Laser Array Traps and Manipulates Particles

Richard Gaughan

A technique that employs an array of vertical-cavity surface-emitting lasers (VCSELs) has demonstrated the ability to trap and manipulate microscopic particles using no moving parts. Developed by researchers at Osaka University in Suita, Japan, the method cooperatively uses multiple beams from the array.

An 8 x 8 array of vertical-cavity surface-emitting lasers traps and manipulates 10-µm-diameter polystyrene beads. Here, the beams cooperate to trap and levitate a bead (1 and 2) and then to translate it to the right (3 and 4). The bead is highlighted with a circle, and a dot marks its initial position. Courtesy of Yusuke Ogura.

Optical traps, created by rapidly focusing laser beams, hold microscopic objects in the region of the focal spot. Creating an optical trap is straightforward, but manipulating trapped objects typically requires the addition of stages, deflection mirrors or other moving parts.

Numerical simulations of a laser array predicted that the axial force on a trapped particle depends more on the number and arrangement of beams incident on the particle than it does on the total beam power. For example, a 4 x 4 array is 16 times more powerful than a single beam, but when the beam spacing is adjusted to slightly more than half the radius of the trapped particle, the axial force is 25 times greater than that of a single beam. To verify the predictions of the model, the researchers constructed an experimental system based on an 8 x 8 array of 854-nm VCSELs from NTT Photonics Laboratory.

The VCSELs feature a 15-µm aperture per pixel and 250-µm pixel spacing. A microlens array couples the beams to a 200-mm-focal-length collimating lens. They are then focused through an Olympus 60X water-immersion, long-working-length microscope objective. The test system holds 10-µm-diameter polystyrene beads immersed in water and sandwiched between a glass slide and a cover glass.

With the VCSEL beams propagating upward through the slide, the beams levitate the beads in the water. At the focal plane, the individual 1.1-mW laser spots are 1.3 µm in radius and spaced 3.75 µm apart. By illuminating sets of the VCSELs in sequence, the experimenters can levitate a single bead and translate it 26 µm before returning it to the surface of the glass slide.

Although the key phenomenon of beam cooperation has been verified, the unknown adhesion force and incomplete purity of the beam mode profile made the measured levitation force in the experiments greater than predicted. For example, a 2 x 2 array at 3.75-µm spacing could levitate a 10-µm bead, but it could not at 6.75- or 7.5-µm spacing. When the focal plane was shifted so that the wider beams could work cooperatively at the surface of the slide, the beads levitated.

Researcher Yusuke Ogura said that advances in array technology are important for further development and that a variety of applications could be enabled by this basic research. Most interesting is the ability to construct a compact and nonmechanical optical trapping system. An added bonus: "We've shown developers of the VCSEL a new application field," Ogura said.