Ultrafast adiabatic manipulation of slow light in a photonic crystal

We demonstrate by experiment and theory that a light pulse propagating through a Si-based photonic-crystal waveguide is adiabatically blueshifted when the refractive index of the Si is reduced on a femtosecond time scale. Thanks to the use of slow-light modes, we are able to shift a 1.3-ps pulse at telecom frequencies by 0.3 THz with an efficiency as high as 80% in a waveguide as short as 19 μm. An analytic theory reproduces the experimental data excellently, which shows that adiabatic dynamics are possible even on the femtosecond time scale as long as the external stimulus conserves the spatial symmetry of the system.