Exceptional Antimodes in Multi-Drive Cavity Magnonics

TL;DR

The paper demonstrates a four-port, three-mode cavity-magnonics system in which two independent microwave drives, with controllable phase and amplitude, create interference-based exceptional points via zeros of the $S$-matrix rather than poles. By introducing a YIG sphere between coupled resonators, the authors realize a tunable antimode spectrum that can transition between level attraction and repulsion, and they show that exceptional antimodes at a single output port enable coherent perfect extinction and active steering of transmission. A dedicated theoretical framework connects $S$-matrix zeros to antimodes and uses an antimode-specific condition $\overline{z}_{+}=\overline{z}_{-}$ to define exceptional points, while experimental calibration and parameter estimation validate the model against measured spectra. The approach offers robust, frequency-flexible access to exceptional points and related high-sensitivity sensing in microwave circuits, with potential for interferometric schemes and practical sensing devices that do not rely on fine-tuning intrinsic system parameters. Overall, the work expands non-Hermitian photonics in cavity magnonics by leveraging multi-drive interference to engineer and harness exceptional points and antimodes for advanced microwave technologies.

Abstract

Driven-dissipative systems provide a natural setting for the emergence of exceptional points -- i.e. non-Hermitian degeneracies where eigenmodes coalesce. These points are important for applications such as sensing, where enhanced sensitivity is required, and exhibit interesting and useful phenomena that can be controlled with experimentally accessible parameters. In this regard a four-port, three-mode, cavity-magnonics platform is demonstrated in which two microwave excitations can be precisely phase shifted and/or attenuated relative to one another. Destructive interference between the hybridised cavity-magnon modes is shown to give rise to antimodes (antiresonances) in the transmission spectrum, enabling coherent perfect extinction of the outgoing signals at selected ports. This interference can be used to actively tune the position and properties of exceptional points, without the fine tuning conventionally required to obtain exceptional points. Such controllable, interference-based engineering of exceptional points provides a practical and flexible pathway toward next-generation, high-sensitivity sensing devices operating at microwave frequencies.

Figures (12)

• Figure 1: (a) Schematic of the experimental platform in the input–output configuration used for the multi-excitation measurements. Two open transmission line resonators are placed in close proximity, allowing inductive coupling between their modes with strength $J$. A YIG sphere is positioned such that it can couple to both resonators. Independent microwave drives can be applied at the four ports $b_{\mathrm{in},1\text{--}4}$, with corresponding output fields $b_{\mathrm{out},1\text{--}4}$. (b) Interaction diagram of the system Hamiltonian. The upper ($a_u$) and lower ($a_l$) resonator modes are coherently coupled with strength $J$, and each couples to the common magnon mode ($m$) with coupling strengths $g_u$ and $g_l$, respectively. Each resonator is also coupled to two independent input–output channels $b_i$.
• Figure 2: $S_{21}$, corresponding to the direct transmission in Eq. (\ref{['eq:tdti']}), of the empty resonator. The transmission in the experiment is significantly less than the simulation primarily because of losses due to the external circuitry connected to the VNA. There is also a static oscillating background due to interference in the cables that is not present in the simulation. In this case it does not greatly obscure fitting to the input-output model.
• Figure 3: Magnetic field intensity profiles for (a) the in-phase dual cavity mode (b) the antiresonance (c) the out-of-phase mode. The in-phase resonators have a magnetic field minima at the midpoint as discussed in the main text while for the out-of-phase resonators the field is at a maximum.
• Figure 4: COMSOL simulation and experimental data showing the effect of tuning the relative phase and amplitude of inputs to ports one and three with the output measured at port two. The experimental data, particularly for $\delta =$ 1.0 and 1.5, contains additional spurious antiresonances occuring due to interference with the external circuitry that is unaccounted for in the simulations.
• Figure 5: Transmission spectra showing the effect of tuning the relative amplitude of the drives $\delta$, the relative phase of the drives $\Phi$, and tuning the coupling strengths, $g_l$ and $g_u$. We assume a pair of $\omega_a/(2\pi)=4.8$ GHz resonators inductively coupled with strength $J/(2\pi)=160$ MHz. The coupling strengths are measured in units of $g/(2\pi)=50$ MHz. Parts (a) and (d) show the effect of increasing $\delta$ with $\Phi=180^{\circ}$. The system exhibits antimode level repulsion for $\delta<1$, and level attraction for $\delta>1$. Parts (b) and (e) show the affect of tuning $\Phi$. Going from $0^{\circ} \ \rightarrow \ 180^{\circ}$ can tune the system between level attraction and repulsion with the linewidth of the horizontal antimode becoming large at intermediate values of $\Phi$. Parts (c) and (f) show the affect of shifting the YIG sphere with $\delta=1$ and $\Phi=180^{\circ}$. The horizontal antimode is then degenerate with the lower cavity mode. With the YIG sphere closer to the lower resonator the antimodes exhibit level repulsion. Shifting the YIG sphere towards the upper resonator tunes the system into a level attraction regime. See the text for more details.