The main problem in laser cooling investigation is to choose materials.One of the keys is the energy width split out between ground state and excited state of luminescence center.Choosing the energy space reasonably will help us to determine laser-cooling media correctly.The energy gap of two states determines two important elements in laser cooling

quantum efficiency and multiphonon relaxation rate.Only as for cooling power in laser cooling

the more the energy gap splits

the higher quantum efficiency is and the more powerful is to fluorescent cooling.When the energy gap reaches a certain width or more

the cooling power will almost keep unchanging.If heat-light converting efficiency is considered in laser cooling

the energy gap in reason should be much narrower.If we confirm that it is the most important to keep cooling power at the highest level

then

it is more appropriate to choose the energy gap at 5000cm

-1

than 10000cm

-1

of Yb

3+

because

the choosing can not only confirm higher heat-light converting efficiency

but also ensure in substance a high cooling power.