For decades, atom physicists have used laser light to slow atoms in a gas, cooling them just above absolute zero to study their strange quantum properties. Now, a team of scientists has managed to cool an object in the same way, but with the absence of light rather than its presence. This technique, which has never been experienced before, could one day be used to cool the components in microelectronics.
In an ordinary laser cooling experiment, physicists project laser light in opposite directions (up, down, left, right, forward, backward) on a puff of gas like rubidium. They tuned lasers accurately, so that if an atom moves toward one of them, it absorbs a photon and receives a slight thrust toward the center. Install it perfectly and the light undermines the kinetic energy of the atoms, cooling the gas at very low temperatures.
Pramod Reddy, a physicist practicing at the University of Michigan at Ann Arbor, wanted to try to cool without the special properties of laser light. He and his colleagues began with a widget made of semiconductor material commonly found in video screens: a light-emitting diode (LED). An LED uses a quantum mechanical effect to transform electrical energy into light. Roughly speaking, the LED acts as a small ramp for electrons. Apply tension in the right direction and push the electrons up and down the ramp like children on skateboards. When the electrons fall on the ramp at a lower energy state, they emit photons.
Crucially for the experiment, the LED emits no light when the voltage is reversed, because the electrons can not cross the ramp in the opposite direction. In fact, inverting the voltage also removes the infrared radiation from the device – the broad spectrum of light (including heat) you see when you look at a hot object through night vision goggles.
This actually makes the device cooler – and that means the little thing can work like a microscopic refrigerator, says Reddy. All that is needed is to place it close enough to another tiny object, he says. "If you take a hot object and a cold object … you can have a radiative heat exchange," says Reddy. To prove that they could use an LED to cool, the scientists placed an element barely a few tens of nanometers – the width of a hundred atoms – at a distance of a few. a heat meter called calorimeter. It was close enough to increase the transfer of photons between the two objects, thanks to a process called quantum tunneling. For the most part, the gap was so small that photons could sometimes cross it.
The cooler LED absorbed more photons from the calorimeter than it did, removing heat from the calorimeter and lowering its temperature to one-ten thousandth of a degree Celsius, reports Reddy and his colleagues. week in Nature. This is a small change, but given the tiny size of the LED, this equates to an energy flow of 6 watts per square meter. For comparison, the sun provides about 1000 watts per square meter. Reddy and his colleagues think that they might someday increase the cooling flow until they reach this power by reducing the size of the slot and evacuating the accumulated heat in the LED.
This technique will probably not replace traditional refrigeration techniques, nor will it be able to cool materials to a temperature below 60 K. However, it could be used one day to cool the microelectronics, according to Shanhui Fan, theoretical physicist at the 39, Stanford University, Palo Alto. , California, which did not participate in the work. In previous work, Fan had used computer modeling to predict that an LED could have a considerable cooling effect if placed at nanometers from another object. Now, he says, Reddy and his team have come up with this idea in an experimental way.