The non-local nature of free-carrier nonlinearity: theory and novel applications

One of the most fundamental nonlinear optical effects is the change of the refractive index of the material induced by light propagating in it. This effect is used in countless applications, most prominently, in modulations of semiconductor-based electro-optic components. These modulations are most frequently based on changing the free charge-carrier density. In this case, the modulation speed is limited by the natural pico-second to nano-second carrier recombination times. However, femto-second modulation times are required for many technological applications as well as for the study of various fundamental physics problems. 
Conventional approaches for shortening the carrier lifetime rely on material-science-based modifications of the material platform. In this talk I will introduce a different approach, based on novel ideas from wave physics, and on an aspect of the nonlinear response of free-carrier which was so far ignored – carrier diffusion, or the non-local nature of the free-carrier nonlinearity. We show that this effect becomes dominant when the free-carrier distribution has nano-scale features, e.g., in the case of transient Bragg gratings. 
Based on this phenomenon, I will show how we can easily achieve sub-picosecond modulation times in semiconductors and metals. I will review the complex analysis and numerics associated with this unusual regime of pulsed wave interactions and give a glimpse into the non-equilibrium dynamics of the hot charge-carriers. Finally, I will demonstrate several novel applications such as time-reversal and short pulse generation.