Not all holographically generated optical fields will create arrays of traps. A large uniformly illuminated volume, for example, will not have the intensity gradients required for three-dimensional trapping. Such non-trapping patterns also have applications, however, and can be created in the same manner as the hexadeca-tweezer with diffractive optics. The resulting optical fields may be useful for forcing particles against a surface and into desired configurations.
Optically induced ordering has been demonstrated by intersecting several discrete beams [35,36,37,38,39,40,41,42]. The resulting periodic intensity patterns are observed to induce order in colloidal monolayers. In this respect, the pattern of light plays the role of a modulated substrate potential and affects the phase behavior of the illuminated two-dimensional system. This effect is believed to be closely analogous to the influence of atomic corrugation on the phase transitions of adsorbed atomic and molecular overlayers [43,44]. The optical substrate's symmetry, periodicity and depth of modulation all can be adjusted experimentally. This system therefore has great promise for exploring the mechanisms of surface phase transitions. Diffractively generated optical substrates, moreover, need not be periodic. Quasiperiodic and aperiodic patterns will be useful for studies of the effect of pinning on monolayer dynamics, a potentially powerful analog for dynamics in superconducting vortex lattices and sliding charge density waves.
Beyond these applications to studies in fundamental condensed matter physics, optical substrates should be useful for assembling composite systems textured on the micron scale. In this case, the patterned illumination acts as a template for depositing particles directly or assisting self-assembly. The resulting mesoscopically arranged structures could be gelled in place and combined to create extended structures and devices. The fabrication of optical circuit elements might be based on such an approach.
Both tweezing arrays and optical substrates can be used to direct the self-assembly of three-dimensionally ordered colloidal crystals. These crystals recently have been shown to have a promising future as photonic circuit elements [45,46] and as quantitative chemical sensors [47,48]. Their full commercial exploitation will require the ability to fabricate large single crystals with desirable symmetry and lattice constants. Diffractive optical arrays can be used to organize the first layer of a growing colloidal crystal and register it with a desired substrate. Subsequent layers then grow epitaxially on the first, leading to much longer-ranged three-dimensional order than might be possible otherwise. This principle has been demonstrated with lithographically patterned substrates [49]. The optical substrates can be adjusted in situ to achieve optical ordering.
Multilayer tweezer arrays could be used to directly form multilayer photonic crystals such as photonic bandgap materials [50], including optical waveguides, tuned defect states in the bandgap, and planned interstitials. The high-dielectric-constant materials required for realizing a full photonic bandgap most likely cannot be trapped with conventional tweezer arrays. Arrays of optical vortex tweezers, however, should be amenable to the task.
Finally, scanned and otherwise time-dependent optical substrates can provide the time-modulated spatially asymmetric potentials required for practical particle size fractionation through directed diffusion [51]. The optical ratchet principle required for practical directed diffusion has been demonstrated with a single scanned tweezer [52,53,54]. Extended optical substrates may turn this demonstration into a practical technique.
This work was supported primarily by the MRSEC Program of the National Science Foundation under Award Number DMR-9400379. Additional support was provided by the National Science Foundation under Grant Number DMR-9320378. Mr. Dufresne was supported by a GAANN Fellowship of the Department of Education under Award Number P200A-10052.