Forty Years of Optical Manipulation

We have, then, a relatively simple optical system with no strong restrictions on laser source or power levels. But how does it work? The high-school explanation is that light carries momentum that can be transferred to particles as the light passes through them; this transfer results in a force that acts along the intensity gradient in the trapping beam toward the point of highest intensity. A typical laser beam has a Gaussian profile, and so the particle will be drawn toward the beam center. This argument fails for smaller particles, where it is more useful to consider them as dipoles that experience a force within an intensity gradient that results in them being drawn toward the beam center.

This “gradient” force is proportional to the intensity gradient of the beam and also to the polarizability of the particle under consideration. An optical trap to measure forces is a beautiful analog to a simple harmonic oscillator. It has a force relationship in accordance with Hooke’s Law. So, if one can measure displacements of a trapped bead with great accuracy, then forces can be measured with high precision. The trapping forces that can be detected range from a few tens of femtonewtons to the more typical tens of piconewtons to in excess of 100 pN. This range of forces makes optical tweezers an ideal candidate for probing many sensitive biological functions, particularly at the molecular level.

Since the initial single-beam trap was introduced, myriad trapping arrangements have been demonstrated for ever-increasingly complex and sophisticated applications. It quickly became very clear that a single-beam trap was a limiting factor in many cases, and it is very useful in many experiments to have two independent beams. This type of dual-beam trap is simple to set up using beamsplitter arrangements. However, for more complicated trap structures with multiple beams, new techniques had to be introduced.

Multibeam techniques

Currently, there are two multibeam techniques that are widely used—scanning and holographic. In scanning techniques, a beam is scanned very rapidly across the particles of interest. As long as the beam returns to a particle before it diffuses from the trapping site, the particle remains trapped, even if the incident light only illuminates it for a fraction of a second. Perhaps the most famous example of this (if not the most practical) is the microTetris demonstration carried out at the Vrije University.

In holographic techniques, an input Gaussian beam has its phase modulated into that of a target intensity at the focal plane of the microscope objective. In this way, complex beam patterns can be created that are static but dynamically altered. Holographic beam shaping has the advantage that particles can be easily manipulated in three dimensions and that patterns of light that are not simple arrays of spots are relatively easy to produce. As a counterpoint to microTetris, Miles Padgett’s group in Glasgow used holographic optical tweezers to demonstrate particles performing the smallest “Strip-the-Willow” dance in the world.

Scanning and holographic techniques are simply manifestations of scientists’ ability to morph light in complicated ways—in what can be thought of as optical sculpting. They are also, of course, a means to an end. The creation of these tailored landscapes depends primarily on the application in which one is interested. They can be as simple or as complicated as one desires (provided that Maxwell’s equations are not violated!). One of the most interesting applications of shaped light fields in recent years has been that of optical sorting, in which a simple interference pattern is used to separate particles based on some physical property, such as shape, refractive index or size.

The technique, which can sort in the absence of fluorescent markers, works through the simple idea that the optical force imparted to the particle as it flows through the landscape is a function of how the particle interacts with the landscape. As the force is related to physical properties, different particles feel different forces and thus follow varying paths through the light field.

For a macroscopic analog, think of a slanted corrugated roof. If you roll a tennis ball down the roof, it will follow one of the channels formed by the corrugations and move off at the slant angle. A soccer ball, on the other hand, doesn’t “see” the corrugations and thus rolls right over them, falling straight down the roof.

Application-based research

Increasingly, applications are driving optical tweezers research. While there have been a growing number of experiments in areas such as colloidal dynamics, statistical mechanics, hydrodynamics, Brownian motion and nanomanipulation, the biggest market for optical tweezers is in biology. Shortly after Ashkin demonstrated the first optical tweezers, he made use of his new tool to make measurements on cells. This has inspired a huge subfield of work that looks at both cellular and molecular properties.

In some of the most remarkable optical tweezers experiments, researchers have studied the function of single molecules such as molecular motors. The Block group in Stanford has achieved measurements of base pair stepping by RNA polymerase (RNAP), in which an RNAP molecule moves along a DNA molecule; the “steps” taken by the RNAP molecules are approximately 3 angstroms in length. This measurement is phenomenally precise: Each displacement is on the order of the size of a hydrogen atom. The researchers accomplished this task by using a 600-nm polystyrene bead. Their work provides the ultimate example of the application of optical tweezers and shows that real insight can be made at the molecular level.