ABSTRACT
Optical cooling of solids, based on anti-Stokes fluorescence, has tremendously advanced in the last years. Since its first experimental demonstration in 1995 [5], the temperature drops achieved have dramatically increased from a few degrees up to more than 150. Operations in the cryogenic regime have been reached in ytterbium (Yb)-doped fluoride single crystals [13, 14, 21]. To date, the lowest temperature achieved is 93 K [12] and novel schemes based on energy transfer processes prospect significant enhancements
of intrinsic efficiencies, pushing down the minimum achievable temperatures. Truly vibration-free, cryogenic laser cooling is a much sought after solution for several space-based technologies. The use of mechanical coolers indeed pushes to complex system solutions in order to minimize microphonic vibrations, which limit the detectors’ performances. Accessing temperatures well below the thermoelectric coolers’ (TECs) cutoff (170 K), the implementation of cryogenic optical coolers would provide an ideal solution for cooling down detectors in the temperature range below 170 K. Moreover, optical coolers would offer an efficient alternative to TECs in the higher temperature range, below 200 K, where TECs exhibit limited wall-plug efficiency. Currently, passive radiators, relying on thermal radiative emission into deep space, provide the main vibrationfree solution for cooling down detectors below 170 K, down to 50 K. The use of these devices, however, imposes several orientation constraints and shielding requirements to build up the refrigeration structure. Furthermore large emitting surfaces are necessary to achieve low temperatures and relatively high cooling powers. The radiated power, and hence the available cooling power, is directly proportional to the panel surface as well as to the fourth power of the temperature to reach. Consequently, due to size limitations, the performances rapidly decrease as the temperature to achieve drops, especially below 100 K. The optical cooling technology offers, instead, advantages of extreme compactness and cooling efficiencies almost independent of dimensions, enabling significant size and weight reduction. In addition to that, optical cooling allows for simpler designs, without orientation and shielding constraints, and possesses a much higher potential for intrinsic efficiency enhancement through the development of novel materials. Additional advantages for space applications, offered by the optical cooling technology, are a long lifetime (only limited by the lifetime of the pumping laser), low electromagnetic interference, low sensitivity to magnetic fields, and extremely high reliability. All these features intensely motivated the research toward device implementation as well as investigation in material development to uplift the intrinsic efficiency of cooling crystals, which are currently of the order of a few percent from room temperature. Various techniques have been exploited and many are under investigation. Laser cavities were introduced to enhance the absorption of
incident laser photons and so the available cooling power at low temperatures [12-14, 21]. Suitable configurations were exploited to reduce parasitic heat loads. Novel alternative schemes based on photon localization effects in nanocrystalline powders were recently suggested to promote the input absorption [19]. The superirradiance regime was proposed to increase the cooling process rate [15]. While several methods have been proposed and can be employed to improve the cooling performances, energy transfer-assisted anti-Stokes processes in co-doped systems have been recently demonstrated to provide a route to enhance the intrinsic cooling efficiency of active materials. The first experimental evidence was achieved with Yb-Tm co-doped systems. Efficiency enhancements, over the single Yb doping performances, have been demonstrated via co-doped YLF single crystals [24]. Preliminary results achieved in our laboratory are presented in the following section. This chapter reports on cooling efficiency measurements performed on various Yb-doped fluoride single crystals grown in our laboratory. The investigation of the cooling process for increasing Yb concentrations in LiYF4 (YLF) single crystals resulted in improved cooling efficiency via significant decrease of detrimental parasitic absorption. Energy transfer processes between Yb ions and low concentrations of rare earth ion impurities were investigated to analyze the effect of detrimental impurities that participate in multiphonon relaxation of excited ions. Studies on Yb-Tm-controlled co-doping led to the first observations of energy transfer-enhanced Yb anti-Stokes efficiency [24]. Preliminary results achieved with YbTm co-doped samples are presented. The following section introduces the anti-Stokes efficiency model for rare earth-doped materials. Experimental results and a description of experimental apparatus and instruments are reported in Sections 8.3-8.5.
