Enabling High-Throughput Screening of Materials for Chiral-Induced Spin Selectivity (CISS) with 2D Material-Optical Metasurface Heterostructures

Chiral-induced spin selectivity (CISS) describes the preferential scattering of spin-polarized electrons through chiral media. CISS has brought a new approach to control spin transport and enable spin-selective chemical reactions. In particular, CISS removes the need for chiral catalysts to generate enantiomeric excess in chiral chemistry. However, spin-polarized currents are difficult to detect, and existing methods (e.g magnetic conducting atomic force microscopy) are not yet adequately high-throughput to efficiently screen CISS materials. Two-dimensional transition metal dichalcogenides (TMDC) present an exciting method for the optical readout of spin-polarized current. TMDC’s unique valley degree of freedom couples the optical spin of emitted photons (i.e., circular polarization) to the valley pseudo-spin (i.e. electron spin) of the K’ and K valleys in the Brillouin zone, presenting a bridge between the types of angular momentum of these quantum particles. This allows quantification of spin-polarization through the emitted photons’ degree of circular polarization (DOP). However, valley coupling degrades severely at higher temperatures due to strong intervalley scattering, traditionally precluding TMDC’s use in investigating spin-polarized electronic transport for CISS-enabled synthesis.<br/><br/>Here, we present a nanophotonic approach to enhance and control the emission properties of TMDCs at elevated temperatures using high-quality-factor (high-Q) spin metasurfaces. An integrated TMDC-metasurface system is tested to read the spin-polarization of the electrons through the enhanced DOP signals at from 4 K to room temperature from various CISS-supporting materials from ferromagnetic films (Co/Pt/Ni oxides, HfO2). Our metasurfaces consist of dielectric nanodisks arranged in a biperiodic lattice and support quasi-bound states in the continuum (q-BIC) resonances upon circularly polarized illumination. More importantly, these q-BIC resonances enhance the spin dissymmetry factor instead of Kuhn’s dissymmetry factor. More specficially, our previous work utilizes metasurfaces that maximize the density of incident circularly polarized light (optical chirality) in the chiral basis to enhance TMDC photoemission, our current work uses the spin basis, allows more efficient coupling to the TMDCs, enhancing the spin-selectivity of the optical transition rate of K and K’ valleys, and thereby minimizing the spin dephasing of metasurface-coupled TMDCs that dominate at elevated temperatures. These resonances can have Q-factors &gt;1000, allowing large electric field confinement and contributing to valley-selective Purcell enhancement of the TMDCs. We use silicon nitride (SiN, n ~ 2) as our metasurface material because of its high refractive index and low ohmic loss properties around the excitation wavelength (760 nm) of A-exciton for a monolayer of molybdenum diselenide (MoSe₂). Our nanodisks have radii of ~ 150 nm and height ~ 200 nm, allowing large optical spin density on top of the metasurface. We then fabricate these SiN nanodisks on a quartz substrate (n ~ 1.45), providing a large permittivity contrast for better electric field confinement near the TMDC. An integrated TMDC-metasurface system is tested to readout the spin-polarization of the electrons through the enhanced DOP signals from 4 K to room temperature from various CISS-supporting materials from ferromagnetic films (Co/Pt/Ni oxides, HfO₂). Moreover, these experiments are repeated in various organic media (acetone, 2-propanol and toluene) to demonstrate the viability of using this platform in these environments for future work for measuring spin-polarized currents from CISS-enabled chiral molecules and macromolecules, such as α-helical oligopeptides, DNA, and helicenes. These experiments allow us to compare and standardize spin polarization measurements en-route to improved CISS-based molecular synthesis.