ELECTRIC AND MAGNETIC DIPOLE 2-PHOTON ABSORPTION IN SEMICONDUCTORS

Nonlinear spectroscopy of the exciton fine structure in GaAs has allow ed us to measure simultaneously both magnetic dipole two-photon absorp tion and electric dipole one-photon absorption. The splitting of the e xcitonic ground state into the spin-allowed orthoexciton and the spin- forbidden paraexciton gives the electron-hole exchange energy directly . The peak assignment is proved by nonlinear magnetoabsorption. Utiliz ing electric dipole two-photon magnetoabsorption within the same exper iment, a rich fine structure of the 2P exciton has been observed that has not been previously resolved.