Photo-Magnonic Crystals for Spin-Wave Control

• Photo-magnonic crystals are engineered periodic structures that combine magnetic and photonic elements to control GHz–THz spin-wave propagation via precise optical techniques.
• These systems utilize tailored geometries such as antidot lattices, multilayer stacks, and laser-induced patterns to create tunable bandgaps and localized modes for customized dispersion engineering.
• They offer promising applications in ultra-fast signal processing, topologically protected devices, and quantum interfaces through integrated optomagnonic coupling and nonreciprocal phenomena.

Photo-magnonic crystals are engineered periodic structures where the propagation and interaction of spin waves (magnons) are controlled via underlying photonic or electromagnetic architectures and are both excited and probed by ultrafast optical techniques. These systems exploit the synergy between photonic band engineering, magnonic metamaterials, and all-optical control, offering a platform for highly tunable, fast, and multifunctional manipulation of spin waves for advanced signal processing, topological physics, hybrid quantum transduction, and on-chip nonreciprocal devices (Lenk et al., 2012, Lenk et al., 2011, Dadoenkova et al., 2018, Gardin et al., 2 Dec 2025, Chang et al., 2019, Graf et al., 2020, Dadoenkova et al., 2017). Their operational regimes span GHz–THz and rely on precise tailoring of internal magnetic and electromagnetic potentials at the sub-micron to nanometer scale.

1. Architectural Principles and Physical Mechanisms

Photo-magnonic crystals are realized by periodically modulating the magnetization and/or the electromagnetic permittivity in ferromagnetic thin films or multilayered stacks—often through antidot (hole) lattices, layer sequencing, or laser-induced transient patterns—to define magnonic (and often photonic) band structures. Crucial features include:

• Antidot Lattices: Circular holes in a continuous magnetic film (thickness $t\sim50$ nm, lattice constant $a\sim1.5$–$3.5$ μm, filling fraction $f=\pi d^2/4a^2\sim0.1$–$0.2$) create spatial modulation of the internal dipolar field $H_\mathrm{dip}(\mathbf r)$, supporting both localized and extended Bloch spin-wave modes (Lenk et al., 2012).
• Layered Multilattices: 1D or 2D periodic stacks of magnetic (e.g., YIG, CoFeB, Ni) and dielectric (e.g., TiO₂, SiO₂) layers create joint photonic-magnonic bandgaps and defect modes, with electromagnetic and spin excitations co-localized for enhanced interactions (Dadoenkova et al., 2018, Dadoenkova et al., 2017).
• Optical Patterning: Interference of ultrafast laser pulses transiently imposes spatial variations in $M_s(x)$, writing dynamic “on-demand” magnonic crystals with arbitrary geometry and temporal duration in the nanosecond regime (Chang et al., 2019).

The dynamics are determined by the interplay between exchange, dipolar, and Zeeman terms in the linearized Landau-Lifshitz-Gilbert (LLG) equation, with the magnonic band structure derived via Fourier-space (plane-wave method) expansion. Bandgaps open at Brillouin zone boundaries (e.g., $k=\pi/a$), and the density of magnon states is engineered by geometry and material properties (Lenk et al., 2011, Lenk et al., 2012).

2. Experimental Techniques: All-Optical Excitation and Detection

Ultrafast all-optical pump–probe setups are central to photo-magnonic crystal studies. Key methodologies include:

• Pump Pulse: Femtosecond ($\sim40$ fs) laser pulses (λ ≈ 800 nm, fluence 5–10 mJ/cm²) demagnetize the film within ~100 fs, generating a broad spectrum of high-$k$ magnons.
• Probe Pulse: Time-delayed weak probe pulse of identical wavelength resolves magnetization precession via time-resolved magneto-optical Kerr (or Faraday) effect with ≈1 GHz temporal resolution, by recording transient $\Delta\theta_K(\Delta\tau)$ (Lenk et al., 2012, Lenk et al., 2011).
• Transient Grating: Interference of two ultrafast beams forms a periodic intensity and, hence, $M_s$ profile (period $\Lambda = \lambda_\mathrm{pump}/2\sin\theta$), establishing a reconfigurable magnonic crystal template (Chang et al., 2019).

This protocol enables non-contact, broadband excitation and detection of spin-wave modes, capturing both bulk and edge states as well as localized modes pinned near inhomogeneities (e.g., antidot edges). Localized excitations can also be monitored via Brillouin light scattering in complex layer structures (Dadoenkova et al., 2017).

3. Dispersion Engineering, Band Structures, and Mode Control

The magnonic and photonic spectra in photo-magnonic crystals are tunable by geometry, material choice, and optical manipulation:

• Bandgap Formation: Periodic modulations induce band folding, yielding gaps at Brillouin zone boundaries, band flattening, and density-of-state peaks (Lenk et al., 2012, Lenk et al., 2011). In layered PMCs, both optical and spin-wave defect modes localize in magnetic layers, with band edges and gap widths strongly tunable by layer thickness asymmetry ($\Delta D$) (Dadoenkova et al., 2017).
• Localization-Delocalization Crossover: The degree of mode localization is set by the filling fraction $f$, intrinsic damping $\alpha$, and dipolar potential depth. Low damping enables Bloch-like propagation across $\sim30$ unit cells; high damping or strong modulation localizes modes at antidot/ridge edges (Lenk et al., 2012, Lenk et al., 2011).
• Micromagnetic and Plane-Wave Modelling: Eigenmode analysis uses plane-wave expansion of $\mathbf m(\mathbf r)$, and micromagnetic simulations (e.g., in mumax3) corroborate spatial profiles, mode splitting, and predicted excitation amplitudes for both static and transient crystals (Chang et al., 2019, Graf et al., 2020).

4. Topological, Hybrid, and Multifunctional Regimes

Recent advances extend photo-magnonic crystals into new domains leveraging bosonic topology and hybridization:

• Topological Boundary Modes: One-dimensional arrays of photo-magnonic cavities (microwave photon mode strongly hybridized with a Kittel magnon in YIG) realize synthetic quantum metamaterials where robust zero-energy boundary modes are protected by many-body bosonic symmetries—particle number, squeezing, and bosonic time reversal (Gardin et al., 2 Dec 2025). The topological invariant (Pfaffian or winding number) predicts the number and robustness of these modes in different symmetry classes.
• Hybrid Optomagnonic Crystals: Patterned YIG waveguides with photonic and magnonic bandgaps are optimized for spatial co-localization of defect modes, enabling direct optomagnonic coupling ($g_0$ in kHz regime), and advancing prospects for quantum microwave-optical transduction (Graf et al., 2020).
• Simultaneous Mode Confinement: Embedding magnetic layers within photonic crystal sections enables coincident localization of electromagnetic and spin-wave states, enhancing nonlinear and magneto-optical interactions (Dadoenkova et al., 2018, Dadoenkova et al., 2017).

5. Magneto-Optical Effects and Nonreciprocal Response

The integration of magnetic and photonic periodicity yields pronounced magneto-optical phenomena:

• Faraday Rotation: In bi-periodic PMCs, the angle of polarization rotation for transmitted light increases linearly with the number of magnetic supercells in passbands and resonates in photonic defect modes ($\theta_F\sim$0.2–0.5° for $K=5$) (Dadoenkova et al., 2018). The effect is further enhanced by linear magneto-electric coupling in YIG, especially for s-polarized light.
• Nonreciprocity and Isolation: Magnetic field inversion reverses Faraday rotation, allowing for reconfigurable, sub-micron optical isolators and circulators. The sensitivity of $\theta_F$ to internal parameters enables sensing applications (e.g., $\delta\omega/\delta M$) and integrated modulation (Dadoenkova et al., 2018).
• Trade-Offs: Enhanced Faraday rotation at defect resonances is generally accompanied by low transmittivity, necessitating design compromises between rotation strength and overall transmission.

6. Ultrafast and Dynamically Reconfigurable Functionality

Optical induction of transient photo-magnonic lattices introduces highly flexible and time-dependent control:

• On-Demand Patterning: Spatial light interference can write and erase 1D or 2D magnonic crystals on ns timescales, with arbitrary periodicity and geometry, enabling reconfigurable filters, delay lines, and logic circuits (Chang et al., 2019).
• Spatiotemporal Tuning: The ability to adjust band structures, mode selectivity, and localization in real time allows for ultrafast control of magnonic signal routing and even phase-encoded information processing.
• Integration with Other Platforms: Prospects include coupling to elastic waves, spin–orbit torques, and programmable phase masks to design neuromorphic or data-parallel magnonic circuits.

7. Applications and Prospective Directions

Photo-magnonic crystals lay the infrastructure for a spectrum of device concepts:

• Signal Processing and Computing: By leveraging extended Bloch and localized modes, on-chip magnonic circuitry for information transfer, wave guiding, and logic can be implemented—reconfigurable on ultrafast timescales (Lenk et al., 2011, Lenk et al., 2012).
• Quantum Information Interfaces: Optimized optomagnonic structures with enhanced $g_0$ may enable coherent coupling between microwave, magnonic, and optical domains at the single-quantum level (Graf et al., 2020).
• Topologically Protected Devices: Synthetic photo-magnonic chains exhibiting symmetry-protected boundary states offer robust, tunable platforms for signal isolation, slow light, and directionally selective amplification with immunity to disorder (Gardin et al., 2 Dec 2025).
• Nonreciprocal Optical Elements and Sensors: The sharp field-dependence and defect-enhanced response of photonic-magnonic crystals facilitate non-reciprocal elements and field/chemical sensors at sub-micron scales (Dadoenkova et al., 2018).

Future directions emphasize the integration of low-damping ferromagnetic materials, advanced nanofabrication for precise band control, and hybridization with superconducting or other quantum systems. Continued development is expected to yield ultrafast, energy-efficient, and topologically protected magnonic-optical platforms for advanced computation and communication technologies.