Observation of ultrafast spin-lattice relaxation in Tm2+-doped CaF2 and SrF2 crystals by optical means

Ultrafast spin-lattice relaxation in the ground state of Tm2+ doped in CaF2 and SrF2 crystals is studied. The magnetization is created and detected by optical pulses, and spin-lattice relaxation times of the order of picoseco nds or femtoseconds, which cannot be measured by the conventional electron spin resonance, can be obtained from the decay curves of the magnetization. It was found that the temperature dependences of the spin-lattice relaxati on rate 1/T-1 near room temperature are deviated from the T-9 dependence, w hich is expected from the well-known Raman process of phonons. Considering the effect of the Debye temperature ED of the host crystal on the Raman pro cess, the observed temperature dependences of 1/T-1 are explained consisten tly both for Tm2+:CaF2 (theta(D)= 513 K) and Tm2+:SrF2 (theta(D)= 378 K).