Optical second-order susceptibility of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As asymmetric coupled-quantum-well structures in the exciton region

We have measured the dispersion of ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As asymmetric coupled-quantum-well (ACQW) structures in the region of room-temperature excitonic resonances. The spectrum of ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ has sharp and strong peaks including the transitions which are allowed by selection rule in a symmetric quantum well (SQW) as well as the transition which is forbidden in a SQW. The peak value of ${\mathrm{\chi }}_{14}^{\mathrm{}\left(2\right)\mathrm{}}$(2ω) of the ACQW which we measured is ten times larger than that of bulk GaAs.