We report type I second-harmonic generation by use of first-order quasi-phase matching in a GaAs/AlAs symmetric superlattice structure with femtosecond fundamental pulses at 1.55 μm. Periodic spatial modulation of the bulklike second-order susceptibility χzxy (2) achieved with quantum-well intermixing for which the group III vacancies were created by As+ -ion implantation. A narrow second-harmonic bandwidth of ∼0.9 nm (FWHM) with an average power of ∼1.5 μW was detected, corresponding to an internal conversion efficiency of ∼0.06%, which was considerably limited by the spectral bandwidth of the fundamental.