Pore formation on n-type GaAs(100) under anodic polarization in 1 M HCl has been studied. Focused ion beam implantation of Si2+ into GaAs was used to write defined surface damage/implant patterns into the substrate. These implant sites represent initiation sites for pore growth and show pore formation at potentials significantly cathodic to the intact surface. Hence, pore growth within the patterns can be achieved selectively if anodic polarization is kept below the pore formation potential of the unimplanted surface. The results show that both the polarization potential and the implantation dose strongly influence the morphology and the photoluminescence behavior of the porous structures. Monte Carlo simulations of the implantation process revealed that the morphology of the etch process and its kinetics are both strongly connected to the implant/damage profile. Furthermore, it is demonstrated that "green"-light-emitting porous GaAs lift-off layers can be produced.