Two-color femtosecond spectroscopy of large-momentum excitons in GaAs

Summary form only given. It is usually assumed that first-order interband transitions in crystalline solids occur almost vertically in the band structure since the photon momentum is negligible compared to the reciprocal lattice vector. Especially, optical excitation and probing of free excitons in direct semiconductors is considered possible only in the center of the Brillouin zone and with photon energies corresponding to the band gap reduced by the binding energy. We demonstrate that these restrictions do not hold in general. Due to the symmetry break at any semiconductor interface, coherent excitons with large wave vectors may be excited in addition to free electron-hole pairs. Such exciton polaritons have been investigated extensively in linear and nonlinear experiments focusing on the frequency regime dose to the fundamental resonance.