This program focuses on how colloidal particles move through extensive potential energy landscapes created with dynamic holographic optical tweezers. Transport through modulated potential energy landscapes is a classic problem in condensed matter physics, with variants arising in systems as diverse as flux flow through type-II superconductors, quasiparticle tunneling in Josephson junctions and procession in biological molecular motors. While much is known about transport in one-dimensional periodic potentials, much more remains to be understood regarding modulated Brownian transport in higher dimensions, particularly in aperiodic, quasiperiodic, random, and time-varying landscapes, all of which figure heavily in natural and industrial settings. Previous efforts to understand such processes have been hampered by the difficulty of controlling most relevant systems' potential energy landscapes while tracking their microscopic components' motions. Dynamic holographic optical tweezers present a unique opportunity to construct arbitrary one-, two-, and three-dimensional potential energy landscapes for micrometer-scale colloidal particles by projecting up to several thousand optical traps in any desired configuration. Unlike most other model systems, colloidal particles' microscopic motions can be tracked with exquisite accuracy using digital video microscopy. The combination of optical manipulation and high-resolution particle tracking provides an extraordinarily flexible and well-characterized model system for studying driven modulated Brownian transport, not only for single particles but also for strongly interacting systems of particles. In addition to the fundamental knowledge gained from such studies, the particular application to mesoscopic transport promises immediate practical applications in nanotechnology, biotechnology, and photonics.
How does an electron find its way through the labyrinth of a metallic glass? How does a DNA molecule thread through a gel? Answering such questions requires a new fundamental insights into how objects traverse complicated potential energy landscapes. And the answers will have immediate practical ramifications for fields as diverse as high-temperature superconductivity, drug discovery, nanotechnology and engineering. This program combines state-of-the-art micromanipulation made possible by the recent introduction of holographic optical tweezers (HOTs) with precision digital video microscopy to provide just such insights. The heart of this program is provided by HOT's ability to create arbitrary custom-designed potential energy landscapes from an ordinary beam of laser light. The technique uses computer-generated holograms to craft the beam into thousands of individual optical traps, each of which can be moved independently in three dimensions under computer control. If a single optical trap can be likened to Star Trek's tractor beam, then holographic optical tweezers more closely resemble the holodeck. Micrometer-scale colloidal particles driven through such latticeworks of light trace out solutions to long-standing fundamental physics questions. In so doing, they also provide the basis for practical applications such as sorting proteins, DNA, nanoclusters, and living cells using light. The new techniques on which this program is based were developed with direct involvement of high school and undergraduate students, as well as graduate students and postdocs. These students' unique training in these methods has helped them to land positions in top-rated schools, as well as long-term employment in industry and academia. This program's methods have been patented, and the patents have led to the foundation of a new industry in optical micromanipulation. Such substantive involvement of industry and students at all levels will continue to be a central theme of this program.
