With leads reversed, experimental IR LED can cool things, no coherent radiation required
The approach exploits photon tunneling and suppressing emission of photons via reverse bias.
|An IR LED about the size of a grain of rice, shown in this scanning electron microscope (SEM) image, was modified by smoothing its surface so that it could be placed in close proximity to a custom-made calorimeter, with a gap of 55 nm between them. The calorimeter's measurements showed that the LED, when run with electrodes reversed, behaved as if it were at a lower temperature, cooling down the calorimeter. (Image credit: Linxiao Zhu)|
In a finding that runs counter to a common assumption in physics, researchers at the University of Michigan (Ann Arbor, MI) ran a light-emitting diode (LED) with electrodes reversed to cool another device mere nanometers away.
The approach could lead to new solid-state cooling technology for future microprocessors, which will have so many transistors packed into a small space that current methods can't remove heat quickly enough.
"We have demonstrated a second method for using photons to cool devices," says Pramod Reddy, one of the researchers.
The first -- 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.
"Usually for thermal radiation, the intensity only depends on temperature, but we actually have an additional knob to control this radiation, which makes the cooling we investigate possible," says Linxiao Zhu, a research fellow in mechanical engineering and the lead author on the work.
That knob is electrical. In theory, reversing the positive and negative electrical connections on an infrared 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.
"The LED, with this reverse bias trick, behaves as if it were at a lower temperature," Reddy says.
However, measuring this cooling, and proving that anything interesting happened, is very complicated. To get enough infrared light to flow from an object into the LED, the two would have to be extremely close together -- less than a wavelength of infrared light. This is necessary to take advantage of near-field (evanescent coupling) effects, which enable more infrared photons to cross from the object to be cooled into the LED.
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, this effect could produce cooling equivalent to 1000 W/m2, or about the power of sunshine on Earth's surface.
This could turn out to be important for future smartphones and other computers. With more computing power in smaller and smaller devices, removing the heat from the microprocessor is beginning to limit how much power can be squeezed into a given space.
With improvements of the efficiency and cooling rates of this new approach, the team envisions this phenomenon as a way to quickly draw heat away from microprocessors in devices. It could even stand up to the abuses endured by smartphones, as nanoscale spacers could provide the separation between microprocessor and LED.
1. Linxiao Zhu et al., Nature (2019) https://doi.org/10.1038/s41586-019-0918-8.