Assembly Technique Leads to Thermally
Responsive Microlens Arrays

Michael A. Greenwood

A team of researchers from China and Japan has developed a technique to assemble thermally responsive microlens arrays that is suitable for mass production.

The one-step technique encapsulates poly-N-isopropylacrylamide (PNIPAAm) for fabrication of monodisperse microcapsules, whose size, embedding efficiency and wall thickness can be controlled. The microcapsules were hexagonally packed to form microlens arrays via a self-assembly process. Because of the thermal responsiveness of PNIPAAm, the imaging capability and light transportation of the arrays could be controlled by temperature.

Image of the letter P from the microlens array was captured below the critical temperature (top picture) and above the temperature (below). The size of the microcapsules is about 800 μm.

Compared with other methods of microlens production, lead investigator Zhong-Ze Gu of Southeast University in Nanjing, China, said that this method has considerable advantages: Core and shell materials can be designed individually and can be assembled together. Also, the method is simple enough to lend itself to mass production.

The apparatus developed by the researchers comprises three components: a flux controller, a droplet generator and a collector.

When tested, the microcapsules created by this method had good monodispersity, uniform thickness and high transparency. The researchers conducted imaging tests at various temperatures and found that, as the heat level increased, all the microlenses became opaque and lost their imaging capability. At temperatures below 34 °C, one microlens had a transmittance rate of about 80 percent and good imaging capability. As the temperature edged over 34 °C, its transmittance dropped sharply, to about 30 percent or lower, and the microlens became cloudy. All the microlenses tested had a uniform diameter of 800 μm and a uniform shell thickness of 20 μm.

The capability of switching the propagation of light on and off during temperature variation holds promise for future applications in optical microshutter arrays as well as others, the researchers said. Liquid crystal and colloidal photonic crystals also can be encapsulated with the same method.

Applied Physics Letters, Sept. 11, 2006, 111121.

Assembly Technique Leads to Thermally Responsive Microlens Arrays

Assembly Technique Leads to Thermally
Responsive Microlens Arrays