Electron Spin Dynamics Of Rare-Earth Ce³⁺ Ions In YAG Crystals

The electron spin is applicable in quantum information storage and processing.Rare-earth ion doped solids are excellent optical materials with abundant energy-level transitions.Among the many rare-earth ions,trivalent cerium ions Ce3+-doped crystal matericals have good electron spin properties,including long spin lifetime and spin decoherence time.Aslo,it is easy to establish and measure the spin using all-optical methods.In this dissertation,the electron spin dynamics of Ce3+ ions in Ce3+:YAG crystals are investigated in various temperatures and magnetic fields by transient spectroscopy techniques.The main research contents and innovative results are summarized as follows:1.Under the excitation of a circularly polarized continuous-wave laser,the electron spin in the sample will be dynamically polarized till to a steady state because of the polarization-selective excitation rule.Periodically alternatin g?+ and ?-circularly polarized continuous-wave laser,the electron spin polarization is periodically switched from one steady state to another.By solving the rate equations,the information of electron spin polarization and its evolution in the ground and excited states can be extracted.An eight-level model which is suitable to analyze the spin information for the 4f and 5d states of Ce3+ ions is theoretically established for the first time.The correlated expressions between the spin information for the ground/excited states and the photoluminescence(PL)signals are obtained.The information for the ground and excited states such as electron spin polarization,formation time of the steady state and spin lifetime can be directly evaluated from the PL transients.2.A measurement system for the time-resolved PL spectroscopy under continuous-wave laser excitation is built.The electron spin dynamics of Ce3+ ions in YAG crystals are investigated based on the above-mentioned eight-level model theory.The magnetic field dependences of the electron spin polarizations for the 4f and 5d states are obtained by analyzing the PL transients.The study shows that:The hyperfine coupling strength for the 4f electrons is?4 times weaker than that for the 5d electrons.A longitudinal magnetic field can effectively suppress the electron spin relaxation due to hyperfine interaction.After suppressing the hyperfine-induced spin relaxation,the electron spin polarization is increased by 16 times for the 4f electrons and 4 times for the 5d electrons.The spin-lattice relaxation time for the 4f electrons is 2.1 ms at 5 K.For the 5d electrons,the dispersion of the nuclear hyperfine field distribution ?B is nearly isotropic and equal to?4.0 mT.In comparision,for the 4f electrons,the dispersion of the nuclear hyperfine field distribution in the three crystallographic directions has:?B[001]=0.8 mT,?B[110]=1.1 mT,?B[110]=2.4 mT.3.The electron spin dynamics of Ce3+ ion ensembles in YAG crystals at room temperature are investigated by time-resolved Faraday rotation spectroscopy for the first time.The research results show that the 5d electron spin dephasing time is as long as 2.5 ns at room temperature.For different magnetic fields,the spin dephasing time has nearly a constant value of 2.5 ns,showing the nearly unaffected properties by inhomogeneous broadening,which is very favorable for the practical spin applications.The electron spin coherence dynamics in high transverse magnetic fields show a beating-like amplitude modulation due to the existence of several slightly different Larmor frequencies,which correspond to different magnetically inequivalent positions of the Ce3+ ions in the crystal lattice.Hyperfine coupling between the electron of Ce3+ ions and Al nuclear spins in the YAG lattice dominates the electron spin relaxation.A longitudinal magnetic field as small as 10 mT can efficiently suppress the hyperfine-coupling-induced electron spin relaxation.