Stimulated Magnonic Frequency Combs Achieve Efficient Control over Spectral Line Number

Magnonic frequency combs represent a burgeoning area within magnon spintronics, offering potential for breakthroughs in information processing and sensing. Xueyu Guo from Huazhong University of Science and Technology, Tianci Gong from the University of Electronic Science and Technology of China, and Guibin Lan from the Chinese Academy of Sciences, along with their colleagues, have now demonstrated a novel method for generating these combs, circumventing the limitations of traditional three-magnon scattering. Their research, detailed in this paper, establishes a stimulated generation mechanism offering precise control over comb characteristics and significantly lowers the power threshold required for creation, paving the way for the practical integration of magnonic frequency combs into future spintronic technologies.

The optical frequency comb functions as a precise ruler for measuring optical frequencies, enabling applications in spectroscopy, atomic clocks, laser ranging, chemical analysis, and telecommunications. Spin waves, or magnons, are emerging as a platform for frequency comb generation due to their intrinsic nonlinearity, soliton formation, strong nonlinear interactions at microwave frequencies, high tunability, and low damping losses. These characteristics make spin waves ideal for realising magnonic frequency combs (MFCs) with applications in on-chip signal processing, neuromorphic computing, and magnetometry.

Recent theoretical studies suggest that interacting propagating spin waves with topological magnetic textures, such as skyrmions, can enhance three-magnon scattering processes. Experimental validation in ferromagnetic thin films demonstrated phase-locked spin-wave dynamics at MHz frequencies, producing harmonic combs extending over six octaves, reaching up to the 60th harmonic. The coherence of these combs was confirmed using quantum sensing with nitrogen-vacancy centres in diamond, resolving phase-stable high-order harmonics and their Rabi oscillations. Stimulated three-wave mixing, utilising a modulation signal, can lower the driving power.

This drive could originate from environmental sources, triggering three-magnon scattering and enabling sensor functionality. Figure 1(a) illustrates the experimental setup, featuring a 5μm×5μm×15nm NiFe (Permalloy) ferromagnetic element lithographically patterned at the centre of a gold microwave antenna. Two synchronised microwave sources generate the excitation (fe) and modulation (fm) signals, combined using a broadband power combiner for independent frequency and amplitude control. An in-plane bias field (Bext) is applied along the antenna’s longitudinal (x) axis, while the microwave-driven Oersted field (brf) oscillates along the y-axis to excite magnons.

Microfocused Brillouin light scattering (μBLS) spectroscopy measures the resulting magnetisation dynamics with high frequency and spatial resolution. Figure 1(b) displays the BLS spectra, averaged across the Permalloy structure, for single-frequency (black curve, fe=4.0GHz, Pe=20 dBm) and dual-frequency excitation (red curve, fe=4.0GHz, Pe=20 dBm with an additional modulation signal at fm=0.5GHz, Pm=30 dBm) under an external field Bext=18 mT. The excitation frequency of 4.0GHz was chosen to align with the ferromagnetic resonance (FMR) frequency, optimising excitation efficiency. The single-frequency spectrum shows a resonance peak at 4.0GHz, while the dual-frequency excitation produces an MFC, a series of equidistant spectral peaks with a spacing of Δf=fm=0.5GHz.

This comb includes frequency components below the FMR frequency, such as the difference frequency (fm−fe) =3.5GHz, typically inaccessible under conventional excitation. To investigate the spatial properties, a laser spot with a diameter of approximately 350nm scanned the Permalloy square with a 250nm step size. The resulting maps reveal distinct localisation patterns. The difference-frequency resonance (3.5GHz) is localised along the edges perpendicular to the external field, indicating edge-localised spin-wave modes arising from a demagnetising field potential well. The sum frequency resonance (4.5GHz) exhibits standing wave behaviour along both the x- and y-axes (kx≠0, ky≠0), a consequence of the square structure’s boundary conditions.

The spatial distributions of all observed peaks share characteristics with these primary modes, supported by micromagnetic simulations. Figure 2(a) presents a normalised two-dimensional frequency-domain distribution obtained from a frequency-swept excitation signal (2.0, 6.0GHz, 20 dBm) without modulation. Figure 2(b) shows the corresponding distribution with the excitation signal combined with a modulation signal (fm=0.5GHz, 30 dBm), highlighting the modulation signal’s role in generating the stimulated MFC. Figure 2(c) displays BLS spectra measured under varying external magnetic fields. Figure 2(d) shows theoretical (blue curve) and experimental (red circles) results for the scattering efficiency η = (|ap| + |aq|)/|ae| as a function of the external magnetic field.

The experimental results demonstrate that a modulation frequency of fm=0.5GHz effectively triggers nonlinear magnonic phenomena, leading to MFC formation. Comparative experiments across a wide excitation frequency range (2-6GHz) with and without modulation, under a constant external field of Bext =18 mT, were conducted. Figure 2(a) reveals a single linear response where input and detected frequencies correspond one-to-one, with a pronounced excitation peak at the quasi-FMR frequency of approximately 3.8GHz. Figure 2(b) shows the spectral response with both excitation and modulation frequencies applied simultaneously, revealing multiple evenly spaced spectral lines with spacing matching the modulation frequency fm, confirming the MFC’s formation.

Similar behaviour is observed for modulation frequencies ranging from 0.4 to 0.6GHz. The modulation frequency is significantly lower than the FMR frequency, distinguishing this approach from four-magnon scattering-induced MFCs, which require modulation frequencies exceeding the FMR frequency. Micromagnetic simulations validate the modulation-induced comb structure, attributing the underlying mechanism to stimulated three-magnon scattering processes, where nonlinear magnon interactions drive energy redistribution among spectral components.

Six-octave magnonic combs confirmed by quantum sensing offer

Experiments conducted on ferromagnetic thin films revealed that phase-locked spin-wave dynamics, driven at MHz frequencies, produce these harmonic combs. The coherence of these combs was rigorously confirmed using quantum sensing with nitrogen-vacancy centers in diamond, which resolved phase-stable high-order harmonics and their Rabi oscillations. The team measured that the comb spacing (Δf) is precisely locked to the modulation frequency, with Δf = fm, while the number of spectral lines scales with the modulation power. This provides two independent control dimensions, enabling precise, lower power, real-time control over the MFC spectra for diverse applications.

Experiments utilized a 5μm×5μm×15nm NiFe (Permalloy) film positioned at the center of a gold microwave antenna on a silicon substrate. Microfocused Brillouin light scattering (μBLS) spectroscopy was employed to measure the magnetization dynamics with high frequency and spatial resolution. Under an external field of Bext=18 mT, the team compared spectra obtained with single-frequency excitation (fe=4.0GHz, Pe=20 dBm) and dual-frequency excitation (fe=4.0GHz, Pe=20 dBm with an additional modulation signal at fm=0.5GHz, Pm=30 dBm). The dual-frequency excitation produced a striking MFC, a series of equidistant spectral peaks with a spacing of Δf=fm=0.5GHz.

Spatial mapping with a 350nm laser spot revealed distinct localization patterns for different frequency components. These edge modes arise from an internal field potential well created by the demagnetizing field, demonstrating the ability to probe and manipulate magnon states beyond traditional limits. This breakthrough delivers a pathway for integrating MFCs into practical spintronic devices and advancing quantum magnonics and hybrid magnon-photonics systems.

Tunable combs via stimulated magnon generation offer new

Theoretical modelling, micromagnetic simulations, and experimental validation corroborate the findings, establishing the feasibility of this approach for practical applications. The study reveals that modulation power directly influences the number of comb teeth in the generated magnonic frequency comb, allowing for deterministic control over the frequency range. Unlike previous methods, this technique enables operation either above or below the ferromagnetic resonance frequency, enhancing flexibility and expanding potential applications. Authors acknowledge that the current work focuses on specific material systems and geometries, and further investigation is needed to explore broader applicability. Future research could focus on integrating these combs into functional devices and exploring their performance in diverse operating conditions, potentially leading to advancements in magnonic sensors and spintronic technologies.