Liquid Enables Tunable Microlenses

Hank Hogan

Microlenses play an important role in numerous photonic applications, including imaging, optical data storage and telecommunications. A liquid device developed at Lucent Technologies Inc.'s Bell Lab in Murray Hill, N.J., promises the ability to change its optical properties on demand, enabling control of the focal spot position in three dimensions.

Tiny droplets of transparent conductive liquid promise to act as tunable microlenses for a multitude of photonic applications. Courtesy of Lucent Technologies' Bell Labs.

To fabricate the microlenses, the Bell Labs researchers placed 6-µl droplets of a 0.01-mol/l aqueous solution of potassium sulfate into 1-mm-diameter holes on an SiN-coated ITO substrate. A ground electrode contacted a chosen droplet, from which the control electrodes were electrically isolated in the dielectric material.

In theory, altering the control electrode voltage would change the shape and location of the droplet and would thereby modify the optical properties of the microlens. In practice, the interaction between the droplet and the surface of the substrate creates problems, and roughness and inhomogeneity make the droplet's reaction vary among a range of outcomes. The droplet also can fall prey to vibration.

To overcome these issues, the researchers added other liquids. Silicone oil acted as an isolation layer between the substrate and the droplet, eliminating surface effects. A second liquid of the same density as the droplet, in which the assembly was immersed, made the microlens resistant to vibration.

In a series of experiments, the group reliably and predictably changed the focal length of the microlens by approximately 20 percent and moved it by about 35 percent of its diameter. With refinement of the technique and materials, changes in focal length of several hundred percent should be possible. There are no limitations on lateral movement.

Substantial work remains to be done before actual components may be realized, and this research and development is under way. More advanced designs, such as smart liquid lenses that sense the position of light and that automatically align themselves, are in the works, said Tom Krupenkin, a member of the technical staff at Bell Labs and one of the developers of the microlenses.