Dielectric Stacks Enable New Class Of Reflectors
Materials research groups in Minsk, Belarus; Essen, Germany; and several other labs around the world have shown that dielectric stacks can be engineered to reflect light effectively over all possible angles of incidence, enabling a new class of nearly ideal reflectors. The novel materials systems could be applied to a wide variety of technologies, since reflective surfaces are crucial both to conventional optics and to such advanced devices as laser diodes.
About The Stacks
Research Methods
Applications
About The Stacks (Back to Top)
A research group led by Dmitry Chigrin, a researcher at the University of Essen (Essen, Germany), worked with research teams at the National Academy of Sciences and Belarusian State University (Minsk) to create the dielectric stacks, according to the Electronic Engineering Times. Through this effort, the researchers both theoretically and experimentally verified a stacked-dielectric structure for optical frequencies. The reflective capabilities of dielectric stacks have simultaneously been discovered and explored by researchers at the Massachusetts Institute of Technology (Cambridge, MA), where an infrared version of a device has been demonstrated, as well as at the University of Bath (England).
The new approach to building reflective surfaces is similar to that used to create Bragg reflectors, which also comprise dielectric stacks. The new structures are composed of quarter-wavelength-thick layers with differing refractive indices. Light at the design wavelength enters the stack from above, at right angles to it. As it proceeds, it is reflected from each of the layer interfaces as a result of the refractive-index differential.
The reflection is reinforced by the optical-path differences among the reflected beams. Generated by the different distances that each beam travels before being reflected, as well as by phase changes introduced at some of the layer boundaries, the path differences cause the reflecting beamlets to interfere constructively. Thus, the whole beam is reflected.
The quarter-wavelength geometry of the structures is vital for the scheme to work properly, since the reflection drops off radically as soon as the incident angle is changed. Because of this, engineers previously had limited options for applications that required wide-angle reflection, but could not employ metal mirrors.
Research Methods (Back to Top)
The Belarusian/German team designed its dielectric stack as a one-dimensional photonic crystal—a structure in which the propagation of particular electromagnetic waves is not allowed because of forbidden bandgaps that are analogous to electron bandgaps in semiconductors. The bandgaps can be opened or closed based on the stack design. For light within the bandgaps, beams are reflected when they come from any possible angle of incidence.
According to Chigrin, the first photonic crystals produced in Minsk in early 1998 were accidents. While working with Andrei Lavrinenko at Belarusian State University, Chigrin explored anisotropic one-dimensional photonic crystals. During this work, the researchers occasionally hit on the right parameters and created sufficiently large index contrasts between the layers to make the photonic-bandgap effect. This result was a surprise, because omnidirectional total reflection in low-dimensional dielectric structures was previously thought to be impossible. To verify their findings, the researchers checked the simplest case—a dielectric lattice of isotropic layers. The experiment worked.
Over the next few months, the researchers probed to find holes in their theory, not believing that so straightforward an effect could have been overlooked for so long. In May 1998, the work was presented to the group seminar of Sergey Gaponenko, head of the research group at the Institute of Molecular and Atomic Physics of the Belarus National Academy of Sciences Sergey. The concept was well received. Researchers have since simulated and built several photonic crystals. One 19-layer device demonstrated total omnidirectional reflection in the low 600-nm (red) range.
Applications (Back to Top)
Gaponenko considers the new structures important for both optics and optoelectronics. The main advantage of the wide-angle reflectors, he says, is that they do not dissipate energy, whereas metallic mirrors both reflect and absorb light. Because of this, metallic mirrors can be destroyed when exposed to high-power fluxes.
"In a simple planar geometry, the structures can be used as filters," Chigrin says. The structures may also be used "as mirrors to improve the performance of devices such as vertical-cavity surface-emitting lasers, or as optical switches and shutters. By rolling the structures into hollow fibers or tubes, the coatings can be used as inside walls of high-finesse waveguides and microcavities."
For more information, call the University of Essen at (+) 201-183-2481.