Near-Infrared Optical Modulation for Ultrashort Pulse Generation Employing Indium Monosulfide (InS) Two-Dimensional Semiconductor Nanocrystals

In recent years, metal chalcogenide nanomaterials have received much attention in the field of ultrafast lasers due to their unique band-gap characteristic and excellent optical properties. In this work, two-dimensional (2D) indium monosulfide (InS) nanosheets were synthesized through a modified liquid-phase exfoliation method. In addition, a film-type InS-polyvinyl alcohol (PVA) saturable absorber (SA) was prepared as an optical modulator to generate ultrashort pulses. The nonlinear properties of the InS-PVA SA were systematically investigated. The modulation depth and saturation intensity of the InS-SA were 5.7% and 6.79 MW/cm2, respectively. By employing this InS-PVA SA, a stable, passively mode-locked Yb-doped fiber laser was demonstrated. At the fundamental frequency, the laser operated at 1.02 MHz, with a pulse width of 486.7 ps, and the maximum output power was 1.91 mW. By adjusting the polarization states in the cavity, harmonic mode-locked phenomena were also observed. To our knowledge, this is the first time an ultrashort pulse output based on InS has been achieved. The experimental findings indicate that InS is a viable candidate in the field of ultrafast lasers due to its excellent saturable absorption characteristics, which thereby promotes the ultrafast optical applications of InX (X = S, Se, and Te) and expands the category of new SAs.

Indium monosulfide (InS) is another promising negative (n)-type semiconductor, which belongs to the III-VI group compounds family [41]. InS has been reported to exist in two different crystalline phases-layered and network structural forms-which are connected by fundamental S-In-In-S units [45]. There have also been several studies concerning the optical and electrical properties of InS [41][42][43][44]. The direct and indirect band-gaps of InS are 2.4 eV and 1.9 eV at room temperature, respectively [41,42], which render the InS crystal more sensitive to near-infrared wavelengths than visible wavelengths. InS also has great potential for next-generation flexible and planar devices. In 2016, Ho et al. experimentally generated single crystals of InS micro stripe, which were grown through a physical vapor transport method. Based on these stripe-like InS crystals, a prototype photo-metal-semiconductor field effect transistor (FET) was successfully constructed [44]. However, the nonlinear saturable absorption properties of InS have rarely been explored and are absent from the literature at this stage of research, to the best of our knowledge. It is expected that InS will have comparable optical performance to other reported SAs (such as BP, TIs, TMDs, etc.) due to its suitable band-gap value and typical layered structure.
In this paper, a passively mode-locked Yb-doped fiber laser based on InS-polyvinyl alcohol (PVA) SA was, to our knowledge, demonstrated for the first time. The InS nanosheets were prepared via a liquid-phase exfoliation (LPE) method. The saturation intensity of the InS-SA thin film was measured to be 6.79 MW/cm 2 , while the modulation depth was 5.7%. With the help of this high-quality InS-SA, stable, mode-locked pulses could be observed. The output laser had a pulse width of 486.7 ps at the fundamental cavity frequency of 1.02 MHz. Harmonic mode-locked pulses were also realized by adjusting the polarization states in the cavity. Our results indicated that InS, as an effective SA, could be applied in the field of ultrafast pulse lasers.

Preparation and Characterization of the InS Sample
The production process of the InS-PVA SA is shown in Figure 1. In our experiment, the commonly-reported LPE method [36] was employed to prepare the InS solution. This solution was further processed into the film-type InS-PVA SA using the spin-coating method [36]. Firstly, to prepare the InS solution, 50 mg InS powder was added to ethanol solution with a concentration of 30% and soaked for 24 h. Then, the solution was placed in an ultrasonic cleaner for 8 h to carry out liquid-phase exfoliation. Subsequently, the ultrasonic solution was allowed to rest for 1 h and the supernatant was decanted to mix with 4 wt% polyvinyl alcohol (PVA) solution. The mixing ratio was 1:1 by volume. The mixed solution was placed back into the ultrasonic cleaner for 3 h to form a uniform InS-PVA solution. Then, 20 µL of the mixed solution was spin-coated onto the substrate (antistatic glass) and placed in a 20 • C incubator for 24 h. Finally, an InS-PVA thin film was formed, and a 1 × 1 mm 2 film sample was cut off from the thin film and transferred onto the end face of a fiber ferrule, forming the SA device. Nanomaterials 2019, 9, x FOR PEER REVIEW 2 of 10 in its infancy. Thus, it is of necessity to study such compounds for their optical performance in ultrafast laser generation. Indium monosulfide (InS) is another promising negative (n)-type semiconductor, which belongs to the III-VI group compounds family [41]. InS has been reported to exist in two different crystalline phases-layered and network structural forms-which are connected by fundamental S-In-In-S units [45]. There have also been several studies concerning the optical and electrical properties of InS [41][42][43][44]. The direct and indirect band-gaps of InS are 2.4 eV and 1.9 eV at room temperature, respectively [41,42], which render the InS crystal more sensitive to near-infrared wavelengths than visible wavelengths. InS also has great potential for next-generation flexible and planar devices. In 2016, Ho et al. experimentally generated single crystals of InS micro stripe, which were grown through a physical vapor transport method. Based on these stripe-like InS crystals, a prototype photometal-semiconductor field effect transistor (FET) was successfully constructed [44]. However, the nonlinear saturable absorption properties of InS have rarely been explored and are absent from the literature at this stage of research, to the best of our knowledge. It is expected that InS will have comparable optical performance to other reported SAs (such as BP, TIs, TMDs, etc.) due to its suitable band-gap value and typical layered structure.
In this paper, a passively mode-locked Yb-doped fiber laser based on InS-polyvinyl alcohol (PVA) SA was, to our knowledge, demonstrated for the first time. The InS nanosheets were prepared via a liquid-phase exfoliation (LPE) method. The saturation intensity of the InS-SA thin film was measured to be 6.79 MW/cm 2 , while the modulation depth was 5.7%. With the help of this highquality InS-SA, stable, mode-locked pulses could be observed. The output laser had a pulse width of 486.7 ps at the fundamental cavity frequency of 1.02 MHz. Harmonic mode-locked pulses were also realized by adjusting the polarization states in the cavity. Our results indicated that InS, as an effective SA, could be applied in the field of ultrafast pulse lasers.

Preparation and Characterization of the InS Sample
The production process of the InS-PVA SA is shown in Figure 1. In our experiment, the commonly-reported LPE method [36] was employed to prepare the InS solution. This solution was further processed into the film-type InS-PVA SA using the spin-coating method [36]. Firstly, to prepare the InS solution, 50 mg InS powder was added to ethanol solution with a concentration of 30% and soaked for 24 h. Then, the solution was placed in an ultrasonic cleaner for 8 h to carry out liquid-phase exfoliation. Subsequently, the ultrasonic solution was allowed to rest for 1 h and the supernatant was decanted to mix with 4 wt% polyvinyl alcohol (PVA) solution. The mixing ratio was 1:1 by volume. The mixed solution was placed back into the ultrasonic cleaner for 3 h to form a uniform InS-PVA solution. Then, 20 μL of the mixed solution was spin-coated onto the substrate (antistatic glass) and placed in a 20 °C incubator for 24 h. Finally, an InS-PVA thin film was formed, and a 1 × 1 mm 2 film sample was cut off from the thin film and transferred onto the end face of a fiber ferrule, forming the SA device.   The characteristics of the prepared InS sample are presented in Figure 2. A scanning electron microscopy (SEM) (Sigma 500, ZEISS, Oberkochen, Germany) image of typical InS flakes is shown in Figure 2a. This measured SEM image shows the typical hexagonal structure of 2D InS flakes, suggesting that the LPE method can be employed to obtain few-layer InS nanosheets. In addition, energy-dispersive X-ray (EDX) spectroscopy was used to characterize the elemental composition of the prepared InS. As shown in Figure 2b, the peaks corresponding to the S and In elements are clear, corresponding with a previous report [42]. In the inset of the figure, the elements In and S are shown to have an atom ratio of 51.61:48.39, which is approximate to 1:1, indicating that the InS flakes had the correct elemental composition and uniform crystal quality. Additionally, X-ray diffraction (XRD) (D8 Advance, Bruker, Billerica, MA, USA) was used to characterize the structure of the prepared InS flakes. As shown in Figure 2c, typical peaks, which match well with standard PDF card #19-0588, were recorded. In particular, the obvious (101) and (004) planes indicated that InS nanosheets with high crystallization quality were prepared in the experiment. As shown in Figure 2d, the InS flakes were also characterized by Raman spectroscopy (LabRAM HR Evolution, Horiba, Kyoto, Japan). Three typical Raman shift peaks located at 148.2 cm −1 (In-In stretching mode), 219.3 cm −1 (S-In bending mode), and 314.5 cm −1 (S-In stretching mode) corresponded with literature data [46,47]. Figure 2e shows a representative transmission electron microscopy (TEM) image (JEM-2100, JEOL, Tokyo, Japan) of the InS solution, which clearly indicates the InS nanosheets had a plicated morphology and an apparent layered crystal structure. . This measured SEM image shows the typical hexagonal structure of 2D InS flakes, suggesting that the LPE method can be employed to obtain few-layer InS nanosheets. In addition, energy-dispersive X-ray (EDX) spectroscopy was used to characterize the elemental composition of the prepared InS. As shown in Figure 2b, the peaks corresponding to the S and In elements are clear, corresponding with a previous report [42]. In the inset of the figure, the elements In and S are shown to have an atom ratio of 51.61:48.39, which is approximate to 1:1, indicating that the InS flakes had the correct elemental composition and uniform crystal quality. Additionally, X-ray diffraction (XRD) (D8 Advance, Bruker, Billerica, MA, USA) was used to characterize the structure of the prepared InS flakes. As shown in Figure 2c, typical peaks, which match well with standard PDF card #19-0588, were recorded. In particular, the obvious (101) and (004) planes indicated that InS nanosheets with high crystallization quality were prepared in the experiment. As shown in Figure 2d, the InS flakes were also characterized by Raman spectroscopy (LabRAM HR Evolution, Horiba, Kyoto, Japan). Three typical Raman shift peaks located at 148.2 cm −1 (In-In stretching mode), 219.3 cm −1 (S-In bending mode), and 314.5 cm −1 (S-In stretching mode) corresponded with literature data [46,47].  The saturable absorption properties of the InS-PVA thin film were also studied based on the previously reported power-dependent transmission technique [48,49]. The seed laser was a selfmade, Yb-doped, mode-locked fiber pulse laser with a central wavelength of 1060.2 nm, repetition rate of 12.36 MHz, and pulse width of 19 ps. The maximum output power was 185 mW after a onestage amplifier. The nonlinear transmittance of the InS thin film at different incident laser power levels is recorded in Figure 2f. The experimental data were fitted using the following formula [50]: The saturable absorption properties of the InS-PVA thin film were also studied based on the previously reported power-dependent transmission technique [48,49]. The seed laser was a self-made, Yb-doped, mode-locked fiber pulse laser with a central wavelength of 1060.2 nm, repetition rate of 12.36 MHz, and pulse width of 19 ps. The maximum output power was 185 mW after a one-stage amplifier. The nonlinear transmittance of the InS thin film at different incident laser power levels is recorded in Figure 2f. The experimental data were fitted using the following formula [50]: where T(I) is the optical transmittance, I is the incident laser power intensity, α s is the modulation depth, I sat is the saturable intensity, and α ns is the unsaturated loss. The fitted saturation intensity and modulation depth were 6.79 MW/cm 2 and 5.7%, respectively.

Experimental Setup and Results
The experimental setup for the passively mode-locked Yb-doped fiber laser is presented in Figure 3. The gain medium was a 25 cm long Yb-doped fiber (Yb-1200) with a dispersion coefficient of 22 ps 2 /km. The gain fiber was pumped by a single-mode 980 nm laser diode, with a maximum output power of 580 mW, through a broadband wavelength division multiplexing system. In order to ensure unidirectional laser propagation, a polarization-insensitive isolator (PI-ISO) was employed. Two polarization controllers were adopted to optimize the polarization states in the laser cavity. A 10/90 output coupler was adopted to extract the energy from its 10% port. The InS-PVA SA was deposited onto the end face of a fiber ferrule adapter and connected between the coupler and PI-ISO to induce mode-locked operation. The total cavity length was about 206.9 m (0.25 m gain fiber and 206.65 m single-mode fiber with dispersion coefficient of 22 ps 2 /km) and the net dispersion value of this laser cavity was~4.55 ps 2 . This cavity operated in an all-normal dispersion regime. where T(I) is the optical transmittance, I is the incident laser power intensity, αs is the modulation depth, Isat is the saturable intensity, and αns is the unsaturated loss. The fitted saturation intensity and modulation depth were 6.79 MW/cm 2 and 5.7%, respectively.

Experimental Setup and Results
The experimental setup for the passively mode-locked Yb-doped fiber laser is presented in Figure 3. The gain medium was a 25 cm long Yb-doped fiber (Yb-1200) with a dispersion coefficient of 22 ps 2 /km. The gain fiber was pumped by a single-mode 980 nm laser diode, with a maximum output power of 580 mW, through a broadband wavelength division multiplexing system. In order to ensure unidirectional laser propagation, a polarization-insensitive isolator (PI-ISO) was employed. Two polarization controllers were adopted to optimize the polarization states in the laser cavity. A 10/90 output coupler was adopted to extract the energy from its 10% port. The InS-PVA SA was deposited onto the end face of a fiber ferrule adapter and connected between the coupler and PI-ISO to induce mode-locked operation. The total cavity length was about 206.9 m (0.25 m gain fiber and 206.65 m single-mode fiber with dispersion coefficient of 22 ps 2 /km) and the net dispersion value of this laser cavity was ~4.55 ps 2 . This cavity operated in an all-normal dispersion regime. Using the described configuration of the Yb-doped ring fiber laser, which incorporated the InSbased SA, stable mode-locked pulses could be observed when the pump power was increased to 231.5 mW. The threshold power in this work was relatively high compared with previously reported results of other SAs [10,11,20,26]. This high threshold power was probably due to the large cavity loss brought by the devices. The output power increased linearly with increasing pump power, as shown in Figure 4. At the pump power of 258.6 mW, the maximum output power obtained in this work was 1.91 mW. The slope efficiency and optical-to-optical conversion efficiency of this cavity were 7.05% and 0.74%, respectively. Higher conversion efficiency and output power could be expected if an output coupler with a higher output coupling ratio was employed. In future experiments, the cavity parameters should be further optimized to enhance the output performance of the pulse fiber laser. Using the described configuration of the Yb-doped ring fiber laser, which incorporated the InS-based SA, stable mode-locked pulses could be observed when the pump power was increased to 231.5 mW. The threshold power in this work was relatively high compared with previously reported results of other SAs [10,11,20,26]. This high threshold power was probably due to the large cavity loss brought by the devices. The output power increased linearly with increasing pump power, as shown in Figure 4. At the pump power of 258.6 mW, the maximum output power obtained in this work was 1.91 mW. The slope efficiency and optical-to-optical conversion efficiency of this cavity were 7.05% and 0.74%, respectively. Higher conversion efficiency and output power could be expected if an output coupler with a higher output coupling ratio was employed. In future experiments, the cavity parameters should be further optimized to enhance the output performance of the pulse fiber laser.
brought by the devices. The output power increased linearly with increasing pump power, as shown in Figure 4. At the pump power of 258.6 mW, the maximum output power obtained in this work was 1.91 mW. The slope efficiency and optical-to-optical conversion efficiency of this cavity were 7.05% and 0.74%, respectively. Higher conversion efficiency and output power could be expected if an output coupler with a higher output coupling ratio was employed. In future experiments, the cavity parameters should be further optimized to enhance the output performance of the pulse fiber laser.  Typical pulse properties are shown in Figure 5. Figure 5a depicts the output spectrum of the mode-locked pulse laser. The spectrum has central wavelengths of 1033.3 nm and 1038.4 nm with 3-dB bandwidths of 0.07 nm and 0.10 nm, respectively. The fundamental frequency pulse repeated at the cavity roundtrip time and the repetition rate was 1.02 MHz with a pulse-to-pulse interval of 0.98 µs, as shown in Figure 5b. A magnified single pulse is depicted in Figure 5c. The pulse envelope was further fitted by a Gaussian function and the fitted pulse duration was 486.7 ps. The radio frequency (RF) spectrum of the output pulse was also characterized, as shown in Figure 5d. The signal-to-noise ratio (SNR) was measured to be~47.3 dB in a small frequency span of 2.5 MHz (0.5-3 MHz) with a resolution of 100 Hz. The RF spectrum in a large range of 12 MHz is presented in the inset of Figure 5d. These results indicated that the pulses obtained in this work had high stability. Typical pulse properties are shown in Figure 5. Figure 5a depicts the output spectrum of the mode-locked pulse laser. The spectrum has central wavelengths of 1033.3 nm and 1038.4 nm with 3-dB bandwidths of 0.07 nm and 0.10 nm, respectively. The fundamental frequency pulse repeated at the cavity roundtrip time and the repetition rate was 1.02 MHz with a pulse-to-pulse interval of 0.98 μs, as shown in Figure 5b. A magnified single pulse is depicted in Figure 5c. The pulse envelope was further fitted by a Gaussian function and the fitted pulse duration was 486.7 ps. The radio frequency (RF) spectrum of the output pulse was also characterized, as shown in Figure 5d. The signal-to-noise ratio (SNR) was measured to be ~47.3 dB in a small frequency span of 2.5 MHz (0.5-3 MHz) with a resolution of 100 Hz. The RF spectrum in a large range of 12 MHz is presented in the inset of Figure  5d. These results indicated that the pulses obtained in this work had high stability. Besides the aforementioned fundamental frequency mode-locked pulses, harmonic modelocked pulses could also be observed at a pump power of 236.1 mW. The high-order harmonic modelocked pulses were achieved by adjusting the polarization controllers (PCs) carefully. Figure 6a,b show the spectrum and pulse train of a 2nd order mode-locked pulse. The spectrum has central wavelengths of 1033.3 nm and 1035.8 nm. The intensity of the 1035.8 nm wavelength was much lower than that of the 1033.3 nm-by about 6.3 dB. Thus, the 2nd harmonic generation actually corresponded to a single emission wavelength. The pulse-to-pulse time was about 0.49 μs, Besides the aforementioned fundamental frequency mode-locked pulses, harmonic mode-locked pulses could also be observed at a pump power of 236.1 mW. The high-order harmonic mode-locked pulses were achieved by adjusting the polarization controllers (PCs) carefully. Figure 6a,b show the spectrum and pulse train of a 2nd order mode-locked pulse. The spectrum has central wavelengths of 1033.3 nm and 1035.8 nm. The intensity of the 1035.8 nm wavelength was much lower than that of the 1033.3 nm-by about 6.3 dB. Thus, the 2nd harmonic generation actually corresponded to a single emission wavelength. The pulse-to-pulse time was about 0.49 µs, corresponding to the 2nd harmonic frequency. Figure 6c presents the spectrum of a 3rd order mode-locked pulse. The spectrum also has two wavelength components. The top of the 1033 nm wavelength component has a collapse, which is different from the 2nd harmonic operation. In Figure 6d, pulse trains corresponding to the 3rd harmonic pulse are presented. As is known, harmonic pulse generation is the result of a balance between output energy and laser gain. In our experiment, due to the limited pump source and low optical efficiency, no higher harmonic generations were able to be obtained. However, based on a higher pump source and low-loss laser cavity, higher harmonic pulses could be obtained in our future works. As mentioned, the total laser cavity length was about 206.9 m. Within a long-length laser cavity, self-mode-locked or Q-switched operations are easily observed due to the Kerr effect within the single-mode fiber under high energy [51,52]. In order to verify the effect of the InS-PVA SA on the mode-locked pulse, the sample was removed from the cavity. In this state, as the pump power was increased from 0 to 580 mW, no matter how we adjusted the PCs, no mode-locked pulse was observed, and only continuous wave operation was found. This indicated that the saturable absorption of InS induced the mode-locked pulses. In addition, it was noted that the band-gap of 1.2 eV, corresponding to the mode-locked operation at 1.03 μm in this work, is lower than the band-gap of InS (direct and indirect band-gaps of 2.4 eV and 1.9 eV, respectively), suggesting that sub-band-gap saturable absorption of InS resulted in the observed mode-locked operation. In fact, pulse fiber lasers induced by a sub-band-gap saturable absorption phenomenon have been reported by different groups [35,53,54]. Ideally, perfect crystals have no sub-band-gap absorption, but in a finite crystal structure, sub-band-gap absorption can be observed due to the edge-state [54]. Therefore, the sub-band-gap absorption phenomenon in this work could also be ascribed to edge-state absorption by the InS-PVA SA.
In Table 1, the optical performance parameters of Yb-doped mode-locked lasers based on different SAs are shown. Various mode-locked modulators, including graphene (oxide), CNT, TMD (MoS2, WS2), TI (Bi2Te3, Bi2Se3), BP, InSe and InS, have been employed as SAs successfully in the 1μm region. The pulse laser constructed in this work had similar optical performance to other modelocked lasers described in the literature. The relatively high pulse threshold of our laser was due to larger intra-cavity loss. Higher output power and a smaller threshold may be expected if the optical As mentioned, the total laser cavity length was about 206.9 m. Within a long-length laser cavity, self-mode-locked or Q-switched operations are easily observed due to the Kerr effect within the single-mode fiber under high energy [51,52]. In order to verify the effect of the InS-PVA SA on the mode-locked pulse, the sample was removed from the cavity. In this state, as the pump power was increased from 0 to 580 mW, no matter how we adjusted the PCs, no mode-locked pulse was observed, and only continuous wave operation was found. This indicated that the saturable absorption of InS induced the mode-locked pulses. In addition, it was noted that the band-gap of 1.2 eV, corresponding to the mode-locked operation at 1.03 µm in this work, is lower than the band-gap of InS (direct and indirect band-gaps of 2.4 eV and 1.9 eV, respectively), suggesting that sub-band-gap saturable absorption of InS resulted in the observed mode-locked operation. In fact, pulse fiber lasers induced by a sub-band-gap saturable absorption phenomenon have been reported by different groups [35,53,54]. Ideally, perfect crystals have no sub-band-gap absorption, but in a finite crystal structure, sub-band-gap absorption can be observed due to the edge-state [54]. Therefore, the sub-band-gap absorption phenomenon in this work could also be ascribed to edge-state absorption by the InS-PVA SA.
In Table 1, the optical performance parameters of Yb-doped mode-locked lasers based on different SAs are shown. Various mode-locked modulators, including graphene (oxide), CNT, TMD (MoS 2 , WS 2 ), TI (Bi 2 Te 3 , Bi 2 Se 3 ), BP, InSe and InS, have been employed as SAs successfully in the 1µm region. The pulse laser constructed in this work had similar optical performance to other mode-locked lasers described in the literature. The relatively high pulse threshold of our laser was due to larger intra-cavity loss. Higher output power and a smaller threshold may be expected if the optical devices are further optimized. This work expanded the category of new SAs, indicating that InS is a promising candidate for ultrafast lasers.

Conclusions
In conclusion, a passively mode-locked Yb-doped fiber pulse laser incorporating an InS-PVA SA was demonstrated experimentally. The nonlinear optical properties of the InS-PVA SA prepared by the LPE method were studied, and the saturation intensity and modulation depth of the InS thin film were 6.79 MW/cm 2 and 5.7%, respectively. When the pump power was increased to 231.5 mW, stable, mode-locked pulses could be observed, and the maximum output power was recorded as 1.91 mW. The mode-locked laser had a pulse width of 486.7 ps at the fundamental cavity frequency of 1.02 MHz, with a SNR of~47.3 dB. In addition, harmonic mode-locked pulses were achieved by rotating the PCs in the cavity. These experimental results indicate that InS can be employed as an effective SA for ultrafast laser generation. Considering its many attractive qualities, such as a suitable band-gap and excellent saturable absorption characteristics, it is expected that InS will also find important application in nonlinear optics where an SA material is required.