Plasmonic metasurfaces: toward the mid-infrared <i>Q</i> -switched mode-locking operation in a simple linear-cavity laser

Plasmonic metasurfaces, featuring broadband nonlinear optical responses and exceptional light-field manipulation capabilities, facilitate the passive Q -switching and mode-locking operation of a laser resonator as a saturable absorber (SA) and thus enable the generation of short- and ultrashort pulses. Here, for the first time, to our knowledge, we successfully fabricate a transmission-type mid-infrared plasmonic metasurface SA composed of gold nanostructures on the fluoride substrate, leveraging localized resonance enhancement. Its nonlinear optical (NLO) properties are characterized by a home-made ∼2.78 μm pulsed Z-scan system, demonstrating a remarkable modulation depth of up to 30%. By integrating the plasmonic metasurface SA into a simple linear-cavity Er:ZBLAN fiber laser, stable Q -switched mode-locking (QML) operation is achieved with a maximum average output power of 224 mW at 2.78 μm. The output exhibits a Q -switched envelope of 768 ns containing mode-locking pulses with 51.42 ps duration at the fundamental repetition rate of 24.5 MHz. This work further expands the application of plasmonic metasurface SAs to the mid-IR wavelength region, by utilizing their enhanced NLO responses and tailored spectral characteristics, enabling them as a core component for next-generation mid-IR ultrafast lasers.