Microstructures Emit Coherent IR Radiation
Daniel S. Burgess
Thermal sources, such as a blackbody or the incandescent filament in a lightbulb, typically produce broadband, quasi-isotropic radiation. But a team at Centre National de la Recherche Scientifique in Châtenay-Malabry and Bagneux, France, and at Commissariat a l'Energie Atomique in Le Barp, France, has demonstrated that this need not be the case by developing a microstructured source that acts like an infrared antenna, generating coherent mid-infrared radiation in narrow emission lobes.
"Contrary to a common belief, light is not always incoherent along the surface of a thermal source," said Jean-Jacques Greffet, a member of the research team and professor at École Centrale Paris who is on sabbatical leave at the University of Rochester in Rochester, N.Y. He explained, however, that this coherence goes unobserved because it is confined to surface waves in the material.
"This is very much like the vibration of the string of a piano," he said. "If one strikes the string at only one point, the whole string vibrates at the same frequency along its full length." Similarly, atoms vibrate in an ordered way along the interface in an ionic crystal, producing an electromagnetic field. "Although the source was random, the vibration along the surface is as ordered as the vibration of a piano string."
To enable them to observe the coherent radiation, the researchers coupled the surface waves to those propagating from the material-air interface. "The idea is that, if one introduces a dust particle on the surface, it will scatter the light and send it away," Greffet said. "What we did was to enhance this effect by designing the best array of 'dust particles' so that they can efficiently scatter the light. This is a grating."
The researchers fabricated the emitter by ruling a periodic structure into a wafer of silicon carbide that was designed to maximize coupling. They coated the wafer with a 1-µm-thick photoresist, which they exposed through a chromium mask with a 3-µm grating. They then coated the wafer and the remaining photoresist with a 40-nm-thick nickel film, after which they dissolved the photoresist. Reactive ion etching produced the 285-nm-deep grooves approximately 6.25 µm apart within a few minutes, Greffet said. Finally, they removed the nickel film with an acid solution.
The periodic structure enabled the device to produce 11.36-µm radiation in two narrow lobes. To verify that the phenomenon was due to coupling of the surface wave, the researchers measured the emissivity for s- and p-polarization, finding no peaks for s-polarization and none for p-polarization in the spectral regions where there should be no surface waves.
Greffet said that the work might enable researchers to tailor the properties of surfaces to improve radiative cooling, for example. He also suggested that the design could be used to produce sources of coherent thermal radiation that emit in only one direction.
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