New York, NY--Physicists at New York University (NYU) have discovered a new method to create extended and knotted 3D optical traps.1 In the technique, a spatial light modulator (SLM) produces a lens focus of "bright" knots, where the maximum of the light intensity traces out a knotted trajectory in space, allowing microscopic objects to be trapped along the path of the knot. The device could become a better way to manipulate cells in lab-on-a-chip setups.
Optical traps such as optical tweezers are used to confine and manipulate small objects—ranging from a few nanometers to several hundred microns in size—in 3D. The trapping of small objects is widely used for a broad range of research applications in biophysics, condensed-matter physics, and medical diagnostics. Ordinary optical traps use Gaussian laser beams that focus to a spot. The beams being used to create the extended optical traps focus to curved lengths instead.
Hologram modifies lens focus
The knotted traps are made by placing a phase-shifting liquid-crystal SLM at the input pupil of an objective lens, imprinting a computer-generated hologram on the laser wavefront focused by the lens. The resulting curve at the focus can cross over and through itself to trace out a knot; in addition, the hologram can redirect the light's radiation pressure to have a component along the curve, moving particles along the curve.
"The knotted optical force fields we created use intensity gradients to hold microscopic objects in place and phase gradients to thread them through the knot," says NYU undergraduate student Elisabeth Shanblatt, one of the researchers. "These optical knots are a special type of a very general class of 3-D optical traps that can be created using holographic techniques."
Extended optical traps are especially useful for moving small objects such as biological cells through microfluidic lab-on-a-chip devices. Previously reported knotted vortex fields can only create intensity minima along the knot, rather than the intensity maxima created using the new computer-generated holograms.
Current loops for nuclear fusion
In another possible application, Shanblatt and fellow researcher NYU physicist David Grier see the possibility of creating knotted current loops of charged particles in high-temperature plasmas intended to exploit energy from nuclear fusion. They believe that projecting a knotted optical force field into a plasma might prove to be a good way to initiate a knotted current loop. If so, the knotted current could then be ramped up by other conventional means. The result could be a stable, high-temperature plasma capable of producing useful power from fusion.
REFERENCE:
1. Elisabeth R. Shanblatt and David G. Grier, Optics Express, Vol. 19, Issue 7, pp. 5833-5838 (2011); doi:10.1364/OE.19.005833.
