Visible Light Reflects Nearly Perfectly

Richard Gaughan

Materials with photonic bandgaps hold great promise for a wide variety of applications, but three-dimensional versions are difficult to manufacture. A few research groups have developed one-dimensional photonic crystals with a structure that varies in one dimension, is identical in the other two dimensions, yet still displays some features of three-dimensional photonic crystals.

Dmitry Chigrin, a researcher at the University of Essen working with A.V. Lavrinenko at the Belarusian State University in Minsk, predicted that a one-dimensional crystal consisting of isotropic dielectric layers could be constructed with a photonic bandgap opened at all external angles of incidence. Unlike scientists at the Massachusetts Institute of Technology in Cambridge, who chose to verify the effect at infrared wavelengths, Chigrin attempted to confirm the theoretical results with a visible wavelength structure.

In collaboration with Sergey Gaponenko's research group at the Institute of Molecular and Atomic Physics of the Belarusian National Academy of Sciences in Minsk, Chigrin has designed a structure consisting of 19 layers of Na₃AlF₆ and ZnSe. For a relatively wide bandgap, the selected coating materials' index of refraction must be very different from each other. With the index of Na₃AlF₆ equal to 1.34 and ZnSe varying from 2.5 to 2.8 over a wide range of visible wavelengths, the scientists predicted that the index difference was sufficient to demonstrate an appreciable region of nearly total omnidirectional reflection.

Traditional dielectric coatings use multiple interference from a large number of layers to achieve the desired reflectance. Because the reflection at each layer must constructively interfere with reflection from the other layers, they are highly wavelength- and angle-dependent. The Belarusian research team's reflector consists of material selected to prohibit the propagation of visible light, and is nearly independent of angle of incidence within a given spectral band. The reflectance for 632.8-nm s-polarized light is more than 99.5 percent for angles up to 85^°. For both p- and s-polarization, the reflectance is greater than 99 percent from 604 to 638 nm.

The results open the door for other coating designs for visible wavelengths, which in turn open up more applications. "The potential applications are enormous," Chigrin said. "Even the simplest planar geometry can provide energy-saving filters, improved vertical-cavity surface-emitting lasers, or optical switches and shutters. By rolling into hollow fibers, the coatings can be used as inside walls of high-finesse waveguides and microcavities."

Chigrin is optimistic about how soon some of these or other applications will be realized. "One-dimensional periodic structures are unique ones which modern technology can produce right now," he said. "It's old fabrication technology, nothing more."