Enhancing Optical Chirality via UV-Resonant Hafnium Oxide Metasurfaces for Enantioselectivity

The chirality, or ‘handedness’, of molecules frequently determines their efficacy and safety, with enantiomers displaying disparate biomedical and chemical behavior. However, the high costs associated with enantiomeric separation or asymmetric synthesis lead pharmaceutical, agrochemical, and industrial markets to predominantly supply racemic mixtures containing off-target enantiomers. More facile methods for chiral separation and synthesis could alleviate this disadvantage, especially in synthetic amino acid and chiral polymer production. To this end, spin-polarized electrons have emerged as powerful symmetry breaking agents in radical-mediated electro and photochemical reactions, but the broad application of electron spin as a chiral reagent is currently limited by insufficient spin polarization and injection. Metasurface enhancement of chiral light-matter interactions can amplify circular dichroism signal,¹ which is associated with spin polarization.² In parallel, ferromagnetic materials support highly polarized spin injection.² Integration of metasurface enhanced chiral light-matter interactions and magnetic properties represent a path to enantiomeric excess in radical-mediated photochemical reactions through chirality induced spin selectivity.^1,2 Here, we present a new ferromagnetic metasurface platform capable of enhancing near-field optical chirality and ultimately facilitating enantioselective synthesis.<br/><br/>We design, fabricate and characterize UV-resonant nanodisk arrays of hafnium oxide, a lossless, high refractive index dielectric in the 200-700 nm wavelength regime. Reports of hafnia’s defect-induced ferromagnetism allow it to simultaneously enhance local density of optical chirality, C, and support spin polarization and coherence. We design hafnia metasurfaces with disk heights of 100 nm and radii ranging from 50-100 nm in order to achieve UV resonances overlapped with the absorption peaks of α-aminonitriles, valuable targets for amino acid production. By manipulating the geometric parameters of these nanodisk arrays, we can spectrally align the magnetic and electric dipolar modes, resulting in Kerker-like conditions to maintain incident light's circular polarization. Full field electromagnetic simulations indicate that these structures enhance C by at least a factor of 100, averaged over the top volume of the metasurface compared to free-space, leading to an equivalent boost in spin-selective injection. We fabricate these hafnia metasurfaces using RF magnetron sputtering, electron beam lithography, and inductively coupled plasma reactive ion etching. Vacuum annealing and reactive sputtering conditions are varied to generate oxygen vacancies in the hafnia, inducing ferromagnetic character. We conduct UV transmission measurements verifying the metasurface’s resonance frequencies and evaluate the varying hafnia samples’ perpendicular magnetic anisotropy and ferromagnetic character through polar magneto-optical Kerr effect experiments. Finally, we demonstrate integration with fluidics to enable a photochemical, radical-mediated Strecker-type synthesis of an α-aminonitrile precursor of phenylglycine. We discuss preliminary results towards enantioselective excess exceeding typical solution phase reactions for this non-proteinogenic amino acid and helical polymers. Our UV-resonant metasurfaces are positioned as versatile platforms for chiral chemical processes, expanding the possibilities for spintronic applications and novel photochemical synthesis methods.<br/><br/>1. Solomon, M. L.; Abendroth, J. M.; Poulikakos, L. V.; Hu, J.; Dionne, J. A. Fluorescence-Detected Circular Dichroism of a Chiral Molecular Monolayer with Dielectric Metasurfaces. Journal of the American Chemical Society, 2020<br/>2. Bhowmick, D. K.; Das, T. K.; Santra, K.; Mondal, A. K.; Tassinari, F.; Schwarz, R.; Diesendruck, C. E.; Naaman, R. Spin-Induced Asymmetry Reaction—the Formation of Asymmetric Carbon by Electropolymerization. Science Advances 2022.