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Liquid flow controls tunable microlens

26 May 2009

Tiny liquid lens offers flexible focusing on a single biochip.

Scientists at Penn State University in the US are seeking licensing partners for a tunable liquid lens that fits onto a single chip. The Penn State device focuses light using a flat lens body rather than a curved surface, offering unique advantages over conventional liquid lenses (Lab on a Chip DOI: 10.1039/b822982a).

"We have created a tunable liquid microlens that can focus and direct light at will within a biochip," Tony Jun Huang, a Penn State professor, told optics.org. "Our tunable microlens allows flexible on-chip manipulation of light for applications such as optical trapping, flow cytometry and single-molecule detection."

Until now, the majority of liquid lenses have relied on refraction via a curved lens surface to focus light. However, deploying such lenses on a chip has proved difficult due to space constraints.

Huang and colleagues have instead turned to gradient refractive index (GRIN) lenses, which focus light via a flat lens body that has a variable refractive index. "GRIN lenses are particularly suitable for on-chip microlenses because their flat structure is much easier to fabricate," explained Huang. "The key innovation is that for the first time, we have made a GRIN lens of liquid and shown precise control over the refractive index distribution within the fluid."

The lens comprises a constant stream of calcium chloride surrounded by two adjustable streams of water. By increasing or decreasing the flow rate of the water, it's possible to vary the amount of diffusion of calcium chloride varies. This alters the refractivity of the fluid and thus the focal length.

"Making a GRIN lens with liquid gives us unprecedented flexibility to control the focusing of the light," explained Huang. "Changing the focal length is as simple as increasing or reducing the flow rates of the two water steams. If we want to direct an output beam in a certain direction, we simply make the flow rate of one water stream higher than the other to create an asymmetrical refractive index profile, which steers the light."

The lens is still in the early stages of development, but Huang and his team believe that its potential is clear. "We are currently optimizing the performance of this lens and will continue exploring its applications," concluded Huang. "Right now, we are looking at possibilities to use this lens for on-chip manipulation of cells and microparticles."

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