  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.
