Science with Optical Tweezers

Figure 1: A strongly focused beam of light creates an optical tweezer. Intensity gradients in the converging beam draw small objects, such as a colloidal particle toward the focus, while the beam's radiation pressure tends to blow them down the optical axis. Under conditions where the gradient force dominates, a particle can be trapped in three dimensions near the focal point.

Figure 2: Creating large numbers of optical tweezers with computer-generated holograms. Projecting a collimated laser beam through the input pupil of a strongly converging lens such as a microscope objective creates a single optical tweezer. The telescope in this implementation creates an image of the objective's input pupil, with the optical axis passing through point A. Multiple beams passing through point A therefore pass into the objective lens to create multiple optical traps. A single TEM₀₀ laser beam can be spilt into an arbitrary fan-out of beams all emanating from point A by an appropriate computer-designed diffraction grating centered there. The example phase grating $\varphi(\ensuremath{\vec \rho}\xspace )$ creates the 20×20 array of traps shown in the video micrograph. These are shown trapping 800 nm diameter polystyrene spheres dispersed in water. Adapted from Ref. (1).

The frontiers of several branches of science and engineering converge in a domain of physical conditions known as mesoscopia. Mesoscopic systems are characterized by length scales ranging from tens of nanometers to hundreds of micrometers, forces ranging from femtonewtons to nanonewtons and time scales ranging upward from a microsecond. In biology, this range covers many of the inter- and intracellular processes responsible for respiration, reproduction, and signalling. In physics and chemistry, it corresponds to the still-puzzling interface between classical and quantum mechanical behavior, made all the more perplexing by the general inapplicability of statistical many-body theory in this realm. The promise of mesoscopic engineering has been held in check by the need for tiny motors to drive micromachines, and for robust human-scale interfaces to atomic-scale nanotechnology. Until quite recently, the options for manipulating, analyzing, and organizing mesoscopically textured matter have been limited. A new generation of techniques based on the forces exerted by carefully sculpted wavefronts of light offer precisely the level of access and control needed for rapid progress across all of these fields.

Many of the most powerful optical manipulation techniques are derived from single-beam optical traps known as optical tweezers, shown schematically in Fig. 1, which were introduced by Arthur Ashkin, Steven Chu, and their coworkers at AT&T Bell Laboratories (3,2). An optical tweezer uses forces exerted by a strongly focused beam of light to trap small objects. Although the theory of optical tweezing is still being developed, the basic principles are straightforward for objects much smaller than the wavelength of light, or much larger. Small objects develop an electric dipole moment in response to the light's electric field. Generally speaking, this is drawn up intensity gradients in the electric field toward the focus. Larger objects act as lenses, refracting the rays of light and redirecting their photons' momentum. The resulting recoil draws them toward the higher flux of photons near the focus (4). This recoil is all but imperceptible for a macroscopic lens, but can have a substantial influence on mesoscopic objects.

Optical gradient forces compete with radiation pressure due to momentum absorbed or otherwise transferred from the photons in the beam, which tends to blow particles down the optical axis like a fire hose. Stable trapping requires the axial gradient force to dominate, and can be achieved if the beam diverges rapidly enough away from the focal point. For this reason, optical tweezers usually are constructed around microscope objective lenses, whose high numerical apertures and well corrected aberrations focus light as tightly as possible.

Optical tweezers can trap objects as small as 5 nm (6,5) and can exert forces exceeding 100 pN (7,9,8) with resolutions as fine as 100 aN (11,12,10). This is the ideal range for exerting forces on biological and macromolecular systems and measuring their responses. Biological and medical applications of optical tweezers have been reviewed extensively (3,14,13); just a few examples suggest the range of activities. Optical tweezers have been used to probe the viscoelastic properties of single biopolymers such as DNA, cell membranes, aggregated protein fibers such as actin, gels of such fibers in the cytoskeleton, and composite structures such as chromatin and chromosomes. They also have been used to characterize the forces exerted by molecular motors such as myocin, kinesin, procecssive enzymes and ribosomes. Such measurements have revealed that cells use mechanical forces not only for mobility, motility, and chromosome sorting during reproduction, but also for regulating gene transcription, inter- and intracellular signalling, and respiration. As a natural extension of these studies, optical tweezers have shown great promise for intracellular surgery, for instance in modifying the chromosomes of living cells (15). On larger scales, optical tweezers are useful for selecting individual microbes from heterogeneous populations. Their ability to transport and modify cells precisely have led to clinical applications in such areas as in vitro fertilization (16).

In the physical sciences, optical tweezers' unique ability to organize matter noninvasively has led to a burst of activity in classical statistical mechanics, including the first direct measurements of macromolecular interactions in solution (17). Each new round of measurements has led to surprises, including the discovery of anomalous attractions between like-charged colloidal particles (18), oscillatory colloidal interactions mediated by the entropy of smaller entities in solution (22,19,20,21), and hydrodynamic fluctuations that may be interpreted as transient violations of the second law of thermodynamics (23).

In all of these cases, and a great many more, fundamental insights emerged from manipulating specially chosen systems at just one or two discrete points. New frontiers of science and engineering would present themselves if optical traps could interrogate more general and more complex systems at many points at once, if they could induce chemical as well as physical transformations, and if they could exert torques as well as forces. Recent advances in physical optics reveal that precisely such multifunctional optical traps can be crafted from single beams of light by subtly modifying their wavefronts. The result optical micromanipulators provide unprecedented access to the microscopic world.