Basic Principles of
Optical Refrigeration
The basic idea of optical refrigeration is that illuminating some solids with laser light makes them get colder. This is of practical interest, since this mechanism has the potential for developing all-solid-state refrigerators that can efficiently cool electronics and detectors to cryogenic temperatures.
One of the simplest systems that could be used as an optical refrigerators is a 3-level “atom” in an otherwise transparent solid, as illustrated in the first figure. This figure shows the energy levels and various transitions of the atom. A ground state manifold is split into two levels labeled “1” and “2”, and there is a single excited state, “3”. Assume that the splitting between levels 1 and 2 is at most a few times the thermal energy of vibrations in the solid, kT, and the energy of level 3 is much greater than this. This ensures that the populations of the two lowest levels rapidly equilibrate with each other. In the absence of sources of excitation, level 3 would be essentially unpopulated.
If a frequency of a laser is tuned to excite the 2 to 3 transition, excited atoms in level 2 will be further excited to level 3. Subsequently, radiative decays through both the 3 to 2 and 3 to 1 transitions can occur. The mean energy of the emitted photons will be larger than that of the absorbed photons. The excess energy is extracted from the thermal energy of the vibrations (phonons) in the solid. The actual absorption of thermal energy occurs when phonons are absorbed to bring the relative populations of levels 1 and 2 back into thermal equilibrium.
In many of our experiments we use an ytterbium-doped fluoride glass (ZBLAN). Theoretical studies show that this material may be useful fro cooling down to 60 K, and experimentally, we have shown cooling at temperatures as low as 100K. The second figure shows the energy levels of the ytterbium ions in the ZBLAN host and the corresponding absorption and emission spectra. To achieved cooling with this material, the laser frequency is tuned to lift ions from the top of the ground-state manifold to the bottom of the excited manifold (step 1). Phonon absorption quickly excites the ion to higher energy (step 2). A subsequent radiative decay produces an ion with an energy in the middle of the ground state manifold (step 3). The cycle is completed when another phonon is absorbed, repopulating the upper level of the ground-state manifold (step 4). The phonon absorption steps (2 and 4) remove thermal phonons and cool the solid.
Optical refrigeration is also possible in direct-bandgap semiconductors. For these materials, the pump radiation is tuned to the minimum energy that permits the creation of electrons and holes. These electron-hole pairs are initially “cold” and sit at the lowest energy portion of the conduction and valence bands. They quickly thermalize absorbing phonons and gaining roughly kT additional energy before they recombine emitting a photon of energy greater than that of the pump radiation. Our theoretical analyses show that GaAs may cool at 10 K or colder, and experimental results from an other group showed localized cooling below 40 K.
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