Laser Trapping of Neutral Particles - Laser Pointer

Meanwhile researchers were making rapid progress in laser pointer 100mw cooling. Phillips and his colleagues discovered that under certain conditions, optical molasses could be used to cool atoms to temperatures far below the lower limit predicted by the existing theory. This discovery prompted Dalibard and Claude Cohen-Tannoudji of the College de France and the Ecole Normale and my group at Stanford to construct a new theory of laser cooling based on a complex but beautiful interplay between the atoms and their interaction with the light fields. Currently atoms can be cooled to a temperature with an average velocity equal to three and a half photon recoils. For cesium atoms, it means a temperature lower than three microkelvins.

Going beyond optical molasses, Cohen- Tannoudji, Alain Aspect, Ennio Arimondo, Robin Kaiser and Nathalie Vansteenkiste, then all at the Ecole Normale, invented an ingenious scheme capable of cooling helium atoms below the recoil velocity of a single scattered photon. Helium atoms have been cooled to two microkelvins along one dimension, and work is under way to extend this technique to two and three dimensions. This cooling method captures an atom in a well-defined velocity state in much the same way atoms were trapped in space in our first optical trap. As the atom scatters photons, its velocity randomly changes. The French experiment establishes conditions that allow an atom to recoil and land in a particular quantum state, which is a combination of two states with two distinct velocities close to zero. Once in this state, the chance of scattering more photons is greatly reduced, meaning that additional photons cannot scatter and increase the velocity. If the atom does not happen to land in this quantum state, it continues to scatter photons and has more opportunities to seek out the desired low-velocity state. Thus, the atoms are cooled by letting them randomly walk into a "velocity trapped" quantum state.

Besides the cooling and trapping of atoms, investigators have demonstrated various atomic lenses, mirrors and diffraction gratings for manipulating atoms. They have also fashioned devices that have no counterpart in light optics. Researchers at Stanford and the University of Bonn have made "atomic funnels" that transform a collection of hot atoms into a well-controlled stream of cold atoms. The Stanford group has also made an "atomic trampoline" in which atoms bounce off a sheet of light extending out from a glass surface. With a curved glass surface, an atom trap based on gravity and light can be made.

Clearly, we have learned to push atoms around with amazing facility, but what do all these tricks enable us to do? With very cold atoms in vapor form, physicists are in a position to study how the atoms interact with one another at extremely low temperatures. According to quantum theory, an atom behaves like a wave whose length is equal to Planck's constant divided by the particle's momentum. As the atom is cooled, its momentum decreases, thereby increasing its wavelength. At sl\fficiently low temperatures, the average wavelength becomes comparable to the average distance between the atoms. At these low temperatures and high densities, quantum theory says that a significant fraction of all the atoms will condense into a single quantum ground state. This unusual form of matter, called a Bose-Einstein condensation, has been predicted but never observed in a vapor of atoms. Thomas]. Greytak and Daniel Kleppner of M.LT. and look T. M. Walraven of the University of Amsterdam are trying to achieve such a condensation with a collection of hydrogen atoms in a magnetic trap. Meanwhile other groups are attempting the same feat in a laser pointer 200mw-cooled sample of alkali atoms such as cesium or lithium.

Atom-manipulation techniques are also offering new opportunities in highresolution spectroscopy. By combining several such techniques, the Stanford group has created a device that will allow the spectral features of atoms to be measured with exquisite accuracy. We have devised an atomic fountain that launches ultra-cold atoms upward gently enough to have gravity turn them around. Atoms for the fountain are collected by a magneto-optic trap for 0. 5 second. After that amount of time, about 10 million atoms are launched upward at a velocity of roughly two meters per second. At the top of the trajectory, an atom is probed with two pulses of microwave radiation separated in time. If the frequency of the radiation is properly tuned, the two pulses cause the atom to change from one quantum state to another. (Norman Ramsey shared the Nobel Prize in Physics in 1989 for inventing and applying this technique.) In our first experiment we measured the energy difference between two states of an atom with a resolution of two parts in 100 billion.

How does the fountain make such precise measurements possible? First, the atoms fall freely and are easy to shield from any perturbation that might alter their energy levels. Second, such measurements are limited in precision by the Heisenberg uncertainty prinCiple. This principle states that the resolution of an energy measurement will be limited to Planck's constant divided by the time of the "measurement." In our case, this time corresponds to the time between the two microwave pulses. With an atomic fountain the measurement time for unperturbed atoms can be as long as one second, a period impossible with atoms at room temperature.

Because the atomic fountain allows extremely precise measurements of the energy levels of atoms, it may be possible to adapt the device to make an improved atomic clock. At present, the world time standard is defined by the energy difference between two particular energy levels in ground states of the cesium atom. Two years after the first atomic fountain, the group at the Ecole Normale used a fountain to measure the "clock transition" in the cesium atom with high precision. These two experiments suggested that a properly engineered instrument might be able to measure the absolute frequency of this transition to one part in 10^16, 1,000 times better than the accuracy of our best clocks. Lured by this potential, more than eight groups around the world are now trying to improve the cesium time standard with an atomic fountain.

Another application being intensively studied is atom interferometry. The first atom interferometers were built in 1991 by investigators at the University of Konstanz, M.LT., the Physikalisch-Technische Bundesanstalt and Stanford.

An atom interferometer splits an atom into two waves separated in space. The two parts of the atom are then recombined and allowed to interfere with each other. The simplest example of such a splitting occurs when the atom is made to go through two separated mechanical slits. If the atom is recombined after passing through the slits, wavelike interference fringes can be observed. The interference effects from atoms dramatically demonstrate the fact that their behavior needs both a wave and a particle description.

More important, atom interferometers offer the possibility of measuring physical phenomena with high sensitivity. In the first demonstration of the potential sensitivity, Mark Kasevich and I have created an interferometer that uses slow atoms. The atoms were split apart and recombined in a fountain. With this instrument we have already shown that the acceleration of gravity can be measured with a resolution of at least three parts in 100 million, and we expect another 100-fold improvement shortly. Previously, the effects of gravity on an atom have been measured at a level of roughly one part in 100.

In recent years the work on atom trapping has stimulated renewed interest in manipulating other neutral particles. The basic principles of atom trapping can be applied to micron-size particles, such as polystyrene spheres. The intense electric field at the center of a focused laser beam polarizes the particle, just as it would polarize an atom. The particle, like an atom, will also absorb light of certain frequencies. Glass, for example, strongly absorbs ultraviolet radiation. But as long as the light is tuned below absorption frequency, the particle will be drawn into the region of highest laser intensity.

In 1986 Ashkin, Bjorkholm, ]. B. Dziedzic and I showed that particles that range in size between 0.02 and 10 microns can be trapped in a single focused laser pointer 300mw beam. In 1970 Ashkin trapped micron-size latex spheres suspended in water in between two fo- cused, counterpropagating beams of light [see "The Pressure of Laser Light," by Arthur Ashkin; SCIENTIFIC AMERICAN, February 1972]. But only much later was it realized that if a single beam is focused tightly enough, the dipole force would suffice to overcome the scattering force that pushes the particle in the direction that the laser beam is traveling.

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 pointer 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.