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.
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.
Described are the prospects for broadband optical refrigeration based on Raman scattering of incoherent light. Laser pumped rare earth fluorescence has been demonstrated and commercial applications are sure to follow. Broadband refrigeration requires strong Raman scattering and large Raman shift. Also required are spectral management and photonic patterning to offset the unfavorable anti-Stoke’s to Stoke’s shift ratio. Materials such as diamond, silicon, and a number of molecular systems are ideal and have low absorption. Optics splits the broadband spectrum into light and dark bands with width corresponding to the Raman shift. Broadband spectrums where photon flux decreases with increasing photon energy are ideal. By tailoring the incoming spectrum, by utilizing extremely transparent strong Raman shift materials and by photonic inhibition of Stoke’s shifted light the prospect become feasible. The Raman optical cross- section increases with decreasing particle size (until the particle become too small to support the Raman-phonons). Where conservation of phonon states in these truncated Brillioun-zone particles requires an increased density (number/cm3) of the allowed-states to compensate for states lost to particle size. Nonetheless, the anti-Stokes to Stokes ratio is approximately one-to-two at laboratory temperature. Thin film deposited diamond is an excellent candidate for refrigeration applications due to its high transparency small grain size and its large Raman magnitude and large shift. Simple one-dimensional photonic structures selectively inhibit the Stoke’s shifted light making refrigeration possible.
The laser cooling of vibrational states of solids has been achieved through photoluminescence in rare-earth elements, optical forces in optomechanics, and the Brillouin scattering light-sound interaction. The net cooling of solids through spontaneous Raman scattering, and laser refrigeration of indirect band gap semiconductors, both remain unsolved challenges. Here, we analytically show that photonic density of states (DoS) engineering can address the two fundamental requirements for achieving spontaneous Raman cooling: suppressing the dominance of Stokes (heating) transitions, and the enhancement of anti-Stokes (cooling) efficiency beyond the natural optical absorption of the material. We develop a general model for the DoS modification to spontaneous Raman scattering probabilities, and elucidate the necessary and minimum condition required for achieving net Raman cooling. With a suitably engineered DoS, we establish the enticing possibility of refrigeration of intrinsic silicon by annihilating phonons from all its Raman-active modes simultaneously, through a single telecom wavelength pump. This result points to a highly flexible approach for laser cooling of any transparent semiconductor, including indirect band gap semiconductors, far away from significant optical absorption, band-edge states, excitons, or atomic resonances.
Optical refrigeration using anti-Stokes photoluminescence is now well established, especially for rare-earth-doped solids where cooling to cryogenic temperatures has recently been achieved. The cooling efficiency of optical refrigeration is constrained by the requirement that the increase in the entropy of the photon field must be greater than the decrease in the entropy of the sample. Laser radiation has been used in all demonstrated cases of optical refrigeration with the intention of minimizing the entropy of the absorbed photons. Here, we show that as long as the incident radiation is unidirectional, the loss of coherence does not significantly affect the cooling efficiency. Using a general formulation of radiation entropy as the von Neumann entropy of the photon field, we show how the cooling efficiency depends on the properties of the light source, such as wavelength, coherence, and directionality. Our results suggest that the laws of thermodynamics permit optical cooling of materials with incoherent sources, such as light emitting diodes and filtered sunlight, almost as efficiently as with lasers. Our findings have significant and immediate implications for design of compact all-solid-state devices cooled via optical refrigeration.
Optical refrigeration using anti-Stokes photoluminescence is now well established, especially for rare-earth-doped solids where cooling to cryogenic temperatures has recently been achieved. The cooling efficiency of optical refrigeration is constrained by the requirement that the increase in entropy of the photon field must be greater than the decrease in entropy of the sample. Laser radiation has been used in all demonstrated cases of optical refrigeration with the intention of minimizing the entropy of the absorbed photons. In this presentation, I will show that as long as the incident radiation is unidirectional, the loss of coherence does not significantly affect the cooling efficiency. Using a general formulation of radiation entropy as the von Neumann entropy of the photon field, I will show how the cooling efficiency depends on the properties of the light source such as wavelength, coherence, and directionality. These results suggest that the laws of thermodynamics permit optical cooling of materials with incoherent sources such as light emitting diodes and filtered sunlight almost as efficiently as with lasers.
This review discusses the progress and ongoing efforts in optical refrigeration. Optical refrigeration is a process in which phonons are removed from a solid by anti-Stokes fluorescence. The review first summarizes the history of optical refrigeration, noting the success in cooling rare-earth-doped solids to cryogenic temperatures. It then examines in detail a four-level model of rare-earth-based optical refrigeration. This model elucidates the essential roles that the various material parameters, such as the spacing of the energy levels and the radiative quantum efficiency, play in the process of optical refrigeration. The review then describes the experimental techniques for cryogenic optical refrigeration of rare-earth-doped solids employing non-resonant and resonant optical cavities. It then examines the work on laser cooling of semiconductors, emphasizing the differences between optical refrigeration of semiconductors and rare-earth-doped solids and the new challenges and advantages of semiconductors. It then describes the significant experimental results including the observed optical refrigeration of CdS nanostructures. The review concludes by discussing the engineering challenges to the development of practical optical refrigerators, and the potential advantages and uses of these refrigerators.
The cooling of solids by optical means only using anti-Stokes emission has a long history of research and achievements. Such cooling methods have many advantages ranging from no moving parts or fluids through to operation in vacuum and may have applications to cryosurgery. However, achieving large optical cryocooling powers has been difficult to manage except in certain rare-earth crystals but these are mostly toxic and not biocompatible. Through study of the emission and absorption cross sections we find that diamond, containing either nitrogen vacancy (NV) or silicon vacancy defects, shows potential for optical cryocooling and, in particular, NV doping shows promise for optical refrigeration. We study the optical cooling of doped diamond microcrystals ranging 10--250 $\ensuremath{\mu}\mathrm{m}$ in diameter trapped either in vacuum or in water. For the vacuum case we find NV-doped microdiamond optical cooling below room temperature could exceed $|\mathrm{\ensuremath{\Delta}}T|>10$ K for irradiation powers of Pin<100 mW. We predict that such temperature changes should be easily observed via large alterations in the diffusion constant for optically cryocooled microdiamonds trapped in water in an optical tweezer or via spectroscopic signatures such as the zero-phonon line width or Raman line.