  Terahertz (THz) radiation can be generated by ultrafast photo-excitation of
carriers in a semiconductor partly masked by a gold surface. A simulation of
the effect taking into account the diffusion of carriers and the electric field
shows that the total net current is approximately zero and cannot account for
the THz radiation. Finite element modelling and analytic calculations indicate
that the THz emission arises because the metal inhibits the radiation from part
of the dipole population, thus creating an asymmetry and therefore a net
current. Experimental investigations confirm the simulations and show that
metal-mask dipole inhibition can be used to create THz emitters.
