Particle Guidance
Maintaining control of micron-sized particles and even molecules and atoms is often on the wish-list of physicists wishing to study the particleproperties at nanometre scales. One way of doing this is to use the force of "radiation pressure" exerted by light on matter. This was experimentally pioneered by Ashkin [A. Ashkin, Phys. Rev. Lett. 24 (4), 156 (1970)] who demonstrated that small particles could be propelled and suspended against gravity using only a laser beam. The topic is of increasing importance in a number of fields, including biomedicine, where optical tweezers can be used to pick up and even spin viruses and cells.
The physics behind the laser-induced forces may be understood as follows. When a laser beam strikes a small transparent particle, the rays of light refract at the interfaces following Snell's law. If the intensity of the light is stronger on one side than the other, momentum is transferred to the particle, pushing it sideways towards the peak intensity of the laser beam. Under these circumstances and in vacuum it behaves like a marble rolling around at the bottom of a bowl. In air, collisions with the gas molecules damp the oscillation, and the particle settles at the centre of the laser beam (i.e. the bottom of the bowl) in its lowest energy position. Momentum is also transferred to the particle in the direction of the laser beam, causing it to accelerate. These two forces permit particles to be held in suspension, cancelling the effects of gravity, and is the basis of optical tweezers. However, these tweezers are intrinsically limited by the diffraction of the laser beam to micrometer length scales, as strong lateral confinement requires tight beam focusing and a long effective length to transport the particles of thousands of Rayleigh lengths -- in other words a large figure-of-merit is required.
In 2002 we reported the first cm-scale (15 cm) guidance of micro-sized particles using a HC-PCF and with a laser power of only 80 mW. The HC-PCF used for that demonstration had a transmission loss of 5 dB/m (the lowest loss reported in 2001 was ~1dB/m) which is much higher than current typical figures (10s dB/km), indicating the possiblity of guidance over distances of 100s of metres.
Photograph of the experimental set-up for partical levitation/guidance. A green laser is directed upward towards a vibrating cell containing the micro-sized particles and is coupled to a piece of HC-PCF. The levitation and guidance is recorded with CCD cameras.
