Dielectric Stack Offers Omnidirectional Reflectance

Specialized bandgap design yields tellurium/polystyrene planar thin film stack with omnidirectional reflectivity.

By: Yvonne Carts-Powell

Researchers at MIT demonstrated a reflector that combines the high reflectivity of dielectric thin-film stacks with the omnidirectional reflectance of metallic mirrors. In most cases, metallic mirrors reflect light over a broad range of wavelengths and polarizations and from arbitrary angles, but at visible and IR wavelengths a few percent of the incident energy is lost to absorption. Multilayer dielectric mirrors, in which the layer thicknesses and indices of refraction are designed to cause light to either reflect or cancel out, are much more efficient.

Because their operation typically depends on making the layers a fraction of the wavelength, dielectric stacks have been used for only narrow wavelength ranges and reflect light within a narrow range of incident angles. In the past, researchers assumed that for an all-dielectric structure to reflect light from all angles, it would have to provide a complete photonic bandgap—in other words, it would require a dielectric with periodicity in three dimensions, as opposed to the single-axis periodicity of thin planar films.

In fact, omnidirectional reflectivity is possible using only thin planar films, as Yoel Fink and collaborators at the Massachusetts Institute of Technology (Cambridge, MA) have demonstrated.¹ The group achieved omnidirectional reflectivity by designing the material's bandgap so that there is a frequency overlap between the gap at normal incidence and the gap at the TM polarization at an incident angle of 90°. The reflectivity range can be tailored by changing the layer thicknesses, which allows different regions of the spectrum to be targeted for different applications. The researchers have filed three patent applications based on this device.

Device design and fabrication
The device is grown using inexpensive vacuum deposition (for the tellurium) and spin casting (for the polymer) techniques. These polymer-processing methods could allow large high-quality devices to be made relatively cheaply.

The demonstrated reflector was a nine-layer stack of 0.8-mm-thick layers of tellurium and 1.65-mm-thick layers of polystyrene deposited on a sodium chloride substrate. Tellurium has a high index of refraction and low loss characteristics at the wavelengths of interest and does not diffuse much into the polymer layer.

The material was modeled without taking into account polystyrene's absorption peaks at about 13 and 14 mm. When researchers compared the predicted and actual reflectances at different polarizations and angles of incidence, they found that as the angle increases, the absorption peak increases for the TM mode, but decreases for the TE mode. This is partially a result of the TM mode's deeper penetration into the stack at higher angles and therefore it has more likelihood of being absorbed. Interestingly, the group found that the film thickness that allows a maximum range width is not the traditional quarter-wave stack, although quarter-wave stacks work well.

Applications
Potential applications abound. The mirror could be used for laser cavities or to create a wide-band hollow waveguide. The demonstrated film was designed to operate over a range from 10 to 15 mm in the infrared spectral region, a range that includes the operating wavelength of carbon dioxide lasers (10.6 mm.) A film could also be used as a heat barrier or thermal collector by reflecting in the thermal IR while transmitting light at visible wavelengths, and energy-conserving windows and radiant barriers are obvious potential uses.

References
1. Y. Fink et. al., Science 282, 27 Nov 1998, p. 1679.

About the author…
Yvonne Carts-Powell is a freelance science writer based in Belmont, MA.