Precise Determination of Minimum Achievable Temperature for Solid-State   Optical Refrigeration
  We measure the minimum achievable temperature (MAT) as a function of excitation wavelength in anti-Stokes fluorescence cooling of high purity Yb3+-doped LiYF4 (Yb:YLF) crystal. Such measurements were obtained by developing a sensitive noncontact thermometry that is based on a two-band differential luminescence spectroscopy using balanced photo-detectors. These measurements are in excellent agreement with the prediction of the laser cooling model and identify MAT of 110 K at 1020 nm, corresponding to E4-E5 Stark manifold transition in Yb:YLF crystal. 
Denis V. Seletskiy, Seth D. Melgaard, Richard I. Epstein, Alberto Di   Lieto, Mauro Tonelli and Mansoor Sheik-Bahae
cond-mat.mtrl-sci physics.atom-ph