Faster, cheaper, safer optical tweezers for the undergraduate laboratory

A tightly focused beam of light can attract and trap micron-sized dielectric particles whose refractive index exceeds that of the surrounding medium. Although single-beam optical traps were developed only in 1986, they have already proven their worth in making possible an increasing number of experiments. Optical traps have found particular use in biophysics, where they allow one to manipulate single molecules of DNA, allowing one access to their physical properties and to the properties of attached molecules of biological interest. They have been used passively, to record the forces induced on a bead, for example, by kinesin molecules and myosin-V. In other applications, tweezers have played an active role, for example, to induce a ‘‘pearling’’ instability in lipid vesicles. The tweezer-induced motion of a bead also can be used to measure local elasticities and viscosities, for example, inside cells. The first designs of optical tweezers used large (>1 W) lasers and expensive optical hardware, which placed them beyond the reach of undergraduate laboratories. Recently, however, Smith et al. developed an apparatus that is simple and cheap enough to be included in an undergraduate laboratory. This article explores improvements to their original design, the cumulative effect of which is to make the apparatus more practical and much cheaper. In addition, the design eliminates several possibilities for injuries, increasing the safety of the experiment. ~After the first version of this work was submitted, Moothoo et al. published a design with similarities to ours. There are, nonetheless, a number of differences worth discussing. In addition, a two-beam trap using a hollow-core fiber has also been described. It shares some of the advantages of the design described here, although lasers of much higher power are required.! In the following, we first briefly review the theory of optical tweezers, mostly to alert the reader to a recent theoretical advance that greatly simplifies calculations. We then discuss our design and its rationale, along with a careful discussion of one application for the tweezers.