New Rules Reveal How an Object’s Scale Affects Its Interaction with Light

Princeton researchers have uncovered rules pertaining to how objects absorb and emit light. Their discovery addresses how the scale of an object affects the way it interacts with light. The new rules will tell scientists how much infrared (IR) light an object of any scale can be expected to absorb or emit.

The movement of light through ordinary-size objects can be described in terms of straight lines, or rays. However, in microscopic objects, properties in the lightwave override the effect of ray optics. Some materials, when observed at the micron scale, have shown IR light radiating at millions of times more energy-per-unit-area than would be possible if ray optics were in effect.

“The kinds of effects [that] you get for very small objects are different from the effects [that] you get from very large objects,” researcher Sean Molesky said.

Princeton researchers, led by Alejandro Rodriguez, have uncovered new rules for how objects absorb and emit light. The work resolves a long-standing discrepancy between large and small objects, unifying the theory of thermal radiation across all scales and boosting scientists’ control of designs that use light-based technology. Courtesy of Casey Horner on Unsplash.

The Princeton team used the 19th-century concept of blackbody — an object that absorbs and emits light with maximum efficiency — to help them uncover how objects interact with light differently, depending on scale. “There’s been a lot of research done to try to understand in practice, for a given material, how one can approach these blackbody limits,” professor Alejandro Rodriguez said. “How can we make a perfect absorber? A perfect emitter?” Previous work has shown that structuring objects with nanoscale features can enhance absorption and emission, effectively trapping photons in a tiny hall of mirrors. But until now, no one has defined the fundamental limits of absorption and emission, leaving important questions about how to assess a design unanswered.

The researchers derived fundamental per-channel bounds on angle-integrated absorption and thermal radiation for arbitrarily structured bodies. They showed that the bounds properly captured the physically observed transition from the volume scaling of absorptivity seen in deeply subwavelength objects (nanoparticle radius or thin film thickness) to the area scaling of absorptivity seen in ray optics (blackbody limits).

The new level of control provided by the rules could help engineers optimize designs mathematically for a wide range of applications. The research could be especially useful for technologies such as solar panels, optical circuits, and quantum computers.

Currently, the team’s findings are specific to thermal sources of light, such as the sun or incandescent bulbs. The researchers hope to generalize their work to encompass other light sources, from LEDs to fireflies to arcing bolts of electricity.

The research was published in Physical Review Letters (www.doi.org/10.1103/PhysRevLett.123.257401).