Optically driven oscillations of ellipsoidal particles. Part I: Experimental observations

Abstract

We report experimental observations of the mechanical effects of light on ellipsoidal micrometre-sized dielectric particles, in water as the continuous medium. The particles, made of polystyrene, have shapes varying between near disk-like (aspect ratio k = 0.2) to very elongated needle-like (k = 8). Rather than the very tightly focused beam geometry of optical tweezers, we use a moderately focused laser beam to manipulate particles individually by optical levitation. The geometry allows us varying the longitudinal position of the particle, and to capture images perpendicular to the beam axis. Experiments show that moderate-k particles are radially trapped with their long axis lying parallel to the beam. Conversely, elongated (k > 3) or flattened (k < 0.3) ellipsoids never come to rest, and permanently "dance" around the beam, through coupled translation-rotation motions. The oscillations are shown to occur in general, be the particle in bulk water or close to a solid boundary, and may be periodic or irregular. We provide evidence for two bifurcations between static and oscillating states, at k ≈ 0.33 and k ≈ 3 for oblate and prolate ellipsoids, respectively. Based on a recently developed 2-dimensional ray-optics simulation (Mihiretie et al., EPL 100, 48005 (2012)), we propose a simple model that allows understanding the physical origin of the oscillations.

Nanometer optical trap based on stimulated emission in evanescence of a totally reflected Arago spot

“…These particles are trapped at the focal point of a tightly focused by optical gradient forces. Several applications of this non-contact particle manipulation have been considered ranging from movement tracking [8][9][10], application or detection of small forces [11,12], altering cell membranes [13][14][15], cells sorting [16][17][18] and particles manipulation [19][20][21][22][23][24], to name a few. This last application, which is the most popular one, includes dielectric spheres, viruses, bacteria, organelles, or DNA stands manipulation.…”

Complex rotational dynamics of multiple spheroidal particles in a circularly polarized, dual beam trap

“…The latter have, only recently, been experimentally observed [22]. In addition, their geometric anisotropy allows angular momentum to be exchanged with optical fields, permitting ellipsoidal particles to be oriented or rotated [23][24][25][26][27][28][29][30][31][32]. A number of applications follow from these properties.…”

Tomographic active optical trapping of arbitrarily shaped objects by exploiting 3D refractive index maps

“…To predict the optical forces on particles with low symmetry, however, requires extensive numerical calculations, such as the T-matrix method 4 and the discrete dipole approximation 5 . Previous works have shown that non-spherical particles can be aligned along a limited equilibrium orientation when trapped with a Gaussian beam 6 7 and their position and orientation were measured by holographic microscopy techniques 8 9 , and have exhibited unstable motion, depending on the sample geometry and optical properties 10 11 . Since optical trapping is an example of light–matter interaction, methods of controlling the stable orientation of arbitrarily shaped particles can be explored either by modifying sample shapes or by engineering the wavefront of light 12 .…”