Subnanosecond relaxation of optically-induced magnetization in aqueous solutions of transition-metal ions

Abstract

Ultrafast spin dynamics in the ground state of the thulium ion doped in calcium fluoride are studied by using ultrashort laser pulses. Quantum-beat free-induction decay in the subnanosecond region and ultrafast spin-lattice relaxation in the picosecond region near room temperature were observed by the polarization spectroscopy with the optical pump-probe technique. Such ultrafast spin dynamics cannot be observed by the conventional electron spin resonance. This chapter reports on optically-induced magnetization and its fast relaxation in aqueous solutions of transition-metal ions at room temperature. The magnetization is created in the ground state by a circularly-polarized pump pulse, and the spin relaxation time can be obtained from the decay curve of the magnetization. In a saturated solution without the external magnetic field, the magnetization precesses around the randomly-distributed internal field due to the surrounding spins, and the decay time of the magnetization becomes of the order of subnanoseconds. Longer decay times are expected when the longitudinal magnetic field larger than the internal field is applied, or when the concentration of the magnetic ion is lowered, where the distance between the ions becomes longer and the magnetic interaction becomes weaker. In a transverse magnetic field, the precession of the magnetization is observed as quantum beat signals. The Fourier transform of the observed signals gives the ESR spectra. This method may be called optically-induced Fourier-transform (FT) ESR spectroscopy.