Light-induced terahertz optical activity.

We propose and demonstrate a simple method to induce optical activity in the terahertz (THz) region using photoexcited carriers in a semiconductor substrate with metal two-dimensional chiral masks, which does not show optical activity without photoexcitation. The three-dimensional chirality induced by the combination of photocarriers and metal mask gives rise to the optical activity. With this simple method, we can develop THz polarization modulation techniques applicable for biology, chemistry, and material sciences.