Light-matter interaction is a key photonics phenomenon that is essential to quantum-information technology. In particular, the coupling of microwave photons to magnons (known as photon-magnon coupling) has received much attention due to its potential applications in coherent information storage. In most studies on magnon-photon coupling, three-dimensional (3D) structures consisting of a cavity resonator and an yttrium iron garnet (YIG) sphere have been used in real experiments wherein the photon and magnon modes were excited by the photon cavity boundary and the spin precession under an external static magnetic field, respectively. However, the main disadvantage of the 3D cavity approach is that the technology is relatively complex and impractical for applications. In the present study, we observed magnon-photon coupling at room temperature in a compact planar-geometry hybrid system consisting of an inverted split-ring resonator (ISRR) and an YIG film.
We experimentally demonstrated strong anti-crossing effects of the ISRR’s photon mode and YIG film’s magnon modes with an effective photon-magnon coupling strength (geff/2π) of 90 MHz at a microwave frequency of f = 3.79 GHz . The spin-number-normalized coupling strength was determined to be geff/2π√N = 0.194 Hz, higher than any reported to date, which result highlights a great potential for quantum information processing. Furthermore, we found that geff/2π can be increased up to 64% simply by changing φ from 90° to 0°, where φ is the angle between the in-plane magnetic field and the transverse direction of the device.
We also observed, in the anti-crossing region, fine photon-magnon coupling features originating from the excitation of different spin-wave modes such as magnetostatic surface waves (MSSWs) and backward-volume magnetostatic waves (BVMSWs). In order to elucidate the underlying physics of such features, we performed delicate measurements of the |S21| power on the f-H plane with 1 Oe fine scans for different field directions ( φ = 0 to 90°). It was demonstrated the coherent magnon-photon coupling for BVMSW modes (0 ≤ φ ≤ 30°) and MSSW modes (30° ≤ φ ≤ 90°) as well as for Kittel modes. Using the coupled-oscillator model, the coupling strength corresponding to each specific spin-wave mode was estimated quantitatively and found to decrease with the mode index.
Our experimental results provide a means for the design of new types of high-gain magnon-photon coupling systems in planar geometry and establish, furthermore, a new approach to the exploration of magnetostatic modes in magnetic systems. Such multi-mode coupling is very interesting in both the fundamental and practical aspects of signal processing, magnon conversion, and quantum memory.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015R1A2A1A10056286).
 B. Bhoi, et al., Sci. Rep., 7, 11930 (2017).