High quality factor metasurfaces for molecular sensing and spectroscopy
Keywords: Materials Science and Engineering.

Light has played a critical role in illuminating complex biological processes, from the discovery of cells in early light microscopes to the biological revolution driven by fluorescence-based DNA sequencing. Nanophotonics, the engineering of light at molecular size scales, promises a new toolkit for more accurate, scalable, and faster biomolecule analysis. Here, I present the design, fabrication, and characterization of high quality factor ("high-Q") metasurfaces - arrays of dielectric nanostructures that both strongly concentrate incident illumination in the near-field, while controlling the amplitude and phase of far-field transmission. I apply these high-Q metasurfaces for 1) rapid infectious disease diagnosis and 2) improved chiral molecule purification. First, I describe high-Q metasurfaces for rapid screening of SARS-CoV-2 gene sequences. Our high-Q guided mode resonators concentrate and confine light in small volumes, enabling the dense patterning of individually addressable sensor elements on the order of 1 million devices per cm^2 for parallelized and multiplexed gene detection. We functionalize self-assembled monolayers of DNA probes to our metasurfaces and demonstrate nucleic acid binding at picomolar concentrations within minutes of sample introduction. Target binding for two different target gene sequences is measured over hundreds of individual sensing elements with sensitivities and specificities up to 94% and 96% respectively. Combined with advances in nucleic acid extraction from complex samples (eg, mucus, blood, or wastewater), this work provides a foundation for rapid, compact, and high throughput multiplexed genetic screening assays spanning medical diagnostics to environmental monitoring. Next, I describe the design, fabrication, and characterization of high-Q guided mode resonators that maintain experimental Q factors exceeding 3,000. Photonic mirror elements are utilized to truncate the physical footprint of free space coupled resonators, without introducing scattering losses that reduce quality factor. Utilizing hyperspectral imaging techniques, we demonstrate the rapid spectral characterization of hundreds of resonators simultaneously. Furthermore, we show the spatial and temporal tracking of resonant wavelength shift signals across a patterned array of individual high-Q pixels as a monolayer of aminosilane molecules is deposited. This study demonstrates a pathway towards a highly parallelized multi-analyte analysis platform. Finally, I describe high-Q metasurfaces that can identify the "handedness" of chiral molecules - critical for determining the enantiopurity of pharmaceuticals and agrochemicals. These metasurfaces support resonant optical modes with both electric and magnetic character, where the appropriate combination produces chiral electromagnetic fields. Adding subtle geometric perturbations to the metasurface enables control of the resonance quality factor and consequently, the magnitude of the chiral electromagnetic field intensities around the metasurface. We achieve enhancement of optical chirality densities exceeding 1000-fold for sensitive circular dichroism spectroscopy (CD). We design Si-based devices for enhanced visible-frequency CD, as well as a diamond-based platform for ultraviolet CD - providing a foundation for few-molecule CD and enantioselective separation of many pharmaceuticals