Fear of the dark: a novel optothermal trapping mechanism

31.10.2012, 16:45

Newsletter 5

A microparticle, laser-trapped and propelled along a hollow photonic crystal fiber core, is observed to come to a halt just in front of a black mark painted on to the outer surface of the fiber. This apparent "fear of the dark" results from the intriguing competition between optical forces and viscous drag caused by a thermally-driven air flow. The order of events is as follows. Light scattered by the particle is absorbed by the black mark, causing local heating and creating a temperature gradient along the fiber. This drives a flow of air towards higher temperature along the core surface. This in turn creates counter-flow of air at the center of the core, thus producing a viscous drag force sufficient to balance the optical radiation force and bringing the particle to a standstill. In contrast to recently reported optothermal techniques, this trapping mechanism permits manipulation of non-absorbing particles. Also, since both radiation and viscous forces scale linearly with optical power, the effect is independent of the optical power. Particle trapping and control in narrow channels has various applications in lab-on-a-chip devices.


Contact: oliver.schmidt@mpl.mpg.de
Group: Russell Division
Reference: O. A. Schmidt et al., Phys. Rev. Lett. 109, 024502 (2012) ); Editors' suggestion & spotlighted in Physics 5, 76 (2012) and Physics Today, July 2012.