Phonon-polaritonic skyrmions: Transition from bubble- to Néel-type

1. Problem Statement

Optical skyrmions are topological light textures with potential applications in robust light-matter interactions and information processing. However, their practical realization in nanophotonics has been severely limited by high inherent losses in plasmonic materials (e.g., gold).

• The Limitation: In plasmonic systems, the regime required for strong confinement and tunability of topological properties is dominated by high optical losses, which shortens the lifetime of guided modes.
• The Gap: While "bubble-type" optical skyrmions (a degenerate case of Bloch and Néel types) have been observed in plasmonic systems, the ability to tune between different topological types (specifically transitioning to Néel-type) has not been achieved due to these losses. Furthermore, precise optical tuning of these features at the nanometric scale has remained elusive.

2. Methodology

The authors propose and demonstrate a platform based on Surface Phonon Polaritons (SPhPs) in thin films of Silicon Carbide (SiC), a polar dielectric, to overcome plasmonic limitations.

• Material Platform: A 200 nm thick free-standing SiC membrane. SiC exhibits a "Reststrahlen band" (792–972 cm⁻¹) where it behaves like a metal due to a negative dielectric constant. Crucially, SiC possesses strong sublinear dispersion in this region, allowing for extreme momentum ($k_{||}$) tuning with minute changes in excitation wavelength.
• Structure Design: Hexagonal chromium ridges fabricated via electron-beam lithography on the SiC membrane. These structures are designed with specific edge shifts (multiples of $\lambda_{SPhP}/6$) to ensure constructive interference of six SPhP waves at the center, forming a skyrmion lattice.
• Excitation: The structures are illuminated with circularly polarized light at normal incidence to launch SPhPs.
• Detection: The team utilized phase-resolved scattering-type Scanning Near-field Optical Microscopy (s-SNOM) in transmission mode.
  • A sharp tip (25 nm radius) scans the surface, interacting with the local near-field.
  • The tip scatters the out-of-plane electric field ($E_z$) to the far field.
  • Pseudo-heterodyne detection with higher-order demodulation is used to extract both amplitude and phase, resolving subwavelength features.
• Analysis: The topological state is characterized by calculating the Skyrmion Number Density ($s$) and the Skyrmion Number Density Contrast (SNDC). The study also introduces new Figures of Merit (FOMs): Inverse Domain Wall Steepness and Domain Wall Size, derived from the polar angle ($\theta$) of the electric field.

3. Key Contributions

• First Observation of Phonon-Polaritonic Skyrmions: The paper demonstrates the generation of skyrmion lattices using SPhPs rather than Surface Plasmon Polaritons (SPPs).
• Demonstration of Topological Tunability: The authors experimentally achieve a continuous transition between bubble-type and Néel-type skyrmions. This is the first time such a transition has been realized in an optical system at the nanoscale.
• Overcoming Loss Limitations: By utilizing the low-loss, highly dispersive nature of SiC, the team bypasses the high-loss constraints of metallic plasmonics, enabling the observation of Néel-type configurations which require strong in-plane field confinement.
• Novel Characterization Metrics: The introduction of "Inverse Domain Wall Steepness" and "Domain Wall Size" provides robust metrics to distinguish topological types, overcoming the sensitivity of traditional SNDC calculations to noise and outliers.

4. Results

• Wavelength-Dependent Transition: By tuning the excitation wavenumber by only 10% (e.g., from 830 cm⁻¹ to 900 cm⁻¹) within the Reststrahlen band, the system transitions from a bubble-type to a Néel-type skyrmion.
  • Bubble-type (e.g., 830 cm⁻¹): Characterized by a vanishingly small in-plane electric field ($E_{||}$). The skyrmion number density forms a sharp ring, and the polar angle ($\theta$) shifts abruptly (0 to $\pi$) across a narrow domain wall. SNDC $\approx$ 1.
  • Néel-type (e.g., 900 cm⁻¹): Characterized by a significant in-plane electric field. The polar angle shifts smoothly over a wider distance, creating a "gearwheel-like" density distribution. SNDC $\approx$ 0.5.
• Experimental Validation: The experimental s-SNOM images of the out-of-plane field ($E_z$) and the calculated skyrmion number density ($s$) closely match theoretical simulations based on Huygens' wave interference.
• Topological Stability: The calculated Skyrmion Number ($S$) for individual skyrmions in the lattice remains close to 1 across all wavelengths, confirming the topological robustness of the structures despite the change in texture.
• Domain Wall Analysis: The data confirms that as the in-plane field increases (moving toward Néel-type), the domain wall size increases and the steepness decreases, validating the analogy to magnetic skyrmions.

5. Significance

• New Paradigm for Topological Photonics: This work establishes polar dielectrics (like SiC) as a superior platform to metals for studying and manipulating topological light textures, offering low loss and high tunability.
• Dynamic Control: The ability to switch topological states via minute wavelength changes opens doors for topology-based information processing, where information could be encoded in the skyrmion type (bubble vs. Néel) or the Chern number.
• Future Applications:
  • Phononics and Twistronics: The platform enables the study of complex structures like skyrmion bags, merons, and "phonon-polaritonic twistronics."
  • Nonlinear Effects: The anharmonicity of phonons in SiC may lead to polaritonic skyrmion-skyrmion interactions, a phenomenon not yet observed.
  • Active Tuning: The approach paves the way for using 2D materials (e.g., graphene) to actively modulate carrier density and rapidly switch skyrmion topologies electrically or optically.

In conclusion, the paper successfully bridges the gap between magnetic and optical skyrmion research by demonstrating a tunable, low-loss, nanometric platform for generating and manipulating complex topological light fields.