Cavity optomagnonics with magnetic textures: Coupling a magnetic vortex to light

Abstract

Optomagnonic systems, where light couples coherently to collective excitations in magnetically ordered solids, are currently of high interest due to their potential for quantum information processing platforms at the nanoscale. Efforts so far, both at the experimental and theoretical level, have focused on systems with a homogeneous magnetic background. A unique feature in optomagnonics is however the possibility of coupling light to spin excitations on top of magnetic textures. We propose a cavity-optomagnonic system with a non homogeneous magnetic ground state, namely a vortex in a magnetic microdisk. In particular we study the coupling between optical whispering gallery modes to magnon modes localized at the vortex. We show that the optomagnonic coupling has a rich spatial structure and that it can be tuned by an externally applied magnetic field. Our results predict cooperativities at maximum photon density of the order of C ≈ 10 −2 by proper engineering of these structures.Introduction.-Optomagnonics is an exciting new field where light couples coherently to elementary excitations in magnetically ordered systems. The origin of this photon-magnon interaction is the Faraday effect, where the magnetization in the sample causes the light's polarization plane to rotate. Conversely, the light exerts a small effective magnetic field on the material's magnetic moments. Shaping the host material into an optical cavity enhances the effective coupling according to the increased number of trapped photons.Recent seminal experiments have demonstrated this coupling [1][2][3]. In these, an Yttrium Iron Garnet (YIG) sphere serves as the host of the magnetic excitations and, via whispering gallery modes (WGM), as the optical cavity. The optomagnonic coupling manifests itself in transmission sidebands at the magnon frequency. So far, these experiments have probed mostly the homogeneous magnetic mode (Kittel mode) where all spins rotate in phase [4]. Very recently, optomagnonic coupling to other magnetostatic modes [5,6] has been demonstrated, albeit still on top of a homogeneous background [7,8].The Kittel mode, although it is the simplest one to probe and externally tune, is a bulk mode and has a suboptimal overlap with the optical WGMs living near the surface. Another caveat is the state of the art in terms of sample size, which is currently sub-millimetric. This results in modest values for the optomagnonic coupling and motivates the quest for smaller, micron-sized magnetic samples, as well as for engineering the coupling between magnetic and optical modes. Increasing the currently observed values of optomagnonic coupling is an urgent prerequisite for moving on to promising applications such as magnon cooling, coherent state transfer, or efficient wavelength converters [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24].In microscale magnetic samples, the competition between the short-range exchange interaction and the boundary-sensitive demagnetization fields can lead to magnetic textures, where the magnetic gr...

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

“…The phenomena of (strong) coupling of magnons-the associated quanta of collective spin wave excitations-to microwave cavity photons resulting in cavity magnon-polaritons (CMPs) has been the subject of numerous works in the past few years [1][2][3][4][5][6][7][8][9]. The ability to couple magnons to different physical systems, through magnetooptical [10][11][12] to optical, or by magnetostrictive interaction to mechanical [13] and cavity photons simultaneously makes CMPs highly interesting for various applications [8]. For instance, it allows for a bidirectional conversion of microwaves to optical light [14], or coupling magnons with superconducting circuits, i.e.…”

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

“…In optomagnonics, cavity photons couple to magnetic excitations via the magnetooptical interactions, which allows characterization using a well-established optomechanical Hamiltonian [15,16], and, as a consequence, one should be able to observe optomechanical effects in magnetic systems [17]. Various phenomena have been proposed for coupled magnon-photon systems inside microwave cavities including Brillouin-scattering-induced transparency in whispering gallery resonators [18][19][20], collective dynamics of spin textures [21,22], and photon-mediated nonlocal interactions [23][24][25][26].…”

Quantum electrodynamics with magnetic textures

“…The latter allows exciting mostly the uniform Kittel mode in which all spins precese in unison. Fewer works have analysed the case of higher-order spin wave modes in confined geometries [14][15][16][17] and individual magnetic solitons such as vortices in soft-magnetic discs [18,19]. Magnetic vortices are extremely stable magnetic textures exhibiting a very rich dynamical behaviour in the sub-GHz to tens of GHz range.…”