Ultrafast Mode Switching of Metamaterials Driven by Intense THz Field-Induced Impact Ionization

Specifically designed resonant THz nano-structures absorb and locally enhance an incident THz pulse, thus, generating fields high enough to cause nonlinear material responses and thus give access to study nonlinear transport phenomena or to realize ultrafast electronic devices, such as efficient photovoltaic nano-cells, electroluminescent emitters, or highly sensitive photon detectors. Here, we study the ultrafast mode switching of metallic split ring resonators (SRRs) on semiconductor substrates, as shown in Fig. 1(a) . Mode switching occurs when the substrate underneath the gap region changes conductivity. For semiconductors, the electric field-induced conductivity is attributed to carrier generation via different mechanisms, e.g., inter-valley scattering (IVS) or impact ionization (IMI) [1] . However, mechanism and time scale by which mode switching occurs is still debated, despite several successful studies conducted to understand the underlying physics of field-induced nonlinear substrate behavior [1] - [2] . In the present work, by using semiconductor substrates with different bandgaps, we first show that impact ionization is indeed the most relevant carrier generation process. Second, we establish a mode switching time of 200 fs.