With the appearance of graphene [1], the exploration for a new generation of two-dimensional (2D) graphene-like saturable absorbers (SAs) has been the focus of intensive research in recent years [2–5]. 2D materials have expended the scope of traditional SAs, such as ${\mathrm{Cr}}^{4+}\text{:}\mathrm{YAG}$, GaAs, and V:YAG, which are wavelength sensitive, responding to specific wavelength band [6–9]. Very recently, a new class of ${\mathrm{sp}}^2$ hybridized metal-free materials, graphite carbon nitride ($\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$), has attracted worldwide attention due to its visible-light-driven bandgap and proper band edges, which make it fairly suitable for water splitting [10], energy conversion [11,12], and pollutant removal [13]. Meanwhile, $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ possesses high stability with respect to chemical, thermal ($<873\mathrm{K}$), and photochemical attack because of its tri-s-triazine ring structure and high degree of polymerization [10]. However, compared with its chemical properties, its optical characteristics have not yet received much attention. Encouraged by the analogous structural and electronic properties of graphene, it seems plausible to obtain positive results using $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ as SAs to generate pulsed lasers.

Herein, for the first time, to our knowledge, we investigated a passively $Q$-switched Nd:LLF laser at 1.3 μm with 2D $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanosheets as an SA by spreading its dispersions over a transparent YAG substrate. Furthermore, the optical properties of the other structural $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ materials, including 12 nm $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanoparticles (NPs) and cashew-shaped mesoporous ${\mathrm{C}}_3{\mathrm{N}}_4(\mathrm{mpg}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4)$, were also investigated. In spite of the great differences in morphology, we found that all $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ samples can be used as broadband SAs due to their absorption from 800 to 3000 nm. It is believed that the $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$-based materials will find wide applications in the generation of $Q$-switched lasers from the infrared to mid-infrared in the near future.

The $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanosheets were prepared through ultrasonic pulverization of bulk $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ powders [14]. In brief, 10.0 g melamine (99%, Aldrich) was placed in a crucible and heated at a rate of $5°\mathrm{C}{\min }^{-1}$ to reach a temperature of 600°C in a muffle furnace, followed by tempering at this temperature for another 2 h. After natural cooling, 200 mg bulk $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ was fully ground and dispersed in 200 mL of distilled water and treated for 12 h with the technique of ultrasonic pulverization. After 30 min centrifugation at 4000 rpm, the stock solution of $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanosheets was obtained by taking the supernatant.

4 g cyanamide was dissolved in 16 mL of 40% dispersion of 12 nm ${\mathrm{SiO}}_2$ particles (Ludox HS40, Aldrich) with vigorous stirring at 100°C to remove water [15]. The resulting white powders were heated at a rate of $2.3°\mathrm{C}{\min }^{-1}$ over 4 h to 550°C and held for another 4 h. The resulting brown-yellow powder was treated with 4 mol/L ${\mathrm{NH}}_4{\mathrm{HF}}_2$ for 2 days to remove the silica template. The remaining powders were filtered and then washed thoroughly with distilled water and ethanol. The final product was obtained by drying overnight at 80°C under vacuum.

Cashew-shaped mesoporous $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ ($\mathrm{mpg}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$) was prepared according to our previous method [16]. 1.0 g of SBA-15 was impregnated in 70 mL of aqueous solution containing 2.0 g of dicyandiamide and sonicated for 2 h. The resulting mixture was then heated to 80°C for 24 h to remove the water, and finally calcined under nitrogen atmospheres at 550°C for 4 h at a heating rate of $2.3°\mathrm{C}{\min }^{-1}$. To remove the silica template, the resulting silica-$\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ powders were treated with 2 mol/L NaOH for 24 h, followed by filtration, washing with water and ethanol several times, and drying at 50°C. Finally, the desired light-yellow powder, denoted as $\mathrm{mpg}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$, was obtained.

Dispersions of 12 nm $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ and $\mathrm{mpg}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ were obtained using the same method as that of $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanosheets, except for the 0.5 h ultrasonic time. After that, a certain amount of these dispersions was transferred to the YAG, followed by drying in air at room temperature several times. All substrates were kept in a dust-free box.

Several types of analytical techniques were used to characterize different $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanomaterials. The microstructures and morphologies of the prepared samples were observed by using a transmission electron microscope (TEM, JEOL JEM 2100) and an atomic force microscope (AFM, Nanoscope IIIa). The crystalline phases of the materials were evaluated by using a Bruker D8 diffractometer with Cu Ka radiation ($\lambda =1.5418\mathrm{\AA }$) in the range of $2\theta =10°–70°$. The Fourier transform infrared spectra (FT-IR, Hitachi U4100) were recorded in a transmission mode from 800 to 3000 nm.

TEM measurements were carried out to characterize morphology and structure of different $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ samples. As shown in Fig. 1(a), bulk $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$, composed of micrometer-sized solid agglomerates, shows a multi-layer structure like graphite. Treated by ultrasonic pulverization, the as-obtained $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanosheets greatly reduced in dimensions and lateral spot sizes on the sub-micrometer scale, as can be seen in Fig. 1(b). AFM observations in Figs. 1(c) and 1(d) further demonstrate that the thickness of $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanosheets is between 1 and 2 nm, implying that the exfoliated nanosheets are composed of only a few layers of carbon nitride. The $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4\text{-}\mathrm{NPs}$ show a mean particle size of 12 nm, as presented in Fig. 1(e), with similar morphologies to those reported in previous literature [15]. The high-magnification TEM image recorded along the (110) direction in Fig. 1(f) reveals the ordered mesostructure of $\mathrm{mpg}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ with the size of 0.5–0.7 μm in width and 0.9–1.0 μm in length. Furthermore, the inset indicates the good dispersity of the cashew-shaped $\mathrm{mpg}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$. All of these TEM images demonstrate the successful synthesis of $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ samples in our study, which is consistent with the previous literature [14–16].

 

Fig. 1. TEM images of (a) bulk $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ and (b) exfoliated $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanosheets, (c) AFM image of $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanosheets deposited on copper grids and (d) the corresponding height image, and (e) TEM image of 12 nm $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ NPs and (f) high-magnification TEM image of $\mathrm{mpg}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$. Inset: TEM image of $\mathrm{mpg}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$.

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As shown in Fig. 2(a), $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ crystal is a layered material based on tri-s-triazine building blocks, and the van der Waals forces between each layer lead to the $\pi \text{-}\pi$ stacking. The x-ray diffraction (XRD) pattern [Fig. 2(b)] of bulk $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ displays two peaks: a low angle reflection peak at 13.1° indexed as a (100) peak, stemming from the lattice planes parallel to the $c$ axis, and a high angle peak at 27.6° indexed as a (002) graphite peak, indicating the typical interplanar stacking peak of conjugated aromatic systems [17]. The weaker peaks of (002) for $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanosheets illustrates that the long-time ultrasonic treatment destroyed the high crystallinity of pristine materials. As for $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4\text{-}\mathrm{NPs}$ and $\mathrm{mpg}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$, the typical (002) peaks also become weaker and wider, indicating that the hard template synthesis resulted in poor crystallinities and geometric confinement in the nanosized pore walls after the nanocasting or nanocoating process. Despite the small differences, all these samples keep the typical crystal structure of original g-${\mathrm{C}}_3{\mathrm{N}}_4$ described elsewhere [10,18,19].

 

Fig. 2. (a) Idealized Kekule model of $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ crystal structure viewed along the $c$ axis and the slightly tilted $b$ axis, and (b) XRD patterns of various $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ samples.

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Absorption spectra of various nanostructured $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ samples were measured by a Hitachi U-4100 UV/VIS/NIR spectrophotometer with a lamp in the region of 800–3000 nm and are shown in Fig. 3(a). It is clear that each $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ sample has a smooth absorption curve in the wide wavelength band ranging from 800 to 3000 nm, suggesting that $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ might be a promising broadband SA. Particularly, the $\mathrm{mpg}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ shows a higher absorption property than that of $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanosheets or $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ NPs, which should arise from the influence of morphology and microstructure. Theoretically, the cashew-like structure and the mesopores inside $\mathrm{mpg}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ would allow multiple reflections and absorptions of light, which may increase the density of photo-carriers, giving rise to the strong absorption. The results show that the $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ samples should be an excellent $Q$-switch for a wide spectral range.

 

Fig. 3. (a) Infrared transmittance of various $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ powders, and (b) nonlinear transmission of 12 nm $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$, $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanosheets and mpg $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ measured by 1.06 μm AO $Q$-switched $\mathrm{Nd}\text{:}{\mathrm{YVO}}_4$ laser.

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To further investigate the saturable absorption features of all three $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ samples, a home-made acousto-optic (AO) $Q$-switched $\mathrm{Nd}\text{:}{\mathrm{YVO}}_4$ laser at 1.06 μm was employed as a pump source, and the obtained results are depicted in Fig. 3(b). The modulation depths of 12 nm $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$, $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanosheets, and mpg $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ were 13.6%, 11.1%, and 11.6%, while their saturation fluences were in the proper order with the values of $5.2$, $3.9$, and $4.3\mathrm{mJ}/{\mathrm{cm}}^2$, respectively. One can see that although there are significant differences in microstructure and morphology among the three $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ SAs, saturable absorption features show little differences, which is attributed to Pauli blocking of the conduction band of the $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ materials and depletion of the valence electrons at high electronic excitation.

By employing the $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ samples as SAs, passively $Q$-switched Nd:LLF lasers at 1.3 μm were demonstrated. As sketched in Fig. 4, the pump source was a fiber-coupled laser diode emitting at 792 nm. The fiber core was 200 μm in diameter with a numerical aperture (NA) of 0.22. Concave mirror M1 with curvature radius of 150 mm was antireflection (AR)-coated at 792 nm and 1.05 μm and high-reflection (HR) coated at 1.3 μm. Plane mirror M2 was employed as an output coupling mirror (OC) with a transmission of 3.8% at 1.3 μm. The high-quality [100]-cut Nd:LLF crystal with Nd-doping concentration of 1 at. % was wrapped in indium foil and tightly mounted in a copper block water-cooled to 13°C. The two faces of the crystal were polished and AR-coated at 792 nm and 1.3 μm. A laser powermeter (MAX 500AD, Coherent, USA) was used to measure average output power. The pulse temporal behavior was recorded by a digital oscilloscope (1 GHz bandwidth and 20 Gsamples/s sampling rate, DPO7104C Tektronix Inc., USA) and a fast pin photodiode detector with a rise time of 0.4 ns.

 

Fig. 4. Schematic setup of diode-pumped Nd:LLF laser.

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Stable passively $Q$-switched lasers were achieved by inserting the $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ films into the cavity. The variation of average output powers versus the incident pump power is depicted in Fig. 5(a). One can see that the highest average output power was obtained by the 12 nm $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ sample. This could be attributed to the lower nonsaturated loss of the 12 nm $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ sample. Under 9.97 W incident pump power, the average output powers achieved with 12 nm $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$, $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanosheets, and mpg $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ were 1.07, 0.96, and 0.98 W, respectively.

 

Fig. 5. Passively $Q$-switched characteristics: (a) average output power, (b) pulse duration, (c) pulse repetition rate, and (d) single pulse energy as a function of incident pump power.

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The pulse durations and pulse repetition rates as a function of incident pump power are exhibited in Figs. 5(b) and 5(c). The pulse durations were decreased with the incident pump power, while the pulse repetition rates varied in an opposite tendency. The shortest pulse duration of 275 ns was realized by $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ under an incident pump power of 9.97 W. The highest pulse repetition rates achieved by 12 nm $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$, $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanosheets, and mpg $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ were 112, 154, and 147 kHz, and the single pulse energies were calculated to be 9.51, 6.2, and 6.68 μJ, respectively.

Figure 6 depicts the typical laser pulse train [Fig. 6(a)], the single pulse temporal profile [Fig. 6(b)], and the laser spectrum [Fig. 6(c)] generated by $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanosheets. No thermal damage was observed during the entire experiment.

 

Fig. 6. Temporal profile of pulse laser: (a) typical pulse train, (b) temporal profile of a single pulse, and (c) laser spectrum.

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In conclusion, our results confirm that the homemade $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ samples exhibit broadband absorption according to the absorption spectra. By employing three $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ samples, passively $Q$-switched Nd:LLF lasers at 1.3 μm were generated. Saturable absorption was observed from 2D $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ nanosheets, 0D $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ NPs and 3D $\mathrm{mpg}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$. Thus $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ could be a promising broadband saturable absorber material.