In this computational study we examine the behaviour of mixed-refractive index colloidal clusters. The clusters consist of bonded collections of spherical nanospheres with different refractive indices. The behaviour of these clusters in optical traps varies depending on the numbers of particles involved and the overall symmetry of the cluster. The symmetry can be varied by changing the refractive indices. It has been shown that breaking the symmetry of a cluster by, for example, introducing a single high refractive index impurity can produce a pronounced impact on the dynamics of the cluster in an optical trap [Davie et al. Proc. SPIE 12649, 126490E (2023)]. In the present study we will explore the dynamics of mixed refractive index clusters counter propagating plane waves and in different types of optical trap. Optical forces are calculated using the discrete dipole approximation; dynamics are computed using Brownian dynamics, with hydrodynamic interactions in the low Reynolds number regime and particle separations are constrained using the SHAKE-HI algorithm.
We consider a system of colloidal particles, with two or more different refractive indices, which stick together to form mixed clusters. The behaviour of such clusters in optical traps will vary depending on the numbers of particles involved and the distribution of their refractive indices. Our own recent studies of heterodimers of such beads suggest there is a rich vein of novel behaviour to explore, including unusual dynamics and optical binding [O'Donnell et al. Proc. SPIE 12436, 124360J (2023)]. In the present computational study we will explore the dynamics of mixed refractive index clusters in different types of optical trap, as a function of number and composition of beads as well as their arrangement. In particular, we will highlighting the difference between symmetrical and asymmetrical arrangements of beads in conventional Gaussian beams as well as in OAM beams. The optics model used is based on the discrete dipole approximation and includes low Reynolds number hydrodynamics with bead separations maintained using SHAKE-HI constraints. Further studies will investigate optical binding between arrays of such clusters in different types of structured fields.
Optical binding occurs when light scattered from a microscopic particle induces forces on other microscopic particles, which includes attractive, binding, forces. Since the original demonstration using dielectric spheres, there has been much interest in altering the shape and symmetry of the bound particles, leading to complex optically driven behaviour typically involving uniform illumination in a restricted region e.g. an extended or line focus or the axis of a Bessel beam. However, in the present computational study we consider confinement in complex optical landscapes comprising one or many focussed beams. In this survey we will explore the behaviours of systems of spherical beads with various refractive indices, as well as systems of dimers, or dumbbells, consisting of pairs of linked spheres and active janus particles comprising pairs of beads with different refractive indices. The relative importance of optical trapping versus optical trapping between the particles will be assessed.
We present a computational study of the trapping, packing and dynamics of clusters of Rayleigh particles in optical vortices. We examine the effect of OAM on the cylindrical packing arrangements and dynamics of the clusters, drawing comparisons with macroscopic systems of beads in fluid vortices.
The use of a patterned surface to create an optical conveyor for spherical, and near spherical particles is explored. Using a surface constructed of a repeating micron-sized motif, we simulate the effects of moving a particle in the near-field region above the surface, as well as exploring optical force changes in the axis perpendicular to the surface resulting from changes in size of the particles, and the choice of incident wavelength.
In this paper the use of a patterned surface to create an optical conveyor for spherical, and near spherical particles is explored. Using a surface constructed of a repeating micron-sized motif, we simulate the effects of moving a particles in the near micron regime above the surface. To reduce optical calculation overheads, a fluid index matched to the substrate of the meta-surface is used, as this allows use to ignore scattering effects due to the presence of a substrate. We also explore optical force changes in the axis perpendicular to the surface that occur due to changes in size of the particles, and the choice of incident wavelength.
In this paper, the use of a 2D meta-surface for the optical trapping and directed movement of spherical nanoparticles is explored. Using the Discrete Dipole Approximation, we calculate the optical forces that occur when a spherical nanoparticle moves across a surface constructed of tessellated equilateral triangles. We calculate that given sufficient optical power, choice of optical materials, and wavelengths, it is possible to use a meta-surface to guide micron sized particles in a direction dictated by the geometry of the surface. Furthermore, we suggest several experimental methods of verifying the results which are also discussed in detail.
We explore a computational, finite element approach to the study of flexible particles in optical tweezers, illustrating the method with optical deformation of membranes and nanowires and an optically driven microscopic swimmer.
Extremely large forces and torques have been reported when light is scattered by 2-dimensional metamaterial surfaces. Potential applications include force actuators in optically driven micro-machines, and diffractive solar sails. Here we consider refractive-index patterning of a dielectric substrate achieved photo-lithographically and demonstrate, using a computational electromagnetic approach, how a periodic structure may be optimised to maximise the optical forces and torques generated. We will show that enhancements of two orders of magnitude in the optical force are available in specific cases.
The non-conservative nature of optical forces has been explored many times previously. Non-conservative optical forces occur in many guises and include lateral forces due to shape asymmetry, polarisation dependant optical torques and spin-dependant effects. When considering the design of optical force actuators, one recent approach has been to exploit the periodic structure of 2-dimensional metamaterial surfaces. In some cases, extremely large optical forces have been reported (Zhang et al, Optics Letters, 39, 4883 (2014)). Most recently macroscopic forces and torques have been demonstrated (Magallanes and Brasselet, Nature Photonics, 12, 461 (2018)). Here we consider refractive-index patterning of a substrate, achieved through a photo-lithographic approach. In this paper we will explore the use of different types of patterning, and show how the use of a periodic structure can enhance the optical forces and torques that may be generated. Using computational electromagnetic techniques we will demonstrate how enhancements of two orders of magnitude in the optical force are available in specific cases. We will examine the influence of such surface structuring on the resultant forces and torques, with a view to optimising such 2-dimensional materials for applications as light-driven actuators. Further, we will demonstrate the sensitivity of the forces generated to variations in the local environment, opening up possibilities for optical sensing applications.
The non-conservative nature of optical forces has been explored previously, with the initial focus on spherical particles, and latterly on particles with less than spherical symmetry. Non-conservative optical forces occur in many different guises, and include lateral forces arising from shape asymmetry, polarisation dependant optical torques and spin-dependant lateral forces. Photo-induced curing of liquid crystalline polymers is a technique that may be used to generate refractive-index patterning of surfaces. Here, we use computational methods to examine the influence of such surface structuring on optically-generated forces and torques, with a view to optimising such materials for applications as light-driven sensors and actuators.
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