As of today, one of the milestones in quantum theory testing is obtaining of macroscopic quantum states, for which very low temperatures are necessary. Such low energies can be present during optical and optomechanical cooling of nanostructures. Here we investigate the deep cooling of ytterbium-doped fluorite nanocrystal via coherent population transfer techniques. We consider two main approaches towards coherent transfer, namely, Raman pulses and stimulated Raman adiabatic passage, and search for the most efficient cooling parameters. Optimization of the process of deep nanocrystal cooling opens up possibilities for various applications and technologies.
Methods of coherent pumping through dipole-allowed 5<i>d</i> levels of RE ion are proposed for laser cooling. The coherent and complete population transfer between the ground and the first excited levels of 4<i>f </i>multiplet is achieved by using the different Raman techniques, namely two-photon scattering, adiabatic passage method, and π-pulse pumping. It is shown that the multiplication of the number of electrons that participate in cooling cycle leads to increasing of the cooling power and to acceleration of the cooling process. The increasing of cooling efficiency of 0.5% compared to the direct pumping between 4<i>f</i> levels is attained through the use of dipole-allowed optical transitions. Performed estimates show that the sample temperature can achieve 94 K for current purity materials. The calculations are obtained for Yb<sup>3+</sup>:CaF<sub>2</sub> system.