We report the use of intermixing techniques to modify GaAs/AlGaAs multiple quantum wells (MQWs). A large shift in the response wavelength of the GaAs/AlGaAs MQW-based infrared photodetector is obtained by proton implantation and then a standard annealing procedure (950 °C for 30 s). The photoluminescence (PL) and photoresponse spectra were measured as functions of ion dose in the range from 5×1014 to 2.5×1015 cm-3. The peak photoresponse wavelength was tunable between 8.2 and 9.8 ?m for the infrared radiation and the energy position of the PL peak from the MQW material changed from 1.62 to 1.645 eV. The effects of the ion implantation and thermal annealing on the device performance have been well characterized theoretically by the inter-diffusion of Al atoms across the GaAs/AlGaAs heterointerfaces and the relaxation energy of free carriers.