Magnon Polaritons Research Articles

Spatial shifts of reflected beams from surface polaritons in antiferromagnets

Goos–Hänchen (GH) and Imbert–Fedorov (IF) shifts of reflected beams and the effects of two Dyakonov surface magnon polaritons (DSMPs) on these shifts are studied in the Otto configuration with an antiferromagnetic substrate. A modified dispersion equation of the DSMPs is proposed to recognize GH and IF shifts related to the DSMPs. Reflected beam shift spectra are simulated for a F e F 2 substrate. Shift features around the two DSMPs are observed by frequency scanning with a fixed incident angle. The GH-shift amplitude corresponding to either DSMP is situated between the immediately adjacent maximum and minimum of a GH-shift curve. The shift-amplitude maximum of an IF-shift curve corresponds to the relevant DSMP accurately. GH and IF shifts also are given by frequency scanning along dispersion curves.

Experimental realization of topologically protected unidirectional surface magnon polaritons on ceramic YIG ferrites

Topologically protected, unidirectional surface magnon polaritons (SMPs) guided along the interface between air and ferrimagnetic yttrium iron garnet slab structures, biased with an external static magnetic field in the Voigt configuration, are realized experimentally by measuring the S-parameters between two small loop magnetic dipole antennas that launch and receive the SMP. Transmission is compared in the two cases where an abrupt change in the form of a step is present and absent in the interface. We find that transmission of the SMP peaks near the same level in each case, a sign of its topological protection. In addition, the dependence of isolation on slab thickness is investigated for a specified antenna configuration.

Bistability of cavity magnon polaritons beyond the Holstein–Primakoff transformation

We report a theoretical study of the bistability of cavity magnon polaritons (CMPs) controlled by dual magnetic nonlinearities. Based on the magnon Kerr effect, we introduce the nonlinear magnon effect in this hybrid system without using the Holstein–Primakoff transformation and find that these two types of nonlinear mechanisms jointly determine the bistability of the system. When the lower-branch polariton is exposed to a microwave driving field, the magnon frequency shift gives rise only to a clockwise hysteresis loop with the appearance of bistability. However, when the upper-branch polariton is exposed to a microwave driving field, the magnon frequency shift produces either a clockwise or counterclockwise hysteresis loop. Interestingly, the bistability vanishes because the Kerr effect cancels the nonlinear magnon effect. Moreover, the simultaneous bistability of both the lower- and upper-branch polaritons occurs when the microwave driving field drives one of the two polaritons. These results for the dual magnetic nonlinearities in CMPs offer more possibilities for information processing and cavity spintronic device design.

Long-Time Memory and Ternary Logic Gate Using a Multistable Cavity Magnonic System.

Multistability is an extraordinary nonlinear property of dynamical systems and can be explored to implement memory and switches. Here we experimentally realize the tristability in a three-mode cavity magnonic system with Kerr nonlinearity. The three stable states in the tristable region correspond to the stable solutions of the frequency shift of the cavity magnon polariton under specific driving conditions. We find that the system staying in which stable state depends on the history experienced by the system, and this state can be harnessed to store the history information. In our experiment, the memory time can reach as long as 5.11s. Moreover, we demonstrate the ternary logic gate with good on-off characteristics using this multistable hybrid system. Our new findings pave a way towards cavity magnonics-based information storage and processing.

Nonlinear level attraction of cavity axion polariton in antiferromagnetic topological insulator

Strong coupling between cavity photons and various excitations in condensed matters boosts the field of light-matter interaction and generates several exciting sub-fields, such as cavity optomechanics and cavity magnon polariton. Axion quasiparticles, emerging in topological insulators, were predicted to strongly couple with the light and generate the so-called axion polariton. Here, we demonstrate that there arises a gapless level attraction in cavity axion polariton of antiferromagnetic topological insulators, which originates from a nonlinear interaction between axion and the odd-order resonance of cavity. Such a novel level attraction is essentially different from conventional level attractions with the mechanism of either a linear coupling or a dissipation-mediated interaction, and also different from the level repulsion induced by the strong coupling in common polaritons. Our results reveal a new mechanism of level attractions, and open up new roads for exploring the axion polariton with cavity technologies. They have potential applications for quantum information and dark matter research.

Phase-Modulated Cavity Magnon Polaritons as a Precise Magnetic Field Probe

We describe and operate a spin-magnetometer based on the phase modulation of cavity magnon polaritons. In this scheme a rf magnetic field is detected through the sidebands it induces on a pump, and the experimental configuration allows for a negligible pump noise and a high-frequency readout. The demonstrator setup, based on a copper cavity coupled to an yttrium iron garnet sphere hybrid system, reached a sensitivity of $2.0\phantom{\rule{0.2em}{0ex}}\mathrm{pT}/\sqrt{\mathrm{Hz}}$ at 220 MHz, evading the pump noise and matching the theoretical predictions. An optimized setup can attain a rf magnetic field sensitivity of about $8\phantom{\rule{0.2em}{0ex}}\mathrm{fT}/\sqrt{\mathrm{Hz}}$ at room temperature. An orders of magnitude improvement is expected at lower temperatures, making this instrument one of the few magnetometers accessing the subfemtotesla limit.

Strong magnon–photon coupling with chip-integrated YIG in the zero-temperature limit

The cross-integration of spin-wave and superconducting technologies is a promising method for creating novel hybrid devices for future information processing technologies to store, manipulate, or convert data in both classical and quantum regimes. Hybrid magnon–polariton systems have been widely studied using bulk Yttrium Iron Garnet (Y3Fe5O12, YIG) and three-dimensional microwave photon cavities. However, limitations in YIG growth have, thus far, prevented its incorporation into CMOS compatible technologies, such as high-quality factor superconducting quantum technology. To overcome this impediment, we have used Plasma Focused Ion Beam (PFIB) technology—taking advantage of precision placement down to the micrometer scale—to integrate YIG with superconducting microwave devices. Ferromagnetic resonance has been measured at milliKelvin temperatures on PFIB-processed YIG samples using planar microwave circuits. Furthermore, we demonstrate strong coupling between superconducting resonators and YIG ferromagnetic resonance modes by maintaining reasonably low loss while reducing the system down to the micrometer scale. This achievement of strong coupling on-chip is a crucial step toward fabrication of functional hybrid quantum devices from spin-wave and superconducting components.

Dyakonov surface magnons and magnon polaritons

We predicted one Dyakonov surface magnon (DSM) and two Dyakonov magnon polaritons (DSMPs) localized at the antiferromagnet (AF)-air interface. Their concise dispersion equations and necessary conditions of existence were obtained. The DSM is the magnetostatic limit of the first DSMP, which are situated inside the AF reststrahlen frequency band. The second DSMP has no magnetostatic limit and lies outside the frequency band. The specifical frequency ranges occupied by the DSM and DSMPs were analytically found. The DSMPs are hybrid-polarization surface polaritons and the DSM is a hybrid-polarization surface magnetostatic mode. The first DSMP possesses large attenuation constants so that it is highly localized at the surface. Compared with it, the second DSMP exhibits relatively small attenuation constants. According to the features of polarization obtained for either DSMP, we used the TE and TM radiations incident on the AF surface to calculate attenuated total reflection (ATR) spectra, respectively. These ATR spectra not only further prove the existence of the DSMPs but also demonstrate their observability in experiment. However, the DSM should be detected by means of a light-scattering technique.

Brillouin precursors from magnon and optical phonon polaritons from a random-walk media

The dispersion of a broadband electromagnetic pulse in a dielectric was theorized by Sommerfeld and Brillouin to separate into parts, with two groups of frequencies traveling faster—or “running ahead”—of the main signal from a Lorentz-like media. The observation of one such group, the Brillouin precursor (BP), arising from magnon and phonon polaritons generated in a random-walk ultra-small particle ferrite media is reported for the first time. Upon input of a broadband low-frequency radio wave in the form of a very low frequency (VLF) square wave to the ferrite particle dielectric core of the Random Walk Antenna (RWA), BP was generated and detected by the VLF antenna. The BP observation propagating in free space and water is supported by two salient percussor features (1) temporal Bessel-like waveform, and (2) algebraic, rather than exponential, attenuation with distance. The Brillouin precursor observed here in the VLF region can lead to precursors in the THz and optical regime with respect to the resonance frequency of the quasi-particles of optical phonons and excitons, respectively

Interferometric control of magnon-induced nearly perfect absorption in cavity magnonics

The perfect absorption of electromagnetic waves has promoted many applications, including photovoltaics, radar cloaking, and molecular detection. Unlike conventional methods of critical coupling that require asymmetric boundaries or coherent perfect absorption that require multiple coherent incident beams, here we demonstrate single-beam perfect absorption in an on-chip cavity magnonic device without breaking its boundary symmetry. By exploiting magnon-mediated interference between two internal channels, both reflection and transmission of our device can be suppressed to zero, resulting in magnon-induced nearly perfect absorption (MIPA). Such interference can be tuned by the strength and direction of an external magnetic field, thus showing versatile controllability. Furthermore, the same multi-channel interference responsible for MIPA also produces level attraction (LA)-like hybridization between a cavity magnon polariton mode and a cavity photon mode, demonstrating that LA-like hybridization can be surprisingly realized in a coherently coupled system.

Magnon photon coupling for magnetization antiparallel to the magnetic field

Current studies of cavity magnon polaritons are focused on ferromagnetic magnons for which the frequency increases with a static magnetic field. In this paper, we propose a ferromagnetic system with magnon frequency decreasing with a static magnetic field. It is achieved by antiparallel alignment between the magnetization and static magnetic field. The magnetization precession is stabilized by a large anisotropic field along the direction of magnetization. The analysis of the Polder tensor shows that the magnon modes in parallel and antiparallel alignments are analogous to those in an antiferromagnet. The strong coupling between a magnon and photon for antiparallel alignment results in an anticrossing gap in the transmission spectrum. Based on the Tavis-Cummings Hamiltonian and Bloch sphere representation, we show that the photon absorption decreases (increases) the spin angular momentum in antiparallel (parallel) alignment. The coupled Hamiltonians of harmonic oscillators are derived and have the same form for both parallel and antiparallel cases. The method developed and results presented are expected to be helpful to realize low-frequency magnon photon coupling that is similar to those in an antiferromagnet.

Tristability of cavity magnon polaritons

We carry out a theoretical study of the nonlinear behavior of cavity magnon polaritons (CMPs) under strong microwave driving, with two kinds of nonlinearity being involved: either (i) the photon Kerr effect (PKE) and the magnon saturation effect (MSE) or (ii) the magnon Kerr effect (MKE) and the MSE. The effects arising in these two cases are similar for the lower polariton mode of CMPs but opposite for the upper polariton mode. In case (i), the dominant nonlinearity for the upper polariton mode of CMPs changes from the MSE to the PKE as the driving intensity increases, which can lead to two hysteresis loops in opposite directions and hence to tristability. In case (ii), however, the upper polariton mode exhibits only the usual bistability rather than tristability since the MKE is dominated by the MSE. In particular, there is no bistable behavior in the upper polariton mode when the two nonlinearities balance each other. The results of this study provide a possible basis for the practical manipulation and control of information via the double nonlinearities in a cavity-magnon system.

Cavity magnon polariton based precision magnetometry

A photon-magnon hybrid system can be realized by coupling the electron spin resonance of a magnetic material to a microwave cavity mode. The quasiparticles associated with the system dynamics are the cavity magnon polaritons, which arise from the mixing of strongly coupled magnons and photons. We illustrate how these particles can be used to probe the magnetization of a sample with a remarkable sensitivity, devising suitable spin-magnetometers, which ultimately can be used to directly assess oscillating magnetic fields. Specifically, the capability of cavity magnon polaritons of converting magnetic excitations to electromagnetic ones allows for translating to magnetism the quantum-limited sensitivity achieved by state-of-the-art microwave detectors. Here, we employ hybrid systems composed of microwave cavities and ferrimagnetic spheres to experimentally implement two types of novel spin-magnetometers.

Universal characterization of cavity–magnon polariton coupling strength verified in modifiable microwave cavity

A comprehensive study of the frequency dependence of the photon–magnon coupling for different magnetic samples is made possible with a tunable 3D-printed re-entrant cavity. Strong coupling is achieved with values ranging between 20 and 140 MHz. The reworked theory, experimentally verified for the first time here, enables coupling values to be calculated from simulations alone, enabling future experiments with exotic cavity designs to be precisely engineered, with no limitations on sample and cavity geometry. Finally, the requirements of the deep strong coupling regime are shown to be achievable in such experiments.

Circulating cavity magnon polaritons

We predict magnon polariton states circulating unidirectionally in a microwave cavity when loaded by a number of magnets on special lines. Realistic finite-element numerical simulations, including dielectric, time-dependent and non-linear effects, confirm the validity of the approximations of a fully analytical input-output model. We find that a phased antenna array can focus all power into a coherent microwave beam with controlled direction and an intensity that scales with the number of magnets.

Laser-induced regular nanostructure chains within microgrooves of Fe-based metallic glass

Femtosecond laser-induced surface structures have been extensively investigated over decades, however, their formation is not well controlled especially for the practical use in the photonic field. We here report the highly regular nanostructure formation within the grooves of the Fe-based metallic glass, upon irradiation of two temporally delayed femtosecond laser beams. For the case of micrometer-sized groove, multiple chains of spindle-like nanostructures are developed, with the spatial periods of 667 nm and 180 nm for the chain and spindle distributions, respectively. Their arrangement properties can be modulated by varying laser parameters. On the other hand, for the case of submicrometer-sized groove, there is only one chain of spindle-like nanostructures formed inside with the improved regular appearance. Two physical effects, including the plasmonic field generation from the groove edges and the surface magnon polariton excitation within the groove, are considered responsible for the phenomena, which consequently result in the spatial interlocking of two types of ripples. The theoretical analyses match well the experimental observations.

Sharply vanishing destructive interference induced by magnon Kerr effect in cavity magnon polaritons

Motivated by the recently discovered nonlinear dynamics of cavity–magnon polaritons (CMPs), bistable CMP behavior is investigated theoretically in the case of two yttrium iron garnet (YIG) spheres (YIG1 and YIG2) in a microwave cavity, which is induced by the magnon Kerr effect of YIG1 originating from the magnetocrystalline anisotropy. The bistability appears in the form of sharp CMP frequency switching. More importantly, when the driving field acts on YIG1 and drives the magnon dark mode, the nonlinear Kerr effect at high excitation plays an important role in creating frequency detuning between the two YIG spheres. This results in sharply vanishing destructive interference between two YIG spheres and simultaneously makes the invisible magnon dark mode visible. However, when the driving field acts on the cavity, the magnon dark mode remains dark and there is no bistability in the CMPs when the driving frequency is tuned to the magnon dark mode, this being because the excitation energy cannot be transferred from cavity to magnon. The present research provides a promising approach for information storage and processing in multimode systems under high excitation.

Manipulation of bistability through the coupling strength in cavity magnon polaritons

Benefiting from the latest experiment of non-linearity in cavity magnon polaritons due to the magnetocrystalline anisotropy in the yttrium iron garnet (YIG) crystal, we theoretically propose a cavity magnonics model that can be used to study the bistable behaviors by changing the coupling strength, which presents the rich and interesting phenomena. In the case of resonance, the critical driving powers of the upper and lower polariton modes have the smaller values for the larger coupling strength but the larger values for the smaller coupling strength. In addition, the upper and lower polariton modes show the repulsive and attractive features, respectively, when the derivative of the coupling strength with respect to the position of the YIG sphere changes from positive to negative. Especially, the bistability of the lower polariton mode disappears when the driving power is strong enough. Our research broadens the means of manipulation of non-linearity in cavity magnon system, which provides a promising application for cavity spintronics and information processing.

Controlling spins with surface magnon polaritons

Polaritons in metals, semimetals, semiconductors, and polar insulators can allow for extreme confinement of electromagnetic energy, providing many promising opportunities for enhancing typically weak light-matter interactions such as multipolar radiation, multiphoton spontaneous emission, Raman scattering, and material nonlinearities. These extremely confined polaritons are quasielectrostatic in nature, with most of their energy residing in the electric field. As a result, these ``electric'' polaritons are far from optimized for enhancing emission of a magnetic nature, such as spin relaxation, which is typically many orders of magnitude slower than corresponding electric decays. Here, we take concepts of ``electric'' polaritons into magnetic materials, and propose using surface magnon polaritons in negative magnetic permeability materials to strongly enhance spin relaxation in nearby emitters. Specifically, we provide quantitative examples with ${\mathrm{MnF}}_{2}$ and ${\mathrm{FeF}}_{2}$, enhancing spin transitions in the THz spectral range. We find that these magnetic polaritons in 100-nm thin films can be confined to lengths over 10 000 times smaller than the wavelength of a photon at the same frequency, allowing for a surprising 12 orders of magnitude enhancement in magnetic dipole transitions. This takes THz spin-flip transitions, which normally occur at timescales on the order of a year, and forces them to occur at sub-ms timescales. Our results suggest an interesting platform for polaritonics at THz frequencies, and more broadly, a way to use polaritons to control light-matter interactions.

Experimental Observation of an Exceptional Surface in Synthetic Dimensions with Magnon Polaritons.

Exceptional points (EPs) are singularities of energy levels in generalized eigenvalue systems. In this Letter, we demonstrate the surface of EPs on a magnon polariton platform composed of coupled magnons and microwave photons. Our experiments show that EPs form a three-dimensional exceptional surface (ES) when the system is tuned in a four-dimensional synthetic space. We demonstrate that there exists an exceptional saddle point (ESP) in the ES which originates from the unique couplings between magnons and microwave photons. Such an ESP exhibits unique anisotropic behaviors in both the real and imaginary parts of the eigenfrequencies. To the best of our knowledge, this is the first experimental observation of ES, opening up new opportunities for high-dimensional control of non-Hermitian systems.