Light-induced non-resonant spin reversal in quantum dot and in quantum ring

Nanostructures like quantum dots and quantum rings subjected to non-resonant laser beam of circular polarization and moderate intensity exhibit characteristic changes in the orbital and in the spin degrees of freedom of the confined electrons. The coupling of the charge density to the electric field component of the light modifies the energy spectrum and, in the presence of spin-orbit interaction, may lead to high polarization of the electron spin. These observations were inferred from the analysis of the Floquet states which are the approximated stationary eigensolutions of the time-periodic Hamiltonian. Here, we study this problem solving numerically the time-dependent Schr"odinger equation for the electron confined in quantum dot and quantum ring and taking into account both, the light-matter coupling and its influence on spin-orbital degrees of freedom. Our results show that the adiabatically switched light pulse would gently settle the system in one of the Floquet states with the reversed spin as compared to the initial state. The reversibility of this process is observed only for moderate intensities of light, however, this requires of about 10³ optical cycles which enormously elongates the spin reversal protocol. Therefore we next have looked for the time-optimized shapes of the light pulse and we found that the duration of the non-resonant pulse can be shorten to below 10² optical cycles (corresponding to several pico seconds). Such optimized pulse can stimulate the selective transition between two electron states which are highly polarized in opposite direction. We show that switching between these states can be realized with fidelity exceeding 99.9%.