Novel mechanisms of optical harmonic generation on excitons in semiconductors

Nonlinear optics of semiconductors is an important field of fundamental and applied research, but surprisingly the role of excitons in the coherent processes leading to harmonics generation has remained essentially unexplored. Here we report on some examples of experimental and theoretical study of the second- (SHG) and third (THG) harmonic generation involving the exciton resonances in the noncentrosymmetric semiconductors ZnO and GaAs. External electric and magnetic fields are used to modify exciton states and their nonlinear optical properties. Depending on the particular symmetry of the exciton states SHG/THG can originate from the electric- and magnetic-field-induced perturbations of the excitons due to the Stark effect, the spin as well as orbital Zeeman effects, or the magneto-Stark effect. A microscopic theory of SHG and THG on excitons is developed, which shows that the nonlinear interaction of coherent light with excitons has to be considered beyond the electric-dipole approximation.