In heterojunctions, the strong electric field present at the interface drives the carriers vertically and therefore opens the way to several unique features observed in the optical spectra and transport experiments. In this work, we report the observation of GaAs free-exciton luminescence by exciting ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ heterojunctions below the energy of the free excitons in GaAs. This anti-Stokes photoluminescence has a long rise time and decay time. We attribute the anti-Stokes photoluminescence to the recombination of the excitons formed bimolecularly from free carriers excited from ionized donor and acceptor levels. This mechanism is enabled by the bending of the band structure across the heterointerface and yields the emission of photons at energies above the injected photons.