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Advances in Raman-based broadband optical refrigeration

This work reports progress towards demonstrated Raman-based optical refrigeration. Previously [1] we introduced Raman refrigeration and its required photonic structures. Building on the previous, the mechanisms and photonic structures are detailed and simplified proof-of-concept devices described. Three wavelength bands, two illuminated and one dark, are considered. The width of each of these bands is predicated upon the magnitude of the Raman shift of the active layer. Optimally, the first illuminated band is approximately one Raman shift (wavelength) in width. This illuminated band is capped above (at shorter wavelengths) by a dark band. At longer wavelengths a second illuminated band again one Raman shift in width is reflected by photonic structuring that forbids light propagation. The dark bands provide an exhaust through which up energy (anti-Stokes) shifted light can be emitted thereby carrying away heat. The Photonic structure prohibits the propagation of a band of long wavelength light thereby both blocking Stoke’s shifted illuminated band light and reflecting incident light having these wavelengths. The naturally occurring solar spectrum with its light and dark bands caused by atmospheric absorption is a good match to diamond-based Raman refrigeration. Diamond also has extremely low absorption and large Raman cross-section especially in small grain form. Proof-of-concept devices employing simple one-dimensional photonic structures are the focus of present experimental effort. The prospect of broadband refrigeration remains a delicate balance requiring limited absorption and increased Raman cross section through phonon engineering of the Raman active layer.