Optical spit roast

Researchers in Germany and the UK have devised a new method for the optical manipulation of microscopic particles and demonstrated it by rotating biological cells under a microscope^†.
 
 

Red blood cells

Optical rotation is not new, but previous techniques have either been technically complicated, fully integrated into the optics of a microscope, or only able to rotate particles in a direction parallel to the microscope's optical axis. That plane of rotation is fine for applications where you want to turn objects around like you would turn a steering wheel in front of you, but what if you wanted to turn them around an axis perpendicular to the viewing direction, perhaps to see the underside of an object? These researchers have come up with the optical equivalent of a spit roast, trapping particles between two light beams and rotating them by turning the handle at one end.

The system is comprised of two Ytterbium-doped fibre lasers, whose beams emerge from fibre ends which face each other across a gap of approximately 200 μm, in the sample plane beneath a microscope. The counter-propagating light beams produce a dual-beam optical trap within the gap, with the optical axis orientated perpendicular to that of the microscope.

The key to the success of this technique is the asymmetry of the optical trap. Each laser produces a symmetrical light beam profile from a single-mode fibre, so in order to produce an anisotropic light trap, one of the fibres was spliced on to a dual-mode fibre, which constituted the output fibre end at one side of the optical trap. Anisotropic dielectric particles trapped between the two counter-propagating beams will come to rest at a position in the trap at which the optical scattering forces balance each other, and they will align themselves preferentially in accordance with the optical force gradients resulting from the anisotropy of the trap.

Sequence of images showing the optical rotation of a red blood cell (scale bar = 10 μm)^†.

The technique was demonstrated using human cells, both individually and in clusters. With the axis of rotation aligned in the direction along which the two fibres faced each other, particles were rotated like a pig on a spit roast by turning the handle at one end, which in this case was the dual-mode fibre. Rotation of the fibre resulted in a corresponding rotation of the trapped particles, which was a direct consequence of the laser light and not due to any hydrodynamic coupling between the fibre and the sample cells.

These optical cell rotators could offer significant improvements in cell manipulation, supplanting more invasive mechanical methods for controlling cell movement. Another advantage it promises is the minimal laser power required, which would make it a favourable technique for the manipulation of fragile specimens susceptible to photo-thermal damage. And there is the added benefit that such a tool would not need to be integrated into the optics of the viewing microscope, so it could easily be fitted to existing instruments as a separate module, making it both versatile and more affordable.

† Moritz K. Kreysing, Tobias Kießling, Anatol Fritsch, Christian Dietrich, Jochen R. Guck, Josef A. Käs (2008). The optical cell rotator Optics Express, 16 (21) DOI: 10.1364/OE.16.016984