Pulsed lasers operating in the near-infrared (NIR) region are of great importance in fields such as optical communications, imaging, and material processing^[1–3]. Owing to the compactness, high efficiency, and low cost, passively $Q$-switched lasers are an ideal technology to achieve pulsed lasers in the nanosecond level^[4]. As a common optical modulator, the saturable absorber is an essential device for the optical pulse generation. In decades, with the development of graphene^[5], two-dimensional (2D) materials have been a hot research topic as a saturable absorber to enable $Q$-switching and mode-locking operations. In comparison with the conventional bulk saturable absorber and ultra-long-period grating^[6], 2D materials possess unique advantages such as short relaxation time, efficient broadband optical response, and low cost^[7–11]. Indeed, black phosphorus (BP)^[12], topological insulators (TIs)^[13], transition metal oxides (TMOs)^[14], transition metal dichalcogenides (TMDs)^[15], MXenes^[16], and metal-organic frameworks (MOFs)^[17] can be applied as the saturable absorber in versatile laser systems. However, these saturable absorbers still suffer some disadvantages.

As a binary TMO, vanadium oxides (${\mathrm{VO}}_x$) are of great interest because they possess controllable electronic and optical properties^[18,19]. As a typical ${\mathrm{VO}}_x$, vanadium pentoxide (${\mathrm{V}}_2{\mathrm{O}}_5$) suffers from low electronic conductivity and structural instability^[20,21]. To overcome the low stability and electronic conductivity, other metal cations are introduced into the ${\mathrm{V}}_2{\mathrm{O}}_5$ interlayer, since those ions can act as “pillars” to improve structural stability and provide a fast diffusion path^[21–23]. Recently, sodium metavanadate (${\mathrm{NaVO}}_3$) is proposed to boost the charge transfer^[23]. On the other hand, the bandgap of ${\mathrm{V}}_2{\mathrm{O}}_5$ is roughly 2.3 eV^[24,25], which cannot absorb the NIR photons, while the bandgap of ${\mathrm{VO}}_x$ with other oxidation states such as ${\mathrm{VO}}_2$, ${\mathrm{V}}_2{\mathrm{O}}_3$, and ${\mathrm{V}}_3{\mathrm{O}}_5$ could be metallic^[25], which can absorb the photons within the full optical span.

Researchers have proposed the composite strategy to boost the nonlinear optical properties^[25–27]. Up to date, a lot of hybridized saturable absorbers were prepared with graphene-${\mathrm{Bi}}_2{\mathrm{Te}}_3$^[28], ${\mathrm{MoS}}_2\text{-}{\mathrm{Sb}}_2{\mathrm{Te}}_3\text{-}{\mathrm{MoS}}_2$^[29], and ${\mathrm{Fe}}_3{\mathrm{O}}_4$-MXene^[30–32], clearly showing the enhanced saturable absorption. In fact, the polyaniline $(\mathrm{PANI})/{\mathrm{NaVO}}_3$ composite was prepared to obtain the large permittivity as well as the optical absorption^[33]. Thus, ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite is expected to enhance the optical features.

In this Letter, to study the nonlinear optical properties of the ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite material, we used the open-aperture Z-scan technique and measured the effective absorption coefficient, modulation depth, and saturation light intensity at 1.3 and 2 µm. The large modulation depth of 13% at 1.3 µm and the good effective nonlinear absorption coefficient of 0.81 cm/GW at 2 µm were shown. These results revealed that the composite material was a good saturable absorber for pulse manipulation. Subsequently, a passively $Q$-switched laser was operated by using ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ as a saturable absorber. When the operating wavelength was 1.3 µm, the shortest pulse width was 129 ns, and the maximum repetition rate was 274 kHz. Our work confirmed that ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite material could be a promising broadband nonlinear material for ultrafast photonics.

We took ${\mathrm{V}}_2{\mathrm{O}}_3$ powder and ${\mathrm{NaVO}}_3$ powder with a mole ratio of 2:3 as the raw materials. After completely ball-milling together, ${\mathrm{V}}_2{\mathrm{O}}_3/{\mathrm{NaVO}}_3$ powder was dispersed in N-methyl-2-pyrrolidone (NMP) and ultrasonically bathed for 6 h. The dispersion solution was centrifuged at 3000 r/min for 15 min. Next, the centrifuged supernatant was dropped slowly onto a quartz substrate with a pipette. The quartz substrate was sucked onto the spinner to ensure that the supernatant was evenly covered on the quartz substrate. Then, the sample was placed in a vacuum drying oven at 80°C for 4 h. Finally, the uniform ${\mathrm{V}}_2{\mathrm{O}}_3/{\mathrm{NaVO}}_3$ nanocomposite was prepared as a thin membrane.

The morphology and structure of the ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite material were studied by different characterization methods. Figure 1(a) shows an image of the scanning electron microscope (SEM), showing the small flake-like shape. Figures 1(b)–1(d) illustrate the energy dispersive spectrometer (EDS) elemental mappings of O, V, and Na, respectively, which display that the element distribution was uniform. A high-resolution transmission electron microscope (HRTEM) picture is shown in Fig. 1(e). The lattice plane separation was 0.3447 nm, corresponding to the $(\overline{2}20)$ crystal plane separation (0.3439 nm) of ${\mathrm{NaVO}}_3$. In the selected area electron diffraction (SAED) pattern of Fig. 1(f), d1 was 0.325 nm, d2 was 0.185 nm, and d3 was 0.145 nm, corresponding to the (121), (013), and (512) crystallographic planes of ${\mathrm{NaVO}}_3$, respectively. Figure 1(g) shows the X-ray diffraction (XRD) pattern of the ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite material. The diffraction peaks of ${\mathrm{V}}_2{\mathrm{O}}_3$ (PDF#34-0187) were marked with asterisks, ${\mathrm{NaVO}}_3$ (PDF#32-1197) with circles, and ${\mathrm{V}}_3{\mathrm{O}}_5$ (PDF#38-1181) with rhombus. It can be seen from the XRD pattern that ${\mathrm{VO}}_x$ and ${\mathrm{NaVO}}_3$ were well composited. Figure 1(h) clearly shows the six Raman peaks of the ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite material. The previously reported vibration modes of ${\mathrm{NaVO}}_3$ were consistent with $170.9{\mathrm{cm}}^{-1}$, $348.3{\mathrm{cm}}^{-1}$, and $631.1{\mathrm{cm}}^{-1}$^[34]. The same vibration modes of ${\mathrm{V}}_2{\mathrm{O}}_3$ and ${\mathrm{NaVO}}_3$ were at $242.9{\mathrm{cm}}^{-1}$ and $507.3{\mathrm{cm}}^{-1}$^[34–36].The vibration mode of ${\mathrm{V}}_3{\mathrm{O}}_5$ was classified as $422.3{\mathrm{cm}}^{-1}$^[37]. The above characterization techniques presented the morphology and structure of ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite material, which fully verified the effective composite of ${\mathrm{VO}}_x$ and ${\mathrm{NaVO}}_3$.

Subsequently, the nonlinear optical properties of ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite were measured by the conventional open-aperture Z-scan technique. The excitation sources were home-made actively $Q$-switched lasers emitting the pulse duration of 50 ns with a repetition rate of 800 Hz at 1.3 and 2 µm, respectively. A convex lens with a focal length of 100 mm was applied to converge the optical beam. Figure 2 displays the typical nonlinear transmission curves versus the relative distance to the focal lens (Z) with both excitation wavelengths. Clearly, the ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite performed the nonlinear saturable absorption. The transmission curve can be fitted by^[38]$$T=\sum _{m=0}^{\infty }\frac{{[-q_0(z,0)]}^m}{{(m+1)}^{1.5}},m\in N,q_0(z,0)=\frac{{\beta }_{\mathrm{eff}}{L}_{\mathrm{eff}}{I}_0}{1+\frac{z^2}{z_0^2}},$$(1)where ${\beta }_{\mathrm{eff}}$ is the effective nonlinear absorption coefficient, $L_{\mathrm{eff}}$is the effective length of the sample, $I_0$ is the peak intensity on the axis, and $z_0$ is the Rayleigh range.

Then, we investigated the variation of the transmittance with the change of laser peak density. The fitting equation of experimental data is as follows^[39]: $$T=1-\mathrm{\Delta }T\exp (-\frac{I}{I_s})-T_{\mathrm{ns}}.$$(2)

Herein, $\mathrm{\Delta }T$ is the modulation depth, $I_s$ is the saturation intensity, and $T_{\mathrm{ns}}$ is the nonsaturable losses. As shown in Fig. 2, as the light intensity incident on the sample increased gradually, the transmittance increased gradually at first and then flattened out gradually. This was because with the increase of incident intensity, the absorption coefficient of composite material decreased and the transmittance no longer increased, that is, the material was bleached.

Table 1 summarizes the nonlinear optical parameters of composite materials at 1.3 and 2 µm. These parameters verified that ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite material possessed good nonlinear absorption and the potential for optical modulators.

The excellent nonlinear optical absorption response of the ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite indicated that it could be used as a saturable absorber to generate $Q$-switched pulses. We demonstrated the passively $Q$-switched bulk lasers operation at 1.3 and 2 µm with the prepared ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite as the saturable absorber for the first time, to the best of our knowledge. In this section, a compact and simple plane-plane resonator with a length of 20 mm was set up to realize the $Q$-switching operations. For the $Q$-switching operation at 1.3 µm, the laser crystal was an $a$-cut 1% (atomic fraction) Nd:GdVO₄ with dimensions of $3\mathrm{mm}\times 3\mathrm{mm}\times 10\mathrm{mm}$. During the experiment, the laser crystal was maintained at 15°C to efficiently remove the thermal load. A fiber-coupled 808 nm laser diode (LD) was employed as the pump. The output coupler (M2) possessed a partial transmission ratio of 3.8%. The whole configuration of the laser cavity can be seen in Fig. 3.

For the continuous-wave (CW) running at 1.3 µm without the saturable absorber inserted into the cavity, the output power increased almost linearly after the threshold. Under the maximum pump power of 3.43 W, the highest output power was 782 mW. Then, the prepared saturable absorber was inserted into the resonator, and the threshold power for the stable $Q$-switching operation was 2.75 W. Note that ${\mathrm{VO}}_x$ is metallic, and ${\mathrm{VO}}_x$ can absorb the photons in the NIR region at 1.3 µm as well as 2 µm. With the increased pump power, the photogenerated electrons and holes separated and transferred in the ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ nanocomposite, leading to the enhanced nonlinear absorption properties. Until the ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ nanocomposite was bleached, when the conduction band was fully occupied by the electrons and the valence band was completely dominated by the holes, further optical absorption became impossible owing to the Pauli blocking principle. The losses can be smaller than the gain in the resonator, generating a giant $Q$-switched pulse. As shown in Fig. 4(a), at the highest pump level, the maximum output power at 1.3 µm was 79 mW. The low output power of the passive $Q$-switching operation was attributed to the induced extra loss of the saturable absorber. The pulse duration and repetition rate as functions of the pump level are illustrated in Fig. 4(b). It can be seen that the pulse width monotonically decreased, which can be related to the gain factor, while the corresponding repetition rate monotonically increased. When the incident pump power was 3.43 W, the minimum pulse width was 129 ns, and the maximum repetition rate was 274 kHz. At this time, the pulse train and single pulse profile are shown in Figs. 4(c) and 4(d), respectively. The maximum peak power of $Q$-switching was 2.24 W, and the corresponding single pulse energy was 289 nJ.

Then, we investigated the nonlinear saturable absorption properties of ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite at 2 µm. In this section, the laser medium was altered by a 1% (atomic fraction) thulium-doped yttrium lithium fluoride (Tm:YLF) crystal with a length of 8 mm. The pump source was a 794 nm LD with a numerical aperture of 0.22. The pump beam was focused into the laser crystal with a spot size of 400 µm in diameter. The output coupler had a transmission ratio of 5% at 2 µm. In Fig. 5(a), the incident pump power threshold for $Q$-switched laser output was 2.6 W. When it increased to 3.8 W, the maximum CW and $Q$-switched output powers were 150.4 mW and 50.7 mW, respectively. The small gain of Tm:YLF and the large loss induced by the saturable absorber resulted in the low output power. Meanwhile, as shown in Fig. 5(b), the minimum pulse width was 292 ns, and the maximum repetition rate was 155 kHz. Figure 5(c) shows the corresponding pulse train diagram, and Fig. 5(d) displays the corresponding single pulse. The corresponding highest peak power was 1.1 W, and the maximum single pulse energy was 327 nJ.

Table 2 summarizes the passive $Q$-switching performances with the ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite saturable absorber and other mainstream saturable absorbers. As shown in Table 2, the composite material can be compared with the mature nanomaterial-based saturable absorbers, indicating that ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite is a promising nonlinear optical material for ultrafast photonic applications.

In summary, the ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite material was synthesized and characterized. We used the open-aperture Z-scan technique to investigate the nonlinear absorption properties of ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ saturable absorber. The composite material displayed a large modulation depth and a high effective nonlinear absorption coefficient for pulsed laser generation. Using ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite saturable absorber, the passively $Q$-switched Nd:GdVO₄ and Tm:YLF lasers were realized for the first time, to the best of our knowledge, emitting the shortest pulse duration of 129 ns. These results indicated that ${\mathrm{VO}}_x/{\mathrm{NaVO}}_3$ composite could be a promising optical modulation device in ultrafast photonics.