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# Emitting Photons Is One Way to Be Cool

Physics 13, 22
A device described in a new proposal would cool an object by causing it to radiate extra heat.

All objects radiate heat while also receiving radiative heat from the environment. A theoretical analysis suggests that a device could boost the outgoing radiation enough to cool an object in a warm environment. When placed on the surface of an object, this “photonic refrigerator” would raise the frequency of radiated photons and thus increase the amount of energy they carry away. Such active cooling devices could be fabricated from layered nanomaterials and installed on electronic components to keep them cool.

The radiation from an object has a characteristic spectrum that peaks at a frequency determined by the object’s temperature. The emission spectrum for an object at $5{\phantom{\rule{0.3em}{0ex}}}^{\circ }\text{C}$, for example, will have a peak frequency of 16 terahertz (THz). If the temperature of its environment is $20{\phantom{\rule{0.3em}{0ex}}}^{\circ }\text{C}$, then the object will absorb radiation having a peak frequency of 17 THz. At every frequency, the number of incoming photons is greater than the number going out, so the object will warm up, assuming no other form of heat transfer is occurring.

Shanhui Fan and his colleagues at Stanford University, California, want to reverse the net effect of this exchange by giving radiated photons extra energy, allowing them to carry away more heat. This cooling device would be in direct contact with the object, acting like a “skin” that changes the color (frequency) of the radiation that the object emits.

To accomplish this feat, the photonic refrigerator would have an index of refraction that oscillates in time. This oscillation, or “modulation,” could be accomplished in a number of different ways. For example, the device could resemble an acousto-optic modulator, which contains a piece of quartz or other transparent material that is vibrated by sound waves. The waves cause the material to periodically contract and expand, resulting in an oscillation of the index of refraction. Photons traveling through the modulator can receive a frequency “kick” by scattering off these waves. A larger frequency shift is possible in certain crystals whose index is modulated by a time-varying electric signal or a beam of light.

Fan’s team imagines engineering the modulation to act on a certain range of photon frequencies. For example, the device could target the 16-THz photons emitted by an object at $5{\phantom{\rule{0.3em}{0ex}}}^{\circ }\text{C}$, boosting their frequency to 18 THz if the device’s index modulated at 2 THz. The number of 18-THz photons emitted by the object would be more than the number of 18-THz photons absorbed from the $20{\phantom{\rule{0.3em}{0ex}}}^{\circ }\text{C}$ environment, so more heat would flow outward at this frequency. At other frequencies, the net heat flow could be inward, but the total effect summed over all frequencies would be to cool the object.

Of course, the cooling doesn’t come for free. The modulation of the index is a form of work performed by, for example, a sound-wave generator or a light source. The team calculated the energy efficiency of their system and found that—in an ideal case—the photonic refrigerator could work at the upper limit of efficiency set by classical thermodynamics.

The team also theoretically analyzed a specific implementation of their concept: a structure consisting of thin layers of two types of insulating materials placed on top of an object. A terahertz light source could modulate the index of certain layers in order to select the desired frequency change. The researchers put in realistic values for the material properties and computed a cooling rate of roughly 300 milliwatts per square meter, which is much smaller than the average refrigerator. Fan admits that the photonic refrigerator would not work well for chilling a gallon of milk, but it might prove useful for very small objects, where traditional cooling devices are not effective. “Computer chips and other electronic components become hot when in use, and this degrades their performance,” says Stanford team member and graduate student Siddharth Buddhiraju. Photonic structures could keep them cool.

Nanophotonics expert Mikhail Kats from the University of Wisconsin, Madison, calls the new predictions “an exciting result.” He says the work follows from recent efforts with optical isolators, which are like one-way devices for light. “Nevertheless, the specific refrigeration mechanism is novel, and the calculated thermodynamic performance is substantial,” he says. Kats also thinks that the layered structures envisioned by the authors seem achievable.

This research is published in Physical Review Letters.

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.

Photonics

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