Reversed Biased LED Shown to Cool Device Nanometers Away
In a finding that seems to run counter to a common assumption in physics, researchers at the University of Michigan ran an LED with electrodes reversed to cool another device that was mere nanometers away. The strategy could lead to new solid-state cooling technology for future microprocessors, which are expected to hold so many transistors in a small space that current techniques can't remove heat quickly enough. In the above image, by Joseph Xu, Linxiao Zhu shows the experimental platform that housed the calorimeter and photodiode.
"We have demonstrated a second method for using photons to cool devices," said Pramod Reddy, who co-led the work with Edgar Meyhofer, both professors of mechanical engineering.
The first method known as laser cooling is based on the foundational work of Arthur Ashkin, who shared the Nobel prize in Physics in 2018. The researchers instead employed the chemical potential of thermal radiation, a concept more often used to explain how a battery works, for example.
"Even today, many assume that the chemical potential of radiation is zero," Meyhofer said. "But theoretical work going back to the 1980s suggests that under some conditions, this is not the case."
Calorimeter Shows Cooling
The U-M team created a calorimeter with a sensing area of just 80 micrometers (0.08 millimeters) across, shown in this electron microscope image. Readings from the calorimeter showed that an infrared photodiode running with the electrodes reversed behaved as if it were at a lower temperature, and thus cooled the calorimeter. (Image courtesy of Linxiao Zhu)
The chemical potential of a battery, for instance, drives an electric current when put into a device. Within a battery, metal ions want to flow to the other side because they can get rid of some energy—chemical potential energy—causing an electic current. Electromagnetic radiation, such as visible light and infrared thermal radiation, typically do 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, which makes the cooling we investigate possible," said Linxiao Zhu, a research fellow in mechanical engineering and the lead author on the work.
In theory, reversing the positive and negative connections on an infrared LED won't just prevent it from emitting light, but it will suppress the thermal radiation that it should produce because it is at room temperature.
"The LED, with this reverse bias trick, behaves as if it were at a lower temperature," Reddy said. However, measuring this cooling and proving that anything unual happened is complicated.
Object to be Cooled Must be Extremely Close
First, to get enough infrared light to flow from an object into the LED, the two have to be extremely close together—a distance of less than a single wavelength of infrared light. This closeness is necessary to take advantage of "near field" or "evanescent coupling" effects, which enable more infrared photons (particles of light), to cross from the object to be cooled with the LED.
The U-M team smoothed the surface of an infrared photodiode about the size of a grain of rice, shown in this electron microscope image. This smoothed surface let them place it close to a custom-made calorimeter with just 55 nanometers (0.000055 millimeters) between them. The calorimeter's measurements revealed that the photodiode, when run with electrodes reversed, behaved as it were at a lower temperature, and thus cooled down the calorimeter.
Reddy and Meyhofer's team had already heated and cooled nanoscale devices, arranging them so that they were only a few tens of nanometers apart—or less than a thousandth of the breadth of a human hair. At this close proximity, a photon that would not have escaped the object to be cooled can pass into the LED, almost like the gap between them did not exist.
Vibrations Have to be Minimized
Vibrations are a major concern for such measurements. The team used an ultra-low vibration laboratory where measurements of objects separated by nanometers become feasible. In this unique lab, vibrations, such as those from other people's footsteps in the building, are dramatically reduced.
"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," Meyhofer said.
However, once the LED is reverse biased, it began acting as a very-low-temperature object, absorbing photons from the calorimeter. While reverse biased, the gap prevents heat from traveling back into the calorimeter via conduction, producing a cooling effect.
The team demonstrated a cooling of 6W per square meter. The researchers theorize that this effect could produce cooling up to 1,000W per meter squared, or a cooling effect equivalent to the power of sunshine on Earth's surface.
Potential Applications for this Cooling Technique
This finding could become critical for future smartphones and other computers. With more computing power expected to be in smaller and smaller devices, removing the heat from a microprocessor is starting to limit how much computing power can be compressed into a given space.
With improvements in the efficiency and cooling rates using this new strategy, the team sees this phenomenon as a potential way to quickly draw heat away from microprocessors in devices. Such a technique could even endure the abuses that smartphones encounter, as nanoscale spacers could separate the microprocessor and LED.
The research was published in the journal Nature on Feb. 14, 2019, titled, "Near-field photonic cooling through control of the chemical potential of photons."
The Army Research Office and the U.S. Department of Energy supported this research. The devices were produced in the U-M Lurie Nanofabrication Facility.