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Lasers & Sources

Laser tweezer benefits from split personality

Eye on Technology - biomedical photonics

From oemagazine June 2002
31 June 2002, SPIE Newsroom. DOI: 10.1117/2.5200206.0001

Laser tweezers offer one of the few ways to non-invasively manipulate mesoscopic objects larger than a few atoms, allowing biologists to study the effects of intracellular processes, drugs, protein development, and DNA, among other applications. Now researchers have gone a step beyond tweezers, developing methods for creating hundreds of traps to manipulate large numbers of particles simultaneously.

In a basic laser tweezer, dielectric particles in a fluid or gas medium polarized by the light's electric field are drawn to the focal point of the beam. The focal point typically is created by tightly focusing the beam of a few milliwatt laser through a microscope objective and onto a sample. Reflecting, absorbing, and low-dielectric particles are driven away from the focal point.

While creating a single laser trap is a relatively simple task, it has proven challenging to create the type of multiple, independent traps that would be useful for larger studies or for independently directing many particles at once for efficient drug assays and other experiments. Groups have made progress, however. Researchers at Harvard University (Cambridge, MA) have used acousto-optic modulators to split a single beam into several steerable beams. Combining two such devices allows the user to steer the beams along the x and y axes. Moving to a tiltable diffractive optical element holographic array on silica substrates, researchers at the University of Chicago (U of C; Chicago, IL) were able to create arrays of traps with limited movement in three dimensions, but not independent of each other.

Now, the U of C group, in collaboration with Arryx Inc. (Chicago, IL), has built on that work to use a computer-controlled 512 x 512 pixel spatial light modulator (SLM) as a reconfigurable phase mask capable of creating hundreds of dynamic, independent laser traps (see figure 1).

Figure 1. The laser-tweezer system uses a laser light source, an SLM, and an inverted light microscope to produce hundreds of independent optical traps in a combination of beads (Gaussian mode), rings (LaGuerre-Gaussian), or a combination of the two.

The BioRyx 200 is based on a CW, 2-W frequency-doubled neodymium-doped yttrium aluminum garnet (Nd:YAG) laser operating at 532 nm. The system incorporates the laser along with directing optics and a Nikon TE200 inverted light microscope to trap particles as small as 150 nm and move them independently in three dimensions. The motion is in steps as small as 5 nm—smaller than what's detectable by the microscope. In operation, the laser light reflects off the SLM, which creates a pattern of bright spots, or traps, in three dimensions. The pattern passes through the microscope objective onto the sample. By controlling the SLM, it is possible to manipulate the particles in the sample (see figure 2).

Figure 2. A computer-controlled SLM dynamically changes the phase of the Nd:YAG wavefront to create individual optical traps based on an object-based, (Windows) graphic user interface. The traps (highlighted with color) can be moved with the click of a mouse to shift mesoscopic particles under study.

"You move the trap on the screen and it moves on the sample," says John Turner, assistant professor of chemistry at Cleveland State University (Cleveland, OH). "The fact that you can move 200 particles at once with x, y, and z positioning using an interface that's like animation software is very exciting."

That software is actually the heart of the system. The group started with adaptive-additive (A-A) algorithms but quickly moved on to other approaches to create the holographic constructs on the SLM when the A-A approach proved too slow. Although the group will not identify which approaches they use, they have demonstrated both bead traps using focused Gaussian-mode beamlets and ring traps using Laguerre-Gaussian-mode beamlets, in which the ring of light can transfer angular momentum (orbital) to physical objects. Single-atom traps using bottleneck modes are also possible but are not a priority for Arryx, according to company co-founder and lead researcher David Grier.

Turner, who investigates Raman imaging, says he could envision many applications for the BioRyx in his work, such as studying Raman signatures during manipulated protein interactions and other biologic processes. According to Kenneth Bradley, vice president of operations at Arryx, biologic assays are the first market for the system, but the technology could aid self-organizing structures in material science, propel MEMs devices used in optical switches or micro-turbines using ring traps, or even be used in vivo as a drug delivery system if the basic technology were miniaturized.