Developing microlenses with focusing capability is becoming increasingly desirable for use in microimaging systems such as those found in mobile phone cameras. A flexible cavity filled with liquid forms a lens which can be deformed and focused simply by the movement of liquid into or out of the cavity. By using an electroactive polymer actuator to apply pressure to the liquid, a research team at Samsung Advanced Institute of Technology has fabricated tunable microlenses which operate at low voltages and can be microfabricated in large arrays.
Schematic diagram of the varifocal microlens, based on a liquid-filled elastomeric cavity. Reprinted with permission from Opt. Lett. 36 (10) 2011 (DOI: 10.1364/OL.36.001920; p. 1920). © 2011 The Optical Society.
In the May 15 issue of Optics Letters (DOI: 10.1364/OL.36.001920; p. 1920), S.T. Choi and co-workers describe their device based on a silicon frame sandwiched between the elastomer polydimethoxylsilane (PDMS) on one side and glass on the other. The silicon is shaped so as to form a 2.4 mm diameter central circle for the lens, linked to four surrounding reservoirs by microfluidic channels. These cavities are filled with a high refractive index optical liquid and then sealed using an ultraviolet curable adhesive. A film of the electroactive actuator poly(vinylidene fluoride-tri-fluoroethylene-chlorotrifluoroethylene) (1.5 μm thick) is laminated onto the PDMS face of the device, while leaving clear the lens. In order to enhance the response of the polymer at low voltages, further films are alternately layered with thin aluminum electrodes to form a multilayered actuator. A complete stack of 15 electrodes is connected vertically by depositing a post of aluminum into a laser-drilled hole.
Applying a driving voltage to the actuators caused them to depress the elastomer beneath them and force the fluid through the channels and into the lens cavity of the device. As a result, the elastomer over the lens is pushed out and the optical length of the cavity is altered. At 40 V, the combined force of the actuators on each of the four reservoirs induced a 37 μm displacement of the lens. The device was integrated into a mobile phone camera to demonstrate tunable focusing, and clear images were taken with the lens focused to 10 cm and infinity.
The multilayering of the polymer actuator is an essential feature which enables it to operate at voltages compatible with hand-held electronics. Together with the fact that they can be easily microfabricated in wafer-sized arrays, these liquid lenses have considerable potential for use in commercial devices, according to the researchers.
