A chiral near‐field with a highly contorted electromagnetic field builds a bridge to match chiral molecules and light wavelengths with large size differences. It significantly enhances the circular dichroism of chiral molecules and has great prospects in chirality sensing, detection, trapping, and other chirality‐related applications. Surface plasmons feature outstanding light‐trapping and electromagnetic‐field‐concentrating abilities. Plasmonic chiral nanostructures facilitate light manipulation to generate superchiral near‐fields. Meanwhile, the nanophotonic structures have attracted significant interest to obtain strong chiral near‐fields due to their unique electromagnetic resonant properties. During the interaction of light and chiral materials, the chiral near‐field not only bridges the light and chiral molecules but is also responsible for the optical activities. This paper reviews state‐of‐the‐art studies on generating or enhancing chiral near‐fields using plasmonic and photonic nanostructures. The principle of chiral near‐fields and the development of chiral near‐fields with plasmonic and photonic nanostructures are reviewed. The properties and applications of enhanced chiral near‐fields for chiral molecule detection, spin‐orbit angular interaction, and the generation of the chiral optical force are examined. Finally, current challenges are discussed and a brief outlook of this field is provided.
Benefiting from the well adjustability and the strong near-field enhancement, surface plasmons are widely used for optical force trap and achieve great performance. Here, we employ the Laguerre-Gaussian beam and a plasmonic gold ring to separate enantiomers by the chiral optical force. Along with the radial optical force that traps the particles, there is also a chirality-sign-sensitive lateral force arising from the optical spin angular momentum, which is caused by the interaction of optical orbit angular momentum and gold ring structure. By selecting a specific incident wavelength, the strong angular scattering and non-chiral related azimuthal optical force can be suppressed. Thus the chiral related azimuthal optical force can induce an opposite orbital rotation of the trapped particles with chirality of different sign near the gold ring. This work proposes an effective approach for catching as well as separating chiral enantiomers.
Optical tweezers are a crucial tool for manipulating nanoscale objects, and have a wide range of applications in various fields. Bowtie-nanohole tweezers, a type of near-field optical tweezers, are particularly intriguing due to their strong near-field enhancement and unique characteristics. In this paper we provide a detailed discussion of the properties of bowtie-nanohole tweezers on trapping and sorting nanoparticles through theoretical and numerical results. It is discovered that the tweezers behave differently when trapping particles with varying refractive indices, leading to a discussion of sorting chiral particles. Moreover, the relative refractive index between the particles and the background solution greatly influences the trapping and sorting abilities of the tweezers. Finally, we investigate the performance of the tweezers at different wavelengths of incident light to determine the optimal working wavelength for trapping or sorting.
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