Figure 4: Active coupling control between outer waveguides in AE configuration.

(a) Schematic of the experimental apparatus. The ultrashort pulse (∼140 fs with central wavelength at 1,310 nm) is split into a strong truncated pump beam that is coupled to the middle waveguide and the weak signal beam that is coupled to one of the outer waveguide. On the pump path, a mechanical chopper is used to turn off and on the injection of the pump pulse to the middle waveguide. On the signal path, a delay line is used to synchronize the entrance of the pump and signal. A full description of the experimental procedure is provided in the Supplementary Method 2 (b) Observed modulation as a result of varied coupling in the presence of a synchronized pump beam, showing a significant reduction of the signal intensity when the pump is present. (c) In the presence of a delayed pump beam (330 fs delay between pump and signal), no change in the signal output is observed. This indicates that the modulation of index originates from the fast Kerr non-linearity rather than long-lived free carriers (FC) effect. A discussion regarding the FC role in this experiment is provided in the Supplementary Discussion 7. Several tests are performed to show repeatability of the process and exclude laser fluctuations (shown here as different coloured data points in b,c).