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 $19\mu$m. 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.

Figure 1

Experimental principle. While a probe pulse traverses a PCW, the absorption of a pump pulse leads to a sudden blueshift of the eigenmodes of the PCW. We detect the probe light at the PCW exit as a function of the delay $\tau$ between pump and probe pulse.

Figure 2

Characterization of the time-varying PCW. (a) Calculated dispersion relation ${\omega }_k$ for two changes $\Delta n$ in the Si refractive index. (b) Measured intensity spectrum $|E_{\mathrm{i}}(\omega )|{}^2$ of the broadband input probe pulse and the resulting output spectra $|E_{\mathrm{o}}(\omega ,\tau )|{}^2$ at 4 ps before and 1 ps after arrival of the pump pulse. (c) Measured group delay of the output pulses of (b) together with curves $L/\partial {}_k{\omega }_k$ as calculated from (a).

Figure 3

Ultrafast adiabatic frequency shifting. (a) Experimentally determined and (b) calculated intensity spectra of the PCW response to a 1.3-ps-long input pulse whose center frequency is indicated by the arrow in Fig. 2b. (c) Measured and calculated output spectra at $\tau =-4$ ps and −1 ps. (d)–(f) Same as (a)–(c) but for a 2.3-ps input pulse.

Figure 4

Model schematic of the experimental situation. The pointlike probe laser with linear polarization ${\mathit{e}}_{\mathrm{i}}$ and a detector are placed next to the input and output facet of the waveguide at positions $\mathit{x}={\mathit{x}}_{\mathrm{i}}$ and ${\mathit{x}}_{\mathrm{o}}$.