Intense terahertz emission from a -plane InN surface

We report a significant enhancement in terahertz emission from the indium nitride (InN) films grown along the a axis ( a -plane InN), relative to the InN films grown along the c axis. The primary radiation mechanism of the a -plane InN film is found to be due to the acceleration of photoexcited carriers under the polarization-induced in-plane electric field perpendicular to the a axis, which effectively enhances the geometrical coupling of the radiation out of semiconductor. In addition, azimuthal angle dependence measurement shows that the p -polarized terahertz output consists of a large angularly independent component and a weak component with a distinctive fourfold rotation symmetry.