1 Introduction

In the past decade, the research about two dimensional (2D) van der Waals materials, such as graphene (Novoselov et al. 2004; Nair et al. 2008; Bao et al. 2009), black phosphorus (BP) (Qiao et al. 2014; Guo et al. 2015; Sun et al. 2015), transition metal dichalcogenides (TMDCs) (Wang et al. 2013, 2018; Tang et al. 2020; Tian et al. 2018; Zeng et al. 2019; Cheng et al. 2020), transition metal carbides and nitrides (MXene) (Jiang et al. 2018, 2020a, b; Zhu et al. 2021), metal-organic frameworks (MOFs) (Lee and Telfer 2023; Gong et al. 2023; Xu et al. 2023a, b) appear like bamboo shoots after a spring rain. Especially, due to their high carrier mobility, wide absorption region and low cost, these materials have been extensively applied in the fields related to light, such as optical switches, optical limiters, photodetectors and saturable absorbers (SAs) (Novoselov et al. 2004; Nair et al. 2008; Bao et al. 2009; Qiao et al. 2014; Guo et al. 2015; Sun et al. 2015; Wang et al. 2013, 2018; Tang et al. 2020; Tian et al. 2018; Zeng et al. 2019; Cheng et al. 2020; Jiang et al. 2018, 2020a, b; Zhu et al. 2021; Lee and Telfer 2023; Gong et al. 2023; Xu et al. 2023a, b). Among these research fields, to study the saturable properties of 2D materials has been a relatively hot research direction. For example, up to now, many kinds of 2D materials such as graphene, BP, TMDCs, MXenes, and MOFs have been successfully employed as SAs to achieve mode-locking or Q-swicthing fiber lasers (Wang et al. 2019a, b, c; Debnath and Yeom 2021; Jiang et al. 2020a, b; Liu et al. 2020). However, due to their diverse nonlinear optical properties, different 2D materials will induce different laser performances in the aspects of pulse duration, output powers, pulse repetition rate, laser wavelength, laser stability etc. Correspondinglly, these different 2D-materials-based mode-locking or Q-swicthing fiber lasers could meet the needs of different applications. Hence, the aspiration to find better laser performances and fulfill specific applications propels researchers to study the properties of new 2D materials.

In recent years, several chromium-based ferromagnetic semiconductors such as Cr2Si2Te6, Cr2O3, Cr2Te3, CrTe2, CrI3 have been prepared to be SAs and show relatively good saturable absorption properties in erbium-doped fiber laser (EDFL) systems (Xu et al. 2023a, b; Li et al. 2023a, b; Lee et al. 2023; Qiao et al. 2023). Among chromium-based ferromagnetic semiconductors, Cr2S3 has attracted huge attentions in the fields of photocatalysis and photodetector for its magnetic and optical properties (Maignan et al. 2012; Hussain et al. 2017; Chu et al. 2019; Cui et al. 2020; Ebrahimi and Yarmand 2020; Xie et al. 2021). Cr2S3 has been reported with two crystallographic forms, trigonal (P\(\overline {3}\)1c space group) and rhombohedral (R\(\overline {3}\)space group) (Cui et al. 2020). As reported in Ref (Cui et al. 2020). , the conductive behavior of Cr2S3 will change from p-type to ambipolar then to n-type with nanosheet thickness increases. Futhermore, Cr2S3 has relatively good stability when exposed to air (Xie et al. 2021). In addition, more recently, the nonlinear optical performances and laser applications of rhombohedral Cr2S3 in 1.5 μm has been demonstrated and show relatively good performances (Yang et al. 2022). However, the nonlinear optical performances of trigonal Cr2S3 and its performances in 1 μm has not been demonstrated up to now.

In this work, open aperture (OA) Z-scan method was employed to investigate the nonlinear optical properties of trigonal Cr2S3 nanoparticles at 1 μm. The related modulation depth, saturable intensity and nonlinear absorption coefficient are obtained to be 11.5%, 0.115 MW/cm2 and − 2.6\(\times {10}^{4}\)cm/GW, respectively. Furthermore, by preparing the 2D Cr2S3 nanoparticles to be SA and inserting into a ytterbium fiber laser (YDFL) cavity, stable mode locking and Q-switching operations were observed, respectively. For the mode-locking laser, stable laser pulse with pulse width of 10.2 ns, central wavelength of 1069.1 nm and 3dB bandwidth of 0.66 nm was obtained. For the Q-switching laser operation, pulse duration of 2.95 µs with pulse repetition rate of 59.6 kHz is obtained. This work indicates that Cr2S3 nanoparticle is a promising material for the SA of fiber laser research.

2 Sample preparation and characterization

In this work, due to advantages of low-cost and high efficient, liquied phase exfoliation(LPE) method was adopted to fabricate 2D Cr2S3 nanoparticles. The steps to prepare Cr2S3 nanoparticles are given as follows: At first, 20 mg Cr2S3 powder dissolved into 20 ml isopropyl alcohol (IPA) solution, then the mixed solution was put into a ultrasonic cleaning machine for 24 h to accelerate the dissolution rate. After that, Cr2S3 nanoparticles could be obtained after the fully dissolved solution centrifuged with a speed of 2500 rpm for 5 min.

Fig. 1
figure 1

(a) SEM image of Cr2S3 nanoparticles. (b) AFM image of Cr2S3 nanoparticles. (c) The height of selected area in Fig. 1(b). (d) A typical TEM image of Cr2S3 nanoparticle. (e) HRTEM image Cr2S3 nanoparticle. (f) FFT image

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In order to check the morphology and the size of Cr2S3 nanoparticles, the sample was observed by a transmission electron microscope (TEM) (JEOL Model JEM-2100 F) and an Atomic Force Microscopy (AFM) as demonstrated in Fig. 1. As can be seen from Fig. 1(a) and (b) that the sizes of Cr2S3 nanoparticles are similar between the SEM and AFM results. Furthermore, three randomly selected area are shown in Fig. 1(b). The corresponding height profiles are given in Fig. 1(c), from which we can see that the heights of the prepared sample are around 5 nm. Besides, a transmission electron microscope (TEM) was also employed to analyze the shape, crystallinity and dimension of Cr2S3 nanosheets (JEOL Model JEM-2100 F). Figure 1(d) shows a randomly selected picture of prepared sample of TEM result. As shown in Fig. 1(d), the dimension of the prepared Cr2S3 sample is about 300–400 nm, which is close to the size of prepared Cr2S3 sample shown in Fig. 1(a) and (b). Figure 1(e) gives a high-resolution TEM (HRTEM) graph of Cr2S3 sample. From Fig. 1(e), it can be seen that the material has the lattice distances of 0.262 nm, which is corresponding to the (\(\stackrel{-}{1}\stackrel{-}{1}2\))plane of trigonal Cr2S3 structure. Furthermore, the (\(\stackrel{-}{1}\stackrel{-}{1}2\))plane is also shown in the electron diffraction pattern of corresponding HRTEM graph as can be seen from Fig. 1(f).

Fig. 2
figure 2

(a) XRD results of Cr2S3 powder. (b) UV–vis-NIR absorption spectrum of Cr2S3 nanoparticles

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As mentioned above, Cr2S3 possesses two kinds of different possible phases. For checking the crystal structure, a X-ray diffraction (XRD) machine (Rigaku SmartLab 9 kW – Advance) was employed to measure the XRD pattern of the Cr2S3 powder. As can be seen from Fig. 2(a), the locations of the diffraction peaks are matched with the peaks of trigonal Cr2S3 (PDF#72-1223, space group P\(\overline {3}\)1c). In addition, to investigate the optical absorption property of Cr2S3 nanoparticles, the Cr2S3 nanoparticles were analyzed by an UV-vis-NIR spectrometer (PERKIN ELMER). The absorbance spectrum of Cr2S3 nanoparticles from wavelength of 300 nm to 1600 nm is shown in Fig. 2(b). As can be seen from Fig. 2(b), the Cr2S3 nanoparticles has a broad absorption from wavelength of 300 nm to 1600 nm, which implys that it has the potential to be SA in the visible to near infrared laser waveband.

Fig. 3
figure 3

(a) OA Z-scan results of the Cr2S3 nanoparticles, (b) Normalized transmittance versus laser intensity at I0 = 0.612 MW/cm2

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In addition, since the nonlinear optical properties of Cr2S3 nanoparticles are still unclear, a normal used nonlinear optical property investigation method which called as OA Z-scan technique was employed to study the nonlinear optical response of the Cr2S3 nanoparticles. In this experiment, the Cr2S3/IPA solution was kept at the 1 mm sample thickness of quartz cuvette, by utilizing a travel direct-drive stage (Thorlabs, DDSM100/M) and setting the step size to be 0.5 mm, the position of Cr2S3/IPA solution can be changed from z = − 20 mm to z = 20 mm. Furthermore, a home-made laser with an pulse width of 350 ps, center wavelength of 1064.1 nm and pulse repetition rate of 11.2 MHz was used as the laser source of Z-scan test. As given in Fig. 3(a), at the incident laser intensity of 0.133, 0.336, and 0.612 MW/cm2, the transmittance of Cr2S3/IPA solution versus the z position were tested, respectively. The normalized values of the OA Z-scan results were fitting by following formula (Xu et al. 2020):

$$\eqalign{ T\left( z \right) = & \Sigma _{n = 0}^\infty {{\rm{(}} - \beta {I_0}{L_{{\rm{eff}}}}{\rm{)}}^n}/{{\rm{(1 + }}{z^2}{\rm{/}}z_0^2{\rm{)}}^n}{{\rm{(}}n + 1{\rm{)}}^{3/2}} \cr & \approx \,1 - \beta {I_0}{L_{{\rm{eff}}}}/{2^{3/2}}{\rm{(1 + }}{z^2}{\rm{/}}z_0^2{\rm{)}} \cr}$$
(1)

where \(T\left(z\right)\) is the normalized transmittance, \(\beta\) is the nonlinear absorption coefficient, \({I}_{0}\) is the peak on-axis intensity at the focus, \({L}_{\text{eff}}=(1-{e}^{-{\alpha }_{0}L})/{\alpha \,}_{0}\) is the effective length, \({\alpha }_{0}\) is the linear absorption coefficient, \(L\) is the thickness of the sample, \(z\) is the position of the Cr2S3/IPA sample, \({z}_{0}=\pi {\omega }_{0}^{2}/\lambda\)is the Reyleigh length, \({\omega }_{0}\) is the beam waist and \(\lambda\)is the wavelength. The value of β are changed from − 5.4\(\times {10}^{4}\)cm/GW to -2.6\(\times {10}^{4}\)cm/GW versus input peak intensity from 0.133 MW/cm2 to 0.612 MW/cm2, respectively. In addition, the NLO of pure IPA solution was also measured by OA- Z-scan method and no NLO phenomenon was shown.

Furthermore, in order to check the modulation depth and saturable intensity of Cr2S3 nanoparticles, the OA results of the Cr2S3 material are also fitted using the most usual one-photon absorption SA model (Xu et al. 2020) as given in Eq. (2):

$$T=1-\varDelta T/(1+I/{I}_{s})-{\alpha }_{ns}$$
(2)

Where \(\varDelta T\) is the modulation depth, \(I\) is the incident laser intensity, \({I}_{s}\) is the saturable intensity, and \({\alpha }_{ns}\)is the nonlinear absorption loss. Figure 3(b) depicts the normalized transmittance versus input intensity laser at the input laser intensity of 0.612 MW/cm2. As given in Fig. 3(b), the modulation depth of 11.5% and saturable intensity of 0.115 MW/cm2 were obtained, respectively. These data derived from the OA Z-scan results represent the nonlinear optical absorption properties Cr2S3/IPA solution in the quartz cuvette. In fact, the Cr2S3-SA device is obtained by droping Cr2S3/IPA solution on a side polished fiber (SPF). So it is important to investigate the nonlinear optical absorption properties of the Cr2S3-SPF-SA device. In this work, a twin-detectors method as shown in Fig. 4(a) was employed to test the optical absorption properties of the Cr2S3-SPF-SA device. Here, the laser source is same with the OA Z-scan experiment. By this method, the modulation depth of 36.1% and saturable intensity of 49.5 MW/cm2 were obtained, respectively. From these results we can see that the Cr2S3-nanoparticles have relatively good nonlinear optical performance. Hence, this study demonstrate that the Cr2S3-nanoparticles has the potential to apply in nonlinear photonics applications.

Fig. 4
figure 4

(a) Schematic setup of twin-detectors method. (b) Nonlinear absorption property of the Cr2S3-SPF-SA

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3 Mode-locking and Q-switching of Cr2S3 nanoparticles based YDFL

The schematic steup of Yb-doped mode-locking or Q-switching fiber laser ring cavity is given in Fig. 5. As can be seen here a ring cavity was adopted. The cavity is composed by a 0.68 m long ytterbium-doped fiber (LIEKKI Yb1200-4/125, group velocity dispersion (GVD) of 24.22 ps2 km− 1), a 980/1060 nm wavelength division multiplexer (WDM), a polarization-independent isolator (PI-ISO), a polarization controller (PC), a Cr2S3-SPF-SA, and an output coupler (OC) with 9:1 coupling ratio. The pump laser source is a 976 nm laser diode (BL976-PAG900, Thorlabs). The length of single mode fiber (SMF, HI1060, GVD: 25.8 ps2 km− 1) which utilized to connect these elements was 17.4 m. To detect the laser pulse train and spectrum, an Oscilloscope (WaveRunner 44MXi, LeCroy, 400 MHz), a photodetector (DET01CFC, ThorLabs, 5 GHz bandwidth) and an optical spectrum analyzer (AQ6370B, 0.02 nm resolution, Yokogawa) were employed.

Fig. 5
figure 5

Schematic setup of Yb-doped mode-locking or Q-switching fiber laser ring cavity

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3.1 Q-switching of Cr2S3 nanoparticles based YDFL

In order to exclude the influence of PC or other factors, the laser cavity was operated without the Cr2S3 nanoparticles-SPF-SA device, in this condition, no matter how to tune the PC, no Q-switching or mode-locking operation appear. Then the pure SPF was inserted into the laser cavity, same to abovementioned situation, just continuous wave laser was observed. Then, a pure SPF-SA was inserted into the laser cavity and the Cr2S3/IPA solution was dropped on the SPF for several times, after drying the solution, a stable Q-switching laser was shown in the oscilloscope as the pump power exceeds 153 mW. As given in Fig. 6(a), the threshold of the Q-swtiching laser is 153 mW. With the pump power increases from 153 mW to the given maximum power of 249 mW, the corresponding average output powers are increased from 4.21 to 6.43 mW, while the pulse widths of Q-switching lasers are decreased from 7.01 to 2.95 µs. Correspondinglly, the pulse repetition rate (PRR) and single pulse energy (SPE) are increased from 55.6 to 59.6 kHz and 75.7 to 107.9 nJ, respectively, as can be seen from Fig. 6(b). Furthermore, the Q-switching laser spectrum at the incident pump power of 249 mW is shown in the inset picture of Fig. 6(b), the center wavelength is located at 1066.8 nm. Figure 6(c) and 6(e) shows the Q-switching laser pulse train and single Q-switching laser pulse at the threshold incident pump power of 153 mW, which can be seen from that the PPR of Q-switching laser is 55.6 kHz, while the pulse width is 7.01 µs. Figure 6(d) and 6(f) shows the Q-switching laser pulse train and single Q-switching laser pulse at the maximum incident pump power of 249 mW, where the corrsponding PPR and pulse width is 59.6 kHz and 2.95 µs, respectively.

Fig. 6
figure 6

(a) Average output power and pulse width of Q-switching laser versus incident pump power. (b) Single pulse energy and PPR of Q-switching laser versus incident pump power. Inset of (b): Laser spectrum at the incident pump power of 249 mW. (c), (d) Typical pulse train and single Q-switching laser pulse at the incident pump power of 153 mW. (e), (f) Typical pulse train and single Q-switching laser pulse at the incident pump power of 249 mW

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3.2 Mode-locking process of Cr2S3 nanoparticles based YDFL

Fig. 7
figure 7

(a) A typical pulse train of mode-locking laser. (b) Single mode-locking laser with pulse width of 10.2 ns. (c) Observed RF spectrum (inset is in 100 MHz scanning range). (d) Optical spectrum of the mode-locking pulse with 3 dB bandwidth of 0.66 nm at central wavelength of 1069.1 nm. (e) Average output powers of YDFL at pump power of 260 mW during 2 h of testing. (f) Laser spectrum of YDFL at pump power of 260 mW during 2 h of testing

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Then, when the incident pump power exceed 249 mW, the Q-switching laser operation change to be mode-locking laser operation. As shown in Fig. 7, the related stable mode-locking laser performances have been investigated at the incident pump power of 260 mW. Figure 7(a) shows a pulse train of YDFL continuous wave mode-locking (CWML) operation with a pulse interval of 89.8 ns. As we can see from Fig. 7(a) that pulse train is quite uniform which demonstrates that the YDFL mode-locking operation is stable. Figure 7(b) gives a typical mode-locking laser single pulse with pulse width of 10.2 ns that obtaining from Oscilloscope. Figure 7(c) illustrates that the pulse radio-frequency (RF) of the mode-locking laser is 11.13 MHz and the corresponding signal-noise ratio (SNR) is 66.9 dB. The RF of 11.13 MHz is also consistent with the pulse interval of 89.8 ns. Moreover, the RF spectrum with (0-100 MHz) wideband is also shown in Fig. 7(c). Figure 7(d) shows the corresponding wavelength spectrum, the central wavelength and 3 dB spectral bandwidth are 1069.1 nm and 0.66 nm, respectively. Furthermore, the related time-bandwidth product (TBP) of this mode-locking laser is calculated to be 1770, which means the mode-locking laser is severely chirped. This may be due to the reasons of self-modulation and normal dispersion when the pump power is relatively high (Zhang et al. 2014; Kang et al. 2014). In order to study the long-term laser stability of mode-locking operation, the average output powers and laser spectrum of mode-locking at the incident pump power of during 2 h of testing are shown in Fig. 7(e) and (f). From these two pictures, we can see that the long-term stability of mode-locking operation is relatively good.

Fig. 8
figure 8

(a) Single pulse energy versus incident pump power from 260 mW to 318 mW. (b) Long time scale pulse train at the incident pump power of 318 mW

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Moreover, the laser performances beyond the incident pump power of 260 mW are given in Fig. 8. As shown in Fig. 8(a), the single pulse energies of mode-locking lasers are increasing from 0.592 to 0.625 nJ versus incident pump powers increase from 260 to 318 mW. Figure 8(b) gives a 10 μm time scale of pulse train at the incident pump power of 318 mW, from which we can see that the Cr2S3-SA based YDFL mode-locking operation is quite stable. This may benefit from the good air stability of Cr2S3 material. To prevent the Cr2S3-SA from being damaged, the maximum incident pump power was set at 318 mW, however, no gain saturation phenomenon was been observed, which mean we can obtain more higher output powers. Furthermore, the mode-locking laser performance could be further optimised (e.g., the threshold of mode-locking laser, output pulse energy, stability). Such as the threshold of mode-locking laser may could be further optimised by changing the polished fibre length in SA device, or changing the sizes of Cr2S3 nanoparticles. The output pulse energy could be optimised by increasing the cavity length and changing the Yb3+ doping concentration in gain fibre. The laser stability could be optimised by adding another PC or adding a cooling measure for D-shape fibre device.

Table 1 shows the Q-switching laser and mode-locking laser performances of several 2D materials SA-based YDFL. From Table 1 we can see that the Cr2S3 nanoparticles SA based Q-switching laser has relatively narrow pulse duration and lower energy laser wavelength in comparsion with the other 2D materials SA-based Q-switching YDFL. For the YDFL mode-locking laser, the Cr2S3 nanoparticles SA based mode-locking laser has relatively high modulation depth and wide pulse duration. In some aspects, researchers pursue ultrafast laser with femtosecond or picosecond pulse duration for some applications, such as nonlinear optics, optical spectroscopy, etc. However, in the fields of medical treatment or micromachining, nanosecond pulse duration lasers with MHz pulse repetition rates are needed. Actually, solid state lasers can achieve Q-switching laser operation with nanosecond pulse duration, but it is a big difficulty for them to obatin the laser PPR with MHz level. In this way, the 2D Cr2S3 SA-based YDFL mode-locking laser with ns level pulse width and MHz level PPR could has a good application prospect in the fields of medical treatment or micromachining.

Table 1 Performances of mode-locking or Q-switching YDFL based on 2D material SAs
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4 Conclusion

In this paper, 2D trigonal Cr2S3 nanoparticles have been prepared by LPE method. The related nonlinear optical properties at 1 μm have been investigated by the OA Z-scan measurement. The nonlinear absorption coefficient value, modulation depth and saturation intensity are measured to be -2.6\(\times {10}^{4}\)cm/GW, 11.5% and 0.115 MW/cm2, respectively. In addition, by employing Cr2S3 nanoparticles as SA, stable mode locking and Q-switching laser operations in YDFL laser systems are observed, respectively. For the Q-switching laser operation, the obtained minimum pulse duration is 2.95 µs with a PRR of 59.6 kHz. For mode-locking laser operation, stable mode-locking laser with central wavelength of 1069.1 nm and pulse duration of 10.2 ns are obatined. In a word, this work indicates that 2D trigonal Cr2S3 nanoparticle is a promising material in the applications of photonic field.