In this study microscopic gas bubbles (7-12 μm diameter) suspended in water were optically trapped in a custom-built microfluidic slide using holographically generated Laguerre-Gaussian (‘doughnut’) beam optical tweezers. The optical potential was then characterized as a function of bubble size, trapping laser power and trapping beam diameter (Laguerre-Gaussian beam mode) using the trap spring constant in the plane transverse to the beam propagation direction, obtained from the position fluctuations of the bubble in the trap measured by video microscopy and particle tracking. It was found that microbubbles were held at the equilibrium position of buoyant and optical forces at a distance from the focus of the beam that increased with laser power, and that optical trapping in this configuration was only possible within a specific range of trap and bubble parameters. Furthermore an optimum size of the doughnut beam to microbubble diameter which maximized the transverse spring constant was found . A ray optics model of the optical forces acting on microbubbles in a focused Laguerre-Gaussian beam was used in order to calculate the trap spring constants and equilibrium trapping position as a function of the different parameters, and highlight key physical behaviours.

Date Published: 2 May 2014
PDF: 10 pages
Proc. SPIE 9126, Nanophotonics V, 91263L (2 May 2014); doi: 10.1117/12.2055889

Published in SPIE Proceedings Vol. 9126:
Nanophotonics V
David L. Andrews; Jean-Michel Nunzi; Andreas Ostendorf, Editor(s)