One is an adaptation of a well-known effect called “optical tweezers” in which objects can be trapped in an area where two laser beams cross. However, this version of the approach would require an atmosphere in which to operate.
The other two methods rely on specially shaped laser beams – instead of a beam whose intensity peaks at its centre and tails off gradually, the team is investigating two alternatives: solenoid beams and Bessel beams.
The intensity peaks within a solenoid beam are found in a spiral around the line of the beam itself, while a Bessel beam’s intensity rises and falls in peaks and troughs at higher distances from the beam’s line.
The approach could be put to use in space and on planetary surfaces
A photon carries a momentum of, so one may anticipate light to “push” on any object standing in its path via the scattering force. In the absence of intensity gradient, using a light beam to pull a particle backwards is counter intuitive. Here, we show that it is possible to realize a backward scattering force which pulls a particle all the way towards the source without an equilibrium point. The underlining physics is the maximization of forward scattering via interference of the radiation multipoles. We show explicitly that the necessary condition to realize a negative (pulling) optical force is the simultaneous excitation of multipoles in the particle and if the projection of the total photon momentum along the propagation direction is small (as in some propagation invariant beams), attractive optical force is possible. This possibility adds “pulling” as an additional degree of freedom to optical micromanipulation.
We introduce optical solenoid beams, diffractionless solutions of the Helmholtz equation whose diffraction-limited in-plane intensity peak spirals around the optical axis, and whose wavefronts carry an independent helical pitch. Unlike other collimated beams of light, appropriately designed solenoid beams have the noteworthy property of being able to exert forces on illuminated objects that are directed opposite to the direction of the light’s propagation. We demonstrate this through video microscopy observations of a colloidal sphere moving upstream along a holographically projected optical solenoid beam.
In all three cases, explained Dr Stysley, the effect is a small one – but it could in some instances outperform existing methods of sample gathering.
“[Current] techniques have proven to be largely successful, but they are limited by high costs and limited range and sample rate,” he said.
“An optical-trapping system, on the other hand, could grab desired molecules from the upper atmosphere on an orbiting spacecraft or trap them from the ground or lower atmosphere from a lander.
“In other words, they could continuously and remotely capture particles over a longer period of time, which would enhance science goals and reduce mission risk.”