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Graphene and other two-dimensional (2d) crystals are promising materials for photonic and optoelectronic applications. A key requirement for these applications is the development of industrial-scale, reliable, inexpensive production processes, while providing a balance between ease of fabrication and final material quality with on-demand properties. Solution-processing offers a simple and cost-effective pathway to fabricate various 2d crystal based photonic devices, presenting huge integration flexibility compared to conventional methods. Here we present an overview of graphene and other 2d crystals based ultrafast photonics, from solution processing of the raw bulk materials, the fabrication of saturable absorbers, to their applications in ultrafast lasers.
© 2013 Optical Society of America
1. Introduction
Ultrafast lasers are essential tools for various applications, ranging from basic research and metrology to medicine, telecommunications, and materials processing [1]. Ultrafast lasers are commonly based on a mode-locking technique, whereby a nonlinear optical element - the saturable absorber (SA) - turns the laser from continuous wave into a train of ultra-short pulses [1]. The current technology mainly exploits semiconductor saturable absorber mirrors (SESAMs) as SA [1–3]. However, SESAMs have a narrow operation bandwidth (∼ a few tens of nanometers [1, 2]) and require complex fabrication and packaging [1]. Carbon nanotubes (CNTs) offer a simpler and cost effective alternative [4–18]. Tunability is possible by combining CNTs with diameter distribution [7] as the operating wavelength is determined by their diameter i.e. bandgap [5]. However, for a given wavelength, CNTs not in resonance are not ”active” and contribute for non saturable losses [4, 19, 20]. In contrast, in graphene there is always an electron-hole pair in resonance for any excitation due to the linear energy dispersion [4,19,20]. Thanks to the ultrafast carrier dynamics [21,22] and large absorption of incident light (∼ 2.3% per layer [23–25]), graphene behaves as a fast SA over a wide spectral range [5, 14, 19, 20]. Graphene is just an example of 2d crystals and the development of other layered materials (LMs) having large nonlinearity is being actively pursued [26–31]. Recently, 2d crystals such as e.g., MoS2 [31–34], Bi2Se3 [35] and Bi2Te3 [36] have shown saturable absorption properties. In particular, the topological insulators (TIs) such as Bi2Se3 and Bi2Te3 are materials with an insulating bulk state and gapless Dirac-type surface/edge states [37] that are receiving great attention in condensed-matter Physics [37] and, recently, have been demonstrated for optical pulse generation [26–30].
The demand for various photonic devices meeting performance criteria as well as economic requirements opens the door to novel processing technologies capable of high-yield, low-cost manufacturing, while delivering high performance and enabling unique functions. In this context, materials with nonlinear optical and electro-optical properties, that can be processed economically in large quantities in liquid environments and then coated/printed on optical elements and/or embedded in polymers, are the ideal choice for such an integration platform. Indeed, solution processing has been demonstrated as one of the key methods to fabricate various 2d crystals for ultrafast laser applications. Here we discuss the potential of solution processed optical materials (e.g., graphene and other 2d crystals), their deposition or coating on optical elements, and/or their incorporation in polymer composites for economic and wide-band ultrafast lasers.
2. Linear optical properties
Although graphene is a single atom thick material (Fig. 1(a)) [24, 38, 39], it can be optically visualized [24,38] and its transmittance can be expressed in terms of the fine-structure constant [23]. The absorption spectrum of graphene is quite flat from ultraviolet (UV) to infrared (IR), with a peak at ∼270 nm, due to the exciton-shifted van Hove singularity in the density of states [40]. This, in principle, allows graphene to be used over a broad wavelength range (e.g., from UV to THz [4, 19, 20]).
Fig. 1: Crystal and band structure of (a) Graphene, (b) MoS2 and (c) Bi2Te3. For Bi2Te3 the shaded regions represent bulk states while the red dashed lines are surface states.
There are many other 2d materials, besides graphene. A number of studies [41–46] have reported exfoliation of LMs to atomically thin layers. Layered materials include h-BN [42,43], transition metal dichalcogenides (TMDs) (e.g., MoS2, Fig. 1(b), MoSe2, WS2 and WSe2) [47, 48], and TIs (e.g., Bi2Te3, Fig. 1(c) or Bi2Se3) [37]. Other classes of LMs also exist such as transition metal oxides (LaVO3, LaMnO3), transition metal trichalcogenides (NbSe3, TaSe3), transition metal chalcogenide phosphides (MnP4, Li7MnP4), and others. Each class consists of a range of materials, with its own set of physical and (opto)electronic properties. Such properties can also be modified when passing from bulk to single layer (SL). For example, bulk 2H-MoS2 has an indirect band gap of 1.29 eV, while SL-MoS2 has a direct band gap (1.8 eV). The absence of interlayer coupling of electronic states at the Γ point of the Brillouin zone in SL-MoS2 [49] results in strong absorption and photoluminescence bands at 1.8 eV [49]. Thus, combined with its electrical properties, these optical features make SL-MoS2 a promising candidate for novel optoelectronic devices, e.g., light-emitting and photovoltaic devices, and photodetectors [50].
Topological insulators, due to their spin-momentum-locked massless Dirac surface/edge states [37], are an emerging class of materials for broadband (opto)electronic applications [51]. Bi2Se3-based transparent conductive electrodes (TCEs) have been recently fabricated via van der Waals epitaxy exhibiting sheet resistances value of ∼330 Ω / square, with a transparency exceeding 70% over a wide range of wavelengths [51].
3. Nonlinear optical properties
Two dimensional crystals also show remarkable nonlinear optical properties [19]. For example, in graphene, interband excitation by ultrafast optical pulses determines a non-equilibrium carrier population in the valence and conduction bands [20]. Two relaxation time-scales are observed via time-resolved experiments [21]: a faster one associated with carrier-carrier intraband collisions and phonon emission, and a slower one, on a picoseconds timescale, which corresponds to cooling of hot phonons and interband relaxation [52,53]. The linear dispersion of the Dirac electrons in graphene implies that for any excitation there will always be an electron-hole pair in resonance [19], thus enabling unique broadband photonics and optoelectronics [19]. If the relaxation times are shorter than the pulse duration, during the pulse, the electrons reach a stationary state and collisions put electrons and holes in thermal equilibrium at an effective temperature [20]. The populations determine electron and hole densities, total energy density, and a reduction of photon absorption per layer due to Pauli blocking [19]. For linear dispersions near the Dirac point, pair carrier collisions cannot lead to interband relaxation, thereby conserving the total number of electrons and holes separately [20]. A three-particle collision is required to move an electron from conduction to valence band [54]. Interband relaxation by phonon emission can occur only if the electron and hole energies are close (within the phonon energy) to the Dirac point [19]. For graphite flakes the situation is different: the dispersion is quadratic and pair carrier collisions can lead to interband relaxation [20]. Thus, in principle, decoupled SLG can provide the highest saturable absorption for a given amount of material [20].
With the advancement in graphene SA (GSA) performance, other Dirac materials (i.e., TIs) have been investigated to explore their saturable absorption properties [26–30]. Other LMs, such as MoS2, have also demonstrated saturable [31–34] and two-photon [31] absorption properties. These ultrafast nonlinear optical responses testify how 2d MoS2 could have huge potential in the development of nanophotonic devices, e.g., optical limiters [19, 55]. In particular, it is possible to use MoS2 [32–34] and other LMs (e.g., WS2) for ultrafast pulse generation. However, currently it is challenging to find laser gain materials with proper wavelength and high-gain to fit their bandgap (e.g., high-gain ∼688nm laser for MoS2).
4. Solution processing
The exploitation of 2d crystals for photonic applications will require cost-effective and large-scale production methods [56, 57], while providing a balance between ease of fabrication and final material quality with on-demand tailored properties [56, 57]. One advantage of LMs with respect to other (nano)-materials, relies on the possibility to produce them by top-down (exfoliation from bulk) techniques [56, 57], such as micromechanical cleavage [58], anodic bonding [59], photoexfoliation [60] and liquid phase exfoliation (LPE) [61]. Amongst these different top-down approaches, LPE of LMs [41, 61–67] is an ideal means to produce dispersions and inks [62]. These as-fabricated dispersions and inks are perfect starting materials for production of thin films [19] and composites [5], suitable for a large range of electronic [62] and photonic [4–6] applications. The LPE process of LMs (Fig. 2(a)) generally involves three steps: 1) dispersion in a solvent (Fig. 2(b)); 2) exfoliation (Fig. 2(c)); 3) “purification” and sorting [56] (Fig. 2(d)). The third step, usually carried out via ultracentrifugation processes [64,68], is necessary to separate exfoliated from un-exfoliated flakes, being also a powerful tool for the separation of large from small [65] and thick from thin [64] flakes.
Fig. 2: Liquid phase exfoliation of LMs.(a) Starting material (e.g., graphite), (b) chemical wet dispersion, (c) ultrasonication and (d) final dispersion after the ultracentrifugation process.
4.1. Dispersion and exfoliation of pristine graphite
Liquid phase exfoliation of graphite is carried out via chemical wet dispersion followed by ultrasonication, either in water [63, 64, 66] or in organic solvents [61, 62, 66, 67, 69, 70].
Ultrasound-assisted exfoliation is controlled by hydrodynamic shear-forces, associated with cavitation [71], i.e., the formation, growth, and collapse of voids or bubbles in liquids due to pressure fluctuations [71]. After exfoliation, the interaction between solvent and graphitic flakes needs to balance the inter-sheet attractive forces [56]. The ideal solvents for graphitic flake dispersion are those that minimize the interfacial tension [mN/m] (the force that minimizes the area of the surfaces in contact) between the flakes and the surrounding liquid [56]. If the interfacial tension between graphitic flakes and solvent is high, the flakes tend to adhere to each other and their dispersion in liquid is hindered by the high work of cohesion (i.e., the energy per unit area required to separate two flat surfaces from contact [72]) between them. Liquids with surface tension γ ∼40 mN/m [72], are the best solvents for the dispersion of graphitic flakes, since they minimize the interfacial tension between solvent and graphene [61].
The majority of solvents with γ ∼40 mN/m (i.e., Dimethylformamide (DMF), N-Methyl-2-pyrrolidone (NMP), Benzyl benzoate, gamma-Butyrolactone, etc.) [61] have some disadvantages. For example, DMF and NMP are included in the candidate list of substances of very high concern [73] and may have teratogenic effects [74]. Moreover, all have high (>450 K) boiling points, thus making difficult the removal of the solvent after the coating/printing process. The exploitation of low boiling point solvents [70], such as acetone, chloroform, isopropanol, etc. can be an alternative. However, the surface tension of these solvents is too low (∼ 25 mN/m) and the exfoliation yield (i.e., percentage of SLG, YM) as well as the concentration of dispersed flakes is by far too low [70] compared to the ones achieved with, for example, NMP [61, 62]. Water has a γ ∼72 mN/m [72], too high (30 mN/m higher than NMP) for the dispersion of graphitic flakes [75]. In this case, the exfoliated flakes can be stabilized against re-aggregation by Coulomb repulsion using linear chain surfactants (e.g., Sodium dodecylben-zenesulfonate [63]), or bile salts (e.g., sodium cholate (SC) [64] and sodium deoxycholate (SDC) [65, 66]), or polymers (e.g., pluronic [76]), etc. However, the presence of dispersant molecules [68] is not the best option in view of their integration in devices when preservation of the pristine electronic structure is important, e.g., in TCEs where the presence of these molecules can decrease the inter-flake conductivity [56].
4.2. Dispersion and exfoliation of graphite oxide
Chemical derivatives of graphene, e.g., graphene oxide (GO) sheets [77], have (opto)-electronic properties complementary to graphene. The oxidation of graphite following the Hummers method [78] makes GO sheets readily dispersible in water [79] and several other solvents [80] with large predominance of SL GO. However, although large (several μm) GO flakes can be produced, these are intrinsically defective and electrically insulating [81] and thus reduced GO (RGO) does not fully regain the pristine graphene electrical conductivity [81]. Nevertheless, RGO can be deposited on different substrates exploiting the coating/printing techniques used for the deposition of pristine graphene films, having the advantage of being dispersible in water without the need of surfactants/polymers. Moreover, GO is ideal for mass production of material that can be used in composites [79] both for mechanical and photonics applications [82–84]. GO is also luminescent due to likely the presence of oxygen-related defect states [19] and could be used in low-cost optoelectronic devices, such as display and lighting applications [19], bio-labelling and bio-imaging [85].
4.3. Dispersion and exfoliation of other 2d crystals
Liquid phase exfoliation can be used to form few-layer sheet of 2d crystals in organic solvents [41–46] and aqueous solutions [86, 87], with [86] or without [87] surfactants, or their mixtures [88]. The exfoliated sheets can then be stabilized against re-aggregation either by electrostatic repulsion by using surfactant molecules adsorption techniques [19, 86] or through interaction with a solvent [41]. In the latter case, reference [41] showed that the best solvents are those having a surface tension matching the surface energy of the target LM. The dispersions can then be exploited for the formulation of inks with a variety of (opto)-electronic properties [62], or be used for processing in thin films and/or composites [41]. Research is still at an early stage, and LPE must be extended to a wider range of materials. To date, exfoliated TMDs both in organic solvents [41] and surfactant aqueous solutions [86] tend to exist mainly as multilayers [41, 86]. Although many 2d crystals (e.g., graphene, BN, MoS2, WS2, etc.) can be produced via LPE from their bulk counterparts, for many of them (e.g., TMDs, oxides and metal halides) intercalation of alkali metals is the more effective exfoliation route [89]. Hydrothermal process has been recently exploited to intercalate (using Li solution) and exfoliate bulk Bi2Te3 [90]. This chemical intercalation method permits to achieve higher levels of intercalated Li, with respect to conventional intercalation methods [89]. Indeed, the highly lithiated powders, obtained from the autoclave method, are then immersed in deionized water facilitating the exfoliation and thus resulting in higher YM of exfoliated Bi2Te3 nanosheets [90]. However, 2d crystals produced by Li (and ion in general) intercalation processes suffer drawbacks related to their sensitivity to ambient conditions and structural changes following intercalation [91]. For example, upon Li intercalation, MoS2 undergoes a first order phase transition from the thermodynamically stable (2H-MoS2) phase to a metastable (1T-MoS2) phase, which is metallic [48].
Topological insulators such as Bi2Te3 and Bi2Se3 can also be produced via chemical synthesis, i.e., Polyol method [92]. This method was historically developed for the synthesis of metal-containing compounds in poly(ethylene glycol)s [92]. The synthesis of TIs via Polyol method is usually carried out by mixing two reactants (e.g., Bismuth (III) nitrate pentahydrate and sodium selenite for the synthesis of Bi2Se3), the precursors, with polyvinyl pyrrolidone and ethylene glycol into a two-neck flask containing a magnetic stirring bar [92]. The as prepared dispersion is keep at high temperature (∼190 °C under stirring for ∼2 hours [93]). The purification of the dispersion is then usually carried out via centrifugation processes, see Section 4.5.
4.4. Co-solvent exfoliation of layered materials
The exfoliation and dispersion of LMs involves the exploitation of either high boiling point solvents [61,62,66,67,69] (generally toxic, see Sect. 4.1) or surfactant/polymer molecules [63–66] in aqueous solvents. Thus, recent research efforts have been driven by the exploitation of co-solvency effect in which the dispersibility of LMs can be greatly improved by using a mixture of solvents [88,94], e.g., water/ethanol [88,94], water/isopropyl alcohol [94], etc. By adjusting the relative concentration of the co-solvents it is possible to tune the rheological properties (i.e., density, viscosity, and surface tension) [95] of the mixture ”on demand”. However, the YM and concentration of the exfoliation process in such co-solvent mixtures is, up to date [88,94], much lower with respect to the ones achieved in NMP [62] and water-surfactant dispersions [65]. Moreover, it is worth noting that exploitation of co-solvent mixtures [88, 94, 95], mostly based on water and alcohols, for the dispersion and exfoliation of LMs has some practical disadvantages. Indeed, the surface tension changes exponentially after the addition of alcohols to water [95] and thus being very sensitive to solvent evaporation [94]. Moreover, all the rheological properties of alcohol-based co-solvents are very temperature sensitive [95]. This is a problem both during processing (the ultrasonication causes a temperature increase of the dispersion) and for the shelf life of the dispersions/inks.
4.5. Sorting strategies
The exfoliation process of LMs produces dispersions with a heterogeneous composition ranging from thick flakes, even un-exfoliated, to thin flakes, ultimately SL. Moreover, the as-produced flakes have, generally, a very broad lateral size distribution. Thus, the control of the morphological properties is fundamental in view of the optimization of the LPE process for industrial applications. Thick flakes can be removed by following different strategies based on ultracentrifugation in a uniform [96] or density gradient medium (DGM) [96]. The first is named sedimentation based-separation (SBS) [96], while the second density gradient ultracentrifugation (DGU) [96]. SBS separates various particles on the basis of their sedimentation rate [96] in response to a centrifugal force acting on them. Sedimentation based separation is the most common separation strategy and, to date, flakes ranging from few nanometers to a few microns have been produced, both for graphene [61, 63–66, 69] and other 2d crystals [41, 97].
High concentration is desirable for large scale production of composites [5] and inks [62]. Following SBS, concentrations of up to ∼17 mg/ml for graphitic sheets [69] and ∼40 mg/ml for MoS2 [97] flakes have been achieved. Higher concentrations (>60 mg/mL) have been reported for dispersion of graphitic flakes in NMP without centrifugation process [69]. However, these dispersions were not stable (concentration decreased to ∼30 mg/ml after 200 h of sedimentation) containing in average a few layer thick flakes [69]. Dispersion with high content of SLGs (up to ∼ 60%) was achieved by mild sonication in water with SDC, followed by SBS [65], while ∼ 33% SLG was reported with NMP [62]. This difference in SLGs yield is related to the difference in lateral sizes of the flakes. The flake size in water-surfactant dispersions is on average smaller (∼30nm [65] to ∼200 nm [63]) than in NMP(∼1 μm [61, 62]). The reason for this relies in the difference in viscosity of the two media. Indeed, the viscosity at room temperature of NMP (1.7 mPa·s [98]) is higher than water (∼1mPa·s [98]). Larger flakes in a higher viscosity medium experience higher frictional force [96] that reduces their sedimentation coefficient (S), making it more difficult for them to sediment. This difference in the viscosity of the medium decreases the SLG yield in NMP compared to water [56]. Control on the number of layers can be achieved via DGU, where nanomaterials in dispersion are ultracentrifuged in a preformed DGM [99]. During the process, they move along the cuvette, dragged by the centrifugal force, until they reach the corresponding isopycnic point, i.e., the point where their buoyant density equals that of the surrounding DGM [100]. The buoyant density is defined as the density of the medium at the corresponding isopycnic point [99]. The buoyant density depends on the dispersion [100], the type of surfactant [68], and may also change with gradient medium [101], or for the same DGM, on the pH [100]. Isopycnic separation has been used successfully to sort nanotubes by diameter [102], metallic vs semiconducting [103] and chirality [68]. In the case of graphitic flakes, uniform coverage of the flakes with surfactant molecules [68, 104] results in an increase of buoyant density with the number of layers. To date, up to ∼80% SLG yield was reported by using isopycnic separation [64]. Isopycnic separation was also recently used to separate GO flakes with different thickness [105].
Another method to sort LMs is the rate zonal separation (RZS) [99, 106]. In RZS the ultracentrifugation is stopped during the transient centrifugal regime, before the (nano)materials arrive at their isopycnic points [106]. RZS exploits differences in the sedimentation coefficient of (nano)materials under ultracentrifugation [106]. Thus, objects with different S values will travel along the cuvette at different sedimentation velocities [99, 106]. This will cause a spatial separation along the cuvette [106]. RZS was used to separate GO flakes with different size [107] (the larger the size, the larger the sedimentation rate) and other nanomaterials [101].
5. Thin films and polymer-composites fabrication
Solution processed graphene, GO, and 2d crystals can be used for electronic and photonic applications, exploiting simple coating/printing process (e.g., spin-coating or drop-casting the as-prepared dispersion/ink on high reflectivity mirrors [19] or quartz substrates [19,83]). However, such substrates are rigid, thus limiting their integration in various waveguide systems (e.g., fiber lasers). A more versatile route for the integration of LMs in photonic devices relies in the incorporation of such nanomaterials in polymer composites. Polymers can be synthesized with customer defined optical characteristics, such as selective transparency bands in different spectral ranges, variable refractive index/birefringence, as well as high laser damage threshold and thermal stability. Moreover, the polymers must be easily processable during device fabrication and be economic [108]. Polymers traditionally used for optical applications (e.g., polycarbonate (PC) [8], polymethylmethacrylate (PMMA) [109] and epoxy resins [109], halogenated [108] or deuterated polyacrylates [108] and fluorinated polyimides [109]) have been developed to address specific issues, such as optical losses [110], heat dissipation [108] as well as environmental stability [108]. Water-soluble polymers, such as polyvinylalcohol (PVA) [5] and cellulose derivatives, such as sodium carboxymethyl cellulose [5], have been widely used for CNT and graphene-based SAs [5, 6, 9] since stable, high-concentration dispersions can be readily prepared [5]. PVA is also very attractive, from the fabrication perspective, because of its mechanical properties [5]. To prepare environmentally stable polymer composites, in particular, against humidity and temperature, LMs can be directly exfoliated in organic solvents [61]. The dispersions are suitable for moisture resistant polymers such as PC and PMMA or copolymers such as styrene methyl methacrylate [5]. Dispersion of LMs in solvents is the first step for the fabrication of polymer composites. The as-prepared dispersion is then mixed with the host polymer. The mixture is then drop-cast or spin-coated to obtain free standing or substrate-bound SA composites with homogeneous, sub-micrometer distribution of flakes, as for the case of graphene [5, 20, 111–113].
6. Applications in Ultrafast Photonics
Solution processing of LMs have been widely used in photonics and optoelectronics, e.g., TCEs for solar cells [114] and electrotactile flexible display [115], photodetection [116], and pulse generation [5, 20, 117], the latter being the most successful example [5, 20, 117–157]. For instance, the first graphene enabled ultrafast fiber [5], solid-state [117,118], and waveguide [119] lasers all have been demonstrated by GSAs fabricated by solution processing technique (i.e., LPE [5, 20, 117], GO [82–84], and RGO [118, 120, 121]). This greatly reflects the fabrication/integration advantages of solution processing over other production methods [56].
Different approaches have been used to integrate GSAs in lasers, such as free-space coupling [83, 118], deposition on fiber ends [122, 123] and inside fibers [124–126]. Thus far, the most popular way to integrate GSAs into fiber lasers is to sandwich a GSA between two fiber connectors (Fig. 3(a)) [5, 10, 20, 66, 82, 117], since this offers ease of integration into various fiber/waveguide systems [5,10,20,66,82,117]. Evanescent field interaction [121,127,128] also has been demonstrated, targeting high-power output.
Fig. 3: Typical GSA mode-locked fiber laser: (a) integrated GSA device. (b) laser setup. WDM: wavelength division multiplexer; (c) pulse duration [160] (d) Tunable fiber lasers [10].
Table 1 summarizes output performance of fiber, solid-state and waveguide lasers mode-locked by GSAs, which are fabricated by solution processing. Lasers mode-locked with 2d crystals fabricated with other methods (and Q-switched lasers) are not listed, as they are beyond the scope of this paper. Since the first demonstration in 2009 [5], the performance of ultrafast laser mode-locked by solution-processed GSAs has steadily improved. For example, the average output power has increased from few hundreds mW [5, 117] to few W [83, 129]. A large range of output parameters, such as operation wavelength, pulse duration, and repetition rate have been achieved. Thus far, the demonstrated wavelengths range from 1 [117, 118] to 2 μm [117, 130]. The most common wavelength of generated ultrafast pulses thus far is ∼1.5 μm, being the standard wavelength of optical telecommunications, with Erbium doped fiber lasers. The output pulse durations range from a few ns [131] to sub-200 fs [112]. The repetition rate spans from few hundreds KHz [131] to ten GHz [123]. Wavelength-tunable [10] and multi-wavelength [128] ultrafast lasers based on GSAs have also been reported. Note that Ref. [119] first demonstrated a monolithic ultrafast Yb-doped waveguide laser with a GSA in an active waveguide laser fabricated by ultrafast laser inscription. The GSA was fabricated by coating a LPE graphene film on an output coupler mirror, which was butt-coupled to a waveguide end using index matching gel [119]. The laser produced ∼1 ps pulses with a 1.5 GHz repetition rate and 202 mW average power [119]. Driven by the development of GSA, just recently solution processed TIs (e.g., Bi2Te3 [27], Bi2Se3 [28]) have been reported for ultrafast pulse generation (Table 1). Tunable output results have been demonstrated [28]. Thus far, TI based ultrafast lasers only have been demonstrated on EDFLs. However, it is expected that TIs will soon be employed on other lasers (e.g., YDFL, TDFL and solid-state and waveguide lasers as well).
Table 1:. Representative output performance of mode-locked lasers using 2d crystals fabricated by solution processing method. T: transmissive type, R: reflective type. PM: Polyol method. EDFL, YDFL and TDFL: Erbium-, Ytterbium- and Thulium- doped fiber lasers, respectively.
7. Perspective
Various lasers (e.g., fiber, solid-state, and waveguide lasers) have been mode-locked using GSAs fabricated via solution processing. Undeniably, due to the versatility and advantages with respect to other processing techniques [56], this method will still be one of the major approaches to fabricate various 2d crystals based photonic devices (e.g., ultrafast lasers). Future investigations would be targeted to improve GSA performance (e.g., insertion losses). This is particularly important in view of new applications. For example, other types of ultrafast laser systems (e.g., semiconductor [158], gas lasers) could integrate solution-processed GSAs. However, such integration requires a significant reduction of non-saturable losses [158]. Because decoupled SLG can provide the highest saturable absorption for a given amount of material [20], the optimization of LPE process to achieve high (> 80%) YM SLGs is needed for this purpose.
Thus far, the output performance (e.g., the output power, pulse duration) of ultrafast lasers based on 2d crystals is inferior with respect to their counterparts (e.g., SESAMs [1–3]). For example, TI-based ultrafast lasers, just recently demonstrated, have unsatisfactory output performance, even far below that achieved with graphene [4, 158, 159] and nanotube SAs [4–6,9,117,160–166]. Nevertheless, improvement can be envisioned by innovative cavity design and device optimization. For example, shorter pulse duration (e.g., sub-100fs generated with CNTs [160], or pulse duration similar to those produced by conventional SAs, such as SESAMs) can be achieved by exploiting cavity optimization (e.g., high-order dispersion management). Such optimization can be simplified by directly spin-coating 2d crystals on high-reflectivity mirrors or even on the dispersion compensation components (e.g., prisms, gratings, photonic-crystal fibers [4]). This avoids the introduction of other optical components and thus additional dispersion in the cavity. High-repetition rate ultrafast lasers [167], in which solution processed 2d crystals can be directly coated on the high-reflectivity mirrors (or the gain section [4]) to reduce the cavity length [119], can benefit from solution processing optimization. The ease of fabrication and integration of GSA also provides flexibility for short cavity designs (e.g., compact waveguide laser [4,119]). It is worth noting, output performance can be improved by external cavity methods [4], such as increasing the power by external amplification [4,6,166] or coherent combination of various lasers [168–170], expanding the wavelength accessibility by nonlinear frequency conversion (e.g., harmonic frequency generation [171–175], parametric oscillation [176,177] and amplification [178], supercontinuum generation [16,166,179]). Moreover, 2d crystal-based inks could be directly coated on the surface/facet of the gain sections (e.g., fiber, waveguide, semiconductor, monolithic solid-state gain materials), taking advantage (e.g., large-area deposition on rigid/flexible substrates) of solution processing to facilitate the realization of these photonic devices.
Currently, huge research efforts have been focused mainly on graphene. However, other 2d crystals will be investigated in the near future for photonic applications. For example, although thus far ultrafast pulse generation from MoS2 and WS2 have not been demonstrated yet, it is anticipated that pulse lasers based on these 2d crystal SA will be demonstrated shortly, given the increasing research activity on these materials [31–34].
Acknowledgments
We thank A.C. Ferrari, L. Colombo, H. Lipsanen, M. Bruna, T. Hasan, D. Popa and members of Cambridge Graphene Centre for useful discussions. We thank funding from the Newton International Fellowship, Teknologiateollisuus TT-100, and Aalto University.
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