Multimode Cavity Research Articles

Optical Broadcasting Employing Incoherent and Low-Coherence Spatial Modes for Bi-Directional Optical Wireless Communications

We introduce recently developed space division multiplexing (SDM)-associated technologies into optical wireless communications (OWC) and propose to use incoherent and low-coherence spatial modes to realize optical broadcasting with reliable performance by mitigating coherent interference. We experimentally demonstrate reconfigurable optical beam steering using a programmable multi-plane light conversion (MPLC)-based mode multiplexer (MMUX) and realize point-to-multi-point optical wireless communications with low-loss up-link optical beam combining and reach extension using multimode fiber (MMF). Both incoherent and low-coherence spatial modes can be generated employing cost-efficient components such as a multimode vertical cavity surface emitting laser (MM-VCSEL) and broadband amplified spontaneous emission (ASE) noise source.

Frequency-diverse multimode millimetre-wave constant-\u03f5r lens-loaded cavity

This paper presents a physical frequency-diverse multimode lens-loaded cavity, designed and used for the purpose of the direction of arrival (DoA) estimation in millimetre-wave frequency bands for 5G and beyond. The multi-mode mechanism is realized using an electrically-large cavity, generating spatio-temporally incoherent radiation masks leveraging the frequency-diversity principle. It has been shown for the first time that by placing a spherical constant dielectric lens (constant-ϵr) in front of the radiating aperture of the cavity, the spatial incoherence of the radiation modes can be enhanced. The lens-loaded cavity requires only a single lens and output port, making the hardware development much simpler and cost-effective compared to conventional DoA estimators where multiple antennas and receivers are classically required. Using the lens-loaded architecture, an increase of up to 6 dB is achieved in the peak gain of the synthesized quasi-random sampling bases from the frequency-diverse cavity. Despite the fact that the practical frequency-diverse cavity uses a limited subset of quasi-orthogonal modes below the upper bound limit of the number of theoretical modes, it is shown that the proposed lens-loaded cavity is capable of accurate DoA estimation. This is achieved thanks to the sufficient orthogonality of the leveraged modes and to the presence of the spherical constant-ϵr lens which increases the signal-to-noise ratio (SNR) of the received signal. Experimental results are shown to verify the proposed approach.

Microwave-assisted modulation of light emission intensity in alkali-pyrotechnic plumes

Creating next-generation pyrotechnic emitters capable of dynamically controllable light output requires a paradigm shift away from emission control via formulation. This work demonstrates the ability to modulate light emission intensity of a pyrotechnic flame with 2.46 GHz microwave energy within a multimodal cavity. Stoichiometric mixtures of three pyrotechnic systems are investigated: magnesium fuel with oxidizers of NaNO3, KNO3, or CsNO3. Using time-resolved visible and infrared emission spectroscopy and high-speed color videography, microwave illumination of flames is found to produce enhanced atomic photo emission from the alkali species. Emission increases of up to ~120% are demonstrated, which are predominantly in the visible and near-infrared wavelengths and have little effect on flame emission chromaticity. At near- and mid-IR wavelengths, gray body continuum enhancement is observed with moderate enhanced emission from CO2 and H2O bands. Sustained light emission from microwave illumination of combustion products long after pyrotechnic extinguishment was also demonstrated. A simplified model of microwave-enhanced visible and near-IR emission is presented and shown to be consistent with the observed trend of elevated emission enhancement for lower wavelength alkali transitions. Sensitivity analysis is performed which suggests a lower equilibrium population of electronically excited alkali atoms is primarily responsible for maximizing the degree of light emission enhancement of applied microwave fields, especially in extinguished low-temperature pyrotechnic plumes. These findings suggest microwave illumination of alkali-containing pyrotechnic flames may be a useful strategy to achieve dynamic control of light emission intensity.

Flexural wave energy harvesting by multi-mode elastic metamaterial cavities

In this paper, we design the elastic metamaterial cavities with multiple cavity modes for harvesting the flexural wave energy. The metamaterial plate is formed by drilling periodic holes in a thin aluminum plate, which exhibits a flexural wave mode band-gap. The metamaterial L1–L4 cavities are designed. The flexural waves are highly confined inside these metamaterial cavities due to the existence of the band-gap. The designed cavities can be utilized to confine and convert the mechanical wave energy to electricity for more than one cavity mode. To experimentally validate the multi-mode cavity design, a metamaterial L3 cavity is fabricated. The three cavity modes of the L3 cavity are observed experimentally, which agrees well with the numerical prediction. Three piezoelectric discs are attached in the metamaterial L3 cavity as the energy harvesters. The electrical outputs are measured to evaluate the performance of the metamaterial energy harvester. Compared to the reference harvester on a bare plate, the average power outputs are amplified by the metamaterial L3 cavity at three resonant frequencies. For the optimal resistance load, more than 40 times of the power amplification is achieved by the L3 cavity energy harvester. As prior energy harvesters based on a metamaterial cavity are usually efficient only for a single frequency, the multi-mode metamaterial cavity design can effectively widen the frequency bandwidth of the energy harvester.

Ultracompact Channel Add-Drop Filter Based on Single Multimode Nanobeam Photonic Crystal Cavity

Ultracompact channel add-drop filter utilizing a single nanobeam photonic crystal cavity is analyzed, designed, and demonstrated. Anti-symmetric multimode periodic waveguide structure is employed to implement mode conversion and thus produce degenerate resonant states in the involved nanobeam cavity. Theoretical analysis results indicate that the inter-modal conversion coefficient and the waveguide-cavity coupling coefficient have significant effects on the lineshape of the filter. By carefully engineering the structure of the multimode nanobeam cavity and its side-coupled configuration, we successfully achieve an ultracompact add-drop filter with device area around 20 μm2. The experimental results on silicon-on-insulator (SOI) platform show that the insertion losses of drop port, the crosstalk of add port of the fabricated filter are -1.1 dB and -14.7 dB, respectively. The modal volume of the involved nanobeam cavity are small as 1.2(λ/n) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , indicating huge potentials in developing low-power consumption tunable devices for future very large scale integration (VLSI) photonic applications.

Anisotropic Conductivity Measurements by Two Kinds of Multimode Rectangular Plate-Laminated Cavities

In this article, we present anisotropic conductivity measurements by using two kinds of multimode rectangular cavities. Plate-laminated waveguides are fabricated by diffusion bonding of etched thin metal plates and their sidewalls have anisotropic conductivity characteristics. The anisotropic conductivity is measured by two multimode rectangular cavities when polarization is vertical or horizontal to the lamination direction. Transmission characteristics of the cavities are measured and the conductivity is calculated by the measured resonant frequencies and unloaded quality factors. The anisotropic conductivities are derived by solving a linear equation. As an example, aluminum alloy (A6063) E-band (60.5–91.9 GHz) rectangular cavities are fabricated and the conductivity is characterized.

Dual-Polarized Wideband Plate Array Antenna With High Polarization Isolation and Low Cross Polarization for D-Band High-Capacity Wireless Application

This letter presents a dual-polarized wideband plate array antenna in D-band for future fixed high-capacity wireless communication. To achieve the high polarization isolation and wideband operation at the same time, a two-stage multimode cavity is introduced, consisting of a polarization isolation cavity and a cascaded dividing cavity. They are combined to optimize and improve the bandwidth of the dual-polarized antenna array. The TE210 and TE120 degenerate modes are selected to resonate in the cavity. They can be used to generate two orthogonal linear polarization modes with the extremely low cross polarization after cascading four integrated horns with a proper placement. To validate the design, an 8 × 8 element array is fabricated by the diffusion bonding of laminated thin copper plates technology. The gain is more than 24.5 dBi from 130 to 149 GHz. The isolation between two channels is higher than 50 dB and the cross polarization level is below −45 dB over the whole band.

Dynamical Casimir effect in cavities with two modes resonantly coupled through a qubit

We revisit the dynamical Casimir effect in high-quality multimode cavities, namely, generation of photons from vacuum due to fast external variation of system parameters. In one-dimensional case the spectrum of modes is equidistant, so for harmonic modulation an infinite number of modes become excited. But only one mode is usually excited in cavities with nonequidistant spectrum, apart from some exceptional configurations for which two or three modes are populated. Here is unveiled another scenario in which a small number of photons is generated from vacuum in two specific modes of a cavity with special spectrum profile. The scheme relies on strongly coupling the field to an initially unexcited qubit, whose frequency undergoes a weak harmonic perturbation. When the ratio between two modes' frequencies is accurately tuned in the vicinity of 3, these modes and the qubit can be driven to tripartite entangled states due to narrow three-photon resonances.

Numerical modelling and investigation of microwave heating and boiling phenomena in binary liquid mixtures using OpenFOAM

To understand Microwave (MW) heating and boiling of binary liquid mixtures, a new multiphysics model was developed. The new model solves Maxwell's equations for the electromagnetic field distribution. Concerning the coupled dispersed and segregated vapor-liquid flow during MW boiling, it was modelled based on a hybrid Two-Fluid-Volume-of-Fluid method. As for the modelling of heat and mass transfer, Two-fluid energy and species conservation equations were solved, with new dispersed and segregated interphase heat and mass transfer closures formulated using appropriate interface jump conditions combined with vapor-liquid equilibrium relations. The new model showed reasonably good agreement with the experimental data. The capability of the model to simulate MW heating, superheating, superboiling, and interphase transfer phenomena in methanol-water mixture (heated in a multi-mode MW cavity) was demonstrated, and the associated physics was investigated. The simulation results revealed three underlying mechanisms that lead to superheating (up to 9.5°C at 300 W) in binary mixtures, namely limited bubble nucleation, inefficient mixing, and higher interfacial saturation temperature compared to that in bulk liquid (about 2°Chigher). Besides, the results also showed that the increase of heating time mainly increases the amount of superheat rather than the superheat temperature. It was also found that MW superheating tends to reduce the mass fraction of the lighter component in the released vapor (by up to 2%). In the subsequent study, the effects of several operating conditions were examined. The simulation results generally showed that higher liquid methanol concentration, higher MW power, rectangular sample geometry, and smaller sample volume lead to higher superheat intensity, but poorer methanol concentration in the vapor released. The new model and findings could open up new avenues for the design and improvement of processes such as MW-assisted synthesis, extraction, biodiesel production, and distillation.

Effect of Microwave Irradiation on Computed Tomography and Acoustic Emission Characteristics of Hard Rock

Microwave-induced fracturing of hard rock can be used to assist mechanical rock breakage and release stress on rock masses in deep underground engineering operations. This is significant to those aiming to improve the construction efficiency and safety of underground tunnelling and ore extraction operations. The research was devoted to exploring the computed tomography (CT) and acoustic emission (AE) characteristics of hard rock subjected to microwave irradiation. For this purpose, microwave irradiation tests at a given power for different irradiation times were conducted on basalt collected from Chifeng, China by utilising a multimode cavity working at a frequency of 2450 MHz. By applying an infrared camera, the temperature distribution on the surface of samples during microwave irradiation was measured. Three-dimensional (3-d) microscopic CT was conducted on samples treated, and not treated, with microwave irradiation to explore the crack propagation regimes in such samples. Based on CT value and P-wave velocities, the damage characteristics of microwave treatment on basalt samples were evaluated. Afterwards, uniaxial compression strength tests were conducted and AE information in the loading process of samples was collected. After conducting microwave irradiation, internal cracks of samples propagated to generate fracture surfaces and surface temperatures of samples were found to have been non-uniformly distributed. The longer the irradiation time, the higher the degree of crack propagation in samples; P-wave velocities both decreased with increasing irradiation duration. With increasing irradiation time, the brittle failure characteristics of basalt shown in the time-dependent change curve of stress on basalt were gradually weakened while ductile failure characteristics were gradually strengthened. The uniaxial compressive strength, corresponding AE energy and cumulative AE energy at peak strength all gradually decreased with increasing irradiation time. The failure stress, the level of stress drop, AE energy, and cumulative AE energy of basalt after different irradiation times reflected consistent fracturing characteristics of basalt, which were interactively validated.

Compact triple-band bandpass filter using multi-mode HMSIW cavity and half-mode DGS

AbstractIn this paper, a triple-band bandpass filter using a multi-mode half-mode substrate integrated waveguide (HMSIW) cavity is presented. In this design, back-to-back connected half-mode defected ground structure (DGS) is loaded into the HMSIW cavity, which generates the first passband. The first four modes of HMSIW cavity are TM110, TM120, TM310, and TM320. They have been used to generate second and third passbands. The slot line perturbation is used to shift the modes close to each other. Here, TM110 and TM120 constitute second passband, and TM310 and TM320 give third passband. Moreover, the notch band is designed using embedded open-circuited stub to improve the attenuation between the second and third passbands. The proposed filter is designed and fabricated on RT/Duroid 5880 substrate with a thickness of 0.508 mm. The measured results are in line with simulated ones for each of the passbands, and the center frequencies are 6.28, 13, and 19.12 GHz.

Photon-Mediated Peierls Transition of a 1D Gas in a Multimode Optical Cavity.

The Peierls instability toward a charge density wave is a canonical example of phonon-driven strongly correlated physics and is intimately related to topological quantum matter and exotic superconductivity. We propose a method for realizing an analogous photon-mediated Peierls transition, using a system of one-dimensional tubes of interacting Bose or Fermi atoms trapped inside a multimode confocal cavity. Pumping the cavity transversely engineers a cavity-mediated metal-to-insulator transition in the atomic system. For strongly interacting bosons in the Tonks-Girardeau limit, this transition can be understood (through fermionization) as being the Peierls instability. We extend the calculation to finite values of the interaction strength and derive analytic expressions for both the cavity field and mass gap. They display nontrivial power law dependence on the dimensionless matter-light coupling.

Addendum: Mapping electron dynamics in highly transient EUV photon-induced plasmas: a novel diagnostic approach using multi-mode microwave cavity resonance spectroscopy (2018 J. Phys: D. Appl. Phys. 52 034004)

A new approach for an in-line beam monitor for ionizing radiation was introduced in a recent publication (Beckers J et al 2018 J. Phys. D: Appl. Phys. 52 034004). Due to the recent detection and investigation of an additional third decay regime of the afterglow of an extreme ultraviolet photon-induced plasma described in a later article (Platier B et al 2020 Appl. Phys. Lett. 116 103703) there is an additional reason for a minimum number of photons for this approach to work. Near or below this threshold, we explain that the response time of the diagnostic method is a limiting factor. Further, a second limit for the number of photons within a pulse is formalized related to the trapping of highly energetic free electrons.

Antiferromagnetic cavity optomagnonics

Currently, there is a growing interest in studying the coherent interaction between magnetic systems and electromagnetic radiation in a cavity, prompted partly by possible applications in hybrid quantum systems. We propose a multimode cavity optomagnonic system based on antiferromagnetic insulators, where optical photons couple coherently to the two homogeneous magnon modes of the antiferromagnet. These have frequencies typically in the THz range, a regime so far mostly unexplored in the realm of coherent interactions, and which makes antiferromagnets attractive for quantum transduction from THz to optical frequencies. We derive the theoretical model for the coupled system, and show that it presents unique characteristics. In particular, if the antiferromagnet presents hard-axis magnetic anisotropy, the optomagnonic coupling can be tuned by a magnetic field applied along the easy axis. This allows to bring a selected magnon mode into and out of a dark mode, providing an alternative for a quantum memory protocol. The dynamical features of the driven system present unusual behavior due to optically induced magnon-magnon interactions, including regions of magnon heating for a red detuned driving laser. The multimode character of the system is evident in a substructure of the optomagnonically induced transparency window.

Magnon-photon strong coupling for tunable microwave circulators

We present a generic theoretical framework to describe non-reciprocal microwave circulation in a multimode cavity magnonic system and assess the optimal performance of practical circulator devices. We show that high isolation (> 56 dB), extremely low insertion loss (< 0.05 dB), and flexible bandwidth control can be potentially realized in high-quality-factor superconducting cavity based magnonic platforms. These circulation characteristics are analyzed with materials of different spin densities. For high-spin-density materials such as yttrium iron garnet, strong coupling operation regime can be harnessed to obtain a broader circulation bandwidth. We also provide practical design principles for a highly integratible low-spin-density material (vanadium tetracyanoethylene) for narrow-band circulator operation, which could benefit noise-sensitive quantum microwave measurements. This theory can be extended to other coupled systems and provide design guidelines for achieving tunable microwave non-reciprocity for both classical and quantum applications.

Loop theory for input-output problems in cavities

The input-output formalism is the basis to study the response of an optical cavity to the external stimulations. The existing theories usually handle cavity systems with only one internal mode. However, there is growing interest in more complex systems, especially the hybrid cavity-matter systems, which contains at least two internal modes, one or more from the optical cavity and the matter, respectively. Here we propose a graphical loop theory to calculate and visualize the reflection and transmission spectrum of such multi-mode cavity, resembling the role of Feynman diagrams in the quantum field theory. This loop theory gives a unified picture to interpret the experimental observations on a hybrid magnet-light system, and is extremely easy to apply to arbitrary complicated problems without any calculations.

Quasiclassical modeling of cavity quantum electrodynamics

We model a collection of $N$ two-level systems (TLSs) coupled to a multimode cavity via Meyer-Miller-Stock-Thoss (MMST) dynamics, sampling both electronic and photonic zero-point energies (ZPEs) and propagating independent trajectories in Wigner phase space. By investigating the ground state stability of a single TLS, we use MMST dynamics to separately study both electronic ZPE effects (which would naively lead to the breakdown of the electronic ground state) as well as photonic ZPE effects (which would naively lead to spontaneous absorption). By contrast, including both effects (i.e., sampling both electronic and photonic ZPEs) leads to the dynamical stability of the electronic ground state. Therefore, MMST dynamics provide a practical way to identify the contributions of self-interaction and vacuum fluctuations. More importantly, we find that MMST dynamics can predict accurate quantum dynamics for both electronic populations and electromagnetic field intensity in the high saturation limit. For a single TLS in a cavity, MMST dynamics correctly predict the initial exponential decay of spontaneous emission, Poincar\'e recurrences, and the positional dependence of a spontaneous emission rate. For an array of $N$ equally spaced TLSs with only one TLS excited initially, MMST dynamics correctly predict the modification of spontaneous emission rate as a function of the spacing between TLSs. Finally, MMST dynamics also correctly model Dicke's superradiance and subradiance (i.e., the dynamics when all TLSs are excited initially) including the correct quantum statistics for the delay time (as found by counting trajectories, for which a full quantum simulation is hard to achieve). Therefore, this work raises the possibility of simulating large-scale collective light-matter interactions with methods beyond mean-field theory.

Data-based analysis of multimodal partial cavity shedding dynamics

Time-resolved X-ray densitometry void fraction measurements and accompanying acoustic emissions have revealed that partial cavity shedding on a hydrofoil can be multimodal, with spontaneous changes in shedding sequence (referred to here as cavitation style) for fixed inlet flow conditions. These spontaneous, intermittent transitions between two physically different shedding styles are examined using data-based image analysis of the cavity flow fields in order to extract the associated physical mechanism leading to style transition. Three data-driven decomposition techniques are compared: proper orthogonal decomposition (POD), dynamic mode decomposition (DMD), and cluster-based reduced-order modeling (CROM), with a primary focus on the latter. The results highlight the utility of CROM over DMD and POD in the context of intermittent event analysis, both in terms of mode interpretability and transition mechanism identification. A frequency-based analysis of the CROM output revealed the existence of a shared harmonic between the different physical cavitation modes which gave further insight to the transition process. The data-based analysis ultimately illuminates the underlying flow mechanism that leads to the style transition, namely the key role of maximum cavity length buildup as it interacts with the vapor cloud collapse downstream of the hydrofoil.

Analysis of multi-mode waveguide cavities containing free space gaps for use in future far-infrared telescopes

In order to investigate the formation and evolution of galaxies, stars and planetary systems, it is necessary to carry out astronomical observations in the far-infrared portion of the electromagnetic spectrum. Missions such as the Herschel Space Observatory (European Space Agency) have already completed observations in this region with great success. Proposed high resolution spectrometer instruments such as SAFARI (a joined European/Japanese (ESA/JAXA) proposal as part of the SPICA mission), aim to build upon the work of previous missions by carrying out observations in the 1.5–10 THz band with unprecedented levels of sensitivity. Spica (SPace Infrared telescope for Cosmology and Astrophysics) is currently a candidate mission as part of ESA’s Cosmic Vision 2015–2025.Future far-IR missions must realise higher levels of sensitivity, limited only by the cosmic microwave background. One solution in achieving these sensitivity goals is to use waveguide coupled Transition Edge Sensor (TES) detectors, arranged in a densely packed focal plane. Additionally, multi-mode pixels can be used in order to maximise the optical throughput and coupling while still defining a definite beam shape. For the SAFARI instrument multimoded horns coupling into integrating waveguide cavities that house the TES detectors and associated absorbing layer are envisioned. This represents a significant technological challenge in terms of accurate manufacture tolerances relative to the short wavelength, however in the case of the SAFARI instrument pixel much work has already been carried out, with prototype pixels having undergone extensive testing at SRON (Space Research Organisation of the Netherlands) Groningen. In order to fully characterise the experimental results, it is necessary also to carry out comprehensive electromagnetic modelling of these structures which is also computationally intensive and requires novel approaches. These waveguide structures (horn and cavity) are typically electrically large however, and so analysis techniques using commercial finite element software prove inefficient (particularly as the structures are multimoded).The mode-matching technique with new analytical features offer a computationally efficient and reliable alternative to full electromagnetic solvers, and in this paper we outline the additions to this technique that were necessary in order to allow typical SAFARI far-infrared pixels to be modeled, including the complete optical coupling calculation of the measurement test setup at SRON and the inclusion of the free space gap within the horn antenna and the integrating cavity. Optical coupling efficiencies simulated using this developed technique show excellent agreement with the experimental measurements.

Highly sensitive temperature sensor based on the reciprocal thermal responses of cascaded dual-cavities

A highly sensitive fiber temperature sensor based on cascaded multimode fiber cavity and polymer cavity between single mode fibers was proposed. A three-beam Fabry-Pérot interferometer was obtained. We demodulated the interference spectrum by means of the fast Fourier transform and the signal filtering. As the thermal response of the polymer cavity and the multimode fiber cavity are negative and positive, respectively, the high temperature sensitivity can be achieved by a subtraction between the two reciprocal thermal responses. Experimental results show that the sensitivity of the temperature sensor is as high as 2.95 nm/°C in the measurement range between −15 °C and 35 °C. The proposed sensor may find widespread application in biomedicine and environmental science due to its simple preparation method and high sensitivity.