All-dielectric chiral-field-enhanced Raman optical activity

The study addresses the long-standing challenge that ROA signals are extremely weak—3–5 orders of magnitude below spontaneous Raman scattering—making them prone to artifacts and requiring long acquisition times. Although surface-enhanced ROA using metallic nanoparticles has been explored, it suffers from inefficient transfer of optical chirality from far-field to near-field, polarization instability due to random nanoparticle motion, and photothermal heating that can alter molecular conformation. The authors hypothesize that an all-dielectric nanophotonic platform can tailor near-field optical chirality and electric fields to substantially enhance ROA while suppressing artifacts. They propose a CMOS-compatible silicon nanodisk array designed to support a dark mode (interference of electric and toroidal dipoles) that efficiently transfers and enhances optical chirality in the near field while maintaining optical isotropy to avoid birefringence artifacts. The goal is to achieve reliable, reproducible, and biocompatible ROA enhancement suitable for trace chiral analytes.

Prior ROA work demonstrates utility across biochemistry, stereochemistry, analytical chemistry, structural virology, and pharmaceuticals but is limited by weak signals and long integration times. Surface-enhanced ROA (SEROA) via plasmonic nanoparticles has shown promise but exhibits key drawbacks: inefficient near-field optical chirality transfer from circularly polarized excitation, polarization instability from nanoparticle motion during long acquisitions, and significant photothermal heating that can damage biomolecules and destabilize signals (e.g., Abdali 2008; Kneipp 2006; Pour 2011; Zong 2018). Conversely, dielectric metasurfaces have enabled enhanced circular dichroism spectroscopy and chiral sensing with reduced losses and improved field control (Solomon 2019–2020; Hu 2020; Garcia-Etxarri 2013; Ho 2017). Superchiral fields have been proposed to enhance enantioselective interactions (Hendry 2010; Tang 2011). Dark modes and optical anapoles in dielectric nanostructures provide routes to strong near-field confinement with suppressed far-field radiation (Zeng 2015; Savinov 2019). These developments motivate a dielectric, dark-mode-based approach for ROA enhancement without plasmonic artifacts.

Design and simulation: A square-lattice silicon nanodisk array (C4 rotational symmetry) with nanodisk radius r, height h, and period p (gap p–2r = 30 nm) was designed to be optically isotropic, avoiding birefringence artifacts. Simulations at 532 nm (balancing Raman efficiency and silicon losses) mapped reflection vs. p and identified a dark mode (partial destructive interference of electric and toroidal dipoles). Near-field maps of electric field and optical chirality vs. r and h pinpointed conditions (e.g., r ≈ 90 nm, h ≈ 180 nm) providing simultaneous enhancement and spatial overlap of electric field and optical chirality, enabling efficient ROA enhancement. Field distributions (E, H, and optical chirality) were computed to verify dark-mode excitation and near-field chirality inversion between LCP and RCP. Fabrication: Arrays were fabricated on a silicon-on-insulator wafer using electron beam lithography, inductively coupled plasma etching, and resist cleaning. Thin-film crystalline silicon was used to reduce losses at 532 nm. Characterization: Reflection spectra under LCP and RCP illumination were measured with a custom reflection setup to confirm optical achirality and resonance near 532 nm. ROA measurement setup: A two-phase virtual-enantiomer ROA system was built, including LP, QWP, HMF, BS, HL, OAPM, RHWP, notch filter, and CMOS camera, to suppress deterministic offsets/artifacts. Validation with achiral sample: Toluene ROA was measured on the array; observed ROA was at the shot-noise level, indicating artifact suppression. Samples and acquisition: - (±)-α-pinene: 25-µm-thick layers (effective volume ~0.1 pL); incident laser power 800 mW; total exposure time 5 h. - (±)-tartaric acid: 25-µm-thick aqueous solution layers (effective volume ~0.1 pL) at 5 M; incident laser power 900 mW; total exposure time 6 h. Raman and ROA spectra were acquired on both a silica substrate and the silicon nanodisk array under identical conditions. Spectra normalization: Raman and ROA spectra were normalized to the maximum intensity obtained on silica. Near-field enhancement estimation removed far-field contributions to report the enhancement within the near-field region of the array. Mirror-symmetry of enantiomer ROA spectra and CID analyses assessed artifact levels and near-/far-field dissymmetric factor consistency.

• The silicon nanodisk array supports a dark mode at 532 nm with strong near-field electric-field enhancement and high optical chirality, with substantial spatial overlap conducive to ROA enhancement. - Reflection spectra for LCP and RCP are nearly identical, confirming the array's optical achirality and isotropy; a reflection valley appears at 532 nm as predicted. - Artifact suppression: Achiral toluene showed ROA at the shot-noise level in the two-phase virtual-enantiomer setup, indicating negligible instrumental or substrate-induced artifacts. - (±)-α-pinene: • Raman intensity on the array exhibited an average near-field enhancement factor of ~10^2 relative to silica (after removing far-field contribution). • ROA intensity showed an average near-field enhancement factor of ~10^2. • ROA spectra of enantiomers were mirror-symmetric, indicating reliable, artifact-suppressed measurement. - (±)-tartaric acid (biological enantiomers): • Raman intensity showed an average near-field enhancement factor of ~10^2. • ROA intensity of (+)-tartaric acid showed ~10^2 average near-field enhancement; characteristic ROA peaks matched those on silica. • Raman peaks on the array matched those on silica, demonstrating biocompatibility and reliability. - The similar enhancement factors for Raman and ROA indicate no appreciable change in circular intensity difference between far- and near-field measurements; the dissymmetric factor of the optical field is maintained upon near-field confinement. - The platform operates with low photothermal effects compared to plasmonic approaches and is CMOS-compatible for integration and scalable fabrication.

The study demonstrates that an all-dielectric, dark-mode-engineered silicon nanodisk array can significantly enhance ROA signals while mitigating the limitations of plasmonic SEROA. By efficiently transferring and enhancing optical chirality from the far field to the near field and providing co-localized electric-field enhancement, the array increases both the Raman scattering rate and the chiral excitation component, yielding ~100× enhancements for both Raman and ROA. Optical isotropy suppresses birefringence-induced artifacts, and the two-phase virtual-enantiomer scheme further removes deterministic offsets, confirmed by shot-noise-limited ROA from toluene and mirror-symmetric spectra of enantiomers. The preserved dissymmetric factor from far- to near-field indicates that confinement by the dielectric array does not distort the chiral structure of the field, supporting accurate ROA measurements. These results address the core challenge of weak ROA signals and the artifact-prone nature of plasmonic enhancement, offering a reliable and biocompatible platform for chiral analysis of trace samples.

The work introduces and validates an all-dielectric chiral-field-enhanced ROA platform based on a CMOS-compatible silicon nanodisk array operating in a dark mode. The device efficiently transfers and enhances optical chirality and electric fields in the near field, delivering ~100× enhancement of both Raman and ROA signals with negligible artifacts, preserved circular intensity difference, and minimal photothermal heating. The approach was validated using both chemical (±)-α-pinene) and biological ((±)-tartaric acid) enantiomers, demonstrating robustness and biocompatibility. This platform provides a cost-effective and reliable route for absolute conformational analysis of trace chiral molecules that are challenging for X-ray crystallography and NMR. Future directions include on-chip integration with photonic components, exploration of other dielectric geometries and wavelengths to target specific molecular bands, further reduction of acquisition times, and broadening the analyte scope to complex biomolecular systems and low-concentration samples.

• Measurements were performed at a single excitation wavelength (532 nm); generality across wavelengths was not experimentally explored. - Demonstrations were limited to two pairs of enantiomers; broader chemical and biological scope remains to be validated. - High laser powers (800–900 mW) and long acquisition times (5–6 h) were required for trace volumes, which may limit throughput and applicability to sensitive samples. - Reported enhancement factors pertain to the near-field region after removing far-field contributions; absolute enhancement in practical settings may depend on sample placement and overlap with the near field.