13 May 2009

Laser on a chip from liquid lens

Tunable fluidic micro lenses can focus and direct light at will to count cells, evaluate molecules, or create on-chip optical tweezers. Just think Jedi light sabers.

Conventional, fixed focal length lenses can focus light at only one distance, according to a team of Penn State engineers. The entire lens must move to focus on an object or to change the direction of the light. Attempts at conventional tunable lenses have not been successful for lenses on the chip. Fluidic lenses, however, can change their focal length or direction in less than a second while remaining in the same place and can go on the chip during manufacture, the researchers said.

“We use water and a calcium chloride solution because they are readily available and safe and their optical properties have been well characterized,” said Tony Jun Huang, Penn State’s James Henderson assistant professor of engineering science and mechanics. “There are lots of possibilities about what fluids we can use. Most solutions change their refractive indices if the concentration changes.”

He said they could use a variety of solutions with water. There are also a number of commercially available “refractive index fluids,” which could potentially provide better optical properties and make these Liquid-Gradient Refractive Index (L-GRIN) lenses work even better.

To create their lens, the researchers have constant, tiny streams of calcium chloride surrounded by two adjustable streams of water. By increasing or decreasing the flow rate of the water, they can shorten or lengthen the focusing distance of the lens. The focal length changes because the amount of diffusion of calcium chloride into the water changes and alters the refractivity of the fluid. The researchers can swing the focal point side to side by changing the flow rate of the water on only one side, skewing the point of focus left or right.

“With these two combined, we will have the capability of directing the light to any given point within the device,” Huang said.

One application would be as optical tweezers positioned directly on a chip the size of a quarter, Huang said. This would eliminate the complex systems now necessary for optical tweezers. Optical tweezers made up of focused laser beams can capture tiny particles like cells, stabilize them, move them, and even rotate them at will.

“Our L-GRIN lens is probably the only microlens that can focus and steer the light simultaneously and yet is still small enough to fit on such a biochip,” Huang said.

For related information, go to www.isa.org/manufacturing_automation.