Near-Ultraviolet Dielectric Metasurfaces: from Surface-Enhanced Circular Dichroism Spectroscopy to Polarization-Preserving Mirrors

Circular dichroism (CD) spectroscopy is an important technique to investigate the structural information about chiral molecules. The intrinsically weak chirality of molecules often requires long optical paths and high-concentrated molecules. Metasurfaces can generate localized strong chiral fields to enhance chiral light-molecule interactions near the surface. However, losses and structural chirality of metasurfaces can diminish the inherent CD of the chiral molecules and induce interfering spectral features that disturb the chiral analysis of the molecules. Herein, we realize achiral titanium dioxide metasurfaces to enhance the near-ultraviolet CD of chiral molecules. The device is completely lossless for wavelengths greater than 360 nm and produces intense chiral hotspots in the near-field. An 80-fold enhancement of optical chirality is numerically demonstrated, giving rise to a 50-fold enhancement of CD. In addition, we show that the metasurface can function as a polarization-preserving mirror and enhance optical chirality in the far-field. Our results provide a scheme to ultracompact CD spectrometers and chiral cavities.