Transmission-Mode Silicon-Rich Nitride Mie-Void Metasurfaces in the Visible

Abstract: Mie-void metasurfaces have so far been developed mainly in reflection, where subwavelength voids embedded in high-index media support localized resonances and spectrally selective optical responses. Yet, many optical systems could benefit from integrating such optical elements operating in transmission mode. Motivated by this great need, we hereby introduce Mie-void metasurfaces operating in transmission. To allow for their operation in the visible range, our Mie-voids are implemented using the silicon-rich nitride (SRN) platform. We show that this transition from reflection to transmission is not a simple change in geometry: placing the voids in a finite film on a substrate introduces slab-guided and Fabry-Perot-like contributions that hybridize with the underlying Mie-void response. Rigorous coupled-wave analysis shows that the dominant spectral transformation occurs when the semi-infinite host is replaced by a finite SRN film, while the substrate acts mainly as a secondary perturbation. Thickness-dependent dispersion maps reveal an avoided crossing between interacting modes, supporting the interpretation of a hybrid transmission regime and identifying film thickness as a clean parameter for tracking the evolution of the coupled modal structure. Experimentally, we realize transmission-mode structural colors by varying the void depth and observe good agreement between measured and simulated spectra and chromaticity coordinates. By spatially programming the void depth, we further demonstrate transmitted-light patterns and image encoding within a single metasurface architecture. These results establish transmission-mode Mie-void metasurfaces as a viable inverse-dielectric platform operating in transmission, with plethora of potential important applications such as transmissive spectral filtering, optical encoding, and display-oriented photonic elements, to name a few.

• The paper demonstrates SRN-based transmission-mode Mie-void metasurfaces with hybrid modal coupling validated through rigorous coupled-wave analysis and experimental measurements.
• It shows that varying film thickness and void depth provides continuous, orthogonal control over resonance positions and resultant structural colors.
• Experimental results confirm strong agreement between simulations and measured chromaticity, enabling precise, pixel-addressable spectral encoding.

Transmission-Mode Silicon-Rich Nitride Mie-Void Metasurfaces in the Visible: A Technical Analysis

Introduction and Motivation

The study "Transmission-Mode Silicon-Rich Nitride Mie-Void Metasurfaces in the Visible" (2604.10488) addresses a critical underexplored dimension in dielectric metasurface photonics: the implementation and characterization of Mie-void metasurfaces in the transmission regime, specifically within the visible spectrum, leveraging a silicon-rich nitride (SRN) platform. Historically, Mie-void architectures have been dominated by reflection-mode geometries, where localized void resonances embedded in a high-index host drive spectral selectivity. However, many optical applications—such as transmissive spectral filters, display components, and compact imaging systems—require efficient operation in transmission mode with robust forward-propagating optical functionality. This work establishes the physical regime and experimental viability of SRN-based transmission-mode Mie-void metasurfaces, elucidating their hybrid modal characteristics and design implications.

Modal Physics and Spectral Evolution

The transition from reflection-mode to transmission-mode operation in Mie-void metasurfaces is shown to be non-trivial: while semi-infinite hosts support well-localized Mie-type void resonances, finite SRN films on substrates introduce coupled slab-guided and Fabry-Pérot-like modes. Rigorous coupled-wave analysis (RCWA) confirms that the primary spectral transformation arises from the confinement and interaction within the finite slab, with the substrate acting as a weak perturbation. The authors map the evolution of eigenmodes as a function of SRN film thickness and identify clear avoided crossings in the transmission spectra—a hallmark of strong hybridization between Mie-void and slab-guided/fabry-pérot mode families.

Thickness is shown to be the most sensitive tuning parameter, producing orderly dispersions and revealing hybrid modal coupling, whereas lateral period mainly influences mode densification via diffractive effects, and the angle of incidence has only weakly perturbative impacts near the main transmission band. Notably, despite the increased modal complexity in finite films, the void-localized resonance component persists as a distinct and tunable feature in the overall response.

Experimental Realization and Validation

Experimental validation is conducted via fabrication of SRN metasurfaces with periodic void arrays (radius 340 nm, period 1 μm, depths modulated from 200–500 nm) defined through grayscale electron-beam lithography and reactive ion etching. Measured transmission spectra across varying depths reveal two dominant coupled resonance branches, consistent with RCWA simulations; the spectral evolution directly translates to controllable structural colors in the visible regime, with good agreement between measured and simulated chromaticity coordinates (CIE 1931). The slight broadening and reduction in spectral sharpness are attributed to fabrication tolerances and surface roughness, but not to modal or platform limitations.

Structural-Color Tuning and Spatial Encoding

A significant implication of the hybridized transmission regime is the ability to spatially program spectral response across the metasurface via local modulation of void depth. The authors realize both simple and intricate image-encoded patterns in transmission, illustrating that the hybrid resonant behavior is robust and pixel-addressable. This capability enables meta-patterning for transmission-mode display, optical encoding, and multiplexed spectral filtering.

Theoretical and Practical Implications

This work rigorously establishes that the inverse-dielectric Mie-void paradigm is preserved—albeit in hybridized form—when ported from reflective, semi-infinite hosts to finite-film, transmission-mode geometries. The transmission response is dominated by hybridization; yet, crucially, the underlying void resonance remains designable, additive, and spectrally controllable, supporting the generality of the Mie-void design intuition. The modal and parametric analysis indicates that film thickness and void depth offer orthogonal and continuous control over coupled resonance position and color, while lateral patterning enables complex spatial encoding. The platform is thus suitable for compact transmissive optical elements, including ultra-thin filters, meta-displays, and information encoding.

This study further suggests that, as metasurface photonics pushes toward ever thinner and more integrated devices, the full modal landscape—including hybrid slab and cavity effects—must be incorporated at the design stage, rather than being relegated as mere perturbations to localized resonance approximations.

Future Directions

Several avenues are implicated by these results:

• Advanced Photonic Integration: Future research will likely target multiplexed metasurfaces integrating Mie-void and positive-resonator geometries for three-dimensional modal engineering.
• Active and Tunable Elements: Combining SRN Mie-void metasurfaces with electro-optic or thermo-optic actuation can enable dynamic spectral filters and reconfigurable display units.
• Quantum and Nonlinear Applications: Given the low optical loss and accessible nonlinearities in SRN, more investigation into transmission-mode nonlinear meta-optics and quantum photonic circuits is warranted.
• Optimization for Imaging Systems: Camera-integrated spectral sensors and augmented reality waveguides will benefit from the demonstrated pixel-level programmability and broad visible tuning documented here.

Conclusion

"Transmission-Mode Silicon-Rich Nitride Mie-Void Metasurfaces in the Visible" (2604.10488) delivers a comprehensive technical foundation for transmission-mode, inverse-dielectric metasurfaces in the visible regime. By elucidating the hybridized modal physics resulting from finite-film architectures, the study extends the design landscape for structural color, filtering, and integrated photonic devices. The persistence and tunability of void-localized resonances within a complex slab-mediated modal environment are confirmed both numerically and experimentally, opening pathways for practical optoelectronic integration and advanced meta-optics applications.