Cryogenic Optical Refrigeration


CryoRay TM  Technology

CryoRayTM coolers remove heat by pumping special materials with precisely tuned laser light.  The basic ideas behind optical refrigeration are not complicated, but may initially seem counterintuitive.  As illustrated in the figure below, laser light at a particular frequency is beamed into a solid cooling material and absorbed.  The solid then fluoresces at a higher frequency, a process called anti-Stokes fluorescence.  The thermal vibrations in the solid (phonons) are annihilated to furnish the energy difference between the higher energy outgoing photons and the lower energy laser photons.  The removal of these phonons cools the solid.  The apparent paradox is that pouring laser power into the solid cools it rather than heating it.  While most materials heat when they absorb light, some materials simply re-radiate light with minimal heating.  These materials, which are said to have high quantum efficiency, are used in lasers and other quantum optics systems.  For optical refrigeration, the physics is pushed a step further, and the laser frequency is tuned to coax high-quantum-efficiency material to actually cool.

Optical refrigeration is possible only in some materials that have the necessary quantum energy-level structure as well as the requisite high quantum efficiency.  The materials which have shown the most success in optical refrigeration are transparent crystals doped with rare earth ions, especially ytterbium.  The low-lying energy levels for these materials are sketched below.  The ground state and first excited state of the rare-earth ions are split by crystal-field interactions.  To cool these materials, laser light is tuned to the energy difference between the top of the ground-state manifold and the bottom of the excited-state manifold.  The solid absorbs the light and excites ions to the bottom of the excited manifold.  The excited ions then absorb thermal energy from the solid before they radiatively decay to the ground-state manifold levels by emitting a photon with energy hvf that is greater than the energy of the pump photon hv.  The cooling cycle is completed when the ions absorb thermal energy from the solid to repopulate the top level of the ground-state manifold.

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