VCSEL array controls micro-particles

17 Jun 2002

Japanese scientists use an 8x8 VCSEL array to trap and manipulate two microscopic beads simultaneously.

Scientists at the University of Osaka, Japan, have found a way to optically trap and move two particles simultaneously. Instead of using individual beams or complex beam shapes, Jun Tanida and colleagues used 64 vertical cavity surface emitting lasers (VCSELs) arranged in a square (Applied Optics 40 5430).

Conventional trapping techniques need a spatial light modulator to shape the beam intensity, and Tanida claims that using the VCSEL array instead will cut hardware costs. His technique should also manipulate more complex shapes and therefore be especially useful in biological applications.

Previously, the power emitted by VCSELs has been thought to be too low for efficient trapping, Tanida told Optics.Org: "There is a preconception that a VCSEL array is a device for optical communication and that trapping requires high power."

In optical trapping, microscopic particles are confined to areas where the radiation intensity is balanced. When the refractive index of the target particle is higher than its surrounding medium, the particle is pushed towards the brighter-illuminated region.

In Tanida's 8x8 VCSEL array, made by NTT Photonics Laboratory, Japan, each of the 64 lasers emitted around 5 mW at 855 nm. Each laser is addressed individually, so that by switching the intensities of adjacent VCSELs, a number of particles can be moved around independently.

Tanida manipulated two 10 micron polystyrene beads at a speed of 0.48 micron per second, with a positional accuracy of 1 micron. He is now working to improve both these parameters.

"Using a VCSEL array means that we can make much more compact manipulation equipment," he said. "However, we will need higher powers to increase the applications of the technique, and a larger number of pixels to increase the flexibility in manipulation."

He added that the team has recently demonstrated that a polystyrene bead can be manipulated in three dimensions using the array.

Tanida is presenting the recent work at this week's Microoptics Conference (MOC'01) in Osaka.