Crinkled films can stop reflection

17 Jun 2002

A new way of introducing nanoridges enables the control of surface optical properties

Swiss researchers have developed thin-film coatings that offer an alternative, inexpensive method for reducing reflection and glare from glass surfaces. Instead of the more traditional approach of pitting a surface using tiny particles or etching techniques, the new films use crinkles or corrugations that are smaller than the wavelength of light to prevent light being scattered or reflected . The researchers claim that their new approach, described in Nature 410, 796-799 (2001), offers performance and cost benefits over pitting techniques and that the films can also be engineered to perform as diffractive thin films, diffusers and directional reflectors that preserve light polarization.

To produce the films, Mohammed Ibn-Elhaj and Martin Schat of Rolic Research in Switzerland deposit a mixture of two liquid-crystal polymer solutions onto a surface. One of the solutions cross-links when it is exposed to ultraviolet radiation to form a plastic-like film. The second solution is trapped in tiny droplets throughout the structure of the film as it forms. The solution can be removed to leave nano- or micropores in the film's surface. The shape of the pores can be controlled, say the researchers, by altering the design of the mixture and the details of the film preparation. For example, if the liquid crystals in their mixture are aligned, ultraviolet irradiation will create roughly parallel, alternating bands of the two components, leaving behind nanocorrugations once the second phase is removed.

Since the alignment can be optically induced, it can also be triggered in a pattern by means of photomasks, giving other possibilities for the shape and size of and also the spacing between the corrugations. The result is a thin film with a controllable nanotopology that enables light radiation striking a surface to be manipulated according to requirements - for example, light can be directionally scattered or diffused, directionally reflected, or not reflected at all. Film topologies, say the scientists, can also be combined with conducting properties - for instance, via metallic top-coatings or conducting polymers - so that their optical functions can be integrated into displays to increase brightness and functionality.