All-optical separation of chiral nanoparticles on silicon-based microfluidic chips with vector exceptional points

Abstract

Surface-enhanced all-optical separation of chiral molecules plays an important role in the field of chiral recognition. However, the originally designed photonic micro/nano-structures are always suffering from a lot of limitations, such as low strengths of chiral optical forces, limited spatial resolutions, and narrow separation areas. Here, we theoretically design a silicon-based microfluidic chip to achieve highly efficient separations of chiral nanoparticles. By breaking the mirror-symmetry of a pair of lossy waveguides, two original orthogonal modes are coupled with each other, triggering the formation of a vector exceptional point. Numerical simulations clearly show that the superchiral gradient field can be generated in the microfluidic chip assisted by the vector exceptional point. Such a surface-enhanced chiral gradient field can induce extremely strong chiral gradient forces, pushing nanoparticles with opposite chirality toward different sides of the extended slot. Furthermore, we construct cascade vector exception points in a single microfluidic chip to fulfill the chiral separation with a larger spatial distance. Based on particle tracking simulations, we numerically demonstrate the feasibility and efficiency of our designed microfluidic chips under the influence of the thermal motion of nanoparticles. Our work proposes an efficient way of separating enantiomers in microfluidic chips with high spatial resolution, and shows an exciting prospect for next-generation chiral separation technologies.