Radiation pressure makes ellipsoidal particles tumble
CNRS, CRPP, UPR 8641 - F-33600 Pessac, France, EU and Univ. Bordeaux, CRPP, UPR 8641 - F-33600 Pessac, France, EU
Received: 3 July 2012
Accepted: 6 November 2012
We report on optical levitation of dielectric particles, of prolate ellipsoidal shape, a few tens of micrometers in length, in a low-aperture laser beam. Ellipsoids of moderate aspect ratio (k < 3) are observed to be trapped on the axis of the laser beam, similarly to simple spheres. Conversely, elongated particles (k > 3) cannot be kept immobile, and rather undergo sustained oscillating motions, comprising both lateral and angular excursions around the beam axis; hence the name “tumble”. The observed tumbling motion, a straightforward manifestation of the non-conservative character of radiation pressure forces, is explained through a 2-dimensional ray optics model of the interaction of light with an ellipsoid.
PACS: 82.70.Dd – Colloids / 42.50.Wk – Mechanical effects of light on material media, microstructures and particles / 87.80.Cc – Optical trapping
© EPLA, 2012