Several approaches have been introduced in recent years for structuring potential energy landscapes on molecular, macromolecular, and cellular levels. Among these are arrays of lithographically defined microscopic posts integrated into hermetically sealed fluidic channels, which provide a periodic and precisely tuned alternative to the gels used for electrophoresis (17). Arrays of interdigitated electrodes (19,18) also have been used to establish periodic potentials through dielectrophoresis. The emphasis in these studies, however, has been on ratchet-like behavior induced by time-dependent potentials.
More recently, techniques have been developed for tailoring extensive potential energy landscapes using forces exerted by light. The most capable of these exploit optical gradient forces, meaning that dipole moments induced in illuminated objects respond to gradients in the illumination's electric field. Such forces are the basis for the single-beam optical trap known as an optical tweezer (20), which acts as a potential energy well for particles with appropriate optical properties. More generally, an extended optical intensity distribution will produce an associated potential energy landscape.
The most straightforward way to project periodic intensity profiles is to create a standing-wave interference pattern from two or more coherent beams of light. Such patterns have come to be known as optical lattices, particularly when applied to controlling the distributions and motions of matter. More general intensity patterns can be created with holographic optical tweezers (HOT) (21,22,23) or with the generalized phase contrast (GPC) technique (24), which establish extended optical trapping patterns using computer-generated holograms.