Feb 14 2019
In a recent discovery that contradicts a standard assumption in physics, a research team at the University of Michigan ran a LED (light emitting diode) with reversed electrodes to cool another device that was only nanometers away.
The new method could pave the way for novel solid-state cooling technology for upcoming microprocessors. Such kinds of microprocessors will include so many transistors sandwiched into a tiny space that present-generation of techniques will not be able to eliminate heat quickly and sufficiently.
“We have demonstrated a second method for using photons to cool devices,” said Pramod Reddy, who co-headed the study with Edgar Meyhofer, both mechanical engineering professors.
Laser cooling—the first to be known in the field—is predicated on Arthur Ashkin’s foundational work. In 2018, Ashkin shared the Nobel Prize in Physics. Instead, the chemical potential of thermal radiation was harnessed by the research team. Thermal radiation is a concept which is often used for explaining the way a battery works, for instance.
Even today, many assume that the chemical potential of radiation is zero. But theoretical work going back to the 1980s suggests that under some conditions, this is not the case.
Edgar Meyhofer, Professors, Department of Mechanical Engineering¸ University of Michigan
For example, a battery’s chemical potential fuels an electric current when inserted inside a device. Metal ions, within the battery, attempt to flow to the other side to get rid of some amounts of energy—chemical potential energy—and it is that energy which is used as electricity. This type of potential is not usually exhibited by electromagnetic radiation, such as infrared thermal radiation and visible light.
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.
Linxiao Zhu, Study Lead Author and Research Fellow, Department of Mechanical Engineering, University of Michigan
That is an electrical knob. Theoretically, reversing the negative and positive electrical connections on an infrared LED will not only merely prevent it from producing light but will also suppress the thermal radiation that it should be emitting simply because it is at room temperature.
“The LED, with this reverse bias trick, behaves as if it were at a lower temperature,” said Reddy.
Conversely, it is extremely complicated to measure this cooling and demonstrate that anything interesting happened.
To make sufficient amounts of infrared light to flow from a single object into the LED, both of them have to be very close together, that is, less than one wavelength of infrared light. This aspect is essential to manipulate “evanescent coupling” or “near field” effects, which allow more number of particles of light, or infrared photons, to travel from the object to be cooled inside the LED.
Meyhofer and Reddy’s research team had an advantage, because the duo had already been cooling and heating nanoscale devices, organizing them in such a way that they were less than a thousandth of the breadth of a single hair, or just a few tens of nanometers apart. At close proximities like these, a photon that generally would not have escaped the object to be cooled can easily travel into the LED, virtually as if there is no gap between them. In addition, the research team accessed an ultra-low vibration laboratory, in which measurements of objects isolated by nanometers become possible because vibrations—like those caused by footsteps by others in the building—are considerably reduced.
The team demonstrated the principle by developing an extremely small calorimeter—a device used for measuring differences in energy—and putting it adjacent to a small LED roughly the size of a rice grain. Both these devices were continuously receiving and emitting thermal photons from one another and also 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.
Edgar Meyhofer, Professors, Department of Mechanical Engineering¸ University of Michigan
However, as soon as the LED is reverse biased, it started behaving as an extremely low temperature object, taking in photons from the calorimeter. Simultaneously, heat is prevented by the gap from traveling back into the calorimeter through conduction, leading to a cooling effect. The researchers demonstrated cooling of 6 W/m2. In theory, this impact can possibly produce cooling corresponding to 1,000 W/m2, or roughly the power of sunshine on the surface of Earth.
These findings can have important implications for upcoming smartphones and other types of computers. With increasing amounts of computing power squeezed in miniaturized devices, eliminating the heat from the microprocessor is starting to restrict the amount of power that can be packed into a specified space.
With better cooling rates and efficiency of this latest method, the researchers believe that this phenomenon can offer a method to rapidly remove heat from microprocessors integrated in devices. Since nanoscale spacers could offer the isolation between LED and microprocessor, this phenomenon could even withstand the abuses endured by smartphones.
The study titled, “Near-field photonic cooling through control of the chemical potential of photons” has been reported in the journal Nature on February 14th, 2019.
The Department of Energy and the Army Research Office supported the study. The devices were developed in the Lurie Nanofabrication Facility at University of Michigan. Reddy is also a professor of materials science and engineering, and Meyhofer is also a professor of biomedical engineering.