The great advantage of using a single beam is that it can be used as an optical tweezers to manipulate small particles. The optical tweezers can easily be integrated with a conventional microscope by introducing the laser light into the body of the scope and focusing it with the viewing objective. A sample placed on an ordinary microscope slide can be viewed and manipulated at the same time by moving the focused laser beam. One application of the optical tweezers, discovered by Dziedzic and Ashkin, has captured the imagination of biologists.
They found that the tweezers can handle live bacteria and other organisms without apparent damage. The ability to trap live organisms without harm is surprising, considering that the typical laser intensity at the focal point of the optical tweezers is about 10 million watts per square centimeter. It turns out that as long as the organism is very nearly transparent at the frequency of the trapping light, it can be cooled effectively by the surrounding water. To be sure, if the laser intensity is too high, the creature can be "optocuted."
Many applications have been found for the optical tweezers. Ashkin showed that objects within a living cell can be manipulated without puncturing the cell wall. Steven M. Block and his colleagues at the Rowland Institute in Cambridge, Mass., and at Harvard University have studied the mechanical properties of bacterial flagella. Michael W. Berns and his co-workers at the University of California at Irvine have manipulated chromosomes inside a cell nucleus.
Optical tweezers can be used to examine even smaller biological systems. My colleagues Robert Simmons, Jeff Finer, James A. Spudich and I are applying the optical tweezers to study muscle contraction at the molecular level. Related studies are being carried out by Block and also by Michael P. Sheetz of Duke University. One of the goals of this work is to measure the force generated by a single myosin molecule pulling against an actin filament. We are probing this "molecular motor" by attaching a polystyrene sphere to an actin filament and using the optical tweezers to grab onto the bead. When the myosin head strokes against the actin filament, the motion is sensed by a photodiode at the viewing end of the microscope. A feedback circuit then directs the optical tweezers to pull against the myosin in order to counteract any motion. In this way, we have measured the strength of the myosin pull under tension.
On an even smaller scale, Spudich, Steve Kron, Elizabeth Sunderman, Steve Quake and I are manipulating a single DNA molecule by attaching polystyrene spheres to the ends of a strand of DNA and holding the spheres with two optical tweezers. We can observe the molecule as we pull on it by staining the DNA vvith dye molecules, illuminating the dye with green light from an argon laser and detecting the fluorescence with a sensitive video camera. In our first experiments we measured the elastic properties of DNA. The two ends were pulled apart until the molecule was stretched out straight to its full length, and then one of the ends was released. By studying how the molecule springs back, we can test basic theories of polymer physics far from the equilibrium state.
The tweezers can also be used to prepare a single molecule for other ex- periments. By impaling the beads onto the microscope slide and increasing the laser power, we found that the bead can be "spot-welded" to the slide, leaving the DNA in a stretched state. That technique might be useful in preparing long strands of DNA for examination with state-of-the-art microscopes. Ultimately, we hope to use these manipulation abilities to examine the motion of enzymes along the DNA and to address questions related to gene expression and repair.
It has only been six years since workers have stopped atoms, captured them in optical molasses and made the first atom traps. Optical traps, to paraphrase a popular advertising slogan, have enabled us to "reach out and touch" particles in powerful new ways. We have shown that if we can "see" an atom or microscopic particle, we may be able to hold onto it regardless of intervening membranes. It has been a personal joy to see how esoteric conjectures in atomic physics have blossomed: the techniques and applications of laser cooling and trapping have gone well beyond our dreams during those early days. We now have important new tools for physics, chemistry and biology.