We note that neither changing the bias between 0 to 0.5 V (Fig. ) nor altering the laser intensity with optical density filters between 0.08 and 0.54 (Fig. ) has a significant effect on the 3dB bandwidth, again suggesting that all of these measurements fall into the set-up limit. In principle, further reduction of the channel length below 1 µm (Fig. ) and channel width to decrease the resistance and capacitance, respectively, could be easily implemented with standard electron beam lithography. For instance, an easily feasible reduction of the channel width from 20 µm to 1 µm would decrease the capacitance by a factor of 20, cf. Figure . However, we neglect such further optimisations at this point due to the speed-limitations of our set-up.