strong second‐harmonic generation (SHG) response and high thermal stability, A3Sc(SO4)3 (A = K, Rb, Cs), are successfully obtained. It is interesting that the slightly distorted [ScO6] octahedrons are in an ordered/oriented arrangement, which is in favor of the generation of SHG response. Especially, K3Sc(SO4)3 not only exhibits a strong phase‐matching SHG response of 1.6 × KDP, a short deep‐ultraviolet (deep‐UV) cutoff edge of 190 nm, a wide transparent window covering from the deep‐UV region to mid IR, a high laser‐induced damage threshold of 165 MW cm−2 and easy growth of large single crystal, but also behaves thermal stable up to 900 °C under an air atmosphere. Crystal structures and theoretical calculations reveal that their strong SHG responses mainly stem from the slightly distorted [ScO6] octahedron. This study highlights A3Sc(SO4)3 (A = K, Rb, Cs) as promising deep‐UV transparent NLO crystals and offers a new approach for the pursuit of high‐performance NLO crystals.

Multifunctional Intelligent Metamaterial Computing System: Independent Parallel Analog Signal Processing

Analog computing based on miniaturized surfaces has gained attention for its high‐speed and low‐power mathematical operations. Building on recent advances, an anisotropic space‐time digital metasurface for parallel and programmable wave‐based mathematical operations is proposed. Using frequency conversions, our metasurface performs 1st‐order and 2nd‐order spatial differentiations, integrodifferential equations, and sharp edge detection in spatially encoded images. The anisotropic nature of the meta‐particle enables independent and simultaneous operations for two orthogonal polarizations. Reconfigurability is achieved through tunable gate biasing of an indium tin oxide layer. Illustrative examples demonstrate that the metasurface's output signals and transfer functions closely match ideal transfer functions, confirming its versatility and effectiveness. Unlike other wave‐based signal processors, the design handles wide spatial frequency bandwidths, even with high spatial frequency inputs.

High-gain far-detuned nonlinear frequency conversion in a few-mode graded-index optical fiber

We present theoretical and experimental evidence of high-gain far-detuned nonlinear frequency conversion, extending towards both the visible and the mid-infrared, in a few-mode graded-index silica fiber pumped at 1.064 μm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu \\hbox {m}$$\\end{document}, and more specifically achieving gains of hundreds of dB per meter below 0.65 μm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu \\hbox {m}$$\\end{document} and beyond 3.5 μm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu \\hbox {m}$$\\end{document}. Interestingly, our findings highlight the potential of graded-index fibers for enabling high-gain wavelength conversion into the strong-loss spectral region of silica. Such advancements require an accurate interpretation of intramodal and intermodal four-wave mixing processes.

Segmented variable-frequency ultrasound synergistic hot-air drying of Rhubarb: Effect on drying characteristics and quality and thermal analysis

This study employed segmented variable-frequency ultrasound synergistic hot-air drying (SVFU-HAD) for Rhubarb slices, selected two sets of time nodes for frequency conversion (60 min, 120 min, and 90 min, 150 min), and two sequences of frequency conversion (high-frequency to low-frequency, and low-frequency to high-frequency). It aimed to investigate the effects of SVFU-HAD on the drying characteristics, quality, and heat transfer of Rhubarb slices. The findings indicated that segmented variable-frequency ultrasound has advantages in increasing drying rate and improving uniformity of cavitation effects compared to constant-frequency ultrasound. Analysis of physical properties revealed that the rehydration performance of dried products subjected to ultrasonic variable-frequency treatment (90 min, 150 min) according to the drying rate was better (RR > 3.3). The transition mode from high-frequency to low-frequency in variable-frequency ultrasonic treatment contributes to maintaining the overall color of Rhubarb. Analysis of chemical properties unveiled that Rhubarb treated with 40 kHz (0 min)–28 kHz (60 min)–25 kHz (120 min) segmented variable-frequency ultrasound contained overall higher levels of tannins, dianthrones and free anthraquinones content, which exceeded the average values by 3.24%, 26.65%, and 14.42%, respectively. In addition, thermal analysis results based on ANSYS Workbench software demonstrated that the drying uniformity of SVFU-HAD is superior to that of hot-air drying and constant-frequency ultrasound synergistic hot-air drying (CFU-HAD). Overall, the SVFU-HAD method employed in this study presents an innovative approach to ultrasound synergistic hot-air drying research with promising potential for enhancing the efficiency and quality characteristics of Rhubarb slices.

Near Ultraviolet-Excitable Cyan-Emissive Hybrid Copper(I) Halides Nonlinear Optical Crystals with Near-Unity Photoluminescence Quantum Yield and High-Efficiency X-ray Scintillation.

Designing and synthesizing multifunctional hybrid copper halides with near ultraviolet (NUV) light-excited high-energy emission (<500 nm) remains challenging. Here, a pair of broadband-excited high-energy emitting isomers, namely, α-/β-(MePh3P)2CuI3 (MePh3P=methyltriphenylphosphonium), were synthesized. α-(MePh3P)2CuI3 with blue emission peaking at 475 nm is firstly discovered wherein its structure contains regular [CuI3]2- triangles and crystallizes in centrosymmetric space group P21/c. While β-(MePh3P)2CuI3 featuring distorted [CuI3]2- planar triangles shows inversion symmetry breaking and crystallizes in the noncentrosymmetric space group P21, which exhibits cyan emission peaking at 495 nm with prominent near-unity photoluminescence quantum yield and the excitation band ranging from 200 to 450 nm. Intriguingly, β-(MePh3P)2CuI3 exhibits phase-matchable second-harmonic generation response of 0.54×KDP and a suitable birefringence of 0.06@1064 nm. Furthermore, β-(MePh3P)2CuI3 also can be excited by X-ray radioluminescence with a high scintillation light yield of 16193 photon/MeV and an ultra-low detection limit of 47.97 nGy/s, which is only 0.87 % of the standard medical diagnosis (5.5 μGy/s). This work not only promotes the development of solid-state lighting, laser frequency conversion and X-ray imaging, but also provides a reference for constructing multifunctional hybrid metal halides.

Mode conversion of fundamental guided ultrasonic wave modes at part-thickness crack-like defects

Guided ultrasonic waves can be employed for efficient structural health monitoring (SHM) and non-destructive evaluation (NDE), as they can propagate long distances along thin structures. The scattering (S0 mode) and mode conversion of low frequency guided waves (S0 to A0 and SH0 wave modes) at part-thickness crack-like defects was studied to quantify the defect detection sensitivity. Three-dimensional (3D) Finite Element (FE) modelling was used to predict the mode conversion and scattering of the fundamental guided wave modes. Experimentally, the S0 mode was excited by a piezoelectric (PZT) transducer in an aluminum plate. A laser vibrometer was used to measure the out-of-plane displacement to characterize the mode-converted A0 mode, employing baseline subtraction to achieve mode and pulse separation. Good agreement between FE model predictions and experimental results was obtained for perpendicular incidence of the S0 mode. The influence of defect depth and length on the scattering and mode conversion was studied and the sensitivity for part-thickness defects was quantified. The maximum mode conversion (S0-A0 mode) occurred for ¾ defect depth and the amplitude of the mode-converted A0 and scattered S0 modes mostly increased linearly as the defect length increased with an almost constant A0/S0 mode scattered amplitude ratio. Similar forward and backward scattering amplitude was found for the mode converted A0 mode. The mode conversion of the S0 to SH0 mode has the highest sensitivity for short defects, but the SH0 mode amplitude only increased slightly for longer defects. Employing the information contained in the mode-converted, scattered guided ultrasonic wave modes could improve the detection sensitivity and localization accuracy of SHM algorithms.

Acceleration-induced spectral beats in strongly driven harmonic oscillators

The harmonic modulation of coherent systems gives rise to a wealth of physical phenomena, e.g., the AC-Stark effect and Mollow triplets, with important implications for coherent control and frequency conversion. Here, we demonstrate a novel regime of temporal coherence in oscillators harmonically driven at extreme energy modulation amplitudes relative to the modulation quantum. The studies were carried out by modulating a confined exciton-polariton Bose-Einstein condensate (BEC) by an acoustic wave. Features of the new regime are the appearance, in the spectral domain, of a comb of resonances termed acceleration beats with energy spacing tunable by the modulation amplitude and, in the time domain, of temporal correlations at time scales much shorter than the acoustic period, which also depend on the modulation amplitude. These features are quantitatively accounted for by a theoretical framework, which associates the beats with accelerated energy-change rates during the harmonic cycle. These observations are underpinned by the high sensitivity of the BEC energy to the acoustic driving, which simultaneously preserves the BEC’s temporal coherence. The acceleration beats are a general feature associated with accelerated energy changes: analogous features are thus also expected to appear under highly accelerated motion e.g., in connection with Cherenkov and Hawking radiation.

New Opportunities in Real-Time Diagnostics of Induction Machines

This manuscript addresses the critical challenges in achieving high-accuracy remote control of electromechanical systems, given their inherent nonlinearities and dynamic complexities. Traditional diagnostics often suffer from data inaccuracies and limitations in analytical techniques. The focus is on enhancing the dynamic model accuracy for remote induction motor control in both closed- and open-loop speed control systems, which is essential for real-time process monitoring. The proposed solution includes real-time measurements of input and output physical quantities to mitigate inaccuracies in traditional diagnostic methods. The manuscript discusses theoretical aspects of nonlinear torque formation in induction drives and introduces a dynamic model employing vector control and speed control schemes alongside standard frequency control methods. These approaches optimize frequency converter settings to enhance system performance under varying nonlinear conditions. Additionally, the manuscript explores methods to analyze dynamic, systematic errors arising from frequency converter inertial properties, thereby improving electromechanical equipment condition diagnostics. By addressing these challenges, the manuscript significantly advances the field, offering a promising future with enhanced dynamic model accuracy, real-time monitoring techniques, and advanced control methods to optimize system reliability and performance.

Wideband coherent microwave conversion via magnon nonlinearity in a hybrid quantum system

Frequency conversion is a widely realized physical process in nonlinear systems of optics and electronics. As an emerging nonlinear platform, spintronic devices have the potential to achieve stronger frequency conversion. Here, we demonstrated a microwave frequency conversion method in a hybrid quantum system, integrating nitrogen-vacancy centers in diamond with magnetic thin film CoFeB. We achieve a conversion bandwidth ranging from 0.1 to 12 GHz, presenting an up to 25th order frequency conversion and further display the application of this method for frequency detection and qubits coherent control. Distinct from traditional frequency conversion techniques based on nonlinear electric response, our approach employs nonlinear magnetic response in spintronic devices. The nonlinearity, originating from the symmetry breaking such as domain walls in magnetic films, presents that our method can be adapted to hybrid systems of other spintronic devices and spin qubits, expanding the application scope of spintronic devices and providing a promising on-chip platform for coupling quantum systems.

Quality improvement of BaGa4Se7 crystal by annealing in BaSe vapor atmosphere

Infrared nonlinear optical crystals are widely used in all-solid-state lasers as pivotal devices for laser frequency conversion. BaGa4Se7 (BGSe) crystal has attracted much attention due to its excellent optical properties. However, the presence of point defects in BGSe increases the absorption loss and reduces the laser-induced damage threshold, thus limiting its practical application in high-power mid- and far-infrared lasers. Therefore, it is crucial to reveal the types of defects and their formation mechanisms in BGSe crystal, which is conducive to the design of rational strategies to improve the crystal quality. In this work, the defect activation energies were calculated by photoluminescence (PL) spectroscopy and the existence of three types of defects, GaBa, VSe and VBa were verified in the as grown BGSe crystal. Therefore, a thermal annealing process under BaSe vapor was rationally designed to compensate for the loss of Ba and Se elements and to reduce the defect concentration in BGSe. In addition, the changes of defects in BGSe crystal were determined by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy. All the experimental results confirm that the defect concentration in BGSe decreases greatly and the crystal quality is significantly improved after annealing in BaSe vapor.

Efficient Parabolic Equation-Driven CNN Propagation Model in Tunnels Based on Frequency Conversion

Efficient Parabolic Equation-Driven CNN Propagation Model in Tunnels Based on Frequency Conversion

Optimizing energy efficiency in multi-chiller systems: A comprehensive Modelica-based approach

To achieve sustainable development of buildings, this paper investigates the energy-saving potential of multi-chiller systems by examining the interactions between sequencing optimization, load optimization, and parameter optimization in systems with varying capacities. A Modelica-based simulation platform, adaptable to various programming languages optimization engines, was developed for this purpose. The study introduces a sequencing optimization threshold function that considers both wet-bulb temperature and cooling load. Additionally, a two-stage optimization strategy that prioritizes parameter adjustments before sequencing is proposed. Case studies demonstrate that this threshold function and reversed optimization order can further enhance overall energy savings. Focusing on a system with three large chillers and one small chiller, the study reveals the energy benefits of activating the small chiller under specific conditions, improving overall system efficiency. The proposed optimization achieved a cumulative energy saving of 650,463 kWh over the summer, with sequencing contributing 5.64 %. Frequency conversion control of the water pump is identified as a significant energy-saving factor. This research provides a comprehensive approach to optimizing diverse chiller systems, highlighting substantial potential for energy conservation.

Low-Power Wireless Sensor Module for Machine Learning-Based Continuous Monitoring of Nuclear Power Plants.

This paper introduces the novel design and implementation of a low-power wireless monitoring system designed for nuclear power plants, aiming to enhance safety and operational efficiency. By utilizing advanced signal-processing techniques and energy-efficient technologies, the system supports real-time, continuous monitoring without the need for frequent battery replacements. This addresses the high costs and risks associated with traditional wired monitoring methods. The system focuses on acoustic and ultrasonic analysis, capturing sound using microphones and processing these signals through heterodyne frequency conversion for effective signal management, accommodating low-power consumption through down-conversion. Integrated with edge computing, the system processes data locally at the sensor level, optimizing response times to anomalies and reducing network load. Practical implementation shows significant reductions in maintenance overheads and environmental impact, thereby enhancing the reliability and safety of nuclear power plant operations. The study also sets the groundwork for future integration of sophisticated machine learning algorithms to advance predictive maintenance capabilities in nuclear energy management.

SIMULATION OF AN AUTOMATIC CONTROL SYSTEM FOR A GRINDING MACHINE

This article discusses the MATLAB and MasterSCADA programs and the possibility of their joint application. To solve this problem, we used packages Fuzzy Logic Toolbox, Neural Network Toolbox, used in MATLAB that allow us to implement a process control model. The program implements the possibility of providing communication with the OPC Toolbox SCADA system, which allows for the interaction of system components using the OPC protocol. A block diagram of the control system of the pulverised unit has been developed, which acts on the control object through actuators. To implement an automated control system for the crumpling process, the following are offered: MATRIX-E/DUPLEX Q412/A near-infrared spectrophotometer, ARIES PLK controller, ARIES KIPPRIBO frequency converters

Terahertz Pseudo‐Waveform‐Selective Metasurface Absorber Based on a Square‐Patch Structure Loaded with Linear Circuit Components

In recent years, metasurfaces composed of lumped nonlinear circuits have been reported to exhibit the capability of detecting specific electromagnetic waves, even when the waves are of the same frequency, depending on their respective waveforms or, more precisely, their pulse widths. Herein, three types of metasurface absorbers (MSAs) are presented which are composed of a square‐patch structure loaded with linear circuit components, including lumped resistors or resistors in parallel with capacitors/inductors, which can mimic the waveform‐selective absorption behavior in the terahertz (THz) region. By judiciously selecting suitable values for the linear circuit components, these MSAs can achieve near‐perfect absorption of incident continuous waves or longer pulses while exhibiting reduced absorption of short pulses at the same THz frequency. These linear circuit structures can be referred to as pseudo‐waveform‐selective MSAs because their waveform‐selective absorption characteristics are primarily derived from the dispersion behavior of the resonator structures, as opposed to the frequency conversion commonly observed in nonlinear circuits. These outcomes and discoveries introduce an additional degree of freedom for waveform discrimination in the THz frequency range, potentially enabling a broader range of applications, including but not limited to detection, sensing, and wireless communication.