PORTLAND, Ore.—Liquid lenses are already used in immersion lithography to boost resolution, but the free-air droplets must be manually adjusted for proper focus. Now Rensselaer Polytechnic Institute (RPI) has shown a liquid lens with a focal length that can be electrically adjusted with tiny microfluidic pistons with no moving parts. RPI claims its electromagnetic liquid lens could boost all types of precision imaging applications, from immersion lithography to implantable retinas to the ultra-low power cameras on cell phones.
RPI's technique, developed by professor Amir Hirsa, uses fluids saturated with iron nanoparticles. By carefully oscillating a ferro-fluidic droplet with a magnetic field, the fluids inside can be pumped up-and-down like a piston, allowing the focal length of a lens to be adjusted using even less energy than a micro-electro-mechanical system (MEMS). Because the fields are generated use alternating currents, the lens is constantly cycling between its closest and furthest focal lengths, but the researchers say that software algorithms can easily eliminate any out-of-focus frames. In its demonstrations, the researchers were able to capture in-focus 30 frame-per-second videos as a proof of concept.
The demonstration set-up housed an opaque ferro-fluidic droplet—the piston—in one hole drilled next to another holding the transparent lens droplet in a solid substrate separating two sealed chambers filled with water (see figure). Electromagnetic pulses then forced the piston droplet to vibrate up and and down in its hole. Magnetic, capillary and inertial forces combined to cause the second droplet—the lens—to follow the movement of the piston droplet, setting it to oscillating between its maximum and minimum focal lengths.
RPI's liquid pistons precisely pump droplets of nanoparticle-infused ferrofluids, which move in and out to focus a liquid lens.
Hirsa's collaborators on the project included MIT Lincolm Lab researcher Bernard Malouin, Michael Vogel, a private research consultant, RPI doctoral candidate Joseph Olles, and former postdoctoral researcher Lili Cheng, now at General Electric Global Research. Funding was provided by the Defense Advanced Research Projects Agency (DARPA).