Scientists developing photonic devices for optical and electronic applications may get a boost from a new process for ‘cutting’ 3-D arrays of holes in a polymer material.
Potentially useful in photonic band gap materials, optical waveguides, lasers and even arrays of tiny chemical beakers, the structures are said to consist of closely packed air bubbles of uniform size.
Mohan Srinivasarao of the Georgia Institute of Technology found a way to create an orderly pattern of air bubbles throughout a polymer film using a simple solvent.
Formation of the structure begins by dissolving a coil-like polymer such as polystyrene in a fast-evaporating solvent. The solution containing the dilute (0.1 to 5 percent by weight) polymer is placed on a glass slide, and moist air is directed across it as the solvent evaporates.
Rapid evaporation of the solvent lowers the temperature of the solution by as much as 25 degrees Celsius. Moisture from the warmer air condenses on the surface of the solution, forming a layer of uniform-size droplets packed tightly together. Because the water is denser than the solvent, the layer of droplets sinks into the sample, allowing another layer to quickly form on top of it.
The process is said to repeat itself for one to two minutes until all of the solvent is evaporated, producing a three-dimensional pattern of closely packed water droplets preserved in the polymer film.
The water then evaporates layer by layer, leaving an interconnected network of air bubbles said Srinivasarao. The bubbles can be created in uniform sizes from 0.2 microns up to 20 microns in diameter, allowing a 30-40 micron thick polymer film to contain as many as 15 layers of bubbles.
‘The beauty of this process lies in its simplicity,’ said Andrew Lovinger, director of the polymers program at the National Science Foundation (NSF). ‘You just let the solvent evaporate at room temperature and in a few seconds you get these beautiful honeycombed polymer films.’
‘This represents an easy way of making materials with the regular structure needed for optical and photonic applications,’ said Srinivasarao. ‘This is completely a self-assembly process.’
If the process is shown to be usable in developing photonic bandgaps or photonic crystals, it could contribute to the development of optical switches and the ability to direct or ‘steer’ light beams, just as electrical switches and conducting materials control and direct electrical current.