All-optical modulators at 10 mu m based on photo-induced intersub-band absorption in multiquantum wells can exhibit very large extinction ratios. However, the frequency response of such modulators is imposed by the electronic band-to-band recombination processes which usually limit the bandwidth to a few tens of megahertz. The authors show that the modulation bandwidth can be substantially improved by proton bombardment of the multiquantum wells. The results of experiments on a 250 period GaAs/Al0.25Ga0.75As quantum well structure agree closely with calculations. Modulation bandwidths larger than 200 MHz appear feasible with implantation doses less than 1013cm-2.