In a finding that shows coherent light is not needed to optically cool things, researchers at the University of Michigan (Ann Arbor, MI) ran a light-emitting diode (LED) with electrodes reversed to cool another device just nanometers away. The approach could lead to new solid-state cooling technology for future microprocessors. The well-known technique of laser cooling is based on the foundational work of Arthur Ashkin, who shared the Nobel Prize in Physics in 2018. It has resulted in compact all-optical cryogenic devices that are already practical for some uses. The Michigan researchers instead harnessed the chemical potential of thermal radiation. In theory, reversing the positive and negative electrical connections on an infrared (IR) LED won’t just stop it from emitting light, but will actually suppress the thermal radiation that it should be producing just because it’s at room temperature. In other words, it behaves as if it were at a lower temperature.
However, measuring this cooling, and proving that anything interesting happened, is complicated. To get enough IR light to flow from an object into the LED, the two would have to be at a subwavelength spacing to take advantage of near-field (evanescent coupling) effects. The group proved the principle by building a minuscule calorimeter and placing it next to a small LED about the size of a grain of rice. The two were constantly emitting and receiving thermal photons from each other and elsewhere in their environments, but once the LED was reverse-biased, it began acting as a very low-temperature object, absorbing photons from the calorimeter. At the same time, the nanoscale gap between them prevented heat from traveling back into the calorimeter via conduction, resulting in a cooling effect. The team demonstrated cooling of 6 W/m2—theoretically, the effect could produce cooling equivalent to 1000 W/m2.
With improvements in efficiency and cooling rates, the team envisions this approach as a way to quickly draw heat away from microprocessors in devices. It could even stand up to the abuses endured by smartphones if nanoscale spacers provided the separation between microprocessor and LED. Reference: L. Zhu et al., Nature (2019); https://doi.org/10.1038/s41586-019-0918-8.