New route to photonic structures

17 Jun 2002

A simple, new evaporation method has produced air-bubble array structures that confer band-gap properties to polymer films.

US scientists have developed a simple technique to produce micron-sized, three-dimensional, polymer structures for use in photonic bandgap materials, optical waveguides and lasers, (Science (292) 79).

Until now, component manufacturers have typically built a honeycomb template by forcing fluid into the gaps between an array of silica spheres. Solidifying the fluid and removing the spheres leaves a skeleton of pores. However, Mohan Srinivasarao from Atlanta's Georgia Institute of Technology and colleagues from the North Carolina State University, Bell Laboratories and Lucent Technologies, claim that their new technique saves time and produces more precise structures.

By evaporating a dilute solution of polystyrene in a highly volatile solvent, while blasting moist air across the polymer's surface, the scientists fabricated a 40-micron thick film that contained up to 15 layers of hexagonally-packed air holes. The scientists were able to alter the structure's dimensions from 0.20 to 20 microns by changing the velocity of airflow across the polymer's surface.

Srinivasarao and colleagues believe that the formation of these polymer structures stems from a phenomenon called "breath figures". Here, patterns develop on a cold surface when it is brought into contact with moist air. In their work, the solvent cools the surface of the polystyrene as it evaporates, which encourages water droplets from the moist air to nucleate and grow. The airflow across the surface helps to arrange the water droplets into hexagonal arrays on the surface. Within seconds, the dense water droplets sink into the polystyrene solution and the process starts all over again. After evaporating the water, the scientists are left with an interconnecting network of air bubbles.

Srinivasarao anticipates that the new structures will be used in optics, where pore dimensions can be tailored to the wavelength of visible light. "We have focused on how to modify the refractive index so we can use these structures as a photonic bandgap material," he said. "What we will be able to do is limited only by our imaginations."