Transition metal dichalcogenides based saturable absorbers for pulsed laser technology

Optical Materials 60 (2016) 601e617

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Transition metal dichalcogenides based saturable absorbers for pulsed laser technology* J. Mohanraj a, V. Velmurugan b, S. Sivabalan a, * a b

School of Electrical Engineering, VIT University, Vellore, 632 014, India Center for Nanotechnology Research, VIT University, Vellore, 632 014, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 May 2016 Received in revised form 5 August 2016 Accepted 6 September 2016

Ultrashort pulsed laser is an indispensable tool for the evolution of photonic technology in the present and future. This laser has been progressing tremendously with new pulse regimes and incorporating novel devices inside its cavity. Recently, a nanomaterial based saturable absorber (SA) was used in ultrafast laser that has improved the lasing performance and caused a reduction in the physical dimension when compared to conventional SAs. To date, the nanomaterials that are exploited for the development of SA devices are carbon nanotubes, graphene, topological insulators, transition metal dichalcogenides (TMDs) and black phosphorous. These materials have unique advantages such as high nonlinear optical response, fiber compatibility and ease of fabrication. In these, TMDs are prominent and an emerging twodimensional nanomaterial for photonics and optoelectronics applications. Therefore, we review the reports of Q-switched and mode-locked pulsed lasers using TMDs (specifically MoS2, MoSe2, WS2 and WSe2) based SAs. © 2016 Elsevier B.V. All rights reserved.

Keywords: Transition metal dichalcogenides Lasers 2D-materials Saturable absorber Short pulse laser

1. Introduction Pulsed lasers have a wide range of applications in the fields of biomedical imaging, optical communication, metrology, spectroscopy and material processing [1e5]. The demand for short and stable laser pulses, for the above mentioned applications, has motivated researchers to explore various pulse generating schemes. This includes active or passive Q-switching (i.e., modulation based on quality-factor of the laser cavity) [6e8] and modelocking techniques (i.e., phase-locking of oscillating cavity modes) [9,10] using various pulse shaping elements inside the laser cavity. In these, passive Q-switching and mode-locking are highly preferred due to their unique advantages such as compact nature, low cost and reliable performance. A saturable absorber (SA) is an important device in a laser cavity which generates short pulses using Q-switching or mode-locking techniques. SAs are broadly classified as real SAs [9,11], devices that exhibit a decrease in nonlinear absorption with an increase in light intensity, and artificial SAs [12e14], devices that utilize nonlinear effects to imitate the action of a real saturable absorber by instigating an intensity-

*

Fully documented templates are available in the elsarticle package on CTAN. * Corresponding author. E-mail address: [email protected] (S. Sivabalan).

http://dx.doi.org/10.1016/j.optmat.2016.09.007 0925-3467/© 2016 Elsevier B.V. All rights reserved.

dependent transmission. Pulsed lasers with an artificial SA have limitations in the reliability and reproducibility due to the change in the state of polarization caused by environmental perturbations like temperature, stress or strain [13]. Among the various types of real SAs, the semiconductor saturable absorber mirrors (SESAMs) [15,16] are widely used in conventional laser systems because of their remarkable properties such as large modulation depth and low saturation absorbing threshold. Though SESAMs have manifold merits, they also hold some drawbacks which include a complicated fabrication procedure, limited bandwidth operation, low recovery time and bulky nature [17]. To overcome these limitations, researchers have explored the saturable absorption property in carbon based nanomaterials such as carbon nanotubes and graphene. S.Y. Set et al. [18] proposed the first SA using single walled carbon nanotubes (SWCNTs) [19,20] for mode-locked fiber laser. Although CNT based SA has enhanced the saturable absorption properties compared to conventional SAs, it has major drawbacks such as narrowband operation and a low damage threshold [11,21e24]. Unlike SWCNT, graphene is a well-known 2D nanomaterial which has been widely investigated for its nonlinear optical properties. Graphene based SA can be used for a wide wavelength ranging from 0.8 mm to 3 mm. It has interesting characteristics such as high thermal stability, fast nonlinear optical response and a broadband absorption [11,25,26]. On the other

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hand, graphene exhibits two main disadvantages like weak modulation depth (typically ~ less than 1 % per layer [17]) and zerooptical bandgap. Therefore, there is much interest in new nanomaterials that can address the above issues. In recent years, several SAs have been fabricated from 2Dimensional materials such as topological insulators (TIs) [27e35], transition metal dichalcogides (TMDs) [36e40] and black phosperous (BP) [41e43]. Among these, TMDs stand out for their distinct characteristics. The family of TMDs consists of more than forty types of layered materials with MX2 stoichiometry, where M is a transition metal (e.g Mo,W) and X is a chalcogen (e.g S or Se) [44]. Fig. 1 (a) shows the schematic structure of a TMD element in which each TMD layer is structured as a trilayered sheet formed by two layers of chalcogen atoms sandwiching a layer of transition metal atoms by strong covalent bonds. Based on the transition metal atoms oxidation states, TMDs can exhibit either metallic (e.g. NbS2) or semiconducting (e.g. MoS2) properties in nature [45]. Semiconducting TMDs (e.g., MoS2, MoSe2, WS2, WSe2) are currently utilized for photonic and optoelectronic device development applications

based on earlier fundamental studies conducted in the 1960s [46,47]. In the recent past, modern fabrication and characterization techniques have opened up new opportunities to investigate the novel characteristics of TMDs to be applied in various fields. Similar to other layered materials, the individual layers in TMD bulk crystals are bonded together by relatively weak van der Waals forces [44] that allows for easier exfoliation into single and few layer forms [48]. The optoelectronic properties of TMDs are strongly layer-dependent [45]. For instance, the bandgap of TMDs generally migrates from indirect to direct and vice versa which is briefed as follows [49]: for MoS2 the bulk indirect bandgap of 1.3 eV (961 mm) converts to a direct bandgap of 1.8 eV (689 nm) in mono-layered form [50], for MoSe2 the bulk indirect bandgap 1.1 eV (1128 nm) gap migrates to a direct bandgap 1.55 eV (800 nm) in single-layered form [51], and for WS2 the bulk indirect bandgap 1.4 eV (886 nm) increases to a direct bandgap 2.1 eV (590 nm) transition in a monolayered form [50]. Such layer-dependent characteristics show that TMDs are comparable or even superior to the zero-gap graphene for a variety of optoelectronic and photonic applications [52]. In addition to the above quality, the mono or few layered TMDs (MoS2,

Fig. 1. (a) Schematic view of a MX2 structure, with the metal atoms (M) in grey and the chalcogen atoms (X) in yellow (reprinted with permission from Ref. [45]. © 2012 Nature Publishing Group), (b) TMDs nano-sheets prepared by mechanical exfoliation technique (reprinted with permission from Ref. [72]. © 2014 Elsevier B.V.), (c) CVD growth process of MoS2 sample through a dip-coated precursor on the substrate with the presence of Ar gas and S vapor (reprinted with permission from Ref. [45]. © 2012 Nature Publishing Group), (d) MoS2 nano-sheets prepared by hydrothermal process using Li-Intercalation (reprinted with permission from Ref. [59]. © 2014 Optical Society of America), (e) Schematic diagram of liquid exfoliation method for preparing MoS2 nano-sheets. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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MoSe2, WS2, WSe2, MoTe2 etc.) have unique optical characteristics such as strong photoluminescence [53e56], ultrafast carrier dynamics [46,57,58] and a high nonlinear optical response [59e61]. These interesting properties pave the way for the development of fast SAs for pulsed laser technology. In this review, we summarize the recent progress of few-layered TMDs based SA for pulsed laser development. Here, we focus on the fabrication techniques, integration schemes, linear and nonlinear optical properties of the TMDs based SAs. Further, we tabulate the properties of various TMDs based SA devices and the performance parameters of short laser pulses that have incorporated TMDs SA in their cavity. Finally, it is concluded that few-layered TMDs dominate other nanomaterials in the development of photonic devices for future applications specifically wideband SAs for short pulsed lasers. 2. Fabrication and characterization of few layered TMDs In this section, we review the various methods reported for the fabrication of mono or few layered TMDs and the different characterization techniques to estimate the number of layers, crystalline nature, absorption spectrum and quality of nanoflakes that are required to develop a reliable SA device for laser applications. 2.1. Mono or few layered TMD fabrication techniques Several approaches have been developed to prepare mono or few layered TMDs from bulk materials such as micromechanical cleavage [62,63], chemical vapor deposition (CVD) [64,65], hydrothermal intercalation/exfoliation [66e68], pulsed laser deposition(PLD) [69,37] and solution processing techniques [70,71]. The above methods are broadly categorized into exfoliation and growth techniques. Exfoliation involves splitting the monolayer or few layer flakes from bulk materials either by mechanical (e.g using Scotch tape) or chemical (e.g Lithium-based Intercalation) or through dispersal with relevant solvents (e.g liquid phase exfoliation (LPE)). The growth techniques involve a proper control of chemical reaction inside the furnace to achieve mono or few layers growth on the specific substrate (e.g CVD,PLD). The mechanical exfoliation (ME) is the first reported technique for producing few layered MoS2 flakes [47]. A new surface of a bulk crystal is pressed against an adhesive tape to extract few layer nanosheets which are shown in Fig. 1 (b) [52,73,74]. In the last few decades, this technique has reported mono layer flakes of high quality that are used for the basic characterization and fabrication of photonic devices [72,75]. However, owing to poor scalability (i.e control over flake thickness and size) [76e78] and repeatability [54,47], this technique is unsuitable for commercial applications. Recently, the advanced mechanical exfoliation technique is named as laser-thinning and it is used in the fabrication of few layered MoS2 down to a single layer using a high power laser [79]. The CVD method involves growing the few layered TMDs on insulating substrates (e.g SiO2) using different solid precursors heated to high temperatures [64,80e82]. The general procedure [65,83e87] for the growth of TMDs on the silicon wafers involves the following steps: 1) MoO3, sulfur powder, SiO2 are placed in the center, upstream and downstream positions of the furnace respectively, 2) The quartz tube, filled with argon gas, is evacuated to a base pressure of 0.1 Torr, 3) The sulfur powder is heated in a heating belt to 100
C, 4) When the pressure is stabilized, the furnace is heated to 550
C at the rate of 25
C/min and maintained for 30 min. These steps result in the even growth of the MoS2 film on a Si substrate. Recently, using other solid precursors, this technique grew few layered MoS2 by depositing the thin layer of Mo metal onto a wafer that is heated with solid sulfur [81]. This

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resultant substrate is dip-coated in (NH)4 MoS4 solution and then heated in the presence of sulfur gas [80]. The above processes are explained schematically in Fig. 1 (c). This technique is specifically used to grow high-quality TMD films with a precisely controlled number of layers. In recent times, large-scale single crystalline WS2 monolayers have been fabricated using the CVD method [88,89]. Hydrothermal synthesis is another growth technique where the preparation of crystals are achieved by intense vapor pressure reaction at a high temperature [45,90]. Using this technique, the fewlayered TMD flakes were fabricated by a hydrothermal reaction between hydrazine (N2 H4) and bulk crystal with intercalated by alkali naphthalenide solution [91,92]. Similarly, the hydrothermal reaction between the sodium molybdate (NaMoO4) and silicotungstic acid (H4[W12 SiO40]) at 240
C for 24 h with thiourea (CSN2 H4), was used to fabricate MoS2 nano flakes [93e95]. The above methods reported high-quality MoS2 nano flakes with typical flake sizes varying from hundreds of nanometres to a few micrometres. In PLD [54,96], the target (i.e bulk material) is placed in a vacuum chamber and a high energy laser pulse is focused on to it. This results in vaporization of bulk material and the deposit of a thin film on the substrate (e.g silicon wafer). Recently, WS2 thin film was fabricated by focusing a laser beam with energy of 2 mJ/pulse on the target using a high energy Nd:YAG laser [97]. This technique controls the ratio of metal to chalcogen in the film since the two ions have different evaporation rates [69]. The next important and commonly used method for the fabrication of few layered nano flakes is the solution processing technique. This is widely adopted because of its simplicity, costeffectiveness and the capability for large scale production of high quality nano flakes under ambient conditions [27]. In this technique, the mono or few layered are exfoliated either by dispersing the bulk material with chemicals via chemical exfoliation [98,99] or by dispersing with suitable solvents through LPE [100,101]. Generally, the chemical exfoliation of TMDs is achieved via lithium intercalation followed by hydrothermal exfoliation since it increases the separation between the layers [98,102,103]. In the hydrothermal exfoliation process, the bulk crystals are mixed with lithium hydroxide, ethylene glycol and filled into a teflon-lined autoclave. It is then heated at 200
C for 72 h to achieve the complete intercalation of bulk crystals through the lithium ions (Liþ) that are dissolved in the solution. The dispersions in the solution are filtered and rinsed with acetone to remove the residual lithium hydroxide and ethylene glycol solution [59,104e106]. The colloidal suspensions of TMD nano flakes are prepared by hydrating the lithiated TMDs powder in deionized water. The intercalation of TMDs by lithium was first demonstrated by Morrison et al. in the 1970s [107]. The steps involves, the bulk TMD was dispersed in a solution of n-butyllithium for more than 24 h to permit the lithium ions to intercalate followed by exposing the intercalated material to water. The above steps are explained schematically in Fig. 1 (d). This results in a vigorous reaction between the lithium and water and the release of hydrogen gas, which eventually results in the separation of the layers [108,109]. Zeng et al. [99] demonstrated an alternative method of lithiation using an electrochemical cell with a lithium foil anode and TMD as cathode. In this method, the intercalation resulted when a galvanic discharge took place in the electrochemical cell. This is faster when compared to n-butyllithium method and the degree of lithiation is also controllable [110]. Lithium-based chemical exfoliation has been demonstrated for TMDs such as MoS2, WS2, MoSe2 and SnS2 [111e113]. Very recently, pulsed lasers have been reported with TMDs based SAs that are developed using lithium-based intercalation techniques [114e116]. The liquid exfoliation is a simple and cost effective method for preparing high quality mono or few layered TMDs nano flakes [36].

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The three step procedure of LPE is schematically represented in Fig. 1 (e) and the details are as follows: 1) The bulk crystals of TMD is mixed with any one of the suitable solvents such as dimethyl formamide (DMF) [117e120], ethanol [39,89,121e128], distilled water [129], lysine [130], N-methyl-2-Pyrrolidine(NMP) [131] or with suitable surfactants such as sodium cholate (SC) [123,133e136] and sodium deoxycholate (SDC) [137,138], 2) Exfoliated via sonication 3) Removal of the unexfoliated thicker flakes via centrifugation or filtration [139,53]. Here, we emphasis that the suitable solvent is chosen such that the surface energy of solvent and the exfoliated TMDs match with each other. Furthermore, the exfoliation studies and the inverse gas chromatography results demonstrate that MoS2, WS2 and MoSe2 have surface energies in the range of 65e75 mJ m2 [140]. Alternatively, the ultrasonic assisted liquid phase exfoliation (UALPE) [141,38] method relies on the formation of micro bubbles in the solvent due to highfrequency pressure variations [142]. In this method, under suitable frequency, pressure and solvent conditions, the micro bubbles become unstable as they grow in size and eventually collapse. The shock waves produced from the collapsed micro bubbles create strong shear forces that are sufficient to overcome the weak van der Waals forces between the layers in bulk TMD crystals, resulting in the exfoliation of thinner flakes. Finally, exfoliated few-layer flakes are separated from the thick flakes, by ultra centrifugation. The other reported few layer fabrication techniques are gas phase growth [143], chemical weathering [144,145] and physical vapor deposition [65]. Due to the complex fabrication steps and the cost factors, these methods have not been exploited for producing SA devices.

profile of the TEM micrographs represents the intensity peaks of the two neighboring M (metal) and X (chalcogen) atoms. For singlelayer MoS2, the estimated intensity ratio is 1.15. This ratio is high for a lower number of layers and it decreases when the number of layers are increased. The morphological (i.e., length and width) investigations of the exfoliated MoS2 nanoflakes are carried out using TEM. The MoS2 sample is two layered and structured as a hexagon symmetrical (2H) as shown in Fig. 2 (b) and by analyzing the intensity profile between the dotted lines, a significant difference in intensity between M and X atoms are observed [36]. On the other hand, for more than two number of layers, no difference in the intensity peaks between the neighboring atoms can be observed since TMD layers are structured as repeated ABAB stacking sequence [53]. Therefore, the high intensity variation shown in Fig. 2 (c) confirms that MoS2 sample is a mono-layer nanoflake. Fig. 2 (d) shows the scanning-TEM (STEM) morphological image of the MoS2 nanosheets along with the lateral profile image Fig. 2 (e) of the respective material. Notably, the pixel resolution along 1e2 and 3e4 in Fig. (d) is directly related to the relative height of the nanosheet in Fig. (e) [36]. From the images we infer, unlike the graphene sheets have folded edges [157], the MoS2 nanosheets exhibit the structural characteristics of sharp edges and prominent geometrical shapes with rigid mechanical property [36]. Additionally, the lateral height profile in Fig. 2 (e) shows that the MoS2 sample attain multiple layered structure. Fig. 2 (f) depict the selected area electron diffraction pattern (SAED) of WS2 nanosheet which reveals multi-layer feature of the sample and the figure inscribed with the marks denotes the diffraction rings to (100), (103), (110) and (112) planes [153].

2.2. Material characterization 2.2.1. Raman spectroscopy Raman spectroscopy is used to study the quality of the samples [54,73] and also to the find the number of layers in the nanosheets [73]. The TMD has four Raman-active modes (E1g, E12g , A1g and E12g ) and two IR-active modes (A2u and E1u) [146,147]. Here, E12g represents in-plane mode induced by the opposing vibration of the two X (chalcogen) atoms with respect to the M (metal) atom and the A1g represents the out-of-plane mode which is generated by the vibration of X atoms in the opposite directions. While transition from bulk to few-layer, based on the number of layers, the frequency shift occurs in E12g and A1g modes [73,147e149]. The A1g modes exhibit blue-shift because of the prevention of atomic vibration by the inter-layer Van der Waals force [150]. However, E12g exhibit redshift since the inter-layer Van der Waals force plays minor role in the structural change [73]. These frequency shift of A1g and E12g modes are utilized to determine the number of layers in the TMD samples [73]. Fig. 2 (a) shows the Raman spectrum of the few-layered MoS2 in which E12g (i.e., In-plane) mode redshifts for exfoliated sample is compared with the bulk MoS2 sample. However, A1g mode (i.e., Out-of-plane) is similar for both the states. From the graph, it can be seen that for bulk MoS2 the values of E12g and A1g vibrational modes are 380 cm1 and 405 cm1 respectively. In the few layer MoS2, the respective vibrational mode E12g shifts to 384 cm1 and eventually the mean frequency difference reduces to 21 cm1. This value imply that the investigated MoS2 sample have average thickness of 1~ 3 monolayers [151]. 2.2.2. Transmission electron microscopy (TEM) TEM is the standard tool to find the length and the width of nanosheets [53,156]. For few layered samples, high resolution TEM is utilized to confirm the single layer formation and also to find the number of layers [36]. In a single layer TMD, the study of intensity

2.2.3. Atomic force microscopy (AFM) An AFM measurement is used to estimate the size and the thickness of the TMD nanosheets. Initially, samples are prepared by dropping the TMD solution on a cleaned Si substrate and based on the solvent, the sample is heated to suitable temperature inside the vacuum chamber to remove the residual solvents. As an example, the AFM image of the prepared MoSe2 sample is shown in Fig. 2 (g). The corresponding height profile diagram to measure the average thickness of the sample is shown in Fig. 2 (h). From the figure, the thickness of sample is determined as 3.7 nm which means that the number of layers is 5e6, and the mono-layer MoSe2 thickness is 0.65 nm. Further, the AFM characterization afford the statistical analysis on the size, width and thickness of the TMD nanosheets [36].

2.2.4. Ultravioletevisible spectroscopy (UV/Vis) UVevisible spectroscopy is employed to analyze the absorption properties of the samples for wider wavelength range of the incident light source. Generally, the absorption peaks are used to infer the basic structure of the sample. For TMDs, the four absorption peaks represent the trigonal prismatic coordination [158,159], which further shows that the TMD nanoflakes are 2H polytype [36]. For example, the MoS2 sample observed four peaks such that (A) ~664, (B) ~605, (C) ~439, and (D) ~395 nm as shown in Fig. 2 (i). The two peaks A and B indicate that the sample is 2H polytype, since it originated through the interband excitonic transitions. The C and D peaks originated due to the transitions between the higher density of the state regions. In addition, the four peaks confirm that the MoS2 sample is well exfoliated as few-layered nanoflakes. Generally, UVevis absorption spectra can also reflect the layer information, for that the excitonic absorption peaks A and B are layer dependent, which show a slight red along with increasing layer number [36,98].

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Fig. 2. (a) Raman spectrum of bulk and few-layer MoS2 (reprinted with permission from Ref. [152]. © 2014 Nature Publishing Group), (b) TEM images of the exfoliated MoS2 nanosheets with 0.5 nm resolution (reprinted with permission from Ref. [36]. © 2013 American Chemical Society), (c) Intensity profile along the dotted line in (b) (reprinted with permission from Ref. [36]. ©2013 American Chemical Society), (d) Morphological image of MoS2 nanosheets using STEM (reprinted with permission from Ref. [36]. © 2013 American Chemical Society), (e) Lateral height profile of image (d) (reprinted with permission from Ref. [36]. © 2013 American Chemical Society), (f) SAED pattern of WS2 nanosheets (reprinted with permission from Ref. [153]. © 2016 Elsevier B.V.), (g) AFM image of the few-layered MoSe2 nanosheets (reprinted with permission from Ref. [154]. © 2016 SPIE), (h) AFM height profile diagram (reprinted with permission from Ref. [154]. © 2016 SPIE), (i) Optical absorption properties of few-layer MoS2 sample (reprinted with permission from Ref. [155]. © 2016 IOP Publishing Ltd), (j) XRD analysis of few-layer and bulk MoSe2 (reprinted with permission from Ref. [154]. © 2016 SPIE).

2.2.5. X-ray diffractometer (XRD) XRD characterization was used to confirm the crystalline nature of the given sample. At first, the bulk TMD material is characterized and then, the supernatant dispersed TMD solution was deposited onto a substrate (eg. silica) and heated at high temperature. Finally, the prepared TMD sample was characterized and the results are shown in Fig. 2 (j). The figure shows the XRD characterization for MoSe2 sample, for bulk (top) the labeled peaks are indexed to the rhomboidal MoSe2 (JCPDS No:29-0914) [130], for exfoliated MoSe2 sample (bottom) a high intensity peak label [002] was observed since other peaks were disappeared. This confirms that the characterized few layer MoSe2 sample exhibits a crystalline nature and it successfully exfoliated from the bulk MoSe2 material. 3. Device integration schemes Several techniques have been implemented to develop an SA device using few layered TMD solution. The SA device was

developed by following schemes such as depositing the TMD solution on the substrates [118,122,133,160,161], fiber ends (e.g drop casting method) [125,72], side polished surface of the fiber (e.g spin coating method) [89,121,124,129] and as polymer composites [117,130e132] sandwiched between two fiber connectors. The selection of a particular integration scheme was based on the choice of the fabrication technique to exfoliate the few layered TMDs. For the liquid phase exfoliation technique, the integration methods that were implemented by researchers were polymer composite [164,141], optical deposition [72] and thermal evaporation [165]. In pulsed laser applications, the polymer composite scheme was widely used because of its fiber-compatibility and simple procedure [134,137,138]. This scheme involves dispersing the prepared few layered TMD with the Polyvinyl alcohol (PVA) aqueous solution, it was then sonicated and the resultant mixer was poured onto the thin glass plate. Subsequently, the glass plate with the mixer was dried at 70
C to form thin TMD film plates. It was then cut into small pieces and inserted between two fiber patch

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cords to construct PVA-TMDs SA as shown in Fig. 3 (a) [38,119,135]. Although this scheme has advantages such as low cost and easy fabrication process at room temperature, in the down side it exhibits low thermal damage threshold [166]. Therefore, incorporating this type of SA in high power bulk lasers was not feasible. To alleviate this issue, optical deposition method was implemented by depositing the few layered TMD flakes directly onto fiber ends or by spin coating on quartz [127,133,161] or onto a BK7 glass [118]or in a gold mirror [122,160]. In the above techniques, the TMD nanoflakes dispersed solution was dripped onto the substrate and rotated at a low speed to get uniform coating of TMD solution. Finally, the prepared sample was dried at constant temperature of 70
C for 2 h [118]. To develop the fiber facet TMD SA, the substrate was replaced with a fiber ferrule and TMD solution is coated using drop casting method. The prepared SA is schematically shown in Fig. 3 (b). The other integration schemes to increase the interaction length between light and TMDs are by coating the few-layered TMDs dispersion along side-polished fiber (SPF) [89,124,128] or taper fiber [123,152] or microfiber [92,105]. Song et al. [167] first demonstrated the deposition along the SPF. The SPF is prepared by polishing one side of a single-mode fiber (SMF) by fixing it in a Vgrooved quartz block. The gradual removal of fiber cladding forms

an SPF and the schematic view is shown in Fig. 3 (c). Then, the TMDs dispersion was deposited on the polished region using spin coating method where the fiber core was close enough to have good evanescent field interaction [85,114,129]. By deep polishing, a Dshaped fiber was evolved and TMDs solution was deposited onto the surface using the optical deposition method. This increased the strong evanescent field interaction with the TMD nanoflakes [39,121,168]. The taper fiber was fabricated using the flame brushing technique [169] i.e the SMF is fixed at both ends using two fiber clamps and the central region is heated with stretching the fiber ends simultaneously. In the tapered fiber, the TMD solution was deposited on the waist region to form the tapered fiber saturable absorber [83,152,170]. Very recently, a novel technique, the ethanol catalytic deposition of MoS2 solution on tapered fiber is demonstrated using erbium doped fiber laser [126]. Using the same flame brushing technique, the microfibers were also fabricated and the MoS2 solution was deposited onto it by the optical force and the same is shown in Fig. 3 (d) [92,97,105]. The integration schemes followed for CVD fabricated TMD nanosheets, involves a direct transfer of flakes onto a desirable substrate. Generally, the spin coating method is used to transfer the flakes on a thin polymer type substrate [75,115]. The coating is

Fig. 3. (a) PVA-composite based TMDs SA between two fiber patch cords (reprinted with permission from Ref. [38]. ©2015 Optical Society of America), (b) Deposition of TMD solution on fiber ferrule (reprinted with permission from Ref. [162]. ©2016 Optical Society of America), (c) Schematic of fabricated WS2 deposited side polished fiber (SPF) SA and its cross section (reprinted with permission from Ref. [114]. © 2015 Optical Society of America), (d) Few-layer MoS2 deposited microfiber (black portion: MoS2) (reprinted with permission from Ref. [105]. © 2014 Optical Society of America), (e) Few layer MoS2 on gold-film mirror (reprinted with permission from Ref. [96]. © 2015 Optical Society of America), (f) Schematic view of the MoS2/PMMA layer on SPF (reprinted with permission from Ref. [85]. © 2014 Optical Society of America), (g) Schematic design of PCF (reprinted with permission from Ref. [163]. © 2015 Nature Publishing Group), (h) PCF cell-type SA (reprinted with permission from Ref. [163]. © 2015 Nature Publishing Group).

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carried out using the following steps: 1) The grown TMD layer on SiO2 was further spin coated with a thin layer of Polymethyl methacrylate (PMMA), 2) TMDs/PMMA layers were etched using a potassium hydroxide (KOH) solution and the lifted-off layer was washed with deionized water, 3) The prepared TMDs/PMMA film was transferred onto the desired substrate or directly on a fiber connecters [86,87] or on an SPF [85] and 4) Finally, the complete device was immersed into the acetone solution to remove the PMMA layer [83,89,94]. The schematic view of TMDs/PMMA film coating on an SPF is shown in Fig. 3 (f). The few layered TMD nanosheets formed by PLD was directly processed and incorporated as an SA device. For example, MoS2 nanoflakes that were grown on the golden mirror could be used directly as an SA and the same is shown in Fig. 3 (e) [160]. A recent novel integration scheme uses photonic crystal fiber (PCF) as an SA device [163]. The cross section of the PCF that was used for the development of SA is shown in Fig. 3 (g) [171]. The PCF cell-type SA fabrication involved three steps: 1) PCF was filled with TMDs nanoflakes solution using the pressurized injection scheme, 2) The solution filled PCF was dried at the temperature 60
C for 3 h to remove the solvent, 3) Finally, it was cut into multiple pieces and each piece was spliced with two SMF on both sides of a solution filled PCF. This acted as the PCF cell type SA which is shown schematically in Fig. 3 (h) [163]. In this section, we have briefed about the various integration schemes used to develop few layered TMD based SA for short pulsed lasers reported to date. We have also discussed the steps involved to fabricate the TMDs SA device in each integration techniques along with their schematic view. In the next section we have dealt with the nonlinear optical properties of TMD nanosheets.

discussed the nonlinear saturable absorption characteristics of few layered TMD materials. In addition, we have described the role of geometrical defects, edge states and boundary effects of layered TMDs on sub-bandgap absorption and have also briefed about other nonlinear effects. To study the nonlinear absorption characteristics of few layered TMDs, either Z-scan [172] or balanced twin-detector set up [173,174] can be utilized. The balanced twin detector set up consists of femtosecond pulsed laser as the source with a variable optical attenuator (VOA) to vary the average power of the source. Further, the source was split into reference arm and a test arm in which SA sample was incorporated. The output of reference and test arms were terminated with separate detectors that were further connected to a power meter. Fig. 4 (a) shows the balanced twin detector set up. The EDF laser oscillator with a pulse repetition rate of ~25 MHz and the pulse duration of ~ 400 fs at 1.55 mm was used. The attenuator controls the input power that passes through the TMD-SA. Using a 3-dB coupler the pulses are separated equally to the test arm, where the TMD-SA sample is inserted and the reference arm. By varying the intensity of illuminating pulses, the absorption of the TMD-SA as a function of incident power has been determined by the difference between the output power of reference and test arms [175]. Fig. 4 (b) shows the Z-scan setup in which the sample was placed in focal plane of high intensity short optical pulses and the transmission was recorded for various positions of the sample. For the constant input optical power, the output intensity from the sample was recorded by changing its positions [176]. The recorded experimental values registered from the above mentioned set ups were verified by the standard SA two level model as mentioned in Eq. (1) [177].

4. Nonlinear optical properties

aðIÞ ¼

As our review focuses on the development of TMD based SA for short pulse laser applications, in this section we have briefly

as 1 þ IIs

þ ans ;

(1)

where as is the modulation depth (saturable loss), ans is the non-

Fig. 4. (a) Balanced twin-detector set-up for measuring nonlinear absorption characteristics of MoS2-SA (reprinted with permission from Ref. [175]. ©2016 Nature Publishing Group) (b) Z-scan measurement setup. BBO: Beta Barium Borate crystal, NPs: Nanoplates (reprinted with permission from Ref. [59]. ©2014 Optical Society of America).

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saturable loss and Is is the saturation intensity. Wang et al. has explored the saturable absorption in TMDs for the first time in few-layered MoS2 sample solution containing 6e7 layer flakes. The results have revealed a stronger nonlinear response (i.e sample transmission increased by 75 %) of MoS2 under intense 800 nm excitation [36]. Additionally, the intraband relaxation time was found to be 30 fs, which confirms that the few layered MoS2 can act as an ultrafast saturable absorber [36]. Recently, K. Wang et al. investigated the nonlinear optical response of MoS2, MoSe2 and MoTe2 using Z-scan technique and determined the broadband absorption characteristics from the visible to nearinfrared region [178]. Different types of TMD SAs were fabricated by different integration schemes and have been characterized at various wavelengths such as ~1 mm [75,106,124], ~1.5 mm [38,132,163] and ~2 mm [122,129] respectively. Table 1 summarizes the measurements of the nonlinear properties of MoS2, MoSe2, WS2 and WSe2 SA which were reported recently. The nonlinear absorption characteristics of TMD-PVA composite based SA is usually measured with balanced twin detector technique. In PVA composite integration technique, the MoS2 SAs modulation depth have been reported from 1.2% [154] to 29.6% [106] with a saturation intensity from 1 MW/cm2 [134] to 137 MW/ cm2 [132] and unsaturable loss varying from 14% [137] to 63.1%

[134]. For WS2, the modulation depths have been reported from 1.2% [164] to 3.1% [141] with saturation intensity from 1.38 MW/ cm2 [134] to 370 MW/cm2 [132] and unsaturable loss varying from 6.9% [137] to 58.3% [134]. For other TMD based SAs (i.e MoSe2, WS2) only a few nonlinear characterization were determined, that exhibits similar properties as MoS2 and WS2. To improve the absorption efficiency, the TMD SAs have to reduce the non saturable loss (i.e <10%) by producing high quality nanoflakes with suitable device integration. An another efficient way to improve the nonlinear optical absorption property is by selecting the suitable polymer with TMD nanosheets. Very recently, MoS2/poly(N-vinylcarbazole)/PMMA combination based material has exhibited the highest nonlinear absorption coefficient of 461 cm GW1 at 1064 nm [179]. The nonlinear saturable absorption characteristics were stronger for the SA based on SPF because of efficient evanescent interaction between light and TMD layers. For MoS2, the reported highest modulation depth is 35 % [83] and the least saturation intensity is 0.34 MW/cm2 [83] with a large unsaturated loss of 75.4 % [121]. The other SA integration schemes show a similar characteristic response for all the reported TMD based pulsed lasers. Fig. 5 shows the absorption characterization of MoSe2 based PVA-SA which has 6e7 layers. Using the balanced twin detector method,

Table 1 Nonlinear absorption characteristics of few-layered TMD flakes based SA devices. Fabrication method

MoS2 LPE

Integration platform

LI MST ME MoSe2 LPE UALPE WS2 PLD LPE

WSe2 LPE

Modulation depth, as (%)

Unsaturated loss, ans (%)

Ref

t (fs)

frep (MHz)

5e6 1e5 e 5e6 e ~4 >6 e e e 4e5 30e50 3e10 ~2 ~6 e 4e5 2e3 e ~15

I I I I I I I I I I I I Z Z I I I I I I

1560 e 1554.4 1042 1562 1940 1480 1560 1550 1550 1544 e 1030 1030 1554 1550 1550 1550 1560 1060

510 560 500 560 650 800 e 600 400 950 500 e e e 400 250 250 10 ns 953 15 ps

e 37 26 26.6 159 55 e 26.7 25 12.8 26 e e e 20 e 20 1 17.96 e

137 129.4 e 8 137 23.1 29.3  103 36.2 158 110 e 8.7 kW/cm2 18 14 27 0.55 0.34 4.53 138 e

2.7 2.15 4 1.2 3.53 13.6 4 0.99 1.2 3.4 1.8 12.45 10.7 29.6 9 28.5 35.4 29 4.48 9.7

44.2 63.1 55.9 e 23.2 16.7 e e 75.4 13.3 51.1 e 50.5 60.5 e e 34.1 63 2 28.8

[132] [134] [180] [154] [163] [122] [125] [162] [121] [168] [92] [94] [95] [106] [96] [84] [83] [165] [171] [75]

PVA Composite

2e3 1e5 6e7

I I Z

1566 e 1566

650 560 750

22.15 37 7.5

19.8 132.5 3.4

0.63 6.73 4.7

3.57 39.2 6.6

[130] [134] [38]

Microfiber PVA composite

SPF Taper fiber

e ~10 1e5 ~3 14e25 14e25 <8 <8 e >5 7e9

I Z I I I I Z Z I I I

1562 1030 e 1560 1555 1053 1030 1558 1064 1925 1560

650 4 ps 560 510 950 15ps 120 150 e 1 ps 400

22.5 89 37 e 12.8 26.9 20 10 e 178 e

25 71.6 148.2 370 e e 1.38 3.83 6.2 e e

1.2 2.06 2.53 2.9 5.1 2.9 3.1 4.9 3.87 10.9 0.5

30 e 58.3 30.9 e e 6.9 3.7 e e e

[97] [164] [134] [135] [124] [124] [141] [141] [116] [129] [170]

PVA composite

1e5

I

e

560

37

270.4

3

61.5

[134]

PVA composite

Microfiber Quartz BK7 glass PVA composite Mirror Fiber end PVA composite Fiber end Gold film Quartz

SPF UALPE UALPE LI

Is (MW/cm2)

l (nm)

D-SF

PLD CVD

Nonlinear characterization setup Z/I

PCF cell type Gold mirror Fiber end

HTG

Layers

PVA composite

Note: We tabulate the number of TMD layers size with respective fabrication techniques and specific integration schemes. Also quoted the nonlinear absorption parameters. HTG, hydrothermal growth; LI, Lithium Intercalation; MST, Magnetron Sputtering Technique; Nonlinear characterization setup: Z/I, Z-/I-scan; t, pulse duration; l, wavelength; frep, pulse repetition rate.

J. Mohanraj et al. / Optical Materials 60 (2016) 601e617

Fig. 5. Nonlinear absorption characteristics of the MoSe2-PVA composite SA measured using twin-detector method (reprinted with permission from Ref. [154]. ©2016 SPIE).

the intensity dependent absorption was plotted and the result is well fitted in Eq. (1). The absorption curve was relatively flat after the input intensity reached 8 MW/cm2. This shows that the modulation depth is 1.2% with saturation intensity of 8 MW/cm2. To compare the overall nonlinear absorption performance among TMDs SA is laborious, since the reported parametric values depends on different fabrication and integration techniques. However, recently Chen et al. reported the comparative studies of TMDs (i.e MoS2, MoSe2, WS2, WSe2) SAs device parameters using LPE fabrication and PVA composite integration technique [134]. It can be concluded from the survey that MoSe2 SA exhibits higher modulation depth (6.73%) compared to WSe2 (3.02%), WS2 (2.53%) and MoS2 (2.15%). In addition, it possess a lower unsaturated loss of 39.1%. To date, most of the reports regarding the saturable absorption property of TMD nanoflakes have been investigated at nearinfrared wavelengths. In this wavelength range, the TMD material bandgap energy is higher than the photon energy. For a perfect semiconductor TMD crystal, the absorption is not possible if the incident photon energy is lower than the crystal bandgap energy [181]. However, the crystal morphological defects, edges and boundary creates a phenomenal effect on the absorption properties of crystal [138]. Wang et al. [37] have explained in detail about the bandgap characteristics of few layered MoS2 with crystal defects. The studies reveal that the increase in defects of both Mo and S atoms can decrease the bandgap. In addition, higher Mo defects can increase the metallic behavior and suppress the saturable absorption effects. However, the excess of S defects decreases the bandgap upto 0.08 eV and maintains the semiconductor behavior of the MoS2 crystal. This characteristic behavior is almost the same for all the reported TMD crystals. Recently, R.I. Woodward et al. [138] exposed the effect of edge defects on the absorption properties of TMD crystals. At the edges, because of the broken symmetry along the atomic layer and the disjoint bonds between the M and X atoms, there is a change in the electronic properties which leads to the formation of edge-states between the crystal bandgap. Roxlo et al. [182] investigated the bandgap absorption property of few layered MoS2 nanoflakes of different sizes using photothermal deflection spectroscopy. These studies reveal that the smaller size nanoflakes exhibit larger ratio between the edge and the surface area which further confirms the significant role of edges in sub-bandgap

609

absorption [183]. The presence of edges in the crystal bandgap is the reason for the absorption of incident photon energies lower than mono layer TMD crystal bandgap. Usually, the absorption can be saturated at high intensity due to Pauli blocking [138]. The recent demonstration of a broadband saturable absorption in TMDs is due to wide spread of edge states along the bandgap of the crystal. However, the density of the edge states are not distributed evenly throughout the crystal bandgap. Therefore the absorption depends upon the edge termination (i.e either M or X sites) and its geometry [184]. Further studies have show that for larger TMD flake sizes, the effect of edge state towards the absorption was minimal [176]. Along with edge states, the other morphological defects such as grain boundaries and vacancies also contributed to bandgap absorption [184,37]. The Mo atoms at the grain boundary always retain the regular coordination. However, the S atoms at the boundary changes from its regular coordination and eventually influence the nonlinear optical absorption property of few layer MoS2 [184]. TMDs also exhibit other nonlinear optical effects in addition to saturable absorption property. In many nonlinear studies on TMD crystals, the absorption effects are not similar (i.e decrease in absorption with the increase in intensity): under high intensity, the crystal absorption property increases when there is an increase in incident intensity, this effect is known as optical limiting [59,97,130,134,185]. N.Dong et al. investigated the nonlinear optical properties of TMDs and observed the optical limiting effect due to nonlinear scattering at 1064 nm [186]. This behavior is attributed to two photon absorption (TPA), which plays an important role during the absorption of photon energy lower than the crystal bandgap and higher than the half bandgap energy. This is because, under high intensity, it exhibits two photon absorption characteristics (i.e two photons excite a single low energy electron). The studies reveals that the multilayer MoS2 possess strong saturable absorption effect and mono-layer exhibits strong TPA effect [187]. Recently, S.Zhang et al. analyzed the optical nonlinear characteristics of mono and few layer WS2 and MoS2 materials. Here, MoS2 exhibit a giant TPA coefficients of (1.0 ± 0.8)  104 cm/GW and the TPA saturation occurs at 25e27 layers [188]. Further, Q.Ouyang et al. observed an another nonlinear effect named as reverse saturable absorption (RSA) in a MoS2 nanoflake films while increasing the input energy to the material [189]. In fiber laser, whenever SAs start to exhibit TPA from the saturable absorption, it influences the cavity elements pulse dynamics that affect the evolution of pulse from Qswitching to mode-locking regimes [190]. The studies exposing the nonlinear absorption property of TMDs are depend on the structural alignment of the films. If the layers aligned horizontally, it exhibits maximum optical performance [191] and on the other hand if aligned vertically, it tends to show electrochemical performance [192]. Very recently, X.Y. Zhang et al. demonstrated the novel idea of integrating both the vertically and horizontally aligned MoS2 layers with a consequence of realizing an excellent nonlinear optical properties [193]. Additionally, the dependency of MoS2 refractive index on the number of layers are demonstrated through the characteristic matrix method [194]. Very recently, G. Wang et al. reported the tunable property of refractive index of TMDs by employing spatial self phase modulation using continuous wave laser beam at 488 nm. It is then concluded that TMDs have potential applications in nonlinear phase modulation devices [195]. Also, studies are ongoing to understand the relationship between the flake size geometry and exfoliation procedure which influence on the nonlinear optical absorption properties. These studies confirm that TMDs exhibit a promising nonlinear absorption property which suggests that TMD SAs are fabricated to use in various laser applications by developing different pulsed lasers.

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5. Pulsed lasers using TMDs Pulsed lasers generate pulses in the order of several microseconds to a few femtoseconds, by means of Q-switching or modelocking techniques. The generated pulse parameters strongly depend on the configuration of the laser cavity and on the properties of the SA such as modulation depth, relaxation time and saturable loss [5,196]. In this section, we review the various Qswitched and mode-locked lasers that use TMD based saturable absorber. 5.1. Q-switched lasers Q-switching mechanism generates pulsed laser light by modulating Q-factor of the cavity using SA. The Q-switched pulses have typical range of parameters such as ns-ms pulse durations, high pulse energies (nJ-mJ level) and kHz repetition rate. Such lasers have wide applications in the field of micromachining, medical diagnosis and material processing. A general schematic diagram of a Q-switched fiber laser with TMDs SA is shown in Fig. 6. Recently, various Q-switched fiber lasers have been reported using nanomaterial based SA. Of these, TMDs SA shows an improved performance in Q-switched lasers. Among the TMDs, MoS2 is the highly preferred material for the demonstration of pulsed lasers. Wang et al. [37] reported the first bulk Q-Switched laser using few layered MoS2 SA followed by Woodward et al. [197] who demonstrated the first Q-switched fiber laser. After this initial report, numerous MoS2 based Q-switched fiber lasers have been evolved for various gain mediums, which include Ytterbium (Yb) [117,138,197], Erbium (Er) [72,84,87,96,117,119,125,134,165] and Thulium (Tu) [117]. Among these, Er is the most preferable gain medium because of the minimum loss in the operating wavelength band and easy excitation of the soliton pulses in single mode fibers [198]. Further, the reports of tunable Q-Switched fiber lasers with the tuning range from 1030 to 1070 nm [138] for Yb gain medium and 1520e1568 nm [119], 1530e1570 nm [84], 1550e1575 nm [165] for Er gain medium have been demonstrated. Very recently, Q-Switched tunable fiber laser (1484e1492 nm) using MoS2 SA with depressed-cladding erbium doped gain medium (DC-EDF) at S-band region was reported [155]. From the above results, it is

further confirmed that the few layered MoS2 SA is highly suitable for broadband operation of lasers. Solid-state Q-switched lasers have the capacity to generate pulses with high output power (>200 mW) compared to fiber lasers (<10 mW). This indicates that bulk lasers require high damage threshold SAs. The fabrication of high power SAs are done by depositing the few layered TMDs on a glass substrate [203,198] or on crystal-line host materials [203,198] that are used as the gain medium. The solid state lasers are generally a free-space cavity that can be devised by incorporating mirrors and a solid-state gain medium [196]. A different solid-state gain media such as Yb:YAG [95], Yb3þ:Ca3 Y2(BO3)4 [94], Yb:LGGG [75], Nd:GdVO4 [37,199], Nd:YAlO3 [118], Nd:YGG [37], Nd:GdTaO4 [161], Nd:GYSO [200], Nd:YVO4 [201], Tm:Ho:YGG [37], Tm:Ho:YAP [120], Tm:CLNGG [160], Tm:GdVO4 [82] and Tm:LLF [133] have been coupled with MoS2 SA to develop Q-switched lasers. Very recently, C. Luan et al. [120] as reported a highest average output power of 910 mW and a peak power of 11.3 W from a Tm:Ho:YAP bulk laser by incorporating a compact MoS2-based saturable absorber. Further, X. Zou et al. reported the highest single pulse energy of 41.5 mJ among MoS2 SA based solid state lasers [133]. These lasers have wide applications in the fields of medicine, material processing, and telecommunications [204e206]. The tungsten disulfide (WS2) is yet another emerging and widely progressing few layered TMD material which is a complement to MoS2. Kassani et al. [114] demonstrated the first WS2 based Q-Switched erbium doped fiber laser. This work triggers further demonstration of few-layered WS2 based Q-Switched lasers using Yb doped [116,141] and Er doped [114,123,134,135,141] gain mediums. Recently, Ahmad et al. [162] demonstrated a Q-Switched fiber laser using WS2 SA at C-band region. By using WS2 SA, the maximum reported average output power is 280 mW [135] and pulse energy of 1.1 mJ [134] at 1.5 mm. Very recently, Lin et al. [116] demonstrated the first WS2-based tunable Q-switching fiber laser with a wide tunable range (1027e1065 nm) at 1.0 mm central wavelength. Further, Wang et al. demonstrated the first Nd:YAG solid state Q-switched laser for three different concentrations of WS2 solutions (i.e.,0.25, 0.5, 1 mg/ml) on quartz cell, which generated pulse widths of 1.28 ms, 1.03 ms and 922 ns respectively [153]. Very recently, N. Zhang et al. as demonstrated the Nd:YVO4 solid state laser using WS2 SA and achieved a stable pulse width of 98 ns [127]. Fig. 7 shows the output characteristics of a tunable laser with WS2 based SA (a) pulse train, (b) single pulse shape with 1.65 ms width, (c) output spectrum with central wavelength of 1048.1 nm and a spectral width of 0.067 nm, (d) RF spectrum with fundamental repetition rate of 81.50 KHz with signal-to-background ratio (SBR) of 50 dB and (e) tunable spectra in the range of 1027e1065 nm. The recent Q-switched fiber lasers demonstrated with other TMD materials are MoSe2 [38,131,134] and WSe2 [134,202]. Woodward et al. [38] developed the MoSe2 SA for the Q-switched Yb, Er and Tm-doped fibre lasers. Recently, Chen et al. reported the WSe2 SA based Er doped Q-switched laser with a pulse width of 3.976 ms, repetition rate of 26.9 kHz and a pulse energy of 17 nJ [202]. In addition, Chen et al. [134] demonstrated the Q-switched fiber laser based on various TMDs such as MoS2, MoSe2, WS2 and WSe2. Table 2 summarizes the parameters and properties of lasers based on the TMDs SAs to date. 5.2. Mode-locked lasers

Fig. 6. The schematic diagram of a ring cavity Q-switched fiber laser. WDM: wavelength division multiplexer; LD: laser diode; SMF: single mode fiber; PC: polarization controller; EDF: Erbium-doped fiber; PII: polarization independent isolator; TMDs-PVA SA: transition metal dichalcogenides-polyvinyl alcohol saturable absorber (reprinted with permission from Ref. [134]. ©2015 Optical Society of America).

Mode-locking mechanism involves loss modulation by SA for a cavity round trip, which results in locking the phases of oscillating longitudinal modes. Thus, the typical phase-locking for each cavity roundtrip generates the ultrashort pulses with the repetition rate in

J. Mohanraj et al. / Optical Materials 60 (2016) 601e617

611

Fig. 7. The output characteristics of the WS2 based Q-switched tunable fiber laser. (a) the output pulse train, (b) single pulse profile, (c) the output spectrum, (d) RF spectrum, (e) the output spectra continuously tuned from 1027 to 1065 nm wavelength (all figures are reprinted with permission from Ref. [116]. © 2015 Optical Society of America).

terms of MHz-GHz, the pulse energy of pJ-mJ and pulse duration in ps-fs [198,196]. These pulse properties find wide applications in biomedical imaging, metrology and sensors [1]. SAs play an important role in the transition of pulse forming regime from Qswitching to mode-locking by the proper control of pump power. In addition, nonlinearity and dispersion effects arise inside the cavity which have an influence on the pulse formation in mode-locking regime. Therefore, the factors that are responsible for transit from Q-Switching to mode-locking regime are pump power, cavity dispersion, nonlinear management and incorporation of high quality SA devices (i.e high modulation depth and low nonsaturable loss) [196]. Zhang et al. have reported the first TMDs based mode-locked Yb doped fiber laser with a pulse duration of 800 ps using MoS2 SA which is developed by depositing few-layered MoS2 nanosheets on the end of fiber ferrule [59]. After the initial report, the numerous MoS2 based mode locked lasers have been reported for Yb [152,106], Er [83,85] and Tm [122] doped gain mediums. The all normal dispersion regime TMD mode locked lasers generates high energy dissipative solitons [59,85,124,152]. Recently, R. Khazaeizhad et al. have realized the passive mode locking in both anomalous- and normal-dispersion regimes using a multilayered MoS2 thin film coated on a D-shaped fiber. This generates pulse

width, repetition rate, spectral width of 521 fs, 25.2 MHz, 10 nm in anomalous-dispersion regime and 11.9 ps, 26 MHz, 18.2 nm in normal-dispersion regime respectively [115]. The harmonic TMDs mode locked lasers are demonstrated to achieve high repetition rate fiber laser. Among them, the MoS2 based mode-locked laser achieved highest repetition rate of 2.5 GHz for 369th harmonic state with 3 ps pulse width [105]. Further, the harmonic bound-state solitons (i.e multiple pulses with stable location) are generated in an Er doped fiber laser with a repetition rate of 125 MHz corresponding to the 14th harmonic of fundamental cavity repetition rate [121]. Among the reported TMD mode-locked lasers, Tian et al. demonstrated the MoS2 based laser with highest output power of 150 mW corresponding to a maximum pulse energy of 15.5 nJ and the shortest pulse duration was recorded as 935 fs [122]. In addition to that, a continuous tuning from 1535 to 1565 nm is demonstrated in erbium doped mode-locked laser by incorporating MoS2-PVA SA device [137]. Recently, C.T. Howe et al. [213] demonstrated both, the Q-Switching and mode-locking, in Yb and Er doped gain medium using MoS2 SA with a tunable wavelength of 1030e1070 nm, 1535e1565 nm respectively. Very recently, H. Ahmad et al. demonstrated the first dual peak dark soliton in a mode-locked laser using MoS2 SA [208]. In bulk laser, the passively Q-switched mode-locked Nd:GdTaO4 [161] and Nd3þ:YVO4 [136] lasers were

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Table 2 Q-switched pulsed lasers with few layered TMD SAs. Laser type

Few Layered TMD fabrication method

MoS2 Yb:Fiber

LPE

Er:Fiber

LPE

CVD

Tm:Fiber Yb:YAG Yb3þ:Ca3 Y2 (BO3)4 Yb:LGGG Nd:GdVO4 Nd:GYSO Nd:YGG Nd:YAlO3 Nd:YVO4 Tm:Ho:YAP Tm:Ho:YGG Tm:CLNGG Tm:GdVO4 Tm:LLF MoSe2 Yb:Fiber Er:Fiber

Tm:Fiber WS2 Yb:Fiber Er:Fiber

Nd:YAG Nd:YVO4 WSe2 Er:Fiber

ME PLD HT-I ST LPE HTG ME PLD HT-I e LPE

PLD HTG LPE UALPE UALPE LPE UALPE LI UALPE LPE

LI UALPE LPE LPE LPE

l (nm)

Laser properties

Ref.

t (m s)

frep (kHz)

P (mW)

Dt (nm)

E (nJ)

ER (dB)

1068 1030e1070 tunable 1067 1520e1568 tunable 1565 1550e1575 tunable 1560 1560 1484e1492 tunable 1560.2 1550 1530e1570 tunable 1563 1550 1567 1550 2032 1030 1030e1043 tunable 1025,1028 dual 1060 1063.4 1074 1420 1080 1064 2129 2100 1979 1902 1918

2.7 2.88 10.8 5.7 7.5 6 9.92 2.79 1.4 7.49 1.72 1.92 3.9 0.66 3.53 2.2 2.06 12 0.42 0.18 0.97 1.8 0.81 0.73 0.23 0.327 0.435 0.41 4.8 0.8 14

67 74 13.4 16.8 11.9 22 41.5 72 101.2 e 161 115 40.9 131 86.4 52 38.4 17 140 333 732 5.61 MHz 342.5 77 233 5 55 149 110 48 13.9

0.5 9.36 0.9 1.5 1.7 3.5 0.8 7.6 0.8 e 4.71 0.9 3.5 20 6.5 e 47.3 250 105 600 227 195 410 52 260 200 910 206 62 304 583

0.07 e 0.18 0.28 0.16 e e e e e 0.06 0.06 e 0.02 e e e e e e e 1.63 e e e e e e 2.5 e e

6.9 126 32.6 160 63.2 150 184.7 160 1.2 e 27.2 8.2 e 152 75 e 1 mJ 15 mJ 0.7 mJ 1.8 mJ e 25.84 1.2 mJ e 1.1 mJ e 5 mJ e 0.7 mJ 2 mJ 41.5 mJ

48 45 44.6 50 54.5 40 48.5 e e e 42.5 41.1 e e 51.6 53 54.6 e e e e e 42.3 e e e e e e e e

[197] [138] [117] [119] [117] [165] [134] [125] [155] [126] [87] [84] [72] [96] [180] [193] [117] [95] [94] [75] [37] [199] [200] [37] [118] [201] [120] [37] [160] [82] [133]

1060 1566 1560 1562 1924

2.8 4.8 6.5 30.4 5.5

74.9 35.4 67 32.8 21.8

8.72 29.2 2.5 102 0.13

0.02 0.02 e e 0.1

116 825 370 57.9 42

e e 31.3 e e

[38] [38] [134] [131] [38]

1027e1065 tunable 1030 1560 1565 1561 1570 1568 1558 1064 1064

1.65 3.2 4 1.3 1.4 e 0.71 1.1 0.922 0.112

97 36.7 78 108 84.8 125 134 97 52.5 392

2.36 0.5 6.41 7.84 0.3 e 2.5 6.4 42.6 173

0.07 e e e e e e e e e

28.8 13.6 1.1 mJ e 139 46.3 19 179.6 e 441

50 53 54.2 e 43.7 e e 44 e e

[116] [141] [134] [123] [162] [135] [114] [141] [153] [127]

1560 1560

4 3.1

85.3 49.6

3.16 1.23

e e

485 33.2

41.9 46.7

[134] [202]

Note: We tabulate the properties of output pulse at a maximum pump power, since the pulse duration and repetition rate are power dependent. HTG, hydrothermal growth; HT-I, hydrothermal Intercalation; ST, solvothermal; Pulse characteristics: t, pulse duration;l, center wavelength; Dt, spectral width; P, output power; E, pulse energy; ER, extinction ratio.

demonstrated using MoS2 SA at 1066 nm. Here, we emphasis that the high non-saturable loss in the existing MoS2 SAs inhibits to demonstrate the mode locking in a solid state laser. To alleviate this issue, a further improvement is needed in modern fabrication and integration techniques of MoS2 SA for bulk lasers. Mao et al. reported the first WS2 SA mode-locked fiber laser using a D-shaped fiber SA and WS2-PVA SA. The D-shaped fiber SA results in a pulse width of 1.32 ps, a repetition rate of 8.86 MHz and spectral width of 2.3 nm. The WS2-PVA SA was reported with pulse width of 1.2 ps, a repetition rate of 8.96 MHz and spectral width of 3.1 nm [39]. Guoyu et al. reported the first Yb doped WS2 SA modelocked laser with pulse width of 2.5 ns and repetition rate of 2.84 MHz [164]. The other reported WS2 based mode-locked lasers

for different gain mediums are Yb-doped [124], Er-doped [97,124,135,163,170] and Tm-doped [129] fibers. Mao et al. demonstrated the first WS2 SA mode-locked dissipative solitons at 1.06 and 1.55 mm [124]. The maximum pulse parameters achieved using WS2 SA are average power of 110 mW [39], a repetition rate of 1 GHz [97] and a pulse duration of 919 fs [135]. W. Liu et al. reported the first WS2 SA based dark soliton generation in a mode-locked laser using erbium doped gain fiber [211]. Very recently, J. Lee et al. demonstrated the first harmonic mode-locked laser based on WS2 SA with a repetition rate of 1.51 GHz at 104th harmonic [128]. Hou et al. demonstrated the WS2 SA solid state laser using Yb:YAG crystal as gain medium with an output power of 270 mW, the peak power of 4.23 kW, a pulse repetition rate of 86.7 MHz and a pulse

J. Mohanraj et al. / Optical Materials 60 (2016) 601e617

width of 736 fs [145]. Mode-locked fiber laser using MoSe2 SA reported a pulse duration of 1.25 ps and repetition rate of 8.1 MHz [130]. Recently, S. Sathiyan et al. demonstrated the first dissipative soliton MoSe2 SA based on all-normal dispersion Yb-doped laser [154]. Very recently, Mao et al. demonstrated the first WSe2 SA based mode-locked laser with a pulse width of 1.31 ps and a repetition rate of 19 MHz [175]. Very recently, J. Koo et al. reported the first bulk-like MoSe2 SA based harmonic mode-locked fiber laser with the highest repetition rate of 3.27 GHz at 212th harmonic resonance [210]. Table 3 summarizes the fabrication method and mode-locked laser pulse parameters and properties based on TMD SAs to date. 6. Conclusion and outlook The study of electronic, structural and optical properties of the

613

TMDs was started early in 1960s [46] but the lack of reliable characterization instruments and insights in their fabrication techniques were acted as a bottleneck for the further exploration in the field of photonics. The recent advancements in the exfoliation and characterization techniques have motivated the researchers to explore the characteristics of TMDs for the saturable absorber device development. The semiconductor TMDs have potential optical characteristics, in order to realize a passive saturable absorber, such as layerdependent nonlinear response, broadband operation and an ultrafast response time. In the family of TMDs, the MoS2 was the first material utilized for a saturable absorber development for pulsed lasers. The other members such as MoSe2, WS2 and WSe2 based SAs were evolved gradually. In the current scenario, there is a strong need to investigate the saturable absorption properties in the other available TMD materials. Very recently, the use of new TMD

Table 3 Mode-locked pulsed lasers with few layered TMD SAs. Laser type

Few Layered TMD fabrication method

l (nm)

Laser properties t (ps)

frep (MHz)

P (mW)

Dt (nm)

ER (dB)

Ref.

1043 1054 1063 1037.5 1037,1039 1569 1568 1568 1570 1535e1565 tunable 1530 1556 1571 1573.7 1584 1570 1570 1558 1568 1905 1060 1064.2

656 800 22.9 ns 1.55 ns e 1.28 4.98 637 fs 1.42 ~1 2 935 fs 116 630 fs 521 fs 11.9 710 fs 3 1.4 843 725 12.7

6.7 6.6 2.2 15.43 17.4 8.3 26 33.5 216 13 125.5 463 7.45 27.1 25.2 26 12.1 2.5 GHz 5.8 9.7 83 88.3

2.4 9.3 9.8 1.5 0.8 5.1 2 e 6.8 e e 5.9 6.9 3.82 0.79 e 1.8 5.4 e 150 156 89

8.6 2.7 0.1 0.9 e 2.6 23.2 12.4 2.8 3 2.1 6.1 11.7 e 10 18.2 4 2.5 2.3 17.3 12.8 0.34

59 50 e 40.5 50 62 63 61 36.1 55 60 97 e 61 54 76 60 e 55 > 55 e e

[152] [59] [106] [207] [208] [83] [85] [85] [86] [137] [121] [132] [168] [209] [115] [115] [104] [105] [180] [122] [161] [136]

LPE LPE

1040 1558 1556.7 1557.3

471 1.45 1.09 737 fs

15.44 8.1 5.03 3.27 GHz

2 0.44 e 22.8

4.26 1.8 2.3 5.1

53.5 61.5 e 61.9

[130] [130] [175] [210]

WS2 Yb:Fiber

LPE

Er:Fiber

LPE

LPE SP-I

1030 1064 1566 1557 1572 1561,1563 1564 1566 1530 1558.4 1560 1560 1565 1550 1560 1941 1058

2.5 ns 630 29.7 2.1 919 fs 369 fs, 563 fs 808 fs 467 fs e 660 452 fs 609 fs 332 fs 1 ns e 1.3 736 fs

2.8 5.6 8.1 8.9 25.2 25,20 19.6 21.1 116.5 1.51 GHz 1 GHz 19.57 31.1 396 352 34.8 86.7

8 76 1.8 110 4 e 2.7 0.32 e 2.08 11.3 1.5 0.43 e 5.28 0.6 270

1.1 0.77 14.5 2.3 5.2 7.5,5.2 5.2 5.6 0.16 3.4 6.6 6.75 8.23 0.032 0.031 5.6 2.1

48 e e 50 75 69,58 60.5 61 94 57.9 48 64 82 56.3 61 72 51

[164] [124] [124] [39] [135] [170] [163] [89] [211] [128] [97] [212] [89] [171] [212] [129] [145]

LPE

1556.7

1.31

3.25 GHz

19

2

e

[175]

MoS2 Yb:Fiber

HT-I

LPE Er:Fiber

CVD

LPE

LI HT-I

Tm:Fiber Nd:GdTaO4 Nd3þ:YVO4 MoSe2 Yb:Fiber Er:Fiber

LPE LPE LPE

PLD CVD MST Tm:Fiber Yb:YAG WSe2 Er:Fiber

Note: We tabulate the properties of output pulse at a maximum pump power, since the pulse duration and repetition rate are power dependent. HTG, hydrothermal growth; LI, Lithium Intercalation; SP-I, solution processing intercalation; MST, magnetron sputtering technique; HT-I, hydrothermal Intercalation; Pulse characteristics: t, pulse duration; l, operating wavelength; frep, pulse repetition rate; Dt, spectral width; P, average output power; ER, extinction ratio.

614

J. Mohanraj et al. / Optical Materials 60 (2016) 601e617

materials such as tungsten ditelluride (WTe2) and molybdenum ditelluride (MoTe2) have been investigated for their nonlinear optical response and successfully demonstrated the mode-locked fiber laser using WTe2/MoTe2 saturable absorbers [214]. Further, the heterostructure TMD materials namely WSe2/MoS2, MoS2/WS2 and MoSe2/WSe2 were used very recently to realize novel passive saturable absorbers [215e218]. In this review, we have discussed the various methods available to fabricate few-layered TMDs from the bulk materials such as micromechanical cleavage, chemical vapor deposition, hydrothermal intercalation, pulse laser deposition, lithium intercalation and liquid phase exfoliation. Among these, the liquid phase exfoliation has been widely implemented to exfoliate few layered TMD because of its easy fabrication steps and cost effectiveness. Further, we reviewed the various techniques that are used to fabricate an all fiber SA using few layered TMD nanoflakes. The integration of TMD nanoflakes with fiber can be done using PVA composite, coating on the fiber end or on the surface of the polished fibers. On the other hand, the free-space SAs were developed by depositing the nanoflakes onto quartz or gold mirror. Recently, PCF based cell type SA was fabricated by depositing the nano flakes inside the air holes [163]. Finally, we summarized the output characteristics of all the Q-switched and mode-locked pulsed lasers that were incorporated with TMDs SAs. From the literature survey, it was found that the lasers that were demonstrated utilizing TMDs SAs have the wavelength range from 1 mm to 2.1 mm, the pulse repetition rate is in the range of kHz to GHz, pulse duration is from ms to fs and average power is in mW. Here, we intend to notice that power that is achieved with the TMD based fiber laser is far less than the conventional solid state lasers. This power can be increased further by decreasing the non-saturable loss that exist on the present TMD SAs. This can be achieved by exploring the new fabrication technique to produce high quality flakes and implementing suitable integration scheme to increase evanescent field interaction between light and the flakes. Most of the pulsed lasers with TMD SAs are functioning in the wavelength range of 800 nme2100 nm. One of the very recent exploitation of TMD SAs in fiber laser is to demonstrate laser pulse generation in visible range. Compared to graphene, TMD materials offer an optimum SA performance in visible range since it exactly coincidence with the TMD materials bandgap resonances. The first visible wavelength Q-switched laser using MoSe2 SA at 635 nm was reported by Wu et al., in 2015 [219]. Very recently, Luo et al. reported the red light Q-switched all fiber laser using the few layered TMDs (MoS2, WS2, MoSe2) SAs [220]. With this initial reports, in future the researchers can focus on the development of visible wavelength lasers for the applications in the fields of laser medicine and underwater detection. Further, there is the huge scope for the investigation on the impact of excitonic effects in the TMD materials that eventually enhance the absorption property of TMD SA. In the current scenario, beyond TMDs, another 2D material such as few layered topological insulators exhibit the wider nonlinear optical absorption and edge-states, this can be used to realize broadband SA. Additionally, the recently explored two dimensional material is black phosphorus, analogous to TMDs. It exhibit tunable bandgap while transition from bulk to monolayer (i.e 0.3 eVe2.0 eV) [221] that shows layer dependent characteristics [222,223] which can be intently explored for tunable optoelectronic and photonic applications. By stacking this new 2D materials with the TMDs the van der Waals heterostructures are achieved which can be further explored for broadband applications. For example, MoS2/graphene heterostructure are implemented for the developments of photodetectors, photodependant memory devices etc [224,225]. Although, TMDs successful in pulsed laser applications, still there is a room to explore its spin and valley dependent

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