Methods for generating and controlling terahertz radiation

An apparatus for generating multicycle pulses of terahertz (THz) radiation, the apparatus comprising:

• a substrate;
• a nonlinear metasurface (NLM) comprising a planar array of subwavelength antennas formed on the substrate having rotational symmetry, Cn, of odd order n and rotational symmetry 2TT/n, which are excitable by pump radiation having wavelengths that are substantially larger than characteristics dimensions of the subwavelength antenna to radiate THz radiation;
• the array having a directrix and a plurality of adjacent sections, including first and second sections separated by third and fourth adjacent sections, each section comprising a plurality of subwavelength antennas, wherein:
• the subwavelength antennas in the first and second sections are characterized by first and second spatial patterns for which angular orientation of the subwavelength antennas in the first and second sections exhibit a substantially continuous change in angular orientation as a function of position along the directrix and for which the second spatial pattern is transformable into the first spatial pattern by a 180° rotation about a normal to the plane of the planar array;
• and
• the subwavelength antennas in the third and fourth sections respectively exhibit third and fourth spatial patterns transformable one to the other by a 180° rotation about a normal to the plane of the planar array.