Microstructured polymer components are often used to direct light in optical devices. As an alternative to mechanical processes, laser light can also be used to produce the microscopic relief structures, and even to remove them without contact.
With a flash of blue-green light, the show begins. Like a line of silent ballet dancers, long-chain macromolecules arrange themselves into a complicated pattern. The molecules rise up to form micrometer-high mountains and open up similarly deep ravines between the peaks. When the laser is extinguished, the chains solidify. An orderly, stable relief has been formed over an area of a few micrometers. The miniature grid, depending on its structure, can be used as an optical component to scatter, split or focus laser beams. The real novelty is that the structure can be shaped to order using nothing but laser light.
“We employ holographic processes to structure the optical components,” explains Joachim Stumpe from the Fraunhofer Institute for Applied Polymer Research IAP in Potsdam-Golm. “The interference created by superimposing two or more laser beams produces a characteristic luminance and polarisation pattern.” The exposed photosensitive polymer films are stimulated with light, causing the macromolecules to migrate, either rapidly in bright sections of the film or not at all in dark areas. The motion of the molecules is driven by photosensitive molecular groups that project beyond the polymer chains like tiny hairs. When they are exposed to light, they begin to bend or stretch in a continuous cycle. The effect is much like a small motor.
“The polymers move like snakes in the underbrush,” says Stumpe, giving a pictorial analogy of the process. “The intensity and polarization of the light determine the direction in which the polymers move, and thus the morphology of the surface structure.” The researchers are able to superimpose several structures on top of one another by adjusting the angle of the laser beam relative to the position of the polymer film. The relief pattern can also be deleted and overwritten. This is accomplished by applying a homogenous single laser beam that distributes the polymers evenly. Repeated exposure to several laser beams creates a new relief.
“The light-sensitive polymers offer considerable scope to produce two- and three-dimensional structures,” says Stumpe. The researchers use different polymers, depending on requirements. The only condition is that the polymer must contain photosensitive azobenzene groups which enable it to move around. If a particular polymer is too costly or not stable enough for an optical component, it can nevertheless be used as a mould. A cast impression of the holographically created relief can be used to transfer the structure onto metal, ceramic or other plastic materials.