Direct observation of terahertz wavefront converted by a metal hole array

Wavefront conversion by a metal hole array (MHA) has been experimentally observed via a visualization of the continuous-wave terahertz (THz) field. At the resonant frequency of the MHA, curved wavefront of the THz wave (125 GHz) emitted from a horn antenna has been converted to a plane wavefront. Near field visualization at the MHA surface (transmission side) revealed that the field re-emitted from the MHA oscillates in phase. We experimentally confirmed that the MHA acts as a planar THz wave collimator.