Fig. 1 Joseph Xu
Linxiao Zhu shows the experimental platform that housed the calorimeter and photodiode. This platform damps vibrations from the room and building, steadily holding the two nanoscale objects 55 nanometers (0.000055 mm) apart.

LEDs Running in Reverse Could Cool Future Computers

Light-emitting diodes hooked up backward absorb heat from objects, but they must be very close by.

Researcher engineers at the University of Michigan ran a light-emitting diode (LED) with electrodes reversed to cool another device mere nanometers away. This approach could lead to new solid-state coolers for future microprocessors which will have so many transistors packed into a small space that current cooling methods can’t remove heat quickly enough.

“We have demonstrated a second method for using photons to cool devices,” says Pramod Reddy, who headed the research team along with Edgar Meyhofer. Both are mechanical engineering professors.

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.

But the Michigan researchers instead harnessed the chemical potential of thermal radiation, a concept more commonly used to explain how batteries work.

“Even today, many assume the chemical potential of radiation is zero,” Meyhofer says. “But theoretical work going back to the 1980s suggests that under some conditions, this is not the case.”

Fig. 2

The UM team made a calorimeter with a sensing area 80 micrometers (0.08 mm) across, shown in this electron microscope image. (Courtesy: Linxiao Zhu)

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 energy—chemical potential energy—which we use as electricity. Electromagnetic radiation, including visible light and infrared thermal radiation, typically does not have this type of potential.

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 anything interesting happened is complicated. To get enough IR light to flow from an object into the LED, the LED and object must be extremely close together—less than a single wavelength of IR light. This is necessary to take advantage of “near field” or “evanescent coupling” effects, which lets more infrared photons (or particles of light) 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 range, 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.

Fig. 3

The UM team modified an infrared photodiode about the size of a grain of rice, shown in this electron microscope image. It smoothed its surface so that they could place it just 55 nanometers (0.000055 millimeters) from a custom-made calorimeter. Calorimeter measurements showed that the photodiode, when run with electrodes reversed, behaved as if it were at a lower temperature, and cooled the calorimeter. (Courtesy: Linxiao Zhu)

The group proved the principle by building a minuscule calorimeter (a device that measures changes in energy) and placing it next to an LED about the size of a grain of rice. The calorimeter and LED were constantly emitting and receiving thermal photons from each other and elsewhere in their environments. “Any object at room temperature is emitting light,” Meyhofer says. “A night-vision camera basically captures the IR light that coming from a warm body.”

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 blocks 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 microprocessors 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.

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