Terahertz microscopy assisted by semiconductor nonlinearities.

Terahertz (THz) imaging is currently based on linear effects, but there is great interest on how nonlinear effects induced by terahertz radiation could be exploited to provide extra information that is unobtainable by conventional imaging schemes. In particular, at field strengths on the order of 100  kV cm-1 to 1  MV cm-1, transmission properties inside semiconductor materials are largely affected at the picosecond time-scale, which raise the prospect of interesting nonlinear imaging applications at THz frequencies. Here, we experimentally investigate a method to map the two-dimensional nonlinear near-field distribution of an intense THz pulse passing through a thin film-doped semiconductor. By inserting a metamaterial structure between the electro-optic sensor and the doped film, the nonlinear near-field dynamics shows a different and enhanced contrast of the sample when compared to its linear counterpart.