Could the Raman Shift Cool Material With Light?

Yujie Ding

Engineers at the Johns Hopkins and Lehigh Universities are pushing the limits of a phenomenon called Raman scattering, and are beginning to ask if it is possible to actually cool a circuit using beams of light.

When light is scattered by an object or a material, most of the light keeps the same amount of energy after it scatters. But a particular form of scattering, called Raman scattering, occurs when the energy is actually absorbed by the atoms and re-emitted.

Electrons are limited to certain energy levels as they oscillate around the atom. When an atom absorbs a unit of energy from light, the electron jumps up to one of these energy levels, and the rest of energy becomes motion of the atom. Altogether, the motion of atoms is measured as temperature.

Things tend to seek the lowest energy state, so the electron eventually drops back down to a lower energy level, and emits a unit of light (called a photon).

However, the energy of the light emitted doesn’t have to be equal to the energy that was absorbed. Usually, some of the atom’s motion remains, and this is why light tends to heat up materials. This is called stokes scattering, when the Raman shift reduces the energy of the light and increases the temperature of the material.

Amazingly, some of the Raman scattering actually goes the other way. The energy of motion of the atom becomes light, and the atom is actually cooled as a result. This positive Raman shift is called anti-Stokes scattering.

Obviously, under normal circumstances, this doesn’t happen often. However, using lasers, scientists have previously been able to selectively energize atoms in a way that causes them to transform heat into light. However, this is typically only done with single atoms, or on clouds of gas that are already cooled to near absolute zero.

The new experiment, led by Yujie Ding, suggests it could be possible to cool materials with light at room temperature.

The experiment focused on a promising material called gallium-nitride, which could potentially revolutionize computer chips. Typically, 35 atoms are heated up for every 1 atom that is cooled down by Raman shift. They were able to reduce this to a shocking ratio of just 2 to 1.

This was accomplished by creating resonances in the lattice of atoms with the light. The resonance caused the atoms to move in a predictable and periodic manner, so that the energy could be applied at the right time in order to reduce the energy of the atoms.

How Raman scattering works on a large scale isn’t yet well understood, so the researchers don’t yet know if it’s possible to cool more atoms than are heated up at room temperature.

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