Differential absorption saturation in laser cooled Yb:LiYF4

The cooling efficiency for optical refrigeration by anti-Stokes Fluorescence was measured in a 10% Yb 3+ :LiYF 4 crystal by Differential Luminescence Thermometry (DLT) and Thermal Lens Spectroscopy (TLS) over a range of pump intensities. Results showed that background and coolant ion absorption coefficients saturated at low intensities, and saturated differently as a function of wavelength. For wavelengths longer than 1000 nm, a higher pump intensity led to improved cooling efficiency through a differential saturation phenomenon. In this range, background absorption saturated more easily than the absorption of coolant ions so that parasitic heating was diminished, yielding higher cooling efficiency for higher pump intensities. This saturation phenomenon therefore offers the prospect of improved performance of electronic devices such as imaging arrays in space which could operate with reduced noise at low temperatures through laser cooling. The measured cooling efficiency was independent of pump intensity at a wavelength of 1000 nm where the saturation intensity of background absorption was found to be the same as that of Ytterbium in LiYF 4 . A novel experimental method for determining the saturation intensities of impurity and coolant ions is described, and consequences for laser cooling and radiation-balanced laser operation of the observed saturation effects are discussed and analyzed.