Microscopic origin of level attraction for a coupled magnon-photon system in a microwave cavity

We discuss various microscopic mechanisms for level attraction in a hybridized magnon-photon system of a ferromagnet in a microwave cavity. The discussion is based upon the electromagnetic theory of continuous media where the effects of the internal magnetization dynamics of the ferromagnet are described using dynamical response functions. This approach is in agreement with quantized multi-oscillator models of coupled photon-magnon dynamics. We demonstrate that to provide the attractive interaction between the modes, the effective response functions should be diamagnetic. Magneto-optical coupling is found to be one mechanism for the effective diamagnetic response, which is proportional to photon number. A dual mechanism based on the Aharonov-Casher effect is also highlighted, which is instead dependent on magnon number.Comment: New Journal of Physics, Focus on Cavity Optomagnonics Issu

Tunable focusing in natural hyperbolic magnetic media

While optical effects such as negative refraction and imaging obtained from slab lenses with plane parallel sides are widely studied using metamaterials, it is less well known that these effects can occur naturally in certain materials. We discuss a class of natural hyperbolic materials that not only display similar effects but also allow one to modify the focal length of a slab lens with an externally applied magnetic field. This is possible because antiferromagnets are gyrotropic and support magnetic polaritons whose frequencies are sensitive to magnetic fields. In addition, a rich caustic structure emerges at low temperatures, when damping should be small. These materials also produce slab focusing at higher temperatures, although the caustic structure disappears

Steering between level repulsion and attraction: broad tunability of two-port driven cavity magnon-polaritons

Cavity-magnon polaritons (CMPs) are the associated quasiparticles of the hybridization between cavity photons and magnons in a magnetic sample placed in a microwave resonator. In the strong coupling regime, where the macroscopic coupling strength exceeds the individual dissipation, there is a coherent exchange of information. This renders CMPs as promising candidates for future applications such as in information processing. Recent advances on the study of the CMP now allow not only for creation of CMPs on demand, but also for tuning of the coupling strength—this can be thought of as the enhancement or suppression of information exchange. Here, we go beyond standard single-port driven CMPs and employ a two-port driven CMP. We control the coupling strength by the relative phase ϕ and amplitude field ratio δ0 between both ports. Specifically, we derive a new expression from input–output theory for the study of the two-port driven CMP and discuss the implications on the coupling strength. Furthermore, we examine intermediate cases where the relative phase is tuned between its maximal and minimal value and, in particular, the high δ0 regime, which has not been yet explored

Using magnetic hyperbolic metamaterials as high frequency tunable filters

Metamaterials have enabled a series of major advances in optical devices in the past decade. Here, we suggest a type of hyperbolic metamaterial based on spin canting in magnetic multi-layers. We show that these structures have unique features in microwave waveguides that act as tunable filters. In the resulting band pass filter, we demonstrate an exceptional frequency tunability of 30 GHz with external fields smaller than 500 Oe. Unlike single metallic ferromagnetic films, we also demonstrate a high-frequency band-stop filter at very low fields

Control of the Coupling Strength and the Linewidth of a Cavity-Magnon Polariton

The full coherent control of hybridized systems such as strongly coupled cavity photon-magnon states is a crucial step to enable future information processing technologies. Thus, it is particularly interesting to engineer deliberate control mechanisms such as the full control of the coupling strength as a measure for coherent information exchange. In this work, we employ cavity resonator spectroscopy to demonstrate the complete control of the coupling strength of hybridized cavity photon-magnon states. For this, we use two driving microwave inputs which can be tuned at will. Here, only the first input couples directly to the cavity resonator photons, whilst the second tone exclusively acts as a direct input for the magnons. For these inputs, both the relative phase $\phi$ and amplitude $\delta_0$ can be independently controlled. We demonstrate that for specific quadratures between both tones, we can increase the coupling strength, close the anticrossing gap, and enter a regime of level merging. At the transition, the total amplitude is enhanced by a factor of 1000 and we observe an additional linewidth decrease of $13\%$ at resonance due to level merging. Such control of the coupling, and hence linewidth, open up an avenue to enable or suppress an exchange of information and bridging the gap between quantum information and spintronics applications.Comment: 9 pages, 6 figure

Emergent asymmetries and enhancement in the absorption of natural hyperbolic crystals

The effects of the anisotropy orientation in hyperbolic media have only recently emerged as a way to control and manipulate several optical effects. Here, we show from both experimental and theoretical evidence that highly oriented-asymmetric absorption can be induced in simple crystal quartz. This can be achieved by controlling the orientation of the anisotropy with respect to the surface of the crystal at infrared regions where crystal quartz behaves as a hyperbolic medium. What is perhaps most intriguing here is that not only is the absorption asymmetric, but it can also be significantly enhanced. Finally, we also show various mechanisms through which the asymmetry in the absorption can be optimized, such as controlling the thickness of the crystal. Such phenomena are key for directional-dependent optical devices and present a pathway for engineering angle-encoded detection and sensing

Oriented asymmetric wave propagation and refraction bending in hyperbolic media

Crystal quartz is a well-known anisotropic medium with optically active phonons in the THz region where hyperbolic phonon-polaritons can be excited. Here, we use this material to illustrate how the behavior of bulk and surface hyperbolic polaritons can be drastically modified by changing the orientation of the crystal’s anisotropy axis with respect to its surface. We demonstrate, both theoretically and experimentally, phenomena associated with the orientation of hyperbolic media. We show the consequences of changes in the crystal’s orientation in various ways, such as the modification of the effective reststrahl regions and associated surface phonon polariton dispersion. Of particular significance, however, is the transmission behavior of radiation passing through a rotated hyperbolic crystal. Here, even a small rotation of the optical axes with respect to the crystal surface can lead to a very large degree of asymmetry in the transmitted intensity. In addition, the refracting angle (which in a hyperbolic medium may correspond to negative refraction and slab lensing behavior) itself becomes asymmetric, so that a slab lens with a laterally displaced image becomes possible. We discuss some of the possible consequences of these types of effects

Steering between level repulsion and attraction : broad tunability of two-port driven cavity magnon-polaritons

Cavity-magnon polaritons (CMPs) are the associated quasiparticles of the hybridization between cavity photons and magnons in a magnetic sample placed in a microwave resonator. in the strong coupling regime, where the macroscopic coupling strength exceeds the individual dissipation, there is a coherent exchange of information. this renders cmps as promising candidates for future applications such as in information processing. recent advances on the study of the cmp now allow not only for creation of cmps on demand, but also for tuning of the coupling strength-this can be thought of as the enhancement or suppression of information exchange. here, we go beyond standard single-port driven cmps and employ a two-port driven cmp. we control the coupling strength by the relative phase phi and amplitude field ratio delta(0) between both ports. specifically, we derive a new expression from input-output theory for the study of the two-port driven cmp and discuss the implications on the coupling strength. furthermore, we examine intermediate cases where the relative phase is tuned between its maximal and minimal value and, in particular, the high delta(0) regime, which has not been yet explored

Nonreciprocity in millimeter wave devices using a magnetic grating metamaterial

The control and manipulation of many of light's fundamental properties, such as reflectivity, has become a topic of increasing interest since the advent of engineered electromagnetic structures—now known as metamaterials. Many of these metamaterial structures are based on the properties of dielectric materials. Magnetic materials, on the other hand, have long been known to interact with electromagnetic waves in unusual ways; in particular, their nonreciprocal properties have enabled rapid advances in millimeter wave technology. Here, we show how a structured magnetic grating can be employed to engineer electromagnetic response at frequencies upwards of hundreds of gigahertz. In particular, we investigate how nonreciprocal reflection can be induced and controlled in this spectral region through the composition of the magnetic grating. Moreover, we find that both surface and guided polaritons contribute to high-frequency nonreciprocity; the nature of these is also investigated. Control of electromagnetic radiation at high frequencies is a current challenge of communications technology where our magnetic gradient might be employed in devices including signal processing filters and unidirectional isolators

Tuning magnetic order with geometry: thermalisation and defects in two-dimensional artificial spin ices

Artificial spin ices are arrays of correlated nano-scale magnetic islands that prove an excellent playground in which to study the role of topology in critical phenomena. Here, we investigate a continuum of spin ice geometries, parameterised by rotation of the islands. In doing so, we morph from the classic square ice to the recently studied pinwheel geometry, with the rotation angle acting as a proxy for controlling inter-island interactions. We experimentally observe a transition from antiferromagnetic ordering in square ice to a slight preference for ferromagnetic vertices in the weakly-coupled pinwheel ice using Lorentz transmission electron microscopy on thermally annealed cobalt arrays. The rotation angle also affects the relaxation timescales for individual arrays, leading to varying degrees of thermalisation, and an apparent change in the nature of the defects supported: from one-dimensional strings in square ice to two-dimensional vortex-like structures for geometries similar to pinwheel. The numerical scaling of these quantities is consistent with that predicted by the Kibble-Zurek mechanism. Our results show how magnetic order in artificial spin ices can be tuned by changes in geometry and suggest the possibility of realising a truly frustrated ice-rule phase in two-dimensional systems. Furthermore, we demonstrate this system as a testbed to investigate out-of-equilibrium dynamics across phases