Optofluidic microlenses of tunable focal length would aid applications in photolithography, optical imaging and on-chip cell sorting and detection. Current techniques for tuning microlenses, through the use of an electric field or electrowetting, for example, often cause optical aberrations, liquid evaporation, slow response times and narrow tuning ranges.

Now, by exploiting multilayer soft lithography — a technique widely used to fabricate highly integrated large-scale microfluidic chips — Yanyi Huang and co-workers from Peking University in China have developed a compound microlens that is flexible, robust and tunable (Lab Chip 11, 2835–2841; 2011).

Credit: © 2011 RSC

The researchers formed a chip containing many microchannels from three horizontal layers of the organic polymer polydimethylsiloxane. Each layer contained a round chamber connected to one of the channels, through which liquid could be injected. In the middle layer, several rectangular chambers were connected to the 700-μm-diameter round chamber to function as integrated pneumatic valves.

Actuation of the valves was used to determine the volume of liquid present in the chambers and thus to tune the focal length of the microlens. The researchers say that the refractive index of the microlens can be adjusted by filling the chambers with different liquids.

The compound microlens has several attractive properties, including self-alignment, large zoom ratio (∼7×), a widely tunable angle of view (15–80°), large focal range (from submillimetres to centimetres), large numerical aperture (up to 0.44), small lens size (a few hundred micrometres in diameter) and a zooming actuation response time of around 100 ms.

According to Huang, the fabrication method not only meets many of the challenges faced by optofluidic microlens builders for over a decade, but also opens many new possibilities for constructing other types of tunable micro-optical components.