Epsilon-near-zero material assisted metasurface saturable absorber for ultrafast Yb-doped fiber lasers

The recent progress in semiconductor processing technology has served as a major driving force behind the rapid development of nanophotonics research. Among the emerging applications, metasurfaces have attracted significant attention as saturable absorbers (SAs) for mode-locked laser systems that generate short pulses. In this study, we present the comprehensive design, fabrication, and experimental demonstration of a metasurface-based SA operating in the 1 μm wavelength range, leveraging both the localized surface plasmon resonance (LSPR) phenomenon and the epsilon-near-zero (ENZ) effect of indium tin oxide. The proposed metasurface SA significantly enhances the nonlinear optical response by combining the effects of LSPR and ENZ phenomena. The design was optimized using finite-difference time-domain simulations, which enabled both structural optimization and comprehensive characterization of optical properties. Based on the simulation results, the metasurface SA was fabricated via a commercial electron-beam lithography process. Linear transmission measurements of the fabricated SA confirmed the presence of LSPR around 1050 nm. Nonlinear transmission measurements revealed a modulation depth of 10.9% and a saturation intensity of 10.98 MW / cm 2 . The metasurface SA was successfully integrated into a mode-locked Yb-doped fiber laser, achieving stable Q -switched mode-locking operation at a central wavelength of 1058 nm. The resulting laser exhibited a spectral bandwidth of 11.7 nm, a repetition rate of 16.15 MHz, and a pulse envelope duration of approximately 2.6 μs. These results validate the reliability and effectiveness of the metasurface-based approach. The experimental findings highlight the strong potential of metasurface SAs incorporating ENZ materials as a versatile platform for next-generation high-performance ultrafast fiber lasers. The integration of plasmonic metasurfaces with ENZ effects offers a promising pathway toward compact, efficient, and tunable ultrafast laser sources for applications in communications, spectroscopy, and precision measurement.