Third-harmonic generation in plasmonic metasurfaces fabricated by direct femtosecond laser printing

Direct ablation-free femtosecond laser printing has been used to fabricate a metasurface in the form of ordered arrays of hollow nanobumps on the surface of a thin gold film. Resonant dips in the reflection spectra of fabricated metasurfaces, as well as a resonant increase in the third-harmonic intensity by two orders of magnitude, at the spectral matching of the observed optical resonances of the structure and the pump wavelength of the fundamental harmonic indicate that such ordered nanostructures allows the existence of high-$Q$-factor collective plasmon resonances, which are associated with the excitation and destructive interference of plasmon-polariton waves.