arXiv subjects

Franz Vilsmeier

Publications and source records attributed to Franz Vilsmeier.

7 recordsLinked to original sources

arXiv2026

Anomalous Reflection of Caustic Spin-Wave Beams in a Magnonic Waveguide

Reflection of waves at interfaces is conventionally governed by Snell's law, which follows from conservation of momentum parallel to the interface. Here we show experimentally that caustic spin-wave beams in anisotropic media obey a fundamentally different reflection mechanism. Applying time-resolved Kerr microscopy to a yttrium iron garnet waveguide, we observe that reflected beams are selected by transitions between caustic points on the anisotropic iso-frequency contour rather than by momentum conservation. As a consequence, the reflected carrier wave vector and wavefront orientation exhibit trends opposite to those predicted by Snell's law. By tuning the magnitude and orientation of an external magnetic field, we continuously control the resulting reflection process and beam routing. Our results establish caustic-point transitions as a distinct reflection law for anisotropic wave beams and provide a route towards reconfigurable magnonic beam steering.

cond-mat.mes-hall↗

arXiv2026

Perspectives on inverse design for AI magnonics

Inverse design - specifying a desired functionality and letting a computational algorithm find the optimal structure - has emerged as a powerful paradigm for magnonic device engineering. In this article, we survey the rapidly growing field of inverse-design magnonics, organising it along two axes: the design variables (topology, material parameters, and magnetic field landscape) and the algorithmic toolbox (gradient-free, gradient-based, and neural-network-based methods) together with the differentiable micromagnetic solvers that enable them. We then identify open frontiers that we consider most promising for the next phase of the field: sensitivity analysis and robust design to bridge the gap between simulation and experiment; input shaping and transducer optimisation; the incorporation of nonlinear spin-wave effects as an explicit design resource; spatially structured amplification; self-adapting media and machine-learning-based design; and the long-term vision of a universal, reconfigurable magnonic platform. We argue that magnonics and artificial intelligence are converging from two directions - machine-learning tools for designing magnonic devices, and magnonic devices as hardware for neuromorphic computation - and propose the term AI magnonics to describe this emerging paradigm.

cond-mat.mes-hall↗

arXiv2026

Brillouin Light Scattering Spectroscopy of Propagating Magnons at Sub-Kelvin Temperatures

Coupling light to magnetic excitations in the form of spin waves underpins both the optical study of magnetism and emerging schemes for quantum transduction, positioning the quanta of these excitations, magnons, as promising carriers for hybrid quantum networks. However, exploiting them in the quantum regime requires millikelvin temperatures to suppress thermal magnon populations, thereby confining such experiments to dilution refrigerators. There, magnons can already be excited and read out electrically, yet an optical interface required for microwave-to-optical photon conversion has been missing. Here, we demonstrate the first optical detection of coherently driven, propagating spin waves via Brillouin Light Scattering (BLS) spectroscopy inside a dilution refrigerator. By simultaneously recording the optical and electrical responses of the same spin-wave mode in a yttrium iron garnet film, we find that the BLS spectra track the electrically measured transmission across a range of applied magnetic fields. For the lowest optical power of 7.9 {\mu}W that still enabled spin-wave detection, we measured a global equilibrium sample temperature of 510 mK via a resistance thermometer, while numerical modelling of the laser-induced heating yields a maximum local temperature of 900 mK at the focal spot. This brings free-space optical access to magnons into the sub-kelvin regime, representing a milestone towards magnon-mediated quantum transduction in hybrid quantum systems.

cond-mat.other↗

arXiv2025

Spin wave propagation in a ring-shaped magnonic waveguide

We experimentally investigate frequency-selective spin wave (SW) transmission in a micrometre-scale, ring-shaped magnonic resonator integrated with a linear Yttrium Iron Garnet (YIG) stripe. Using super-Nyquist-sampling magneto-optical Kerr effect microscopy (SNS-MOKE) and micro-focused Brillouin light scattering ({\mu}-BLS), we probe SW dynamics in the dipolar regime under in-plane magnetisation. Spatially resolved measurements reveal a sharp transmission peak at 3.92 GHz for an external field of 74 mT, demonstrating strong frequency selectivity. Our results show that this selectivity arises from scattering and interference between multiple SW modes within the ring. These modes are governed by the anisotropic dispersion relation, transverse mode quantisation due to geometric confinement, and inhomogeneities of the effective magnetic field. In addition, the anisotropy enforces fixed group velocity directions, leading to caustic-like propagation that limits efficient out-coupling. Fourier analysis reveals discrete wavevector components consistent with quantised transverse eigenmodes. Additional {\mu}-BLS measurements at 70 mT show a shift of the transmission peak, confirming that the filtering characteristics are tunable by external parameters.

cond-mat.mes-hall↗

arXiv2024

Comparative analysis of spin wave imaging using nitrogen vacancy centers and time resolved magneto-optical measurements

Spin waves, the fundamental excitations in magnetic materials, are promising candidates for realizing low-dissipation information processing in spintronics. The ability to visualize and manipulate coherent spin-wave transport is crucial for the development of spin wave-based devices. We use a recently discovered method utilizing nitrogen vacancy (NV) centers, point defects in the diamond lattice, to measure spin waves in thin film magnetic insulators by detecting their magnetic stray field. We experimentally demonstrate enhanced contrast in the detected wavefront amplitudes by imaging spin waves underneath a reference stripline and phenomenologically model the results. By extracting the spin wave dispersion and comparing NV center based spin wave measurements to spin wave imaging conducted through the well-established time-resolved magneto-optical Kerr effect, we discuss the advantages and limitations of employing NV centers as spin wave sensors.

cond-mat.mes-hall↗

arXiv2024

Spatial Control of Hybridization-Induced Spin-Wave Transmission Stop Band

Spin-wave (SW) propagation close to the hybridization-induced transmission stop band is investigated within a trapezoid-shaped 200\,nm thick yttrium iron garnet (YIG) film using time-resolved magneto-optic Kerr effect (TR-MOKE) microscopy and broadband spin wave spectroscopy, supported by micromagnetic simulations. The gradual reduction of the effective field within the structure leads to local variations of the SW dispersion relation and results in a SW hybridization at a fixed position in the trapezoid where the propagation vanishes since the SW group velocity approaches zero. By tuning external field or frequency, spatial control of the spatial stop band position and spin-wave propagation is demonstrated and utilized to gain transmission control over several microstrip lines.

physics.app-ph↗

arXiv2023

Caustic spin wave beams in soft, thin films: properties and classification

In the context of wave propagation, caustics are usually defined as the envelope of a finite-extent wavefront; folds and cusps in a caustic result in enhanced wave amplitudes. Here, we tackle a related phenomenon, namely the existence of well-defined beams originating solely from the geometric properties of the corresponding dispersion relation. This directional emission, termed caustic beam, is enabled by a stationary group velocity direction, and has been observed first in the case of phonons. We propose an overview of this "focusing" effect in the context of spin waves excited in soft, thin ferromagnetic films. Based on an analytical dispersion relation, we provide tools for a systematic survey of caustic spin wave beams. Our theoretical approach is validated by time-resolved microscopy experiments using the magneto-optical Kerr effect. Then, we identify two cases of particular interest both from fundamental and applicative perspectives. Indeed, both of them enable broadband excitations (in terms of wave vectors) to result in narrowband beams of low divergence.

cond-mat.mes-hall↗