Solid state optical crycoolers: Developments and prospective

Optical refrigeration (solid-state laser cooling) is based on the principle of anti-Stokes fluorescence where incident light from a coherent (low entropy) source such as laser is upconverted into high entropy fluorescence via absorption or removal of vibrational energy (phonons) [1]. This phenomenon has been termed “a laser running in reverse” as it mimics the reverse operation of a solid-state laser. Since its first experimental observation in 1995 [2], optical refrigeration in a variety of rare-earth doped glasses and crystals has been demonstrated [3]. A major milestone was achieved in 2010 by cooling a 5% Yb:YLF crystal to an absolute temperature of 155K from room temperature [4]. This represented the first demonstration of an all-solid-state cryocooler, whereby achieving lower temperatures than that of standard multi-stage Peltier coolers. Exploiting the E4-E5 electronic resonance in the Yb<;sup>3+<;/sup> Stark manifold was essential in achieving these results [4]. Subsequently, the Yb:YLF cooler was employed to cool a semiconductor load (GaAs) to 165K [5]. Most recently, we have cooled a 10% doped Yb:YLF to115K from room temperature. Figure 1(a) reflects this progress history since 1995 by depicting the lowest temperature achieved in Yb doped glasses and crystals versus year. Fig. 1(b) shows the expected cooling efficiency in the record 10% doped Yb:YLF sample as a function of excitation wavelength and temperature. The minimum achievable temperature in this sample is ~<;90K and is expected to be further lowered towards 70K by improving the purity during crystal growth [6].In this talk, we present an overview of the recent advances in optical refrigeration and outline a prospective study for practical application of this technology. Various pumping schemes (intracavity [7] and external cavity [8] enhancements), implementation of a practical thermal link, and various techniques for improving the overall efficiency will be dis- ussed. As the only all-solid-state cryocooling technology with no moving parts (vibration free), and immune from electromagnetic interference, it can boost numerous applications ranging from spaceborne sensors to high-T_c superconducting electronics prevalent, for example, in the medical devices.