Virtual states, dynamic localization, absolute negative conductance and stimulated multiphoton emission in semiconductor superlattices

We report the observation of dynamic localization, absolute negative conductance (ANG) and multiphoton stimulated emission in sequential resonant tunnelling semiconductor superlattice bow-tie antenna coupled to intense terahertz electric fields. Perhaps the most remarkable observation is that with increasing terahertz field strength the conductance near zero d.c. bias decreases towards zero and then becomes negative. The results presented here compare favourably with a model in which virtual states, familiar from nonlinear optics, take a role similar to the unperturbed quantum well states.