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John O | February 2019

Running an LED in reverse could be new solid-state cooling technology for microprocessors

By Josh Perry, Editor


Researchers at the University of Michigan (Ann Arbor, Mich.) discovered that running a light-emitting diode (LED) with the electrodes reversed could cool a second device that was nanometers away, according to a report from the university.


Linxiao Zhu shows the experimental platform that housed the calorimeter and photodiode. (Joseph Xu/University of Michigan)


The interesting breakthrough could provide the roadmap to new solid-state cooling technology for microprocessors that are packed with transistors in small packages and produce heat that can’t be managed through current techniques.


This is the second method that researchers developed for using photons to cool devices. The first was laser cooling and the second harnesses the chemical potential of thermal radiation and tunes it with electricity.


“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 article explained.


Reversing the electrodes makes the LED behave as if it were at a lower temperature.


The article continued, “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.”


Keeping the devices nanometers from each other enables the photons to move as though there is no gap. A nanoscale calorimeter was created and placed next to an LED about the size of a grain of rice. There was constant emitting and receiving of photons between the devices.


“But once the LED is reverse biased, it began acting as a very low temperature object, absorbing photons from the calorimeter,” the article noted. “At the same time, the gap prevents heat from traveling back into the calorimeter via conduction, resulting in a cooling effect.”


Experiments demonstrated 6 W/m2 of cooling. As electronic devices continue to shrink, this type of cooling solution could enable more processing power.


The research was recently published in Nature. The abstract stated:


“Photonic cooling of matter has enabled both access to unexplored states of matter, such as Bose–Einstein condensates, and novel approaches to solid-state refrigeration. Critical to these photonic cooling approaches is the use of low-entropy coherent radiation from lasers, which makes the cooling process thermodynamically feasible. Recent theoretical work has suggested that photonic solid-state cooling may be accomplished by tuning the chemical potential of photons without using coherent laser radiation, but such cooling has not been experimentally realized.


“Here we report an experimental demonstration of photonic cooling without laser light using a custom-fabricated nanocalorimetric device and a photodiode.


“We show that when they are in each other’s near-field—that is, when the size of the vacuum gap between the planar surfaces of the calorimetric device and a reverse-biased photodiode is reduced to tens of nanometres—solid-state cooling of the calorimetric device can be accomplished via a combination of photon tunnelling, which enhances the transport of photons across nanoscale gaps, and suppression of photon emission from the photodiode due to a change in the chemical potential of the photons under an applied reverse bias.


“This demonstration of active nanophotonic cooling—without the use of coherent laser radiation—lays the experimental foundation for systematic exploration of nanoscale photonics and optoelectronics for solid-state refrigeration and on-chip device cooling.”

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