Abstract
Optical trapping is a well-established technique that is increasingly used on biological substances and nanostructures. Chirality, the property of objects that differ from their mirror image, is also of significance in such fields, and a subject of much current interest. This review offers insight into the intertwining of these topics with a focus on the latest theory. Optical trapping of nanoscale objects involves forward Rayleigh scattering of light involving transition dipole moments; usually these dipoles are assumed to be electric although, in chiral studies, magnetic dipoles must also be considered. It is shown that a system combining optical trapping and chirality could be used to separate enantiomers. Attention is also given to optical binding, which involves light induced interactions between trapped particles. Interesting effects also arise when binding is combined with chirality.
Highlights
When a laser beam is scattered by a dielectric microparticle, resulting in light refraction on entering and leaving the particle, a small amount of momentum is transferred from the photons to the matter
Optical trapping of microscale particles via this mechanism was first reported in the 1970s [1] and duly led to the initial observation of a single beam optical trap in 1986 [2]
While all the enantiomers are attracted to the high-intensity part of a circularly polarized trapping beam, the left-handed molecule may be more inclined, in relation to its mirror image, to reside in the high intensity region of the beam [105]. This prospect does not account for the intermolecular interactions known as optical binding, which is the subject of the following section
Summary
When a laser beam is scattered by a dielectric microparticle, resulting in light refraction on entering and leaving the particle, a small amount of momentum is transferred from the photons to the matter. Photonics 2015, 2 developed from them, utilized the gradient force exerted by a single, tightly focused Gaussian laser beam to trap particles in solution through what has become known as the ―optical tweezer‖ effect. Since these initial findings, optical technology has evolved significantly, and traps that facilitate three dimensional manipulation of particles are readily available. Beyond the capacity to isolate and spatially arrange microparticles, modern optical traps can be utilized to quantitatively measure displacement and applied force with nanometre and piconewton resolutions, respectively This level of precision, along with the inherent non-contact and non-invasive characteristic of the technology, make optical tweezers appealing for the study of biological systems [20]. Our focus is on the novel opportunities afforded by the optical binding of chiral molecules and nanoparticles
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