Nonlinear nanophotonics associated with resonant metasurfaces aims at enhanced nonlinear light-matter interaction and thus giant nonlinear optical responses in the far field by virtue of artificially engineered meta-atoms and nanostructures at the subwavelength scale [1,2], which has attracted much attention in the past decades and opened exciting avenues for developing nonlinear optics related applications including spectroscopy [3], imaging [4], and sensing [5], among others. Typically, plasmonic nanoparticles could support localized surface plasmon resonances (LSPRs), which are dominated by the collective oscillations of conduction electrons generally excited by incident electromagnetic waves [6,7]. LSPR can significantly enhance local fields in the vicinity of nanoparticles, leading to large light-matter interaction in both linear and nonlinear regimes [8,9]. Unfortunately, the potential of LSPR-based metasurfaces with low Q-factors (e.g., Q<10) for nonlinear optics is limited due to the intrinsic Ohmic losses present in metals [10–12]. Recently, metasurfaces that support surface lattice resonances (SLRs) induced by coupling LSPR and Rayleigh anomaly (RA) have been demonstrated to provide an alternative way for achieving low losses and high Q-factors [13–15], and have been widely employed to boost nonlinear optical processes such as harmonic generation [16–18].

Based on the modulation of the quality factors Q within the cavity, passively Q-switched fiber lasers generate a sequence of short pulses, offering cost-effective and reliable solutions for achieving high pulse energy. Consequently, they are widely used in many fields, for example, material processing [19], laser ranging [20], and environmental sensing [21]. As a key nonlinear component in passive Q-switching, saturable absorbers (SAs) have been extensively researched and reported, for example, graphene [22] and semiconductor saturable absorption mirrors [23].

In recent years, due to the near-field enhancement effects of the underlying resonance mechanisms, a prominent saturation absorption response has been observed on metal- and semiconductor-based metasurfaces such as gold [9,24–28] and GaAs [29] nanostructures. Moreover, the latest publications including some findings by us have reported mode-locked and Q-switched fiber lasers using gold metasurfaces as SAs [9,28,30], which exhibit advantages of tunable and flexible design associated with resonances, light weight and easy integration, as well as high modulation depth.

Aluminum (Al) is a trivalent light metal material with the advantages of low cost, high stability, and good compatibility with existing semiconductor technology [8,31]. Additionally, investigations on Al metasurfaces have already revealed that Al itself has significant second- and third-order nonlinearity [32,33] and has been extensively studied in various fields, such as harmonic generation [34–36], wave mixing [37], and multiphoton photoluminescence [38]. Similar to gold, Al also possesses ultrafast dynamics [39]. The above-mentioned aspects prove the great potential of Al metasurfaces for ultrafast optics and laser optics. However, until now, there has been no report on Al-metasurface-based SA devices and their further applications.

In this work, we have designed and fabricated Al metasurfaces with different periods, so as to excite LSPR and SLR around 1 μm. Their optical properties including linear transmission and nonlinear saturation absorption are both investigated and compared. The experimental results prove that the SLR metasurface has a narrower resonance linewidth and a more remarkable modulation depth (Md=9.6%) than the LSPR metasurface, as a result of low loss and high Q-factor merits existing in the SLR mode. Furthermore, we utilized the SLR-based Al metasurface as an SA to construct a polarization-maintaining (PM) Q-switched fiber laser at 1 μm, achieving pulse output with tunable wavelength over 28 nm (1020–1048 nm). Moreover, we analyzed the influence of pump power and central wavelength of the filter on repetition rate and pulse width of Q-switched pulses, respectively. Our trial of employing an Al resonant metasurface in a pulse fiber laser opens an exciting door for emerging SA nanodevices and may find more applications ranging from optical computing [40] to ultrafast switching and modulation [41].

Figure 1(a) displays schematically our designed Al resonant metasurface capable of shaping light as needed, namely, a periodic Al nanoarray located on the substrate SiO2, and the schematic diagram of the single-nanorod unit cell is shown on the right side of Fig. 1(a). To match the required resonances at approximately 1 μm (operation wavelength of the fiber laser), metasurfaces are always excited by Y-polarized normal incidence (i.e., along the long axis direction of the single nanorod). To reveal the relationship between resonance linewidth and mode coupling, we first fix the parameters of the Al metasurfaces as width w=60nm, length l=240nm, and height h=45nm, and only simultaneously change the periods P in both X and Y directions. The period-dependent transmission spectra of the metasurfaces are illustrated in Fig. 1(b), calculated based on the finite element method. When the period P is small (such as P=400nm), the Al metasurface can support LSPR mode, which is characterized by a large resonance linewidth (typically, on the order of a hundred nanometers). According to previous research [17], a Rayleigh anomaly appears at the position λRA=nsub×P (in our work, nsub=1.45) when a periodic structure is excited by normal incidence. Generally speaking, RA can be coupled with LSPR when the two modes are close to each other, resulting in more localized field energy, suppression of large radiation losses in metal structures, and ultimately SLR mode with a narrower linewidth. As displayed in Fig. 1(b), with the increase of the period, the coupling between RA and LSPR becomes more evident, accompanied by a sharp decrease in resonance linewidth. However, as the period continues to increase until RA and LSPR are far apart, the narrow resonance linewidth disappears again. So, we should choose the period of the metasurface reasonably according to the needs. When the period of the nanostructure approaches 700 nm, the RA appears at the wavelength of 1015 nm. Subsequently, the resulting metasurface that supports SLR mode exhibits a much narrower linewidth (45 nm, fitted by a Fano formula [42]) and a higher transmittance at resonance, and the transmission spectrum is shown by the black solid inverted triangles in Fig. 1(c). We should mention that, in order to match the resonance wavelength around 1 μm, slight adjustments are made to the parameters of the metasurface with the period of 400 nm. In detail, the length of the nanorod is adjusted to l=290nm, and its transmission spectrum is depicted by the blue hollow triangles in Fig. 1(c), exhibiting an LSPR linewidth of approximately 270 nm (fitted by a Lorentz formula). In addition, the electric near-field distribution in Fig. 1(d) reveals that the SLR metasurface provides more significant field enhancement with (|E/E0|max)SLR∼2(|E/E0|max)LSPR.

In order to further explore the optical properties of Al metasurfaces in experiment, the above-mentioned two resonant metasurfaces were fabricated via electron beam lithography (EBL) (see fabrication in Appendix A). The size of both metasurfaces is 100μm×100μm and the scanning electron micrograph (SEM) images are shown in Fig. 2(a). As depicted on the right side of Fig. 2(a), both metasurfaces are composed of periodic and homogeneous nanorods as designed, confirming robust nanofabrication. Figure 2(b) illustrates the linear transmission spectra of the two metasurfaces, which were measured by a supercontinuum light source (more details can be found in Appendix B). The experimental curves are in good agreement with the simulation in terms of resonance profiles and positions. Notably, the SLR metasurface has a much narrower linewidth (about 75 nm) compared to the LSPR metasurface (about 300 nm), which implies a larger local near field and a greater optical nonlinearity.

A mode-locked laser (central wavelength of 1035 nm, pulse width of 122 fs, and repetition rate of 50.1 MHz) is applied to investigate nonlinear absorption responses of our metasurfaces (more details can be found in Appendix B). And the diameter of the laser beam focused on the samples is approximately 50 μm, less than our samples. As displayed in the Fig. 2(c), both types of resonant metasurfaces exhibit anticipated saturation absorption features, characterized by an increase in transmission as the incident fluence rises. Thanks to greater near-field enhancement, the SLR metasurface exhibits a more remarkable saturation absorption (modulation depth Md=9.6%) than the LSPR metasurface (Md=1.9%) at a low incident fluence (25μJ/cm2). In comparison to the work previously reported, such as a gold LSPR metasurface (Md∼1.7%) [30], graphene-plasmonic hybrid metasurface (Md∼5.5%) [26], gold-indium tin oxide (ITO) strong coupling system (Md∼6.6%) [25], or some 2D nanomaterials (for instance, graphene, exhibiting Md typically less than 5% [43,44], and MoS2 exhibiting Md of ∼2.9% [45]), our SLR-based Al metasurface demonstrates more prominent third-order optical nonlinearity. On the one hand, our demonstration has fully proven that Al resonant metasurfaces possess significant saturation absorption characteristics in the near infrared due to strong optical nonlinearity of Al material in addition to near-field enhancement of the nanostructures, holding great potential as saturable absorbers in achieving pulse lasers. On the other hand, our work further provides a way to tailor and enhance saturation absorption by utilizing coupled resonance modes, thus leading to both boosted modulation depth and decreased linear insertion loss (both beneficial for a practical SA device).

The ytterbium (Yb)-doped polarization-maintaining fiber laser cavity, as shown in Fig. 3, is constructed to investigate whether the designed SLR-based Al metasurface can serve as a stable SA for a pulse fiber laser. A segment of approximately 0.6 m Yb-doped fiber (Coherent PM-YSF-HI-HP) is applied as the laser gain medium, pumped by a 976 nm laser, which is coupled into the resonator via a 980 nm/1030 nm wavelength division multiplexer (WDM). The isolator ensures the unidirectional operation of the laser. The output port of the optical coupler (10% coupling ratio) provides the laser output and characteristic measurement, while the other port helps the lasing return to the cavity for sufficient feedback. The metasurface is utilized as the passive saturable absorber. Collimators and lenses are used to achieve collimation and focusing of the free-space light. The operation center wavelength can be adjusted with a wavelength and flat-top bandwidth (BW) tunable optical bandpass filter (a center wavelength adjustable range of 1005–1085 nm and a BW adjustable range of 1.4–40 nm). Unless otherwise specified, the parameters of the filter are fixed at a center wavelength of 1030 nm and a bandwidth of 2 nm. Note that in experiment both collimators and associated PM fibers need to be carefully aligned. On the one hand, the incoming light upon the metasurface should be polarized under the optimum condition, so as to best excite the resonance mode of the metasurface. On the other hand, both collimators are aligned to each other to reduce the overall coupling losses between the fiber optical path and free-space optical path.

As the pump power ramps up to about 140 mW, the fiber laser starts to operate in a stable Q-switching state. And the Q-switched pulses are always in a linearly polarized light state, observed by a polarization analyzer. Figure 4 shows the results of Q-switched pulses with a pump power of about 200 mW, monitored by a digital oscilloscope together with a photodetector, an optical spectrum analyzer, and a radio frequency analyzer. With the help of the metasurface, a stable Q-switched pulse train is generated, exhibiting a repetition rate of 33.7 kHz, corresponding to a time interval between adjacent pulses of 29.7 μs, as illustrated in Fig. 4(a). The uniform pulse train shows little amplitude fluctuation. Figure 4(b) indicates that the full width at half maximum (FWHM) of the individual pulse is 2.1 μs. The relevant optical spectrum is displayed in Fig. 4(c), with a central wavelength of 1031.734 nm and a 3 dB bandwidth of 0.088 nm. To investigate the stability of the laser, we measured the corresponding radio-frequency (RF) spectrum. The frequency component at 33.7 kHz shows a signal-to-noise ratio (SNR) over 40 dB with measurable resolution BW (RBW) of 100 Hz and video BW (VBW) of 300 Hz, as shown in Fig. 4(d). The SNR of about 40 dB is typical and similar to those Q-switched fiber laser cavities employing graphene [46], gold-ITO metasurfaces [25], or MoS2 [47] as SAs. Additionally, the inset of Fig. 4(d) portrays the RF spectrum over a wide span of 500 kHz with both RBW and VBW set as 300 Hz, where high SNRs are also present at harmonic frequencies, indicating again the good stability of the laser. On the contrary, the Q-switched pulses did not appear when the Al metasurface was not incorporated in the laser cavity, which confirms that the Al metasurface is responsible for the stable Q-switching operation.

We also investigated the output properties as a function of pump power. As depicted in Fig. 5(a), the laser goes through four different states depending on injected pump power. Initially, the laser cavity does not oscillate due to excessive loss (gray background). And then, it begins to oscillate in a continuous-wave state when the pump power reaches 85 mW (green background). Third, the intermediate state, namely, the unstable Q-switched pulse output state, appears as the pump power further increases from 100 to 140 mW (orange background). Finally, stable Q-switching operation occurs when the pump power exceeds a threshold value of 140 mW (blue background). The average output power after oscillation increases almost linearly from 0.08 to 8.60 mW with the input pump power from 85 to 280 mW, leading to a slope efficiency of 4.23%. The efficiency is closely related to the loss of the cavity and can be effectively upgraded by improving the coupling ratio of the spatial optical path and reducing the insertion losses of various components in the fiber optical path. As illustrated in Fig. 5(b), with the increase of the pump power from 140 to 280 mW, the repetition rate increases from 25.5 to 46.6 kHz, and the pulse width decreases from 2.9 to 1.5 μs. Under high incident fluence, the transmission of the Al metasurface increases, indicating that the SA is partially bleached, which leads to a decrease in intracavity loss and an increase in Q value, and thus a reduction in the pulse width of the Q-switched pulses [48]. Consequently, the corresponding output pulse energy increases from 81.3 to 184.5 nJ, as displayed by red triangles in Fig. 5(a). In the total tunable pump power range mentioned above, the Q-switching operation holds good robustness, which indicates that our metasurface exhibits excellent thermal stability.

The lasing wavelength can also be tuned by adjusting the operation center wavelength of the intracavity filter. As illustrated in Fig. 6(a), the laser supports continuous tuning of the emission wavelength from 1020 to 1048 nm, when the pump power is fixed at 200 mW, covering a spectral range of 28 nm. Our tunable wavelength range is narrower than those by employing graphene or MoS2 as SAs (about 50 nm) [49,50]. This difference may be attributed to the wavelength-dependent modulation depth of our metasurface, where modulation depth decreases far away from resonance wavelength. In other words, this limitation is particularly understandable when taking the limited resonance linewidth (i.e., high Q-factor) of our metasurface into account. Besides, the tunable ranges of other intracavity passive components may impose additional limitations on the wavelength tunability of the laser. Furthermore, the repetition rate and pulse width of the output pulses also change with the filter wavelength, due to wavelength-dependent losses of the metasurface and other devices, as described in Fig. 6(b).

Similar to the SLR metasurface, our LSPR metasurface can also help acquire a stable Q-switching operation (see Appendix C). However, due to the lower transmittance from the LSPR metasurface at resonance, the laser cavity begins to oscillate at a higher pump power of 120 mW, with a smaller slope efficiency of 3.06%. In addition, the laser cavity based on the LSPR metasurface with weaker saturation absorption response requires greater pump power variation from a continuous-wave state (pump power about 120 mW) to a stable Q-switched pulse output state (pump power about 220 mW). With the increase of the pump power from 220 to 300 mW, the repetition rate increases from 28.9 to 38.8 kHz, the pulse width decreases from 3.3 to 2.1 μs, and the corresponding output pulse energy increases from 101.7 to 157.8 nJ. Overall, the SLR metasurface is more ideal for Q-switching operation in terms of pump threshold and laser output performances, which is attributed to lower unsaturable loss and more pronounced modulation depth compared to the LSPR metasurface. Note that a thin alumina layer covers the surface of both Al metasurfaces, preventing further oxidation of the Al nanoparticles and ensuring good stability for our metasurfaces. Indeed, the reproducible measurement over time indicates that the influence of alumina on device functionality is negligible.

In summary, we have numerically designed and experimentally fabricated two nonlinear Al single-nanorod metasurfaces with different periods that support LSPR and SLR modes at about 1 μm. Compared to the LSPR metasurface, the SLR metasurface, in spite of less nanoparticle filling density, exhibits a more significant modulation depth of 9.6% under a low incident fluence (25μJ/cm2) in the experiment. We attribute it to the higher Q-factor, stronger light-matter interaction, and greater localized near-field enhancement in the SLR mode. We further incorporate the SLR metasurface in a PM fiber laser cavity, where stable wavelength-tunable Q-switched pulses can be successfully generated, covering a tunable spectral range of 28 nm from 1020 to 1048 nm. Additionally, the influences of pump power and central wavelength of the inserted filter on the repetition rate and the temporal width of Q-switched pulses have been examined, providing more adjustable details for characteristics of output pulses. Typically, when the pump power is 200 mW and the filter is set to 1030±1nm, a stable Q-switched pulse train with center wavelength of 1031.724 nm, 3 dB spectral bandwidth of 0.088 nm, repetition rate of 33.7 kHz, pulse width of 2.1 μs, pulse energy of 141.7 nJ, and SNR of greater than 40 dB at the fundamental frequency can be realized. Our work reveals a remarkable third-order nonlinear optical effect of Al resonant metasurfaces, which can act as saturable absorbers for pulsing optics and nonlinear optics. Our wavelength-tunable Q-switched fiber laser based on the designed SLR Al metasurfaces can provide a flexible, simple, compact, and low cost pulse source for applications in communication, bio-sensing, spectroscopy, and measurement, among others. The powerful manipulation potential of multifunctional metasurfaces over light including phase modulation [25] may even offer new perspectives for extending laser output dimensions from not only the temporal domain but also to the spatial domain [51,52]. Furthermore, combining the ultrafast dynamics of the supporting material, applications of our proposed Al SLR-based metasurfaces in ultrafast optics such as for ultrafast switching, modulation, and mode-locking can also be envisioned in the future. Last but not least, saturable absorbers with low power consumption, miniaturized footprint, and CMOS-compatible fabrication could be good candidates for integrated photonic neural networks and optical computing [53].