Emergent properties in optically bound matter

Abstract

with the permission of OSA. The paper can be found at the following URL on the OSA website:https://doi.org/10.1364/OE.16.006921. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law. Additional information:Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract:Sub-micron particles have been observed to spontaneously form regular two-dimensional structures in counterpropagating evanescent laser fields. We show that collective properties of large numbers of optically-trapped particles can be qualitatively different to the properties of small numbers. This is demonstrated both with a computer model and with experimental results. As the number of particles in the structure is increased, optical binding forces can be sufficiently large to overcome the optical landscape imposed by the interference fringes of the laser beams and impose a different, competing structure.

Directed assembly of optically bound matter

“…During our studies we also observe emergent dynamic behaviour in longer chains such as chain collapse, tilting and off-axis drift [25]. Perhaps the most striking dynamical effect observed was what we have dubbed "Newton's Cradle" motion, in which particles are ejected from the ends of chains with velocity components both in and out of the plane of the interface; at a certain height and distance from the optically bound chain the free particle then rejoins the chain under the influence of both gravity and optical binding/scattering forces.…”

“…of dimensions comparable to the wavelength of the light. Our approach is broadly similar to that used in previous work [25], including theoretical work by other authors to include considerations such as optical forces acting on particles [26] and multiple particle interactions [27]. In addition, important roles are played by two factors that are not normally considered in GLMT calculations; these are the effects of scattered light reflected by the prism surface and the focused nature of the illuminating Gaussian beams.…”

“…In the case of an extremely broad laser spot, such effects can be ignored and the beam can be approximated as an ideal evanescent wave (using the formulation used previously in [25] and derived in [28]). If the laser is more tightly focused the particles will be exposed to different evanescent field strengths depending on their position on the prism surface.…”

“…Such models typically deal with small ensembles of particles due to the complex nature of multiple scattering processes. Recent experiments [25] have shown that, for large chains of particles, multiple scattering effects become increasingly important relative to the background optical potential. There is still much work to be done to bridge the gap between these simple model systems and the more complex structures observed in reality, and our aim in this paper is to further explore the transition between these two regimes using a simple counter-propagating evanescent trap in combination with optical tweezers [12].…”

Multipole expansion of Bessel and Gaussian beams for Mie scattering calculations

“…Hence there is a need for efficient ways to calculate the beam shape coefficients for a given beam type. This is particularly important for time evolution simulations where large numbers of scattering calculations must be performed [7][8][9] The BSCs can be determined for an arbitrary field E͑r͒ by exploiting the orthogonality of the vector spherical harmonics and performing an eigenfunction transform, as outlined below in Eq. (2).…”