The intensity of the resonant emission of the
1s orthoexcitons in
Cu2O is observed in a weak magnetic field under two-photon resonant excitation at
1.6K to investigate the quantum interference within the Zeeman sublevels in the Faraday configuration. A specific decrease of $\ensuremath{\sim}15%$ in the emission intensity is observed in the region of $\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. This structure is interpreted as a manifestation of the quantum interference effect between the Zeeman sublevels of the
1s orthoexcitons, known as the Hanle effect in atomic spectroscopy. The homogeneous width of the exciton level is estimated to be $5.5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{eV}$. The contribution of the quadrupole exciton polariton is discussed by taking into account the geometry-dependent characteristics of the exciton-polariton on the direction of the magnetic field. The Hanle structure observed is attributed to the interference of the polariton wave packets of the longitudinal excitons, where the phase coherence is transferred via transverse excitons driven by resonant two-photon coupling with the radiation field.
Two-photon absorption spectroscopy was performed in GaAs quantum wells with photon energy \ensuremath{\Elzxh}\ensuremath{\omega} near half the band gap
Eg. For the case of incident light polarized perpendicular to the confinement direction \ensuremath{\epsilon}^\ensuremath{\perp}z^, the onset of two-photon absorption occurs at the 2P state of the lowest-energy heavy-hole exciton and the absorption increases linearly with 2\ensuremath{\Elzxh}\ensuremath{\omega}-
Eg. For \ensuremath{\epsilon}^\ensuremath{\parallel}z^, the two-photon spectrum is stepped with superpositions of sharp 1S exciton features at odd \ensuremath{\Delta}N light-hole--conduction-band transitions. These results are used to accurately determine exciton binding energy, successive subband energy splittings, and band offsets.