Researchers devised a method in which running a light emitting diode (LED) with electrodes reversed was able to cool another device nanometers away. They harnessed the chemical potential of thermal radiation — a concept more commonly used to explain how a battery works.
It is assumed that the chemical potential of radiation is zero; however, 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 energy — chemical potential energy — that is used 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 there is an additional way to control the radiation that makes cooling possible; that method 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 behaves as if it were at a lower temperature.
Measuring this cooling is extremely 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 that enable more infrared photons, or particles of light, to cross from the object to be cooled into the LED.
The researchers built a minuscule calorimeter — a device that measures changes in energy — and put 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. 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.
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 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, this phenomenon could serve as a way to quickly draw heat away from microprocessors.