The exploration of new two-dimensional (2D) materials, such as graphene ^[1–3], topological insulators ^[4], and transition-metal dichalcogenides ^[5] has attracted wide attention in recent years because of their exceptional chemical and physical properties. These layered materials exhibit a common characteristic of possessing strong atomic forces between the atoms within a layer and weak van der Waals forces between adjacent atomic layers. This results in their special electronic and optoelectronic properties. Because of the large nonlinear optical (NLO) susceptibility, high carrier density, and mobility, they have been utilized as multiple optical components such as passive saturable absorbers for Q-switching and mode-locking, optical limiters, and photodetectors ^[6–12].

Graphitic carbon nitride (g-C3N4), a metal-free 2D conjugated semiconductor, has attracted research attention since 2009 because of its sustainable photocatalytic ability ^[13,14]. g-C3N4 possesses highly stable chemical, thermal, and photochemical properties for its tri-s-triazine ring structure and a high degree of polymerization ^[13]; therefore it can be used in photocurrent, photoreactivity, electrocatalysis, and bioimaging applications ^[13,15,16]. Compared with the chemical properties, the optical characteristics of g-C3N4 have received less attention. In the last two years, due to the saturable absorption (SA) property of g-C3N4, researchers have realized its applications in passive Q-switching and mode-locking of infrared lasers at wavelengths 1, 1.3, 2.84, and 2.95 μm ^[17–20]. However, SA in the visible waveband has never been observed for pure g-C3N4 ^[21,22].

In this study, we used the Z-scan method to investigate the NLO properties of g-C3N4 at different wavelengths, with picosecond laser pulses as the excitation source. For the first time to our knowledge, strong SA was observed in a g-C3N4 dispersion in the visible waveband. At the same time, under long wavelength (650 and 1064 nm) excitation, two opposite absorption phenomena, namely, SA and optical limiting (OL), were found when we changed the input intensity. The nonlinear absorption coefficient β and the nonlinear refractive index n2 were calculated from the experimental results of open aperture (OA) and closed aperture (CA) Z-scan measurements. For a g-C3N4 dispersion with 60% initial transmittance, the β values are −2.05, −0.34, −0.11, and −0.06cm·GW−1 at 355, 532, 650, and 1064 nm, respectively. This illustrates that the SA ability of g-C3N4 is stronger at shorter wavelengths. Currently, diode-pumped continuous-wave visible solid-state lasers that can generate various colors are undergoing rapid development ^[23]. By directly inserting nanomaterial produced saturable absorbers, such as Au nanorods, MoS2, or Bi2Se3, visible laser pulses can be obtained conveniently ^[24–26]. Besides, 2D nanomaterials have also been applied to the passive Q-switching of visible fiber lasers ^[27,28]. As a new, powerful SA material, g-C3N4 is hoped to produce pulsed visible lasers with the advantages of high efficiency, affordability, and compactness with potential applications in medical treatment, material processing, microscopy, and scientific research, etc.

Bulk g-C3N4 was synthesized by direct heating of dicyandiamide ^[29]. First, dicyandiamide was heated at 550°C for 4 h in static air at a ramp rate of 2.3°C/min. Then, it was cooled down to the room temperature at a cooling rate of 1°C/min. The resultant yellow agglomerate was finely ground to powder, as shown in Fig. 1(a).

The g-C3N4 dispersion was prepared by the liquid exfoliation method. The as-obtained powder was dispersed in deionized water with ultrasonic pulverization for 2 h. Then the suspension was allowed to settle for two days to deposit the undispersed particles. The final supernatant g-C3N4 dispersion was used in the Z-scan experiment. The g-CN-1, g-CN-2, and g-CN-3 samples [as shown in Fig. 1(b)] had different g-C3N4 contents, and their linear optical transmittances at 532 nm were 75%, 60%, and 43.5%, respectively. The samples prepared were stable and could be stored without any change in appearance for more than one month. This feature makes them suitable for time-consuming measurements and is also advantageous for future applications.

Atomic force microscopy (AFM) and X-ray diffraction (XRD) were used to analyze the morphology and structure of the g-C3N4 sample. Before the AFM measurement, the sample was deposited on a Si/SiO2 substrate and dried for 12 h. As shown in Fig. 2, the thickness of the g-C3N4 nanosheets is about 4–6 nm. Assuming an interplanar stacking distance of 0.326 nm ^[30] for the aromatic units, this thickness corresponds to a packing of 12–18 layers.

The XRD result was obtained on a Bruker D8 advance X-ray diffractometer under Cu–Kα radiation (λ=0.15418nm). As presented in Fig. 3, there are two primary diffraction peaks located at 13.1° and 27.6°. The (100) peak at 13.1° reveals the in-plane structural packing motif, and the (002) peak at 27.6° shows typical graphite-like stacking of the conjugated aromatic C-N segments ^[30,31]. The result reflects the graphite-like structure of the g-C3N4 powder and confirms that the influence of amino on the structure of g-C3N4 can be neglected.

Using 1064 nm light as the excitation source, the Raman spectrum of g-C3N4 powder was recorded. As shown in Fig. 4, the peak at 707cm−1 attributes to the heptazine ring breathing mode. The peaks around 1232cm−1 correspond to the stretching vibration mode of C-N heterocycles ^[32]. The 1566cm−1 fingerprint peak of the NH2 bending mode does not appear, which proves the high purity of the g-C3N4 powder.

Figure 5(a) shows the transmission spectrum of the g-C3N4-2 sample in the range of 250–1750 nm. It can be seen that the ultraviolet (UV) absorption cutting edge of g-C3N4 is shorter than 250 nm. The UV absorption peak at 300 nm is due to the π bond transition of g-C3N4. The absorption bands around 965, 1180, and 1450 nm are attributed to the O─H bond vibrations of water in g-C3N4 dispersion. Figure 5(b) is the Fourier transform infrared (FTIR) transmission spectrum of g-C3N4 powder in the range of 2.5–15.4 μm measured with a Perkin Elemer 100 FTIR spectrophotometer. In the infrared direction, the cutting edge extends beyond 15.4 μm (650cm−1). This result means that a wide waveband from visible to infrared is available for optical applications. The broad absorption band from 3000 to 3300cm−1 originates from the stretching vibration of the N─H bond, associated with uncondensed amino groups ^[33]. The other broad absorption bands in the 1200–1600cm−1 region are assigned to the typical stretching vibration modes of C-N heterocycles. The sharp peak at 810cm−1 attributes to the breathing vibration of triazine units ^[34,35].

The Z-scan method is a popular technique for characterizing the NLO properties of materials, including nonlinear absorption, scattering, and refraction ^[36]. Based on different materials and absorption mechanisms, the nonlinear absorption can be multiphoton absorption, reverse saturable absorption (RSA), and free-carrier absorption, etc. In this study, a Z-scan apparatus was used to study the NLO behavior of g-C3N4 dispersions at four wavelengths, namely, 355, 532, 650, and 1064 nm.

The experimental setup is shown in Fig. 6. The fundamental light source is a mode-locked Nd:YAG dye laser (PY61C-10, Continuum Inc, USA), which can operate at a near-infrared wavelength of 1064 nm. After frequency doubling and third harmonic generation with NLO crystals, 532 nm and 355 nm excitation wavelengths can be obtained. Further, after stimulated Raman scattering of pure water to the 532 nm output, the 650 nm excitation wavelength is obtained. Thus four representative wavelengths, namely, deep violet (355 nm), green (532 nm), red (650 nm), and near infrared (1064 nm), can be used for the Z-scan experiments. The pulse widths are 20, 30, 35, and 40 ps for 355, 532, 650, and 1064 nm, respectively. The pulse repetition rate is 10 Hz. The focal length of the focusing lens is 40 cm. The experimental samples, i.e., the g-C3N4 dispersions with different concentrations, were transferred to quartz cuvettes of 1 mm thickness. During the experiments, each Z-scan for the g-C3N4 dispersions was companied by a reference scan for the quartz substrate with deionized water. The final Z-scan results were obtained by dividing the g-C3N4 data by the quartz substrate with deionized water data. In this way, the thermal effects, nonlinear effects from the quartz substrate, and the deionized water were removed.

Figure 7 presents the OA Z-scan results of g-C3N4 dispersions (g-C3N4-1, g-C3N4-2, and g-C3N4-3) at the excitation wavelengths of 355, 532, 650, and 1064 nm. The linear optical transmission values of g-C3N4-1, g-C3N4-2, and g-C3N4-3 are 55%, 43.6%, and 30% at 355 nm; 75%, 60%, and 43.5% at 532 nm; 81.8%, 65.5%, and 50% at 650 nm; and 82.4%, 73.5%, and 65% at 1064 nm, respectively.

Figures 7(a)–7(d) show the variation of normalized transmission as a function of the incident pulse energy density (J·cm−2) for different g-C3N4 samples. All samples exhibit a gradual increase in the transmittance with increasing incident energy at 355 and 532 nm, indicating typical SA. This is the first time that SA is observed in the visible waveband for g-C3N4. Furthermore, for the same sample, the strongest SA response is seen at the shortest excitation wavelength, 355 nm. Normalized transmittance TN is used to scale the output clamping characteristics of different g-C3N4 samples. A larger TN implies better SA. When the 355 nm input fluence at the focal point (Z=0) is fixed at 0.47J·cm−2 (corresponding to an intensity of 23GW·cm−2), the TN values of g-C3N4-1, g-C3N4-2, and g-C3N4-3 samples can reach 128%, 147%, and 162%, respectively. This indicates that the SA ability increases with decreasing linear optical transmission. Under the same experimental conditions, the TN value of black phosphorus can only reach 115% at a much higher incident intensity of 115GW·cm−2. For 532 nm excitation, contrary to earlier reports on OL phenomena ^[21,22], all of the three g-C3N4 samples exhibit the SA effect under picosecond pulses. When the input fluence at the focal point is fixed at 1.66J·cm−2 (corresponding to an intensity of 55GW·cm−2), the TN values of g-C3N4-1, g-C3N4-2, and g-C3N4-3 samples are 125%, 163%, and 192%, respectively. Similarly, for 355 nm excitation, the SA ability increases with decreasing linear optical transmission. At 650 nm, the g-C3N4 dispersions exhibit two opposite NLO phenomena, SA and OL. When the input fluence at the focal point is fixed at 9.23J·cm−2 (corresponding to an intensity of 230GW·cm−2), the TN values of g-C3N4-1, g-C3N4-2, and g-C3N4-3 samples are 123%, 160%, and 75%, respectively. With decreasing linear optical transmission, the NLO responses of g-C3N4 dispersions shift from SA to OL. This means that the nonlinear absorption parameters of the g-C3N4 nanosheets have changed significantly with the changing initial linear optical transmission, or in other words, the concentration of the g-C3N4 dispersion. Meanwhile, at the wavelength of 1064 nm, no obvious SA behavior is shown by the three g-C3N4 dispersions. After increasing the fluence of the 1064 nm laser to 7.83J·cm−2, the samples show two different NLO phenomena, namely, weak SA and strong RSA. The TN values of g-C3N4-1, g-C3N4-2, and g-C3N4-3 are 105%, 86%, and 74%, respectively. Furthermore, the NLO absorption properties of g-C3N4 shift from SA to RSA with decreasing initial transmission. Such behavior indicates that g-C3N4 is highly suitable for the protection of infrared lasers. Overall, with increasing concentration, the g-C3N4 dispersion shows a lower initial linear transmittance, larger transmission variation magnitude, and more significant NLO absorption. Therefore, the g-C3N4-3 sample shows the strongest SA effect at low excitation energy densities (355 and 532 nm) and the strongest RSA effect at high excitation energy densities (650 and 1064 nm). As a layered material, g-C3N4 is based on tri-s-triazine building blocks, and the van der Waals forces between each layer lead to the π–π stacking ^[17]. It possesses higher charge carrier density due to an obvious increase of state density at the conduction band edge with respect to the bulk counterpart ^[37]. Its SA feature comes from the Pauli blocking effect of the conduction band and the depletion of the valence electrons under intense laser field.

Figures 7(e)–7(h) present the OA Z-scan results of the g-C3N4-2 sample at different excitation wavelengths and different energy intensities. At 355 nm, the values of TN are 147%, 149%, and 152% for the energy intensities (Z=0) of 23, 47, and 70GW·cm−2, respectively. At 532 nm, the TN values are 128%, 147%, and 163% when the energy intensities are 20, 37, and 55GW·cm−2, respectively. It can be seen that at these two wavelengths the SA ability of g-C3N4 dispersion grows with increasing incident intensity. Under the excitation of 650 and 1064 nm lasers, the g-C3N4-2 dispersion exhibits SA and OL behaviors at different incident intensities. Therefore, in the near-infrared waveband, g-C3N4 can serve as an optical modulation material depending on the practical working conditions. From the excited state nonlinear absorption theory ^[38,39], it can be known that at 355 and 532 nm, σ1>σ2 for g-C3N4-2, where σ1 and σ2 are the absorption cross sections of the first and the second excited states, respectively. Under such conditions, only the SA effect occurs [Figs. 7(e) and 6(f)]. At 650 and 1064 nm, σ1<σ2. Under this condition, the SA effect will change to the RSA effect when the incident intensity reaches a certain threshold, which is inversely proportional to the value of σ2−σ1. Figure 5(a) reveals that for g-C3N4-2, σ1(650nm)>σ1(1064nm), so if σ2 is assumed to be unchanged from 650 to 1064 nm, then σ2−σ1 at 650 nm will be smaller than that at 1064 nm and correspondingly the SA→RSA inversion intensity threshold at 650 nm will be larger. This is in accordance with the experimental phenomena: the inversion threshold at 650 nm is larger than 230GW·cm−2 [Fig. 7(g)], while the one at 1064 nm is between 137 and 156GW·cm−2 [Fig. 7(f)].

The normalized transmittance for the OA Z-scan experiment can be expressed as ^[36] where β is the nonlinear absorption coefficient, I0 is the peak intensity at the focus point (z=0), Leff=(1−e−α0L)/α0 is the effective length, L is the length of the sample, α0 is the linear absorption coefficient, z is the position of the sample, and zR is the Rayleigh length.

Taking the g-C3N4-2 sample as an example, the value of the nonlinear absorption coefficient β is fitted to be (−2.05±0.4)cm·GW−1 under different 355 nm intensities [Fig. 7(e)]. At 532 nm, the β value is fitted to be (−0.34±0.02)cm·GW−1 under different 532 nm intensities [Fig. 7(f)]. For 650 nm, the β value is −0.11cm·GW−1 at 230GW·cm−2 and 0.014cm·GW−1 at 307GW·cm−2. At 1064 nm excitation [Fig. 7(h)], the fitted β value is −0.06cm·GW−1 at 137GW·cm−2 and 0.016cm·GW−1 at 196GW·cm−2. The β values at all of the four wavelengths are listed in Table 1, as well as the corresponding saturation intensities Is. Is is defined as the optical intensity required to increase the saturable transmission to a half. For g-C3N4-2, the Is values in Table 1 are obtained from Figs. 7(a)–7(c) and 7(h) (at 137GW·cm−2), respectively. Based on a 30 ps, 532 nm laser, the Is of g-C3N4-2 is 11.4GW·cm−2, which is much larger than the values of graphene oxide (0.86GW·cm−2) ^[40], black phosphorus (7.6MW·cm−2) ^[41], and MoS2 (1GW·cm−2) ^[42] under similar conditions. The large Is represents high energy storage ability, which indicates that g-C3N4 will be a powerful Q-switcher material based on the theory of passively modulated pulsed lasers ^[47]. From Fig. 7(b), it can be known that g-C3N4-2 possesses a large saturable loss (36%) and a small non-saturable loss (4%). The other favorable characteristics includes short recovery time (4 ps) ^[48] and high laser damage threshold (>8MW·cm−2 for mid-infrared solid-state laser Q-switching) ^[19].

At the same visible excitation wavelength of 532 nm, the nonlinear absorption properties of the g-C3N4-2 sample and several representative nanomaterials are compared in Table 2 ^[40–46]. It can be seen that at a similar laser pulse duration (∼30ps) and energy range of 10−100GW·cm−2, only g-C3N4-2 presents an SA response (β<0). At the same time, compared with other popular 2D materials, such as graphene ^[49], black phosphorus (BP) ^[41], and boron carbon nitride (BCN) ^[46], g-C3N4 exhibits a different SA characteristic, i.e., a larger SA coefficient in the visible waveband (355, 532, and 650 nm) than in the infrared region (1064 nm). All of these properties indicate that g-C3N4 is a powerful 2D SA material in the visible waveband. In earlier reports, nanosecond lasers were used to study the NLO effects of g-C3N4 at 532 nm. No NLO response was observed under a power intensity of 0.8GW·cm−2, and the OL effect was observed under a power intensity of 4GW·cm−2 ^[21,22]. We used picosecond lasers as the excitation light source; they can supply a much higher power intensity than what was produced from the nanosecond lasers in earlier works. Thus, the SA effect is easier to realize. Besides, the dispersion concentration has a positive influence on the NLO performance; the NLO phenomena are more remarkable at higher dispersion concentrations.

The nonlinear refractive index can be extracted by dividing the CA data by the OA data. The results are shown in Fig. 8. The valley-peak configurations reveal that the nonlinear refractive index is positive, which shows that self-focusing occurs in g-C3N4 dispersions when they are irradiated by high power laser pulses. The fitting formula is ^[38] where x=z/zR, Δϕ=kn2I0Leff is the on-axis nonlinear phase shift at the focus, and k is the wavelength number. Using Eq. (3), we can obtain the nonlinear refractive index n2 under different conditions. At 355 nm and an input fluence of 0.47J·cm−2, the n2 values of g-C3N4-1, g-C3N4-2, and g-C3N4-3 samples are 0.59×10−15, 0.87×10−15, and 1.52×10−15cm2·W−1, respectively. For 532 nm excitation, when the input fluence is 1.66J·cm−2, the n2 values of g-C3N4-1, g-C3N4-2, and g-C3N4-3 samples are 0.57×10−15, 1.42×10−15, and 1.44×10−15cm2·W−1, respectively. At 650 nm and an input fluence of 9.23J·cm−2, the n2 values of g-C3N4-1, g-C3N4-2, and g-C3N4-3 samples are 0.15×10−15, 0.40×10−15, and 0.51×10−15cm2·W−1, respectively. For 1064 nm excitation, when the input fluence is 7.83J·cm−2, the n2 values of the three samples are 0.27×10−15, 0.34×10−15, and 0.42×10−15cm2·W−1, respectively. A high dispersion concentration and short excitation wavelength result in a high nonlinear refractive index.

The imaginary part of the third-order NLO susceptibility, Imχ(3), is related to the nonlinear absorption coefficient, and the real part, Reχ(3), is related to the nonlinear refractive index. They can be expressed by the following equations ^[50]: where n0 is the linear refractive index, and ω is the light frequency. The figure of merit (FOM) for the third-order optical nonlinearity is defined as FOM=|Imχ(3)/α0|. The Reχ(3), Imχ(3), and FOM values of the g-C3N4-2 sample are listed in Table 1 for different wavelengths.

g-C3N4 nanosheets were fabricated from bulk C3N4 by mechanical exfoliation. The nonlinear absorption and nonlinear refractive properties of g-C3N4 nanosheets were investigated at the wavelengths of 355, 532, 650, and 1064 nm with the Z-scan technique. For the first time to our knowledge, SA behavior in the visible waveband is observed from g-C3N4. At the same time, we found that the SA effect would change to the OL effect with increasing excitation intensity. It indicates that g-C3N4 can be used for different applications and be altered depending on the practical working conditions. In summary, this research shows that g-C3N4 is an excellent and promising wide waveband NLO material, whose applicable waveband includes the visible region.