Super-Planckian Material Exhibits LED-Like Light

This advanced Super-Planckian material exhibits LED-like light when heated. (Source: RPI)

Could there be a new kind of light in the universe? Since the late 19th century, scientists have understood that, when heated, all materials emit light in a predic­table spectrum of wavelengths. Now, researchers at the Rensselaer Poly­technic Institute present a material that emits light when heated that appears to exceed the limits set by that natural law.

In 1900, Max Planck first mathematically described a pattern of radiation and ushered in the quantum era with the assumption that energy can only exist in discrete values. Just as a fireplace poker glows red hot, increasing heat causes all materials to emit more intense radiation, with the peak of the emitted spectrum shifting to longer wave­lengths as heat rises. In keeping with Planck’s Law, nothing can emit more radiation than a hypo­thetical object that absorbs energy perfectly, a blackbody.

The new material disco­vered by Shawn Yu Lin defies the limits of Planck’s law, emitting 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. The new results show a spike in radiation at about 1.7 microns, which is the near-infrared portion of the electro­magnetic spectrum. “Our results offer the most convincing evidence of super-Planckian radiation in the far-field,” said Lin. “This doesn’t violate Planck’s law. It’s a new way to generate thermal emission, a new under­lying principle. 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.”

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

Lin has been working to establish this advance for 17 years, since he created the first all-metallic photonic crystal in 2002, and the two papers represent the most rigorous tests he has conducted. “Experi­mentally, this is very solid, and as an experi­mentalist, I stand by my data. From a theoretical perspec­tive, no one yet has a theory to fully explain my discovery,” Lin said. In both the imaging and spectro­scopy study, Lin prepared his sample and a 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 degrees Kelvin, about 620 degrees Fahrenheit.

Now, Lin presents spectral analysis taken in five positions as the aperture of an infrared spectro­meter moves 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, occurs at 1.7 micro­meters. Recently presented images taken with a near-infrared conven­tional charge-coupled device, a camera that can capture the expected radiation emission of the material. 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 centi­meters distance, a result showing 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, military infrared-based object tracking and identi­fication, producing high efficiency optical sources in the infrared driven by waste heat or local heaters, research requiring environ­mental and atmo­spheric and chemical spectro­scopy in the infrared, and in optical physics as a laser-like thermal emitter. (Source: RPI)

Reference: S.-Y. Lin et al.: An In-situ and Direct Confirmation of Super-Planckian Thermal Radiation Emitted From a Metallic Photonic-Crystal at Optical Wavelengths, Sci. Rep. 10, 5209 (2020); DOI: 10.1038/s41598-020-62063-2

Link: Nanophotonics, Dept. of Physics, Rensselaer Polytechnic Institute, Troy, USA

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