Researchers at the University of Central Florida in Orlando have constructed a tunable-focus lens by using an electric field to vary the concentration of a liquid crystal material, which creates a gradient in refractive index.The redistribution is performed via diffusion, which optics professor Shin-Tson Wu explained enables the lens to avoid aberration and light scattering. A problem with the approach is that the response time is proportional to the diffusion distance. Changing the focus would take minutes in a large lens. The researchers estimate that a lens with a 50-μm aperture would have a response time of about 1 s at room temperature.In their adaptive lens, they use a transparent mixture of liquid crystal and the monomer N-vinylpyrrolidone. When subjected to an inhomogeneous electric field, the liquid crystal material moves toward the high electric field region, while the monomer moves away from it. They selected a liquid crystal material that would not undergo molecular reorientation as a consequence of the field.They constructed an ITO electrode on a curved surface and filled the volume with a transparent polymer. A flat electrode defined the other side of the cell, and the liquid crystal and monomer mixture was introduced into the 25-μm gap.Using a HeNe laser and a CCD camera from Santa Barbara Instrument Group Inc. in California, they measured the focal point of the lens and showed that it moved in response to changes in the applied voltage. The response time was about three minutes, which the researchers attributed to the 9-mm aperture of the lens.The device also displayed a relatively high operating voltage and relatively low optical power. The voltage problem may be addressed by using a liquid crystal material with a larger dielectric constant. Using a liquid crystal material with a high refractive index and a monomer with a low refractive index may tackle the optical power issue.Wu said that improvements will render the tunable lens useful in applications such as zoom lenses in cell phones equipped with cameras.Applied Physics Letters, May 8, 2006, 191116.