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Abstract
In our works, germanium monosulfide (GeS) was employed as a saturable absorber for obtaining passively mode locked fiber laser. Unlike two-dimensional transition metal dichalcogenides, we report the nonlinear optical properties of GeS and its application as novel saturable absorbers in generating mode-locked lasers. At present, reports of this 2D material applied to ultrafast lasers are rare. By inserting a saturable absorber in the fiber laser cavity, we obtained pulse duration of 854 femtosecond and 560 picosecond laser pulse output in the erbium-doped fiber laser and the ytterbium-doped fiber laser, respectively. The experimental results display that GeS sheets have excellent saturable absorption properties, which open the new avenue for group IV monosulfide to laser photonic devices and will have a wide range of potential applications in the field of ultrafast photonics.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Semiconductor saturable absorber mirror (SESAM) invented in the early 1990s by Bell Labs researchers, is one of the key components for many passively mode-locked ultrafast laser sources. But it still has many problems such as narrow operating bandwidth (<100 nm), slow response time, and intricate manufacturing process costs. In recent years, new two-dimensional (2D) nanomaterials e.g. graphene [1–5], black phosphorus (BP) [6–8], transition-metal carbides (MXene) [9–11], transition metal dichalcogenides (TMDs) [12–23] and metal-organic framework (MOFs) [24,25] have been widely suitable as saturable absorbers (SAs). Their performance in the field of lasers also has been attracting more and more attention. The 2D material as a SA for achieving mode-locking laser has also been deeply explored by research communities. It is precisely because of its relatively superior optical properties that attract the constant investment of researchers, so that the new 2D nanomaterials have significantly been developed in the field of laser. Germanium sulfide (GeS) belongs to group IV monosulfide MX(M = Ge, Sn; X = S, Se, Te). MX has quickly attracted widespread attention due to its environmental protection, excellent electrical properties, and abundant earth reserves. These two-dimensional MX materials have long been found to have excellent application prospects in various fields such as optoelectronics, photodetectors, energy conversion, and storage. The reason is that the MX material has the same wrinkle structure as BP, which produces a large surface to volume ratio and chemical stability. Moreover, the MX has a bandgap value of 1.0 to 2.0 eV, which is suitable for use in the above fields. However, as far as we know, no related articles have been reported on the study of the nonlinear optical absorption characteristics of MX.
In this article, GeS sheets were successfully prepared and used as a SA to obtain Er-doped fiber laser (EDFL) and Yb-doped fiber laser (YDFL). By using GeS sheets as a SA, both passively mode-locked operation in EDFL and YDFL with very stable operation were successfully obtained. Our work suggests that GeS sheets could be new material for broadband nonlinear optics and be used for ultrafast Margins. However, to the best of our knowledge, the nonlinear optical absorption properties of MX have rarely been reported, and only one article has been reported [26].
2. Preparation and characterization of GeS sheets
2.1 Preparation of GeS sheets
GeS sheets are prepared by liquid-phase exfoliation method [27] using GeS powder in this experiment. The particular process is as follows, 0.05 gram of GeS powder is dispersed in a mixture of 30 ml alcohol and 10 ml deionized water, then the dispersion was sonicated continuously for 20 h using a high power (400 W) ultrasonic cleaner. After that, the dispersion was centrifuged at 5000 rpm for 5 min to clear unexfoliated bulk. Then the obtained supernatant is a GeS sheets suspension. To confirm the crystal structure of GeS, the crystalline phase was measured by X-ray diffraction using a Rigaku SmartLab X-ray diffractometer. The morphology and chemical composition of samples were analyzed by a field emission scanning electron microscope (SEM, JEOL Model JSM-6490) equipped with an energy-dispersive spectrometer (EDS). The thickness of the GeS sheets was measured by using an atomic force microscopy (AFM, Veeco Nanoscope V). Transmission electron microscopy (TEM, Talos F200S) image was recorded under the Schottky field emission electron gun acceleration voltage of 200 kV.
2.2 Nonlinear optics characterization of GeS sheets
We measured the saturable absorption properties of the GeS-SA using the balanced twin-detector measurement system shown as Fig. 1. The input laser source was a homemade mode-locked EDFL (central wavelength: ∼ 1560 nm, repetition rate: ∼ 11.8 MHz, pulse width: ∼1 ps). It was connected to a low dispersion fiber laser amplifier to achieve the required laser power. The amplified power is divided by a 50:50 fiber coupler, and the optical power is injected into the D-shaped fiber with GeS-SA. A power meter (PM20CH) is used to measure the laser power. The transmission of D-shaped fiber with GeS-SA can be obtained by calculating the ratio of the power in the two fiber arms. The cross-sectional area of D-shaped fiber core is 30.19 µm2. It is easy to determine the pump peak power density after measuring the pulse duration and the repeat frequency of the ultrafast laser source. The relationship between the transmittance of the GeS-SA and its incident peak power density is shown in Fig. 1(b). As the type of fiber optical elements are same, we didn’t consider the transmittance difference between the two arms of the D-shaped fiber when they were both without GeS-SA. It may bring some measurement deviations in our measurement experiment. The experimental data can be fitted by the following equation:
where I, Isat represent the input intensity, saturation intensity, respectively. αs and αns are the saturable and non-saturable absorption coefficients. For the GeS-SA sample that was used in this work, the modulation depth, nonsaturable intensity and saturation intensity were measured to be approximately 39%, 41.13% and 0.99 MW/cm2, respectively. These results showed that GeS-SA could be used to construct passively mode-locked fiber lasers.
Fig. 1. (a) Balanced twin-detector measurement system for measuring the saturable absorption properties of the GeS-SA. (b) Dependence of the transmittance of the GeS-SA on the incident peak power density.
2.3 Characterization of GeS sheets
Figure 2(a) is the atomic structure model of double-layer GeS which is a kind of monochalcogenides. It has an orthorhombic structure with a space group of Pnma. Because of the similar structure to black phosphorus, GeS is considered as a binary compound analogue of BP. The thickness of the monolayer GeS is 0.52 nm. In order to investigate the crystal structure of the powder, X-ray diffraction (XRD) pattern was measured and shown in Fig. 2(b). It shows a strong diffraction peak at 2θ=34.4°, which is indexed to the (400) crystal planes of GeS, indicating the well stacked layer structure of GeS. In order to observe the morphology, SEM was carried out, and the image is shown in Fig. 2(c). The layered structure of GeS can be observed clearly in Fig. 2(c) and the crystal size is approximately 2 µm.
Fig. 2. (a) Atomic structure model of double-layer GeS. (b) XRD pattern and (c) SEM image of GeS powder.
GeS powder was dispersed in a mixture of water and ethanol and then be treated using an ultrasonic exfoliation method. The obtained upper dispersion is a kind of brown dispersion as shown in Fig. 3(a) inset. The XRD profile of exfoliated GeS sheets was also obtained and shown in Fig. 3(a). The strong diffraction peak corresponding to (400) planes was still observed, indicating the stacked layered structure was reserved after ultrasonic treatment. Because of the excellent electrical conductivity of FTO, it was selected as the substrate for SEM measurement. GeS dispersion was dropped and dried on the fluorine-doped tin oxide (FTO) substrate for measurement, and the obtained SEM images are shown in Figs. 3(b) and 3(c). The size of GeS is about 1µm after ultrasonic treatment, much smaller than GeS powder. The composition of the sample was analyzed using EDS and the result is shown in Fig. 3(d). The spectrum shows strong peaks ascribed to Ge and S elements with an atomic ratio of 1.0839:1 which is very close to the stoichiometric ratio of GeS (Sn peaks are due to the FTO substrate).
Fig. 3. (a) XRD pattern of GeS sheets, inset is the photograph of GeS sheet dispersion. (b, c) SEM images of GeS sheets on FTO substrate. (d) EDS spectrum of GeS.
Figures 4(a) and 4(b) are TEM images of GeS sheets. They show that the size of GeS sheets are 1 µm below. The elemental mapping images shown in Figs. 4(c) and 4(d) demonstrate the uniform distribution of Ge and S, indicating the homogeneous composition. AFM images were presented in Fig. 5(a). As can be seen from the Fig. 5(a), the outline of the sample is clear. The height profile [Fig. 5(b)] indicates that the sheet has a thickness of 95 nm.
Fig. 4. (a) XRD pattern of GeS sheets, inset is the photograph of GeS sheet dispersion. (b, c) SEM images of GeS sheets on FTO substrate. (d) EDS spectrum of GeS.
3. Photonics applications
3.1 Experimental details
Fiber lasers are demonstrated based on the absorption characteristics of GeS sheets. The fiber laser structure diagram is shown in Fig. 6. As shown, a 980 nm LD was used as a pump source by a wavelength division multiplexer (WDM). The role of polarization independent isolation (PI-ISO) is to make the laser transmit in one direction in the laser cavity. The polarization controller (PC) is used to adjust the polarization state in the laser cavity. A 10:90 fiber output coupler with a HI 1060 fiber pigtail was used to export the laser signal. The steps on how to polish the D-shaped fiber are as follows. The single mode fiber is burnished by the grinding wheel with diamond sandpaper, and then is burned using the electrode discharge to improve the smoothness. The D-shaped fibers having an interaction length of 10 mm and the distance from the fiber core boundary to the side polished area is 1µm. The steps on how to polish the D-shaped fiber are described as follows. The single mode fiber is burnished by the grinding wheel with diamond sandpaper, and then is burned by using the electrode discharge to improve the smoothness. We dropped GeS sheets solution onto the side polished area of the D-shaped fiber to form SAs. Then, we insert GeS-SA between WDM and PC. The total cavity length of the YDFL is 18.12 m with a 1 m long Yb-doped fiber (250 dB/m @ 980 nm). In the EDFL with a total cavity length of 23.01 m, the gain fiber (4.45 dB/m @ 980 nm) is 3 m long. We mainly use an oscilloscope (Tektronix, DPO4104B) connected to a 4 GHz high-speed InGaAs photodetector, an optical spectrum analyzer (Yokogawa, AQ6370D), a radio frequency spectrometer (Rigol, DSA800) and optical power meter (Thorlabs, PM20CH) to measure the characteristic information of the mode-locked fiber laser.
Fig. 6. The schematic diagram of the ring cavity of the mode-locked fiber laser: LD: 980 nm laser diode with a maximum output power of 650 mW; YDL: Yb-doped fiber laser; EDL: Er-doped fiber laser; WDM: wavelength division multiplexer; PC: polarization controller; PI-ISO: polarization independent isolator; OC: 10% optical coupler.
3.2 Mode-locked fiber lasers based on GeS-SA
3.2.1 The characterization of Er-doped mode-locked fiber laser
Figure 7 illustrates the Er-doped mode-locked fiber laser performance based on GeS. The EDFL starts to self-starting mode locking operation as we raise the pump power from 85 mW to 445 mW. Figure 7(a) shows the optical spectrum of EDFL, which is central at 1556.92 nm with a 3 dB bandwidth of about 3.8 nm. The optical spectrum has obvious Kelly sidebands, which is the result of the interactions of the dispersion and nonlinearity in the laser cavity. Figure 7(b) shows the pulse duration of 854 fs for EDFL based on the full width at half-maximum(FWHM), which was measured by an optical autocorrelator (APE, Pulsecheck). Figure 7(c) demonstrates the optical spectrum long-term stability by measuring every 1-hour over 6 hours. There is almost no change in the optical spectrum, indicating that our fiber lasers have excellent long-term spectral stability. Figure 7(f) gives the relation between the ouput power and the pump power. As shown, when the pump power is increased from 85 mW to 445 mW, the average output power (2.75 mW to 13.72 mW) increases linearly with a power conversion efficiency of approximately 3%. The relatively low average output power in the mode-locked state is primarily due to the strong absorption of the GeS sheets.
Fig. 7. Characteristics of Er-doped mode-locked fiber laser: (a) output optical spectrum of mode-locked pulsed laser. (b) Pulse duration of 854 fs. (c) long-term stability: optical spectra measured every 1 h over 6 hours. (d) the output power versus pump power.
Figure 8(a) shows stable pulse trains with a pulse period of 110 ns, which matched the laser cavity length of 23.01 m. Figure 8(b) exhibits the pulse train with a 40 ms span indicated the stable mode-locking state. In order to verify both the stability of the laser mode-locking regime, the radio frequency (RF) spectrum was measured at both 120 MHz-broad frequency span and 15 MHz frequency span with 100 Hz resolution bandwidth. As depicted in the Fig. 8(d), the fundamental frequency of the EDFL is 9.05 MHz with a signal noise ratio (SNR) of 70 dB, indicating high quality in terms of good mode-locked stability.
Fig. 8. Characteristics of laser pulse train: (a)mode-locked pulse train under 1 microsecond. (b)output pulse train under 40 milliseconds. (c) RF spectrum with a SNR of 70 dB. (d)RF spectrum in a span of 120 MHz.
3.2.2 The characterization of Yb-doped mode-locked fiber laser
In the experiment, the laser output performance of the GeS-SA without inserting into the laser cavity was first studied. When we increased the pump power from 0 to 650 mW, the laser always outputs the continuous wave laser when we carefully adjusted the PC. Then, the GeS-SA is inserted into the laser cavity. By carefully adjusting the PC, a stable mode-locked operation is achieved when the pump power is 200 mW.
Figures 9(a)–9(d) gives the optical spectrum, single-pulse shape, the long-term stability, and variation curve of average output power with the pump power of the mode-locked YDFL, respectively. As shown in Fig. 9(a), the typical spectrum with a step-to-step width of 0.02 nm were observed with a central wavelength of 1036.88 nm. Figure 9(b) is the single pulse curve which is a Gaussian profile, which illustrated that the real pulse width is approximately 560 ps. The rise time of the oscilloscope we used is 350 ps (Tektronic DPO4104B, 1 GHz) and the measurement value of the laser pulse is very near to the oscilloscope limitation. So, the laser pulse duration maybe is more smaller than the measurement value. As Fig. 9(c) shows, we also measured the long-term stability of the optical spectrum at 20-minute intervals over 2 h. The relationship between the average output power of the fiber laser and the pump power is displayed in Fig. 9(d). It is clear that there is a linear correlation between the average output power and the pump power, and the slope efficiency is 3.48%.
Fig. 9. Characteristics of Yb-doped mode-locked fiber laser: (a) optical spectrum. (b) pulse trace with a FWHM of 560 ps. (c) long-term stability: optical spectrum measured every 20 minutes. (d) the output power versus pump power.
As shown in Fig. 10(a) and Fig. 10(b), we measured the typical pulse trains of the mode-locked laser at different bandwidth. As shown in the Fig. 10(a), the laser pulse interval is 87 ns and the corresponding pulse repetition rate is 11.40 MHz with 1kHz resolution bandwidth, which is consistent with the total length cavity of 18.12m. Figure 10(b) gives the pulse train with a 20 µs span indicated the stable mode-locking state. Here, we also measured the RF spectrum of mode-locked pulses under different bandwidths to study the output SNR of mode-locked pulse laser. A fundamental peak was located at the cavity repetition rate of 11.40 MHz with a SNR of 63 dB, as shown in Fig. 10(c). The wideband RF spectrum under 200 MHz span is presented in Fig. 10(d), which indicates the mode-locked pulses exhibit good stability.
Fig. 10. Characteristics of laser pulse train: (a) output pulse train recorded at a division of 100 ns/div. (b)output pulse recorded at a division of 2 µs/div. (c) RF spectrum with the SNR of 70 dB. (d)RF spectrum in a span of 200 MHz.
4. Conclusion
In summary, we have successfully demonstrated that GeS sheets can be used as an efficient saturable absorption material to generate ultrafast laser pulses in YDFL and EDFL. In YDFL, the center wavelength of the mode-locked laser pulse was 1038.8 nm, with a pulse duration of 560 ps, and its repetition rate and SNR were 11.40 MHz and 63 dB, respectively. In addition, the EDFL also worked in very stably mode-locked pulses operation with a center wavelength of 1556.92 nm and its pulse duration was 854 fs under a repetition rate of 9.05 MHz with an SNR of 70 dB. Our results exhibited that GeS sheets hold many attractive merits as a new type of nonlinear saturable absorption material for applications in ultrafast and nonlinear optics.
Funding
Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20170302153540973, JCYJ20170412111625378).
Disclosures
The authors declare no conflicts of interest.
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