Dynamic Nuclear Polarization and Nuclear-Induced Frequency Focusing in Gallium Arsenide under periodic optical electron spin pumping

Periodic optically pumped electron spins in gallium arsenide polarize the nuclear spin system by a process called dynamic nuclear polarization. These polarized nuclei will create an effective magnetic field that acts on the electron spin system and affects the electron system such that the Larmor precession frequency will synchronize to discrete values; this effect is known as nuclear-induced frequency focusing. These discrete frequencies correspond to integer or half-integer multiples of the optical pulse repetition frequency. The observation of nuclear-induced frequency focusing and its dependence on pump wavelength is corroborated by numerical calculations using a model incorporating optical orientation and the optical Stark effect (Phys. Rev. B 101, 205203 (2020)). This model can be used to predict the time-dependent frequency focusing behavior of the nuclear spin system.