03 Oct 2006
Microencapsulation is a simple and effective way of mass producing arrays of tiny spherical lenses say experts in Asia.
Researchers from Southeast University, China, and Osaka University, Japan, have found that a single-step process typically used to make foods and medicine is perfect for fabricating microlenses. The lenses, which could be used to couple light into a fiber or as part of a spatial light modulator, have the added benefit of being switchable, thanks to their thermally responsive core. (Appl. Phys. Lett. 89 111121)
"For most optical elements, the switching of light is usually realized by a mechanically or electrically controlled shutter," Zhong-Ze Gu of Southeast University's bioelectronics lab told optics.org. "However, in our experiment we found that such a capability can be fulfilled by the microlens itself by simply changing the temperature."
The lenses are made by a process known as microencapsulation. Here, core (poly-N-isopropylacrylamide), shell (polystyrene) and flow (polyvinyl alcohol) liquids are fed into a droplet generator and rotary evaporated to give a self-assembling array of spherical lenses. Matching the density of the three liquids helps to ensure uniform shape and shell thickness. The size of the lenses is controlled by adjusting the flow rate.
The team discovered that it could reduce the diameter of its lenses from 1680 to 470 microns by increasing the flow rate of the polyvinyl alcohol solution from 16 to 1000 microlitres per second. The flowrate of the core and shell liquids was fixed at 4 and 6 ml/h respectively.
Lens "switchablity" is initiated by a change in temperature, which alters the structure of polymer chains within the core. "Once the lens temperature has reached 34 degC, it takes just 30 seconds for the transmission to fall from 80% to below 30%," explained Gu. "The next step is to encapsulate various types of liquid crystal in the microlens and use an electrical field or ultraviolet light as the switching mechanism."