Alto Osada

Bidirectional conversion between microwave and light via ferromagnetic magnons

Coherent conversion of microwave and optical photons in the single quantum level can significantly expand our ability to process signals in various fields. Efficient up-conversion of a feeble signal in the microwave domain to the optical domain will lead to quantum-noise-limited microwave amplifiers. Coherent exchange between optical photons and microwave photons will also be a stepping stone to realize long-distance quantum communication. Here we demonstrate bidirectional and coherent conversion between microwave and light using collective spin excitations in a ferromagnet. The converter consists of two harmonic oscillator modes, a microwave cavity mode and a magnetostatic mode called the Kittel mode, where microwave photons and magnons in the respective modes are strongly coupled and hybridized. An itinerant microwave field and a traveling optical field can be coupled through the hybrid system, where the microwave field is coupled to the hybrid system through the cavity mode, while the optical field addresses the hybrid system through the Kittel mode via Faraday and inverse Faraday effects. The conversion efficiency is theoretically analyzed and experimentally evaluated. The possible schemes for improving the efficiency are also discussed.

Cavity Optomagnonics with Spin-Orbit Coupled Photons.

We experimentally implement a system of cavity optomagnonics, where a sphere of ferromagnetic material supports whispering gallery modes (WGMs) for photons and the magnetostatic mode for magnons. We observe pronounced nonreciprocity and asymmetry in the sideband signals generated by the magnon-induced Brillouin scattering of light. The spin-orbit coupled nature of the WGM photons, their geometrical birefringence, and the time-reversal symmetry breaking in the magnon dynamics impose the angular-momentum selection rules in the scattering process and account for the observed phenomena. The unique features of the system may find interesting applications at the crossroad between quantum optics and spintronics.

Experimental mapping of magnetostatic mode structures of a ferrimagnet spheroid

The exploration of the interaction of light with spin waves in ferromagnets within an optical cavity might lead to new chiral photonic devices and be a stepping stone towards the coherent optical manipulation of magnons in the quantum regime [1]. The developments made so far in cavity optomagnonics have been focused on the fundamental magnetostatic mode of an yttrium iron garnet (YIG) sphere, so-called ‘Kittel mode’ [2, 3] and lead to the observation of a magnon-induced Brillouin scattering between optical whispering gallery modes following selection rules dictated by the conservation of the orbital momentum. Higher-order magnetostatic modes, with reduced mode volume and different orbital angular momentum, could couple more efficiently with the optical whispering gallery modes hosted by the YIG sphere, lying in the vicinity of the sphere surface equatorial plane. Though the resonance frequencies of these modes can be predicted theoretically [4], small deviations due to the actual sample properties or environment, as well as possible hybridization of the modes, could cause strong misinterpretations. Hence, unambiguously experimentally identifying higher-order magnon modes in a spheroid is required to properly scrutinize their interaction with light.