Photonic Crystal Emits Super-Planckian Thermal Radiation at Optical Wavelengths

Since the late 19th century, scientists have understood that, when heated, all materials emit light in a predictable spectrum of wavelengths. But a material that appears to exceed the limits set by Planck’s law has been experimentally demonstrated by researchers at Rensselaer Polytechnic Institute (RPI).

The new material, discovered by professor Shawn Yu Lin, emits a coherent light similar to that produced by lasers or LEDs, but without the costly structure needed to produce the stimulated emission of those technologies.

An advanced “Super-Planckian” material exhibits LED-like light when heated. Courtesy of Rensselaer Polytechnic Institute.

For his research, Lin built a 3D tungsten photonic crystal with six offset layers, in a configuration similar to a diamond crystal and topped with an optical cavity that further refined the light. The photonic crystal shrinks the spectrum of light that is emitted from the material to a span of about 1 μm. The cavity continues to squeeze the energy into a span of roughly 0.07 μm.

Lin and his team compared the thermal radiation from the microcavity/tungsten photonic crystal (W-PC) and a blackbody. Both were measured from the same sample and in situ. Lin prepared his sample and blackbody control (a coating of vertically aligned nanotubes on top of the material) side-by-side on a single piece of silicon substrate, eliminating the possibility of changes between testing the sample and control that could compromise the results. In an experimental vacuum chamber, the sample and control were heated to 600 K (about 620 °F).

Lin’s spectral analysis was taken in five positions as the aperture of an infrared (IR) spectrometer moved from a view filled with the blackbody to one of the material. Peak emission, with an intensity of 8 times greater than the blackbody reference, occurred at 1.7 μm.

Recent unrelated research has shown a similar effect at a distance of less than 2 thermal wavelengths from the sample, but Lin’s is the first material to display super-Planckian radiation when measured from 30-cm distance (about 200,000 wavelengths) — a result showing that the light has completely escaped from the surface of the material.

Although theory does not fully explain the effect, Lin hypothesizes that the offsets between the layers of photonic crystal allow light to emerge from within the many spaces inside the crystal. The emitted light bounces back and forth within the confines of the crystal structure, which alters the property of the light as it travels to the surface to meet the optical cavity.

“We believe the light is coming from within the crystal, but there are so many planes within the structure, so many surfaces acting as oscillators, so much excitation, that it behaves almost like an artificial laser material,” Lin said. “It’s just not a conventional surface.”

The new material could be used in applications like energy harvesting and IR-based object tracking and identification. It could be used to produce high-efficiency optical sources in the IR, driven by waste heat or local heaters; in research requiring environmental and atmospheric and chemical spectroscopy in the IR; and in optical physics as a laser-like thermal emitter.

“This doesn’t violate Planck’s law. It’s a new way to generate thermal emission, a new underlying principle,” Lin said. “This material, and the method that it represents, opens a new path to realize super-intense, tunable LED-like infrared emitters for thermophotovoltaics and efficient energy applications.”

The research was published in Scientific Reports (www.doi.org/10.1038/s41598-020-62063-2).