Forty Years of Optical Manipulation

It is strange to think that light—that most ephemeral of things—can have any mechanical effect. But it has long been known that the universal palette can, in fact, push and pull on physical objects. The idea was put on sound mathematical footing with the development of the theory of electromagnetism by James Clerk Maxwell, who described what we now call radiation pressure.

Radiation pressure is the most intuitive form of optical force: Light incident on a surface produces a force on that surface. As P.N. Lebedev notes in his experimental verification of this hypothesis, “The value of this beam pressure is rather small.” This is something of an understatement. The maximum pressure exerted by the sun on a reflecting object is on the order of 1 µN/m². Measuring such a tiny force at the turn of the century, as Lebedev somehow managed to do, was an impressive feat.

The lack of attention that this subject received over the subsequent few decades can perhaps be explained by the fact that it is difficult to disentangle the forces due to the light from thermal forces induced by the light beam. The thermal forces often swamp any effect that one might seek to measure. In addition, the tiny forces that available light sources could generate would also make experiments challenging.

Ashkin lays the groundwork

Forty years ago, Arthur Ashkin addressed the thermal problem using a relatively new light source called the laser. With it, Ashkin realized that he could use particles that were transparent and therefore non-absorbing at the wavelength of the light source. This de-coupling of optical and thermal forces paved the way for a number of significant breakthroughs. Ashkin was especially interested in developing techniques to manipulate atoms. His work on optical forces culminated in his first observation of the laser cooling of atoms. This application of the technique led to two Nobel Prizes—one given to Steven Chu, Bill Phillips and Claude Cohen-Tannoudji in 1997 for the development of an atomic trap based on laser cooling, and another given to Eric Cornell, Wolfgang Ketterle and Carl Wieman in 2001 for using advanced techniques based on the same principles to create a Bose-Einstein condensate. Laser cooling research is ongoing, with recent breakthroughs in the development of Fermi gases.

Although Ashkin’s research played an important role in the development of these techniques, his best known work is in optical forces on microscopic particles. In this area, he should be rightly seen as having founded a new field. For the bulk of the 1970s, Ashkin and his collaborators embarked on a series of pioneering experiments that showed the capabilities of these optical forces. Of particular importance were demonstrations of trapping using two counter-propagating beams and using a single beam that was obtained by propagating a beam vertically and using gravity to balance the radiation pressure force. Much of Ashkin’s early work was devoted to understanding basic forces and ways to improve stability. However, finding lasting applications or a committed research community proved difficult.

It wasn’t until 1986, when Ashkin demonstrated a single-beam gradient trap, that a more powerful technique was born. The trap differed from the single-beam radiation trap in that it was able to act in the same direction as gravity, forming a true three-dimensional device. In all previous optical trapping work, the gradient force, in which a particle moves along an intensity gradient toward the point of highest intensity, acted only in two dimensions, thereby confining the beam on the beam axis. Trapping in the third dimension—in the direction of the beam propagation—was due to either gravity or the presence of a second counter-propagating beam.

The trick was to focus the beam very tightly to produce a gradient force in the propagation direction that was able to counteract any scattering force. This was achieved by using a high-numerical-aperture microscope objective. Thus, optical tweezers were born. The use of the microscope objective would immediately signal the power of the technique, as it could be coupled to any conventional microscope. In addition to demonstrating simple trapping, the original work also illustrated the general frame of reference for the technique, in which particles within the size regime from 25 nm to 10 µm could be confined.

Optical tweezers

One of the great powers of optical tweezers is the simplicity with which a system can be set up. In fact, basic systems can be developed by motivated high school pupils. This simplicity has aided the take-up of the technique by scientists outside of the optics community. Much more sophisticated systems, including commercial products, are widely used for advanced studies.

Typically an optical tweezers system uses a laser source passed through two telescope systems—the first to expand the beam to slightly overfill the back aperture of the microscope objective and the second to make the beam at the microscope conjugate with the beam on a steering mirror. This helps to aid the optical alignment and allows simple beam steering in the image plane of the microscope objective. A CCD camera can then be used to image the sample plane, and, typically, a sample is loaded on a microscope slide. The choice of laser source is often critical for experiments. For those wishing to look at biological samples, infrared lasers are important due to the reduced photodamage they cause compared to visible light sources. Typical power levels for trapping are not terribly high, with only a few milliwatts of power needed to trap, although most experiments will use tens of milliwatts or more.