Monolithically integrated nonlinear-linear metasurface for Fano-resonance-enhanced terahertz generation and in situ broadband achromatic manipulation.

Compact integrated terahertz (THz) systems require direct coupling between THz generation and wavefront manipulation. However, in most existing THz platforms, these two functions are still implemented separately. This study proposes and validates a monolithically integrated nonlinear-linear metasurface platform that combines Fano-resonance-enhanced THz generation with in situ broadband achromatic focusing. The nonlinear generation unit is based on an asymmetric plasmonic gold nanorod metasurface, where Fano-resonance-enhanced pump absorption and localized electromagnetic field confinement enable the generation of a broadband THz signal centered near 3 THz under infrared excitation. For broadband in situ wavefront manipulation, an all-dielectric silicon metasurface is integrated to achieve achromatic focusing across the 2.8-3.2 THz band. The integrated device maintains the focal position deviation within 10% of the designed focal length, with a focusing efficiency of approximately 40% over the operating band. This work provides a compact and scalable strategy for integrating nonlinear THz sources with dispersion-engineered wavefront manipulation, offering new opportunities for on-chip THz photonic systems, imaging, and spectroscopy.