Inverse-Designed Superchiral Hot Spot in Dielectric Meta-Cavity for Ultra-Compact Enantioselective Detection

TL;DR

This work tackles the challenge of enhancing chiroptical signals for enantioselective detection by designing a dielectric metasurface that converts linearly polarized light into a superchiral near-field hotspot. A two-step topology optimization strategy combines a differentiable RCWA-based neural-network phase with adjoint-FDTD refinement to maximize local optical chirality $C$, achieving theoretical enhancements up to $10^{4}$ and experimental demonstrations near $10^{2}$ with a silicon-on-sapphire cavity. The dielectric platform offers low losses and high quality factors, enabling strong subwavelength confinement of chiral fields and scalable fabrication. Collectively, the approach provides a practical route to ultra-compact chiral sensing devices and demonstrates the valuable role of ML-assisted inverse design in photonic nanostructures.

Abstract

Chiral nanophotonic structures have garnered considerable interest in recent years due to their potential to enhance the efficacy of chirality-sensitive biomolecular detection. Designing metaplatforms to enhance chiroptical signals under linearly polarized excitation is particularly appealing due to the minimal chiral background and the ease of controlling excitation polarization. Here, a novel two-step inverse design scheme for dielectric lossless metasurfaces with superchiral hot spots is proposed. The method extends the local density of field enhancements for non-chiral fields into the chiral regime and significantly surpasses previous enhancements in super-chiral field generation. It has been demonstrated that by leveraging the excitation of high quality factor modes with small mode volumes, it is theoretically possible to convert linearly polarized plane waves into a superchiral hot spot with record-high enhancement in the near-field optical chirality up to 104. A prototype is successfully implemented using advanced nanofabrication technologies. The optical characterization of the prototype demonstrates a 102-fold enhancement in optical chirality. The findings of this study unveil novel prospects for chiral spectroscopy with ultra-compact devices, underscoring the role of machine learning and physics-based inverse design in the development of cutting-edge, functional photonic structures.

Figures (16)

• Figure 1: Superchiral cavity design. (a) Top view of the designed metasurface, with an inset showing the 2.6-$\micro$m-wide meta-unit cell. (b) Schematic side view of the designed metasurface representing a 200-nm-thick patterned silicon film on a sapphire substrate. The structure is designed to be illuminated under normal incidence with a plane wave propagating along the $z$-axis with linear polarization along the cavity (along $x$-axis). Horizontal (H) and vertical (V) polarizations correspond to the electric field vector orientation along $x$- and $y$- axes, respectively
• Figure 2: 2-step inverse design method. In the first step, the random noise vector is propagated through fully connected neural network (NN) layers, and its dimensions are expanded with convolutional layers. The NN output is a binary, pixelated image representing the Si-pattern shape in the unit cell of the metasurface. The optical response of the metasurface is calculated with RCWA. Using the stochastic gradient optimizer with fully differentiable RCWA, the NN weights are optimized to achieve maximum chirality $C$ enhancement at the center of the unit cell. In the second step, the pre-optimized topology is further fine-tuned using an adjoint-variable method with the direct FDTD solver. (b-d) Simulated pseudo-color maps over the top surface of the unit cell, showing the normalized near-field optical chirality density $C$ (b), and the normalized magnitudes of the $\mathbf{E}$ and $\mathbf{H}$ near-fields (c), (d), respectively, under monochromatic plane wave excitation at a wavelength of 1310 nm. All maps are normalized to those for circularly polarized light propagating in free space. The gray lines outline the shape of the metasurface pattern. Scale bar is 500 nm.
• Figure 3: Fabricated sample and experimental results. (a-e) The experimental (solid line) and FDTD-simulated (dashed line) spectra of the total transmission of the metasurface under a plane wave illumination at normal incidence with H- (a), V- (b), R- (c), and L- (d) polarization. The spectra are normalized to the transmission spectra of an unstructured Si film on the same substrate. (e) The experimental (solid line) and FDTD-simulated (dashed line) CD spectra in transmission, calculated from Equation (\ref{['eq:cd']}). Gray vertical line marks 1270 nm wavelength. (f) SEM image of the fabricated metasurface. Dashed lines outline the unit cell of the periodic metasurface. Dotted lines highlight the extracted edges of the patterned silicon film. (g) Schematic representation of the method used to build the FDTD model of the fabricated structure.
• Figure 4: Resonance spectral characterization. (a) The experimental (solid line) and FDTD-simulated (dashed line) CD spectra in transmission, calculated with Equation \ref{['eq:cd']}; (b) Spectra of maximum local chirality value (pink line) and average local chirality (light blue line) across the unit cell on the surface of the structure. The values are normalized to the local chirality value for a circularly polarized plane wave in free space.
• Figure 5: FDTD-simulated near fields of the fabricated metasurface. (a-c) Simulated pseudo-color maps over the top surface of the unit cell, showing the normalized near-field optical chirality $C$ (a) and the normalized magnitudes of the E and H near fields (b and c). The gray dashed line shows the location of the $xz$ plane in (d-f). (d-f) Simulated pseudo-color maps in the $xz$ plane crossing the center of the cavity, showing the normalized near-field optical chirality $C$ (d) and the normalized magnitudes of the E and H near fields (e and f), under monochromatic plane wave excitation at a wavelength of 1267 nm. All maps are normalized to those for circularly polarized light propagating in free space. The gray lines outline the shape of the pattern. The scale bar is 500 nm.