ALBUQUERQUE, N.M. Applying photonic bandgap theory to a common light emitter tungsten Sandia National Laboratories has developed a way to dramatically boost the efficiency of light bulbs.
The discovery could also be applied to increase the performance of thermal photovoltaic systems. For 10 years, optical researchers have been developing micromachined materials with a forbidden region for optical fields, a property analogous to the electronic bandgaps in semiconductors.
Researchers Shawn Lin and Jim Fleming speculated that a photonic bandgap might be able to control the heat emission of tungsten filaments heated by an electrical current.
Since heat is simply visible light at a longer wavelength, the two reasoned that a tungsten material with a bandgap in the infrared region of the spectrum would be unable to dissipate energy as heat, and that much more of the power in the current flowing through it would be converted to radiation at visible wavelengths.
Test structures were built on silicon wafers using chemical vapor deposition of tungsten. A periodic pattern was etched into the silicon wafers and filled with tungsten to create a photonic bandgap structure. Because of the difficulty of building systems that operate in the visible spectrum, the test structures operated at longer infrared wavelengths, but the principle is the same.
The researchers discovered that the system would absorb longer-wavelength radiation and re-emit it in the allowed wavelength range. While the experiment confirms the original hunch, exactly how it works is difficult to explain based on current theories of photonic bandgap materials. While it appears that the system is able to redirect lattice vibrations that would normally produce black-body radiation into a higher, selective-wavelength range, the physical explanation of how that occurs is still unknown, the researchers said.
If suitable tungsten microstructures could be manufactured, the researchers estimate, the effect would result in light bulbs with a conversion efficiency of 60 percent, rather than the current 5 percent figure.