  Emerging technology based on artificial materials containing metallic
structures has raised the prospect for unprecedented control of terahertz waves
through components like filters, absorbers and polarizers. The functionality of
these devices is static by the very nature of their metallic or polaritonic
composition, although some degree of tunability can be achieved by
incorporating electrically biased semiconductors. Here, we demonstrate a
photonic structure by projecting the optical image of a metal mask onto a thin
GaAs substrate using a femtosecond pulsed laser source. We show that the
resulting high-contrast pattern of photo- excited carriers can create
diffractive elements operating in transmission. With the metal mask replaced by
a digital micromirror device, our photo-imprinted photonic structures provide a
route to terahertz components with reconfigurable functionality.
