Laser cooling of solids

Abstract

Laser cooling of solids, sometimes also known as optical refrigeration, is a fast developing area of optical science, investigating the interaction of light with condensed matter. Apart from being of fundamental scientific interest, this topic addresses a very important practical issue: design and construction of laser pumped solid-state cryocoolers, which are compact, free from mechanical vibrations, moving parts, fluids and can cause only low electromagnetic interference in the cooled area. The optical cryocooler has a broad area of applications such as in the development of magnetometers for geophysical sensors, in biomedical sensing and can be beneficial for satellite instrumentations and small sensors, where compactness and the lack of vibrations are very important. Simply, a laser cooler works on the conversion of low energy pump photons into high-energy anti-Stokes fluorescence photons by extracting some of the phonons (heat energy) in a material. That is, the process of laser cooling of solids is based on anti-Stokes fluorescence also known as luminescence upconversion, when light quanta in the red tail of the absorption spectrum are absorbed from a pump laser, and blue-shifted photons are spontaneously emitted. The extra energy extracted from the solid-state lattice in the form of the phonons is the quanta of vibrational energy which generates heat. The idea to cool solids with anti-Stokes fluorescence was proposed in 1929 by Peter Pringsheim and first demonstrated experimentally by Epstein's research team in 1995. In 1999, Steven Bowman proposed to use the optical refrigeration by anti-Stokes fluorescence within the laser medium to balance the heat generated by the Stokes shifted stimulated emission in a high-power solid-state bulk laser. Such a laser without internal heating named radiation-balanced or athermal laser was experimentally demonstrated for the first time in 2002. At the present time laser cooling of solids can be largely divided into three main areas: laser cooling of rare-earth doped solids, laser cooling in semiconductors and radiation-balanced lasers. All three areas are very interesting and important and will be considered in this paper.

Heat meets light on the nanoscale

“…However, as the chemical potential increases, the entropy of radiation decreases and tends to zero when the lasing condition (µγ → ω) is reached [9,28]. The low entropic content of lasers and other coherent sources makes possible optical refrigeration, that is, anti-Stokes fluorescent cooling [50][51][52], which is discussed in more detail in section 3.2. Figure 2a also shows that high-temperature emission is characterized by lower entropy content than the low-temperature one for any value of the chemical potential of photons.…”

“…The photon up-conversion via anti-Stokes luminescence instead makes use of a non-equilibrium photon emission as a result of radiative recombination of electron-hole pairs assisted by electron-phonon scattering, as schematically shown in Figure 9a. This process results in the emission of photons carrying chemical potential (see section 1.1) and with energies that are higher than the energy of photons in the optical pump, which forms a basis for optical refrigeration (also called laser refrigeration or anti-Stokes fluorescent cooling) [50][51][52]. Optical refrigeration can be achieved by irradiating a luminescent material at a frequency on the low energy tail of its absorption band, which is followed by spontaneous emission of more energetic photons in the process of the anti-Stokes luminescence (Figure 9b).…”

“…Landau helped to resolve the controversy with the second law violation by proper accounting for the entropy of light emitted in the process of spontaneous isotropic antiStokes fluorescence [6] (see section 1.2). In turn, progress in materials engineering yielded materials with high internal quantum efficiency, which enabled successful experimental demonstrations of the optical refrigeration, mostly of rare-Earth-doped grasses [42,51,52], starting with the experiments of Epstein and co-authors [42]. The efficiency of the laser cooling process scales with the material absorption efficiency η abs and the external quantum effi-…”

Cooling profiles of laser induced temperature fields for superconducting vanadium nitrate products

“…C p is the specific heat of at constant pressure (J kg −1 K −1 ), C is the species concentration in the fluid/solid (kg m −3 ), k is the thermal conductivity (W m −1 K −1 ), T is the temperature profile (K), D is the mass diffusion coefficient (m 2 s −1 ), The initial boundary shown above is expected to illustrate the crucial order-parameter phase of the two dissimilar films via the application of the anti-Stokes fluorescence for superconducting material [19]. The resolution of Eqs.…”

Laser cooling of solids containing local centers with electric dipole allowed transitions: a feasibility study

“…Since the first successful experimental results were obtained with Yb 3+ -doped glass [1], many exciting achievements were reported: laser cooling was observed in different materials including rare earth doped insulating glasses and crystals, organic dye solutions [2], and semiconductors [3,4]; see [5][6][7][8] for reviews. The most successful results were obtained with 2 F 7/2 -2 F 5/2 transitions of Yb 3+ ions in insulating glasses and crystals.…”