The ins and outs of adjustable microlenses

Hank Hogan, hank.hogan@photonics.com

Adjustable microlenses now have a new knob that can be turned, courtesy of a research team led by Yanchun Han of Changchun Institute of Applied Chemistry. The group demonstrated two new types of variable-focus liquid microlenses, one constructed with sidewalls that curve in and the other with sidewalls that curve out.

The slope of the sidewalls is adjustable during fabrication, while the curve of the lens can be changed during operation. Thus, any application requiring adjustable optics could have new tools to bring things into focus.

Researchers in China have developed two new types of variable-focus liquid microlenses. The first (a-c) has sidewalls that curve in, while the second (d-f) has sidewalls that curve out. Changes in pressure move the meniscus from concave (a, f) to convex (c, d), changing the focal length. Reprinted with permission from Langmuir.

A variable-focus liquid microlens exploits the flexible nature of a liquid-air interface. As pressure changes, this boundary moves, bending from concave to flat to convex. Because of refractive index differences between the liquid and air, the interface acts like an optical surface. The result is an adjustable lens with a variable focus, with its optical performance determined by the characteristics of the liquid and the lens housing.

The researchers’ innovation involves that housing. They fabricated microlenses with curved sidewalls, using a housing made of polydimethylsiloxane (PDMS), a transparent rubber.

In one case, they molded the PDMS around a water droplet sandwiched between two plates with identical wettability. Bridging the gap between the plates, the droplet narrowed toward the center and flared at either end. The curvature of the resulting PDMS sidewall depended upon the surface wettability of the plates.

In the second case, they molded the PDMS around a solid microsphere. This housing had sidewalls that curved out, with the curvature determined by the radius of the microsphere.

When they put a liquid, such as water, in the microlenses, they could flex the interface, or meniscus, between it and the air from concave to flat to convex by changing the pressure. The slope of the sidewall, along with the pressure, determined the curvature of the meniscus and the focal length of the lens.

The two kinds of microlenses, the group reported in a Dec. 9, 2009, Langmuir online paper, have opposite tuning tendencies. The focal length of the first type – the one with sidewalls that curve in – goes more negative as pressure is increased. In contrast, the focal length of the second type, with sidewalls that curve out, goes more positive as pressure is increased.

The researchers showed good agreement between simulations and demonstrations for both types. They noted that the lenses can be adjusted over a wide dynamic range, with focal lengths spanning from ±2 mm to ±∞ for each type in their demonstration microlenses.

They also noted that the lenses’ sensitivity to pressure changes depends on the curvature of the sidewalls, which can be set as needed, within limits, during fabrication. Thus, a lens can be made so that it covers the required focal range for an application.