Lateral forces on circularly polarizable particles near a surface

Abstract

Optical forces allow manipulation of small particles and control of nanophotonic structures with light beams. While some techniques rely on structured light to move particles using field intensity gradients, acting locally, other optical forces can ‘push' particles on a wide area of illumination but only in the direction of light propagation. Here we show that spin–orbit coupling, when the spin of the incident circularly polarized light is converted into lateral electromagnetic momentum, leads to a lateral optical force acting on particles placed above a substrate, associated with a recoil mechanical force. This counterintuitive force acts in a direction in which the illumination has neither a field gradient nor propagation. The force direction is switchable with the polarization of uniform, plane wave illumination, and its magnitude is comparable to other optical forces.

An electromagnetic multipole expansion beyond the long-wavelength approximation

“…Plasmonic or highindex dielectric nanoparticles are frequently used for this purpose 1,2 . The multipole expansion provides insight into several optical phenomena, such as Fano resonances 3,4 , electromagnetically-induced-transparency 5 , directional light emission [6][7][8][9] , manipulating and controlling spontaneous emission [10][11][12] , light perfect absorption [13][14][15] , electromagnetic cloaking 16,17 , and optical (pulling, pushing, and lateral) forces [18][19][20][21][22] . In all these cases, an external field induces displacement or conductive currents into the samples.…”

Spin-dependent optics with metasurfaces

“…Moreover, such asymmetric effects can be demonstrated for even a single particle placed on the surface of metal surface [98][99][100]. As a direct implication, the particle is optically pushed in opposite directions for the incidence of different circular polarizations as demonstrated as the mechanical effects from spin-orbit coupling [101,102]. When the slit becomes much smaller than a wavelength, it reradiates as a localized dipole moment [100].…”

Elliptical orbits of microspheres in an evanescent field

“…The behavior is unique in that it depends neither on the nature of the microparticles nor that of the excitation; rather, angular momentum is introduced by the particle's interaction with the anisotropic fluid and optical trap environment. Overall, we find this motion to be highly tunable and predictable.optical forces | colloids | microparticles | evanescent field | elliptical motion R ecently, much work has gone into the investigation of optical forces on micro-and nanoparticles near surfaces, primarily in the context of an electric field localized by a microstructured surface (1-5).Surface-based geometries have raised theoretical excitement due to, for instance, their ability to significantly enhance optical forces (6-8) as well as the emergence of lateral forces due to the extraordinary momentum and spin in evanescent waves (9)(10)(11)(12). From an applied point of view, such geometries can enable miniaturization and parallelization of efficient optical traps enabling integration into optofluidic devices (13,14).…”

“…Surface-based geometries have raised theoretical excitement due to, for instance, their ability to significantly enhance optical forces (6-8) as well as the emergence of lateral forces due to the extraordinary momentum and spin in evanescent waves (9)(10)(11)(12). From an applied point of view, such geometries can enable miniaturization and parallelization of efficient optical traps enabling integration into optofluidic devices (13,14).…”

“…Despite this, little quantitative study has been done on the dynamics of optically driven particles near a surface. In studies introducing new schemes to optically manipulate matter, the particle's response function is often ignored (11,12,(26)(27)(28)(29).…”