Researchers discover new way to cool solids with light

Assistant Professor Gaurav Bahl
Assistant Professor Gaurav Bahl
MechSE PhD student Yin-Chung Chen and Assistant Professor Gaurav Bahl recently devised a new method to cool solids using light. Their paper published in the high-impact scientific journal Optica is titled “Raman cooling of solids through photonic density of states engineering.”
The research for the paper was conducted at Illinois, and was funded through an Office of the Vice Chancellor for Research Campus Research Board Grant and an Army Research Office grant. Bahl and Chen have outlined for the first time the conditions required to refrigerate a solid using Raman scattering—a fundamental light-scattering process that occurs in all materials. They showed that a transparent material simply patterned into a photonic crystal (a periodic array of nanoscale holes), could be cooled when illuminated by specific laser wavelengths.
Heat in a solid is stored in the form of phonons—quantized vibrational degrees-of-freedom of a material. Colder objects have less heat energy (fewer phonons), while hotter objects have more. Refrigeration occurs when this phonon energy is removed and delivered elsewhere, faster than it can be regained from the ambient environment. 
Although attempting to remove heat with light may seem counterintuitive, this research and previous studies have proved otherwise.  

“When photons, or quantized particles of light, interact with an object, they exchange momentum and vibrational energy with the object through the addition or removal of phonons,” said Bahl. “If we are smart enough to engineer a system to only allow photon-scattering that results in the removal of phonons, then we will be able to cool the material using light.” 
To date, the most efficient method by which light has been used to cool solids is through optical fluorescence. While this can be used to cool specialized glasses to sub-cryogenic temperature (-183 degrees Celsius), it only applies to select materials. Furthermore, fluorescence cannot cool most semiconductors (like silicon) that are commonly used in electronics and optics. Thus, the search for laser-cooling mechanisms that may be applicable to any material is still ongoing.
Bahl previously explored cooling of solids through Brillouin scattering of light, another universal mechanism available in all states of matter, in which light adds or removes low-frequency phonons from the material [Bahl et al, Nature Physics 8(3), pp. 203-207 (2012)]. That 2012 study laid the foundation for the current work on Raman scattering, as an extension of the Brillouin scattering concept but with extremely high-frequency phonons. 
“The per-photon cooling efficiency achieved through Raman scattering can be one hundred thousand times greater than achieved through Brillouin scattering. If we can successfully harness Raman cooling then we can revolutionize this field of research,” said Bahl.
Although heat is made of phonon “quasiparticles,” the impact of their proposal is quite real. 
“We now have a path to discover a new refrigeration mechanism that could be applied to any transparent material, with any geometry and any laser wavelength,” said Bahl.