Separating and trapping of chiral nanoparticles with dielectric photonic crystal slabs

Optical enantioseparation of chiral nanoparticles in macroscopic scale within a microfluidic channel

In this Letter, we have proposed an all-optical scheme for chiral particle separation with a microcylinder-pair system (MCPS) with a micrometer scale channel, applicable in microfluidic environments. By illuminating the MCPS with two counter-incident plane waves of orthogonal polarization, the electromagnetic chirality gradient can be generated. The MCPS can also enhance chirality-dependent lateral optical forces of the coupled fields so that the setup can shift trapping equilibrium positions for opposite-handedness nanoparticles and make the sideways motion observable. Our thorough numerical calculations demonstrate that the chiral-separation MCPS here can offer high efficiency in selectively trapping and transporting enantiomers in an optofluidic system. Furthermore, the MCPS can realize the sorting and collecting of enantiomers with a directional flow in a larger spatial distance from the macroscopic perspective. We believe that our research provides a new, to the best of our knowledge, and promising method for chiral optical tweezers and optofluidic transporting applications.

Artificial neural network assisted the design of subwavelength-grating waveguides for nanoparticles optical trapping

In this work, we propose artificial neural networks (ANNs) to predict the optical forces on particles with a radius of 50 nm and inverse-design the subwavelength-grating (SWG) waveguides structure for trapping. The SWG waveguides are applied to particle trapping due to their superior bulk sensitivity and surface sensitivity, as well as longer working distance than conventional nanophotonic waveguides. To reduce the time consumption of the design, we train ANNs to predict the trapping forces and to inverse-design the geometric structure of SWG waveguides, and the low mean square errors (MSE) of the networks achieve 2.8 × 10-4. Based on the well-trained forward prediction and inverse-design network, an SWG waveguide with significant trapping performance is designed. The trapping forces in the y-direction achieve-40.39 pN when the center of the particle is placed 100 nm away from the side wall of the silicon segment, and the negative sign of the optical forces indicates the direction of the forces. The maximum trapping potential achieved to 838.16 kBT in the y-direction. The trapping performance in the x and z directions is also quite superior, and the neural network model has been further applied to design SWGs with a high trapping performance. The present work is of significance for further research on the application of artificial neural networks in other optical devices designed for particle trapping.

Precise size sorting of nanoparticles by bound states in the continuum in a dual finite grating

We consider two parallel dielectric gratings (dual grating) which support accidental bound states in the continuum (BICs) mostly localized between gratings. As distinctive to true periodical BICs in an infinite dual grating, the enveloping intensity of quasi-BICs in a finite dual grating behaves as a standing wave. That behavior is a key property to trap nanoparticles into selected cells of the dual grating sorted by sizes of nanoparticles dragged by liquid flowing between gratings. For excitation of quasi-BIC with high quality factor by an electromagnetic plane wave with normal incidence and power 1 mW/µm2 we show high efficiency of sorting of nanoparticles by sizes.

Achiral nanoparticle trapping and chiral nanoparticle separating with quasi-BIC metasurface

Dielectric metasurfaces based on quasi-bound states in the continuum (quasi-BICs) are a promising approach for manipulating light-matter interactions. In this study, we numerically demonstrate the potential of silicon elliptical tetramer dielectric metasurfaces for achirality nanoparticle trapping and chiral nanoparticle separation. We first analyze a symmetric tetramer metasurface, which exhibits dual resonances (P1 and P2) with high electromagnetic field intensity enhancement and a high-quality factor (Q-factor). This metasurface can trap achiral nanoparticles with a maximum optical trapping force of 35 pN for 20 nm particles at an input intensity of 100 mW. We then investigate an asymmetric tetramer metasurface, which can identify and separate enantiomers under the excitation of left-handed circularly polarized (LCP) light. Results show that the chiral optical force can push one enantiomer towards regions of the quasi-BIC system while removing the other. In addition, the proposed asymmetric tetramer metasurface can provide multiple Fano resonances (ranging from R1 to R5) and high trap potential wells of up to 33 kBT. Our results demonstrate that the proposed all-dielectric metasurface has high performance in nanoparticle detection, with potential applications in biology, life science, and applied physics.

Optical Trapping Separation of Chiral Nanoparticles by Subwavelength Slot Waveguides

Enantiomer separation opens great opportunities to develop the technologies of pharmaceutics, chemicals, and biomedicine, but faces daunting challenges. Here, we discover a considerable chiral-dependent trapping force to separate nanometer-scale enantiomers in a new silicon-based waveguide platform. The electromagnetic chirality gradient of strongly confined evanescent fields can be largely enhanced by the counterpropagating slot waveguides so that the resulting chiral gradient forces can shift the trapping equilibrium positions of dielectric gradient forces. Especially, there exists a transitional width for the slot waveguides to exchange the trapping equilibrium positions between two opposite enantiomers. Our thoroughly numerical investigations demonstrate that the chiral-separable slot waveguides here can offer high efficiency and feasibility of separating chiral nanoparticles, and may pave a route toward new on-chip chiral optical tweezers or optofluidic transport systems for large-scale chiral separation.