Photonic Cooling with an LED

In a finding that runs counter to a common assumption in physics, researchers at the University of Michigan ran a light emitting diode (LED) with electrodes reversed in order 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.

This calorimeter with a sensing area 80 micrometers across, is able to show that an infrared photodiode running with electrodes reversed behaved as if it were at a lower temperature and cooled the calorimeter. (Source: L. Zhu)

“We have demonstrated a second method for using photons to cool devices,” said Pramod Reddy, who co-led the work with Edgar Meyhofer, both professors of mechanical engi­neering. The first – known in the field as laser cooling – is based on the foundational work of Arthur Ashkin, who shared the Nobel prize in Physics in 2018. The researchers instead harnessed the chemical potential of thermal radiation – a concept more commonly used to explain, for example, how a battery works. “Even today, many assume that the chemical potential of radiation is zero,” Meyhofer said. “But theo­retical work going back to the 1980s suggests that under some conditions, this is not the case.”

The chemical potential in a battery, for instance, drives an electric current when put into a device. Inside the battery, metal ions want to flow to the other side because they can get rid of some chemical potential energy and we use that energy as elec­tricity. Electro­magnetic radiation, including visible light and infrared thermal radiation, typically does not have this type of potential. “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,” said Linxiao Zhu, a research fellow in mechanical engi­neering.

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 said. However, measuring this cooling and proving that anything interesting happened is hideously 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 single wave­length of infrared light. This is necessary to take advantage of near field or eva­nescent coupling effects, which enable more infrared photons, or particles of light, to cross from the object to be cooled into the LED.

Reddy and Meyhofer’s team had a leg up because they had already been heating and cooling nanoscale devices, arranging them so that they were only a few tens of nano­meters apart. At this close proximity, a photon that would not have escaped the object to be cooled can pass into the LED, almost as if the gap between them did not exist. And the team had access to an ultra-low vibration laboratory where measure­ments of objects separated by nanometers become feasible because vibrations, such as those from footsteps by others in the building, are dramatically reduced.

The group proved the principle by building a minuscule calorimeter, which is a device that measures changes in energy, and putting it next to a tiny LED about the size of a grain of rice. These two were constantly emitting and receiving thermal photons from each other and elsewhere in their environments. “Any object that is at room tempera­ture is emitting light. A night vision camera is basi­cally capturing the infrared light that is coming from a warm body,” Meyhofer said.

But once the LED is reverse biased, it began acting as a very low temperature object, absorbing photons from the calorimeter. At the same time, the gap prevents heat from traveling back into the calori­meter via conduction, resulting in a cooling effect. The team demonstrated cooling of 6 watts per meter squared. Theoreti­cally, this effect could produce cooling equivalent to 1,000 watts per meter squared, 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. (Source: U. Michigan)

Reference: L. Zhu et al.: Near-field photonic cooling through control of the chemical potential of photons, Nature 566, 239 (2019); DOI: 10.1038/s41586-019-0918-8

Link: Nanoscale Transport Lab, University of Michigan, Ann Arbor, USA

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