Manipulating crystals with light

When light is scattered by an object there is an exchange of momentum which results in a net force being applied to the scatterer. Although relatively weak, optical forces can be significant for micro-objects in intense light fields. For example, milliwatts of laser power generate optical forces sufficient to levitate and transport 10 mu m particles. Applied to biological matter, optical force techniques offer the important advantage of permitting contactless, nonmechanical, in situ manipulation. By appropriate selection of the laser wavelength, radiation and thermal damage can be avoided. We describe a compact laser seeding apparatus by which individual crystal micro-seeds can be selected and transferred into a growth solution. The device allows for the rapid, reproducible transfer of single seeds. Other plausible uses of the seeding apparatus are seeding directly into capillary tubes and transfer of microcrystals into cryo-loops. (C) 1999 Elsevier Science B.V. All rights reserved.