Fig. 1. Geometrical patterns and resonance characteristics of our designed Al metasurfaces. (a) Schematic of the Al resonant metasurfaces. (b) Period-dependent transmission spectra of the metasurfaces under Y-polarized incidence. The following parameters of the metasurfaces are kept constant: w=60  nm, l=240  nm, and h=45  nm. The black (white) dashed line corresponds to the position of Rayleigh anomaly (the LSPR resonance mode) at different periods. (c) Linear transmission spectra of Al metasurfaces with P=400  nm (blue triangles) and P=700  nm (black inverted triangles). Magenta (red) solid curve corresponds to results fitted by Lorentz (Fano) formula. (d) Electric near-field distributions at resonance wavelength 1035 nm at the X-Y plane of the half height of Al nanostructures (P=400  nm and P=700  nm), which is normalized to the incident electric field E0. The nanostructures contain rounded corners with the radius of 5 nm.

Fig. 2. SEM images and linear and nonlinear optical properties of the two metasurfaces in experiment. (a) SEM images of the fabricated metasurfaces. (b) Transmission spectra of Al metasurfaces with P=400  nm (blue triangles) and P=700  nm (black inverted triangles). Magenta (red) solid curve corresponds to results fitted by Lorentz (Fano) formula. (c) Nonlinear transmission measurement that exhibits the considerable saturation absorption of Al metasurfaces with P=400  nm (blue inverted triangles) and P=700  nm (red triangles).

Fig. 3. Setup of the Q-switched Yb-doped PM fiber laser. WDM, 980 nm/1030 nm wavelength division multiplexer; YDF, ytterbium-doped fiber; ISO, isolator; OC, 10/90 optical coupler; Col., collimator; Filter, with adjustable center wavelength in the range of 1005–1085 nm and an adjustable flat-top bandwidth in the range of 1.4–40 nm.

Fig. 4. Experimental results of Q-switching operation with a pump power of about 200 mW. (a) Pulse train. (b) Single pulse. (c) Optical spectrum. (d) RF spectrum near the fundamental frequency. Inset: wideband RF spectrum.

Fig. 5. Experimental results of the fiber laser with tunable pump power. The blue background area indicates stable Q-switching operation regime. (a) Output power and pulse energy and (b) repetition rate and pulse width as a function of pump power.

Fig. 6. Experimental results of Q-switching operation with a tunable filter. (a) Spectra and (b) repetition rate and pulse width of the laser as a function of the filter operation wavelength. The filter BW of 2 nm is fixed.

Fig. 7. Schematic diagram of (a) linear transmission measurement and (b) nonlinear absorption measurement.

Fig. 8. Experimental results of the fiber laser using the LSPR-based Al metasurface with tunable pump power. Four colors represent four different laser states: no oscillation (gray background), the continuous-wave state (green background), the unstable Q-switched pulse output state (orange background), and the stable Q-switched pulse output state (blue background). (a) Output power and pulse energy as well as (b) repetition rate and pulse width as a function of pump power.