Plasmonic Metasurfaces as Broadband Saturable Absorbers for Ultrafast Fiber Laser

As artificial materials composed of subwavelength structures, metasurfaces have a strong ability to control linear and nonlinear light fields, which greatly promotes the development of nanophotonics. Recently, plasmonic metasurfaces have been proven to behave as saturable absorbers (SAs) with a modulation performance much higher than that of other SAs, exhibiting excellent nonlinear polarization transfer functions. However, due to the polarization dependence of plasmonic resonance, the working bandwidth of metasurface saturated absorbers is usually very narrow, which is not favorable for the generation of wideband ultrafast laser. Here, we propose a silver double-nanorod plasmonic metasurface for stable broadband saturated absorption, which is attributed to the unique gap resonance mode of the double-rod structure. The dipole resonance on the carefully arranged silver nanorods and gap mode between the nanorod pair are simultaneously excited by the pump light, improving the response bandwidth of the metasurface saturable absorber. By inserting the metasurface into fiber laser cavity, stable pulse sequences are obtained operating at 1.55 and 1.064 μm, respectively. Our work not only further releases the potential of metasurfaces in the field of ultrafast lasers but also provides new ideas for the design of broadband nonlinear devices.