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

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Near-Infrared Optical Modulation for Ultrashort Pulse Generation Employing Indium Monosulfide (Ins) Two-Dimensional Semiconductor Nanocrystals nanomaterials Article Near-Infrared Optical Modulation for Ultrashort Pulse Generation Employing Indium Monosulfide (InS) Two-Dimensional Semiconductor Nanocrystals Tao Wang, Jin Wang, Jian Wu *, Pengfei Ma, Rongtao Su, Yanxing Ma and Pu Zhou * College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China; [email protected] (T.W.); [email protected] (J.W.); [email protected] (P.M.); [email protected] (R.S.); [email protected] (Y.M.) * Correspondence: [email protected] (J.W.); [email protected] (P.Z.) Received: 14 May 2019; Accepted: 3 June 2019; Published: 7 June 2019 Abstract: 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. Keywords: indium monosulfide; saturable absorber; mode-locked fiber laser; pulse laser 1. Introduction Pulse fiber lasers have widespread and important applications in medicine, remote sensing, telecommunication, and material processing, etc. [1–3]. In recent years, passively mode-locked fiber lasers based on novel two-dimensional (2D) materials as saturable absorbers (SAs) have attracted much attention. As is widely known, extensive investigations on graphene [4–11] have greatly promoted the exploration of new 2D materials. Starting from the enthusiastic study of graphene, various kinds of materials, such as carbon nanotubes (CNT) [12–15], transition metal dichalcogenides (TMDs) [16–21], topological insulators (TIs) [22–26], and black phosphorus (BP) [27–31], have been employed as SAs and have successfully demonstrated pulse laser operation from visible to mid-infrared optical regions. Recently, III–VI group compounds (MX; M = Ga, In; X = Te, Se, S) have also gained widespread attention and have been applied in the areas of optoelectronic devices, nonlinear optics, ultrafast laser, and terahertz generation due to their large nonlinear effect, high damage threshold, dramatic photo-response and suitable band-gap [32–45]. For example, in 2018, Xu et al. presented a mode-locked laser in the 1 µm region based on InSe as a SA [36]. Other compounds, such as GaS and GaSe, were also shown to be suitable for photo-electric devices [37–40]. It is obvious that exploration into the practical Nanomaterials 2019, 9, 865; doi:10.3390/nano9060865 www.mdpi.com/journal/nanomaterials Nanomaterials 2019, 9, 865 2 of 10 Nanomaterials 2019, 9, x FOR PEER REVIEW 2 of 10 inapplications its infancy. of Thus, III–VI it group is of compoundsnecessity to exhibitsstudy such significance, compounds but isfor still their in itsoptical infancy. performance Thus, it is in of ultrafastnecessity laser to study generation. such compounds for their optical performance in ultrafast laser generation. IndiumIndium monosulfide monosulfide (InS) (InS) is is another another promising promising negative negative (n)-type (n)-type semiconductor, semiconductor, which which belongs belongs toto the the III–VI III–VI group group compounds compounds family family [41]. [41]. InS InS has has be beenen reported reported to to exist exist in in two two different different crystalline crystalline phases—layered and and network structural forms—wh forms—whichich are are connected by fundamental S–In–In–S unitsunits [45]. [45]. There There have have also also been been several several studies studies conc concerningerning the the optical optical and and electrical electrical properties properties of InS of [41–44].InS [41– 44The]. Thedirect direct and and indirect indirect band-gaps band-gaps of ofInS InS are are 2.4 2.4 eV eV and and 1.9 1.9 eV eV at at room room temperature, temperature, respectivelyrespectively [41,42], [41,42], which which render render the the InS crystal more sensitive to near-infrared wavelengths than visible wavelengths. wavelengths. InS InS also also has has great great potential potential for for next-generation next-generation flexible flexible and and planar planar devices. devices. In 2016,In 2016, HoHo et etal. al. experimentallyexperimentally generated generated single single crystals crystals of of InS InS micro micro st stripe,ripe, which which were were grown throughthrough a a physical physical vapor vapor transport transport method. method. Based Based on these on these stripe-like stripe-like InS crystals, InS crystals, a prototype a prototype photo- metal-semiconductorphoto-metal-semiconductor field effect field transistor effect transistor (FET) (FET)was successfully was successfully constructed constructed [44]. [However,44]. However, the nonlinearthe nonlinear saturable saturable absorption absorption properties properties of InS of ha InSve have rarely rarely been been explored explored and andare absent are absent from from the literaturethe literature at this atthis stage stage of research, of research, to tothe the best best of ofour our knowledge. knowledge. It Itis is expected expected that that InS InS will will have have comparablecomparable optical performance to other reported SAs (such as BP, TIs, TMDs, etc.) due to its its suitable suitable band-gapband-gap value and typi typicalcal layered structure. InIn thisthis paper,paper, a a passively passively mode-locked mode-locked Yb-doped Yb-doped fiber fiber laser laser based based on InS-polyvinyl on InS-polyvinyl alcohol alcohol (PVA) (PVA)SA was, SA to was, our to knowledge, our knowledge, demonstrated demonstrated for the for first the time.first time. The The InS nanosheetsInS nanosheets were were prepared prepared via viaa liquid-phase a liquid-phase exfoliation exfoliation (LPE) (LPE) method. method. The saturation The saturation intensity intensity of the InS-SA of the thin InS-SA film wasthin measuredfilm was measuredto be 6.79 MWto be/cm 6.792, while MW/cm the2 modulation, while the modulation depth was 5.7%. depth With was the 5.7%. help With of this the high-quality help of this InS-SA, high- qualitystable, mode-lockedInS-SA, stable, pulses mode-locked could be pulses observed. could The be outputobserved. laser The had ou atput pulse laser width had ofa pulse 486.7 width ps at the of 486.7fundamental ps at the cavityfundamental frequency cavity of 1.02frequency MHz. of Harmonic 1.02 MHz. mode-locked Harmonic mode-locked pulses were pulses also realized were also by realizedadjusting by the adjusting polarization the statespolarization in the cavity. states Ourin th resultse cavity. indicated Our results that InS, indicated as an eff ectivethat InS, SA, as could an effectivebe applied SA, in could the field be applied of ultrafast in the pulse field lasers. of ultrafast pulse lasers. 2.2. Preparation Preparation and and Characterization Characterization of of the the InS InS Sample Sample TheThe production production process process of of the the InS-PVA InS-PVA SA SA is isshown shown in inFigure Figure 1. 1In. Inour our experiment, experiment, the commonly-reportedthe commonly-reported LPE LPE method method [36] [36 was] was employed employed to toprepare prepare the the InS InS solution. solution. This This solution solution was was furtherfurther processedprocessed intointo the the film-type film-type InS-PVA InS-PVA SA usingSA using the spin-coating the spin-coating method method [36]. Firstly, [36]. Firstly, to prepare to preparethe InS solution,the InS solution, 50 mg InS 50 powdermg InS waspowder added was to added ethanol to solutionethanol withsolution a concentration with a concentration of 30% and of 30%soaked and for soaked 24 h. Then,for 24 the h. Then, solution the was solution placed was in an placed ultrasonic in ancleaner ultrasonic for 8cleaner h to carry for out8 h liquid-phaseto carry out liquid-phaseexfoliation. Subsequently, exfoliation. Subsequently, the ultrasonic the solution ultrasonic was allowed solution to was rest allowed for 1 h and to rest the supernatantfor 1 h and wasthe supernatantdecanted to was mix decanted with 4 wt% to mix polyvinyl with 4 alcoholwt% polyvinyl (PVA) solution. alcohol (PVA) The mixing solution. ratio The was mixing 1:1 by ratio volume. was 1:1The by mixed volume. solution The wasmixed placed solution back was into placed the ultrasonic back into cleaner the ultrasonic for 3 h to cleaner form a for uniform 3 h to InS-PVA form a uniformsolution. InS-PVA Then, 20 solution.µL of the Then, mixed 20 solution μL of wasthe mixed spin-coated solution onto was the spin-coated substrate (antistatic onto the glass)substrate and (antistaticplaced in aglass) 20 C and incubator placed forin a 24 20 h. °C Finally, incubator an InS-PVA for 24 h. thin Finally, film an was InS-PVA formed, thin and film a 1 was1 mm formed,2 film ◦ × andsample a 1 × was 1 mm cut2 film off from sample the was thin cut film off and from transferred the thin film onto and the transferred end face of aonto fiber the ferrule, end face forming of a fiber the ferrule,SA device.
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