Nonlinear Metasurface Fresnel Zone Plates for Terahertz Generation and Manipulation

We introduce a nanoengineered nonlinear metasurface based optical element that acts as an emitting Fresnel zone plate of terahertz (THz) waves. We show that the nonlinear zone plate generates broadband THz radiation and focuses each generated frequency on a different focal point along the optical axis. Therefore, a narrow beam waist and spectral selectivity of both the bandwidth and central frequency are achieved. Furthermore, we measure and analyze the temporal structure of the focused THz electric field and show that it comprises of few cycles with an axially varying carrier frequency in agreement with the calculated dispersion of the zone plate. This demonstration of controlled emission and focusing of THz waves opens the door for the development of a wide variety of additional holographic metasurface-based THz emitters and can lead to the development of efficient, active, integrated, and ultracompact optical devices for the THz spectral region.