Fig. 4.

Terahertz generation from nonlinear metamaterial photonic crystals (NLMPC). a Structure of the fabricated NLMPC. Arrows indicate the orientation of the split-ring resonators, similarly to Fig. 3a. b Reflection image of the fabricated NLMPC. c Simulation of the spatiotemporal profile of the THz emission pattern from the NLMPC in a when excited by the near-infrared pulse at normal incidence. d Frequency domain spatial profile of c. Dashed lines mark the Raman–Nath diffraction orders. e Measured THz emission pattern from NLMPC fabricated according to a, with varying incidence angles, in the time domain and f in the frequency domain. Dashed lines indicate calculated Raman–Nath relation for different diffraction orders