Meterscale Strong Coupling between Magnons and Photons

Jinwei Rao, C. Y. Wang, Bimu Yao, Z. J. Chen, K. X. Zhao, and Wei Lu

Phys. Rev. Lett. 131, 106702 – Published 7 September 2023

Abstract

We experimentally realize a meterscale strong coupling effect between magnons and photons at room temperature, with a coherent coupling of $\sim 20 m$ and a dissipative coupling of $\sim 7.6 m$ . To this end, we integrate a saturable gain into a microwave cavity and then couple this active cavity to a magnon mode via a long coaxial cable. The gain compensates for the cavity dissipation, but preserves the cavity radiation that mediates the indirect photon-magnon coupling. It thus enables the long-range strong photon-magnon coupling. With full access to traveling waves, we demonstrate a remote control of photon-magnon coupling by modulating the phase and amplitude of traveling waves, rather than reconfiguring subsystems themselves. Our method for realizing long-range strong coupling in cavity magnonics provides a general idea for other physical systems. Our experimental achievements may promote the construction of information networks based on cavity magnonics.

Received 4 May 2023
Revised 13 July 2023
Accepted 9 August 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.106702

Physics Subject Headings (PhySH)

Magnetization dynamics Magnons Spintronics

Magnetic insulators

Ferromagnetic resonance Microwave techniques

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Jinwei Rao ^1,*, C. Y. Wang¹, Bimu Yao ^2,1,†, Z. J. Chen¹, K. X. Zhao¹, and Wei Lu ^1,2,‡

¹School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
²State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

^*raojw@shanghaitech.edu.cn
^†yaobimu@mail.sitp.ac.cn
^‡luwei@shanghaitech.edu.cn

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Vol. 131, Iss. 10 — 8 September 2023

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Images

Figure 1
(a) Top: Strong photon-magnon coupling achieved in a passive cavity because of the direct mode overlap. Bottom: Photon-magnon cooperativity smaller than unity [21], when the distance between subsystems is extended, hindering the realization of strong coupling. (b) Schematic picture of our experimental setup, where the cavity is a planar structure. A phase shifter and an attenuator are inserted between the cable and the cavity. They are used to modulate traveling waves in the coaxial cable. (c) Planar active cavity embedded with an amplifier. Positions A and B are selected to demonstrate strongly coherent and dissipative photon-magnon coupling, respectively. (d)(e) Measured transmission spectra ( $| S_{21} |$ ) of the cavity with the transistor off and on, respectively. When the transistor is on, the cavity mode shifts from $ω_{c}^{'} / 2 π = 4.0 GHz$ to $ω_{c} / 2 π = 3.82 GHz$ .
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Figure 2
(a),(b) Measured transmission spectra of our system with coherent photon-magnon coupling, when sweeping up and down the field detuning ( $Δ$ ), respectively. Two frequency jump points, respectively, occur at $Δ_{u}$ and $Δ_{d}$ . White dashed lines indicate the dispersion of the magnon mode. (c) Frequencies of the synchronization mode extracted from (a) and (b) are plotted as a function of $Δ$ . (d) Calculated relative phase ( $θ$ ) between two modes at different $Δ$ values. The black lines are theoretical calculations using Eqs. (1a) and (1b). The dashed lines in (c) and (d) represent the unstable values of $θ$ and $ω_{s}$ . Orange and green arrows indicate the processes of sweeping up and down $Δ$ .
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Figure 3
(a) YIG sphere position for realizing the strongly dissipative photon-magnon coupling. $\vec{h}$ and $\vec{e}$ represent the microwave magnetic and electric fields, respectively. (b) Measured transmission spectra of our system with dissipative photon-magnon coupling at different detuning. (c) Extracted synchronization mode frequencies from (b) plotted as functions of $Δ$ . The solid line is the theoretical calculation. (d) Calculated $θ$ at different $Δ$ values by using Eq. (1a). Blue color in (c) and (d) indicates the detuning range of the mode coalescence.
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Figure 4
(a),(d) Coherent and dissipative coupling strengths as linear functions of the transmission coefficient $σ$ of the attenuator. Their top axes indicate the equivalent length of coaxial cable for each $σ$ . Black solid lines are linear fitting results. The orange (blue) arrow indicates the intersection of the coherent (dissipative) coupling strength and the magnon damping. (b),(c) Bistability of $ω_{s}$ measured with $σ = 1$ and 0.63. Orange and green dots represent synchronization mode frequencies from upward and downward sweep of $Δ$ . (e),(f) Measured $ω_{s}$ in the purely dissipative coupling case with $σ = 1$ and 0.63. Black solid lines in (b), (c), (e), (f) are calculated by using Eq. (1).
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