Pass me that DNA snippet, please

These days, you can read about nanotechnology everywhere—from scientific journals to mainstream newspapers and magazines. Materials built from molecular-sized building blocks, manipulation of DNA strands, MEMS (microelectromechanical systems), microfluidic systems, and related technologies will change the way you fabricate products. This statement is not just speculation: Nanomaterial-based devices already exist. Take, for example, mass-produced MEMS accelerometers and microchannel bioanalysis ICs.

As you know, however, you need tools with which to build and test your projects, and these tools exist in the form of "optical tweezers" for manipulating nanosized objects (references 1 and 2). What are optical tweezers? Remember your basic physics: Photons have momentum, and momentum can impart force and thus move things. All that remains, then, is to be able to concentrate, control, and direct this photon-based momentum, so that you can capture and then move nanosized object. These tasks are difficult to perform, but lasers, their associated optics, and electronic-control technologies let you employ this radiation pressure from the platform.

You may have observed the optical-tweezers effect on a much larger scale. The so-called tail of a comet is actually loose debris that the intense stream of photons from the sun pushes away from the comet. This tail can be either ahead of or behind the comet, depending on where the comet is and which way it is moving. Some science-fiction writers and space-propulsion engineers have suggested that long-range space travel should use this "solar wind," acting as a huge sail, to propel a space vehicle attached to it. Although acceleration would be low, the "fuel" for this engine would never run out, and accelerating it even a little for a long enough time would cause it to move at high speed. This idea is not new: Mathematician and astronomer Johannes Kepler noticed this comet effect and proposed sailing from the Earth to the moon on light in 1609, the same year he published his laws of planetary orbit and motion.

When you work with photons at the quantum-physics level, how things work is never as simple as just "photon push comes to molecule shove." In addition to momentum transfer due radiation pressure, a microparticle also experiences a significant force through the reflection or refraction that occurs when it changes the direction of a beam of light that impinges on it. The resulting gradient forces let the tweezers optically move matter. In one lab application, tweezers have even pushed microparticles against cogwheel teeth to drive a micromachined gear train.

Optical tweezers are the latest in a series of systems that use light to control matter, beginning with optical traps that designers developed in the 1970s. These basic levitation traps send a stream of photons upward to counter the downward pull of gravity on microparticles; subsequent designs used two beams of light with counter-rotating circular polarization to trap particles, independently of gravity's pull. But this setup doesn't allow for 3-D trapping and particle motion, and the two beams must be aligned with each other and become stabilized. Despite its technical challenges, some applications still use this technique.

In 1986, the same team that worked on the levitation and two-beam traps devised a better approach. To trap a particle with a single beam, you have to confine it at right angles to the beam with a transverse force and along the direction of the beam using a longitudinal force. To create the transverse force, use a laser beam with its maximum intensity at the center of the beam, as is usually the case (Figure 1a). When the particle moves to the left of the beam center, it refracts more light from right to left and transfers momentum in that direction. By Newton's third law of motion, the particle then experiences an equal and opposite force, pushing it back to the center of the beam. Meanwhile, if the beam is tightly focused, the particle experiences forces that drive it toward the focus of the light source (Figure 1b). The result is a tiny, trapped object that you can also move by adjusting the light path within limited boundaries and conditions. In theory, beam power need be only about 1 mW, although, in practice, a power of about 10 mW compensates for path losses, distortion, and beam imperfections.

Nanomaterials research also brings challenges in basic materials measurements. You can't use a miniature version of a common load cell to measure piconewton forces. But researchers have combined their analysis of the trap roll-off shape and resulting trap force, or "stiffness," versus distance from the trap center with image analysis to measure these forces without physical contact. For example, one team projected an image of a micron-sized polystyrene bead through a microscope-based imaging system and thus measured with an accuracy of 10 nm, the position of a sphere. This setup had a trap stiffness of 50 pN/micron and had an overall resolution of 0.5 pN. Other designs have used laser interferometers to more accurately measure the bead position but at a considerable increase in system complexity and control requirements.

You may wonder whether optical tweezers are like most physics projects, requiring millions of dollars' worth of extraordinarily complex, large, and power-consuming equipment. The answer may surprise you. You can build optical tweezers from commercially available, standard optical parts, including a 50-mW laser, lenses, and mirrors mounted on an optical bench, for about $2500 (references 1 and 2). To demonstrate to yourself and others what you've built, you need to tweeze the object as well as see what you are doing, so the do-it-yourself design includes a dielectric mirror assembly and CCD camera (Figure 2).

The design matches basic component cost against practical implementation issues. Although the project could use an inexpensive diode laser that costs only a few dollars, the output beam of such a laser usually has an elliptical cross-section and would thus need complex lenses to shape it into the desired circle. Instead, the bill of materials lists the Cirulase, a $200, 50-mW, 780-nm laser diode that has a microlens bonded directly to the diode-output facet and provides a more symmetrical output beam that is easier to shape to a near-perfect circular beam with a low-cost aspheric lens.

Author Information

You can reach Executive Editor Bill Schweber at 1-617-558-4484, fax 1-617-558-4470, e-mail bschweber@edn.com.