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1. Highly Efficient Anisotropic Chiral Plasmonic Metamaterials for Polarization Conversion and Detection

   Bai, Jing ; Yao, Yu ( August 2021 , ACS nano)

   null (Ed.)

   Plasmonic chiral metamaterials have attracted broad research interest because of their potential applications in optical communication, biomedical diagnosis, polarization imaging, and circular dichroism spectroscopy. However, optical losses in plasmonic structures severely limit practical applications. Here, we present the design concept and experimental demonstration for highly efficient subwavelength-thick plasmonic chiral metamaterials with strong chirality. The proposed designs utilize plasmonic metasurfaces to control the phase and polarization of light and exploit anisotropic thin-film interference effects to enhance optical chirality while minimizing optical loss. Based on such design concepts, we demonstrated experimentally optical devices such as circular polarization filters with transmission efficiency up to 90% and extinction ratio >180, polarization converters with conversion efficiency up to 90%, as well as on-chip integrated microfilter arrays for full Stokes polarization detection with high accuracy over a broad wavelength range (3.5–5 μm). The proposed design concepts are applicable from near-infrared to Terahertz regions via structural engineering. 

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2. Highly Efficient Anisotropic Chiral Plasmonic Metamaterials for Polarization Conversion and Detection

   Bai, Jing ; Yao, Yu ( August 2021 , ACS nano)

   null (Ed.)

   Plasmonic chiral metamaterials have attracted broad research interest because of their potential applications in optical communication, biomedical diagnosis, polarization imaging, and circular dichroism spectroscopy. However, optical losses in plasmonic structures severely limit practical applications. Here, we present the design concept and experimental demonstration for highly efficient subwavelength-thick plasmonic chiral metamaterials with strong chirality. The proposed designs utilize plasmonic metasurfaces to control the phase and polarization of light and exploit anisotropic thin-film interference effects to enhance optical chirality while minimizing optical loss. Based on such design concepts, we demonstrated experimentally optical devices such as circular polarization filters with transmission efficiency up to 90% and extinction ratio >180, polarization converters with conversion efficiency up to 90%, as well as on-chip integrated microfilter arrays for full Stokes polarization detection with high accuracy over a broad wavelength range (3.5–5 μm). The proposed design concepts are applicable from near-infrared to Terahertz regions via structural engineering. 

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3. Asymmetric reactions induced by electron spin polarization

   https://doi.org/10.1039/d0cp03129a  

   Bloom, B. P. ; Lu, Y. ; Metzger, Tzuriel ; Yochelis, Shira ; Paltiel, Yossi ; Fontanesi, Claudio ; Mishra, Suryakant ; Tassinari, Francesco ; Naaman, Ron ; Waldeck, D. H. ( January 2020 , Physical Chemistry Chemical Physics)

   Essential aspects of the chiral induced spin selectivity (CISS) effect and their implications for spin-controlled chemistry and asymmetric electrochemical reactions are described. The generation of oxygen through electrolysis is discussed as an example in which chirality-based spin-filtering and spin selection rules can be used to improve the reaction's efficiency and selectivity. Next the discussion shifts to illustrate how the spin selectivity of chiral molecules (CISS properties) allows one to use the electron spin as a chiral bias for inducing asymmetric reactions and promoting enantiospecific processes. Two enantioselective electrochemical reactions that have used polarized electron spins as a chiral reagent are described; enantioselective electroreduction to resolve an enantiomer from a racemic mixture and an oxidative electropolymerization to generate a chiral polymer from achiral monomers. A complementary approach that has used spin-polarized, but otherwise achiral, molecular films to enantiospecifically associate with one enantiomer from a racemic mixture is also discussed. Each of these reaction types use magnetized films to generate the spin polarized electrons and the enantiospecificity can be selected by choice of the magnetization direction, North pole versus South pole. Possible paths for future research in this area and its compatibility with existing methods based on chiral electrodes are discussed. 

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4. Interplay of structural chirality, electron spin and topological orbital in chiral molecular spin valves

   https://doi.org/10.1038/s41467-023-40884-9  

   Adhikari, Yuwaraj ; Liu, Tianhan ; Wang, Hailong ; Hua, Zhenqi ; Liu, Haoyang ; Lochner, Eric ; Schlottmann, Pedro ; Yan, Binghai ; Zhao, Jianhua ; Xiong, Peng ( August 2023 , Nature Communications)

   Chirality has been a property of central importance in physics, chemistry and biology for more than a century. Recently, electrons were found to become spin polarized after transmitting through chiral molecules, crystals, and their hybrids. This phenomenon, called chirality-induced spin selectivity (CISS), presents broad application potentials and far-reaching fundamental implications involving intricate interplays among structural chirality, topological states, and electronic spin and orbitals. However, the microscopic picture of how chiral geometry influences electronic spin remains elusive, given the negligible spin-orbit coupling (SOC) in organic molecules. In this work, we address this issue via a direct comparison of magnetoconductance (MC) measurements on magnetic semiconductor-based chiral molecular spin valves with normal metal electrodes of contrasting SOC strengths. The experiment reveals that a heavy-metal electrode provides SOC to convert the orbital polarization induced by the chiral molecular structure tospinpolarization. Our results illustrate the essential role of SOC in the metal electrode for the CISS spin valve effect. A tunneling model with a magnetochiral modulation of the potential barrier is shown to quantitatively account for the unusual transport behavior.

    

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5. Colossal anisotropic absorption of spin currents induced by chirality

   https://doi.org/10.1126/sciadv.adn3240  

   Sun, Rui ; Wang, Ziqi ; Bloom, Brian P ; Comstock, Andrew H ; Yang, Cong ; McConnell, Aeron ; Clever, Caleb ; Molitoris, Mary ; Lamont, Daniel ; Cheng, Zhao-Hua ; et al ( May 2024 , Science Advances)

   The chiral induced spin selectivity (CISS) effect, in which the structural chirality of a material determines the preference for the transmission of electrons with one spin orientation over that of the other, is emerging as a design principle for creating next-generation spintronic devices. CISS implies that the spin preference of chiral structures persists upon injection of pure spin currents and can act as a spin analyzer without the need for a ferromagnet. Here, we report an anomalous spin current absorption in chiral metal oxides that manifests a colossal anisotropic nonlocal Gilbert damping with a maximum-to-minimum ratio of up to 1000%. A twofold symmetry of the damping is shown to result from differential spin transmission and backscattering that arise from chirality-induced spin splitting along the chiral axis. These studies reveal the rich interplay of chirality and spin dynamics and identify how chiral materials can be implemented to direct the transport of spin current.

    

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