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.
"We have demonstrated a second method." Pramod Reddy, who collaborated with Edgar Meyhofer, both professors of mechanical engineering.
The first-known in the field of laser cooling is based on the foundational work of Arthur Ashkin, who shared the Nobel Prize in Physics in 2018.
"Even today, many assume that the chemical potential of radiation is zero, "Meyhofer said.
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 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 the cooling we investigate possible, "said Linxiao Zhu, a research fellow in mechanical engineering and the lead author of the work.
That knob is electrical. In theory, reversing the positive and negative electrical connections on an infrared LED does not just stop it from emitting light, but it actually suppresses the thermal radiation that it should be producing because it's at room temperature.
"The LED, Reddy said.
However, this cooling-and proving that something interesting happened-is hideously complicated.
To get enough infrared light to flow from an LED into the light, the two would have to be extremely close together as a single wavelength of infrared light.
Reddy and Meyhofer's team had a leg up because they have already been arranged and less than a thousandth of a hair's breadth, so they were only a few tens of nanometers apart. At this close proximity, a photon would not have escaped the object to be cooled in the LED, almost as the gap between them did not exist. And the team had access to an ultra-low vibration laboratory.
The group proved the principle by building a minuscule calorimeter, which is a device that measures changes in energy, and next to a tiny LED. These two are broadcasting in a different atmosphere.
A night vision camera is capturing the light that is coming from a warm body , "Meyhofer said.
But once the LED is reversed biased, it began acting as a very low temperature object, absorbing photons from the calorimeter.
The team demonstrated cooling of 6 watts per meter squared. Theoretically, this effect could be 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 improvements in 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.
Nature Nature on Feb. 14, 2019,
It could even be up to the end of life with smartphones, as nanoscale spacers could provide the separation between microprocessor and LED.
titled, "Near-field photonic cooling through the control of the chemical potential of photons."
Interference as a new method for cooling quantum devices
Near-field photonic cooling through control of the chemical potential of photons, Nature (2019). DOI: 10.1038 / s41586-019-0918-8, https://www.nature.com/articles/s41586-019-0918-8