Graphene is a flat monolayer of carbon atoms tightly arranged into a two-dimensional honeycomb lattice, and is a basic building block for graphite. Since its successful isolation in 2004, graphene has quickly emerged as the most promising material for nanoelectronics and nanophotonics. The electronic and optical properties of graphene as well as the roadmap for its technological development are summarized in a few recent review papers.^[25–28] Graphene was first employed for the mode-locking of lasers independently by two groups.^[29–31] Compared with carbon nanotubes, which have similar atomic arrangement and have been identified earlier as a broadband saturable absorber material,^[31–33] graphene exhibits additional advantages in terms of operating bandwidth, uniform nonlinear optical response over broad spectral region, as well as ease of integration into photonic components and systems. While carbon nanotubes are most often used in the form of polymer composites, the fact that large-scale, high-quality crystalline graphene can be grown on copper foil using chemical vapor deposition (CVD) method and can subsequently be flexibly transferred to arbitrary optical substrates makes graphene highly adaptable for various laser formats. Thus far, graphene has been used to mode-lock fiber,^{[29–31,34–39]} solid-state,^[40–44] waveguide,^[45] and semiconductor lasers.^[46,47] A few comprehensive reviews of graphene-based saturable absorber devices are available.^[48–50] Graphene derivatives, including graphene oxide and reduced graphene oxide, also exhibit ultrafast optical properties and can enable pulsed laser mode-locking. However, the necessity of additional chemical/physical processing renders the fabrication less well controlled and in the meantime unavoidably introduces a considerable amount of defects and gap-states.^[51] Among various 2D materials, graphene has thus far demonstrated performance levels most comparable to or even better than conventional semiconductor devices and it has also shown the best adaptability for fulfilling requirements of a wide range of laser formats.

Similar to broadband operations of SWNTs,^[52–54] graphene absorbers have been shown to enable broadly tunable Q-switched and mode-locked operations.^[34,35] Yang and co-workers used a CVD-grown multilayer graphene SA (> 5 layer) to mode-lock three different lasers, including ytterbium-, erbium-, and thulium-doped fiber cavities, convincingly validating the intrinsic broadband feature of graphene flakes.^[36] Sotor et al. demonstrated a passively synchronized thulium- and erbium-doped fiber laser by a common graphene SA, where soliton operations were simultaneously achieved at 1559 and 1938 nm, respectively.^[37] Such synchronous lasers could be extremely useful for different-frequency generation (DFG) for mid-infrared operation beyond 3 μm. In terms of repetition rate scaling, the highest repetition was achieved based on a short-cavity fiber laser by Martinez and co-workers, where a repetition rate of 20 GHz is achieved through careful optimization of the gain and loss of a miniaturized FP cavity.^[38] Zhang and co-workers demonstrated for the first time mode-locked operation other than conventional soliton regime, i.e., dissipative soliton operation, validating the versatility of graphene SA for various pulsation regimes.^[39]

While early studies are mainly focused on the conventional near-infrared band, recent efforts have shifted towards the use of graphene for long-wavelength lasers, especially those around 2 μm and beyond.^[55–63] The exploration of graphene-based mid-infrared fiber lasers started in 2012, with several groups reporting pulsed laser operations around 2 μm. For fiber lasers, liquid phase exfoliated (LPE) graphene-based composite films were first used to demonstrate a Q-switched laser^[55] and a mode-locked thulium fiber laser,^[56] respectively. Wei and co-workers demonstrated the first graphene-based pulsed laser operation at a wavelength close to 3 μm, where graphene was deposited onto a fiber ferrule mirror using the optically-driven deposition method.^[57] The laser emitted 2.9 μs pulses with a pulse energy up to 1.67 μJ. By optimizing the fiber laser cavity and using a multi-layer graphene-covered gold mirror, mode-locked operation at a wavelength of 2.8 μm with a pulse duration ∼ 42 ps was demonstrated (see Fig. 1).^[58] This is the longest laser wavelength at which graphene-based passive mode-locking has been demonstrated to date. The first use of CVD-grown graphene for mode-locking solid-state lasers in the mid-infrared was demonstrated by Xie and co-workers, where a Tm: CLNGG gain media was used to generate 60 mW, 729 fs pulses at a wavelength of 2018 nm.^[43] Graphene was transferred onto a dielectric mirror and the device was operated in the reflectance mode as a cavity end mirror.^[43] By transferring graphene to a transparent dielectric substrate, a Tm:Lu₂O₃ crystal laser was mode-locked in such a way that shorter (∼ 410 fs) and more powerful (∼ 270 mW) pulses were obtained.^[44] Thus far, the longest wavelength, graphene-enabled, mode-locked, solid-state laser is based on a Cr:ZnSe laser, operating at 2500 nm, in which graphene transferred onto a CaF₂ substrate acted as an absorber in the transmission mode.^[59] The laser features a further reduced pulse duration of 226 fs, with 80 mW output power. Via careful intra-cavity dispersion compensation, Sorokina and co-workers achieved the shortest mid-infrared pulse duration for graphene-based pulsed lasers. The 41 fs duration corresponds to only 5.1 optical cycles.^[60] Recently, Cho and co-workers employed a CVD grown monolayer graphene SA to mode-lock a solid-state Cr²⁺:ZnS laser. The laser exhibits a broad tuning range from 2120 nm to 2408 nm (see Fig. 2). This represents the widest tuning range for graphene-based, mode-locked laser (300 nm, > 10% of the center wavelength).^[62] In addition, the relaxation dynamics of monolayer graphene is measured at 2.35 μm where a fast recover time ∼ 240 fs and a slower component ∼ 2.4 ps were obtained.^[62] Graphene-based SA devices can take other forms such as those directly deposited on SiC substrates. Q-switched operation of a Tm:YAG laser was demonstrated using such an approach.^[63]

Fig. 1.

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Fig. 1. (color online) Graphene mode-locked mid-infrared pulsed fiber laser. (a) Er3+-doped ZBLAN fiber laser incorporating a multi-layer graphene mirror as a saturable absorber. (b) Measured autocorrelation trace showing a FWHM (full-width-half-maximum) pulse width of ∼ 65 ps. Assuming a sech2 profile, this yields a pulse duration of ∼ 42 ps. (c) Optical spectrum of the mode-locked laser, showing a mid-IR center wavelength of ∼2.78 μm.[58]

Fig. 2.

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	Fig. 2. (color online) (a) Schematic of the graphene mode-locked broadly tunable Cr:ZnSe laser. (b) The optical spectra showing an ultra-broad tuning range of ∼ 300 nm, from 2120 nm to 2408 nm. (c) The autocorrelation trace showing a pulse duration ∼ 340 fs.[62]

Flexibility in controlling the optical parameters, such as linear absorption, relaxation times, modulation depth, etc., is an important aspect for engineering practical SA devices and the availability of a large parameter space would allow for adequate optimization of SA-enabled, pulsed laser regimes. Indeed, SESAM has been the most capable of SA devices in terms of parameter customizations. The gate-tunable feature of graphene makes it an excellent choice for an electrically-tunable SA, which may provide multiple benefits for laser operations, including actively controlling the operating regime of a pulsed laser.^[64] Rotermund, Yeom, and co-workers demonstrated an electrically manipulable in-line graphene SA device by integrating graphene-based field effect transistors on a side-polished fiber (SPF) (see Fig. 3(a)).^[65] The device exhibits significant (> 90%) optical transmission change and shows continuous tuning of the nonlinear saturable absorber characteristics, i.e., the modulation depth (see Fig. 3(b)). Due to good compatibility with fiber-optic system, the in-line graphene SA was used in an Er fiber ring cavity for a switchable pulsed laser where the control voltage can be employed to switch between the Q-switched state (pulse duration, 3.5 μs; repetition rate, 25.4 kHz; gate voltage −0.18 V) and the mode-locked state (pulse duration, 423 fs; repetition rate, 30.9 MHz; gate voltage −1.05 V). Sennaroglu and co-workers reported a graphene supercapacitor device as a voltage-controlled SA.^[66] The device was then operated at bias voltages of 0.5 V−1 V to generate sub-100 fs pulses from a Cr⁴⁺:forsterite laser operating at 1255 nm (see Figs. 3(c) and 3(d)). It should be pointed out that although modulation depth is sensitively dependent on the control voltage, the relaxation time constants, which may have strong impact on the initialization of mode-locked operations, are not directly amendable by the externally applied voltage. For SESAMs, both low-temperature growth and post-growth implantation have been applied to tune the relaxation time, through the introduction of lattice defects.^[67,68]

Fig. 3.

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Fig. 3. (color online) (a) Schematic diagram of gate-variable all-fibre graphene device. The side-polished fiber (SPF) was fabricated using a standard single-mode optical fiber where two metal electrodes were deposited with 50-nm thickness at both sides of the side-polished region. After transferring the graphene layer, ion liquid was applied to the graphene. Two electrodes and a Pt wire were used as source, drain and gate, respectively.[65] (b) Gate-controlled optical properties of all-fiber devices with bi-(left panels) and quad-layer (right panels) graphene. The bottom panels correspond to electrical transport properties of devices with bi- and quad-layer graphene.[65] (c) Schematic of the voltage-controlled graphene SA based on the supercapacitor structure.[66] (d) Variation of the normalized change of the optical transmission of the graphene as a function of the wavelength at different bias voltages in the 0–3 V range.[66]

Novel approaches to tune the relaxation time of graphene are to be developed and would further expand the versatility of graphene-based optical devices, pertinent not only to saturable absorbers but also to photodetectors, solar cells, as well as light-emitting devices. Apart from the active approaches, passive ways of adjusting the graphene SA’s parameters have also been demonstrated through fine tuning the thickness of a spacer between a single-layer graphene and a high-reflection mirror (see Fig. 4). Due to the interference between the incident and reflected light field at the mirror, graphene’s field intensity enhancement can be continuously adjusted between 0 and 400%. With such a flexible graphene SA, a VECSEL is successfully mode-locked and wavelength-tunable operation can be achieved across 935 nm to 981 nm.^[47]

Fig. 4.

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Fig. 4. (color online) (a) Schematic of the graphene SA mode-locked VECSEL setup (left). Linear absorption and field intensity enhancement at the single-layer graphene location corresponding to the DBRs without SiO2 (), a λ/12 layer of SiO2 (), a λ/8 layer (), and a λ/4 layer () (right-top). Mode-locking results showing the autocorrelation trace, corresponding to a pulse duration of 466 fs, and the optical spectrum (right-bottom).[46] (b) The schematic of the V-cavity with the gain chip at the fold and the graphene SA mirror and outcoupler at either end (left). The optical spectrum of the VECSEL cavity operating at 9.3 W, with a spectral bandwidth of 6.92 nm (middle) and the corresponding autocorrelation trace (right).[47]

Transition metal dichalcogenides – layered crystals comprising X−M−X sheets (where , Se, Te and , W) bound by weak van der Waals forces – have enjoyed a renaissance as a consequence of the explosive growth in nanoelectronics and nanophotonics research.^[69–72] Distinct from graphene, TMDs can be semiconducting exhibiting a layer-dependent bandgap, ranging from indirect in bulk form to direct in monolayer crystals.^[73] On one hand, the reduced dimensionality and the associated reduced dielectric screening of Coulomb interactions between charge carriers lead to extremely strong excitonic effects in TMDs.^[74,75] Theoretical calculations have predicted very large exciton binding energies in the range of 0.5–1 eV for monolayer TMDs.^[76] The excitonic nature of photoexcitation in TMDs provides enhanced light absorption which is very useful for saturable absorbers where higher modulation depth is needed, such as in the case of fiber lasers. On the other hand, the breaking of inversion symmetry and strong spin–orbit coupling in TMDs lead to valley-selective circular dichroism, and have resulted in a number of novel device concepts.^[69–71] It should be mentioned that TMDs exhibit a rather rich and complex photophysics, i.e., multiple physical mechanisms may operate simultaneously to govern the ultrafast and nonlinear optical response of the material. Recently, several groups experimentally identified the presence of giant and ultrafast bandgap renormalization (BGR) effect in TMDs upon intense ultrashort pulse irradiation. The effect which is induced by the Mott transition from excitonic states to free-carriers in the 2D system opens up an exciting new route for realizing optical controlled functionalities, i.e., ultrafast all-optical modulators (see Fig. 5).^[77–79] On the other hand, Kumar and co-workers observed a strong density-dependent decay of the excitons in monolayer MoSe₂ in the initial phase (within 50 ps) of the transient relaxation, i.e., higher exciton injection density (or pump fluence) would lead to a faster initial decay component. The effect can be well accounted for by considering an exciton–exciton annihilation rate of , suggesting strong interaction between excitons in this strongly confined system. Such a feature was not observed in a bulk sample under similar conditions.^[80] A similar effect was also observed in monolayer MoS₂, where an exciton–exciton annihilation rate of was experimentally derived.^[81] These transient dynamics of the excitonic features could have a profound impact on TMD-based optical and optoelectronic devices.^[77–81] Quite a few studies of the ultrafast photocarrier dynamics in TMDs have been reported based on optical pump-probe spectroscopy.^[82–86] Ultrafast photo-responses of TMDs are found to depend on a number of factors, including microscopic morphologies, substrates, excitation species, as well as the number of layers. The importance of these studies is to reveal fundamental behaviours of photoexcitation species in these emerging 2D systems on one hand, and on the other hand to provide guidelines for realizing robust and reliable tuning of the relaxation times of such materials, which play a significant role in the build-up phase of mode-locked pulses. For mechanically-exfoliated MoS₂ flakes, the relaxation times are found to be relatively long, in the range of on a SiO₂/Si substrate,^[82] and for a suspended sample.^[83] The dynamics shows completely different features for liquid phase exfoliated TMD nanosheets, which exhibit an ultrafast initial photobleaching (PB) signature followed by a transition to photo-induced absorption (PA) signature.^[84] By using an ultrashort pump pulse of ∼ 10 fs and by using pump fluence above the 2D Mott density of MoS₂, i.e. in the range of , Nie and co-workers revealed that the carrier thermalization and subsequent cooling dynamics are on < 20 fs and ∼ 0.6 ps timescales respectively.^[85] Wang and co-workers studied carrier dynamics of few-layer MoS₂ and found that the carrier recovery time increases dramatically from ∼ 50 ps in monolayer to ∼ 1 ns in 10-layer MoS₂.^[86] The results are consistent with defect-assisted carrier recombination and represent a meaningful way of tuning the photocarrier recombination through scaling the layer number in few-layer MoS₂. (see Fig. 6). These extrinsic factors pose a challenge for practical exploitation of TMDs. Compared with pump-probe investigations typically carried out in the non-degenerate configuration, transient response in the degenerate configuration may directly yield insights about their saturable absorber behaviour.^[87] Broadband nonlinear absorption measurement performed on a CVD-grown single crystal MoSe₂ flake (on a silver mirror) recently yielded uniform nonlinear optical response over the broad wavelength range covering the A-exciton resonance, where a constant modulation depth of ∼ 80% and saturation intensity ∼ 2.5 MW/cm² were inferred (Fig. 7).^[88] An immediate benefit of such an unexpected spectral response of TMDs is the possibility of engineering a device with precisely pre-defined nonlinear optical properties over a broad optical range, which has proved difficult for most semiconductor-based devices including SESAMs and SWNTs.

Fig. 5.

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Fig. 5. (color online) (a) Reflectance contrast spectra of a WS2 layer at different time delays after excitation, vertically offset for clarity (n.p. denotes the spectrum without pump excitation), showing the evolution of the spectra. The initial bleaching of the exciton peak and inversion both develop instantaneously within the experimental resolution of 0.4 ps, indicating that initial relaxation processes occur on the 100 fs timescale. After ∼ 250 ps, the optical response fully returns to the initial condition. (b) Illustration of the transition from the low-density excitonic regime to a dense electron–hole plasma.[78]

Fig. 6.

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Fig. 6. (color online) (a) Pump-probe spectroscopy performed with 390 nm pump and 660 nm probe on a mechanically exfoliated MoS2 sample (transferred onto a SO2/Si substrate), showing a carrier lifetime of .[83] (b) Time-resolved differential transmission spectra performed on a liquid phase exfoliated few-layer WS2 sample with degenerate 635 nm pump and probe beam, showing a fast PB signal (∼ 5 ps), followed by an emerging PA signature.[84] (c) Transient differential transmission performed with 10 fs visible pulses on a few-layer MoS2 sample grown by CVD method. Carrier thermalization occurs within 20 fs and the subsequent cooling of Fermi–Dirac carrier distribution occurs on the ∼ 0.6 ps timescale.[85] (d) Pump–probe spectroscopy performed on mechanically exfoliated MoS2 sample with 452 nm pump and 905 nm probe, showing the dependence of the timescales on the layer number. Different curves are scaled in magnitude for clarity.[86]

Fig. 7.

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Fig. 7. (color online) (a) Schematic setup of a microscopic nonlinear optical absorption setup. Inset shows optical micrograph of MoSe2 on silver mirror (scale bar, 20 μm). (b) Nonlinear reflectance and (c) normalized absorption of monolayer MoSe2 on silver mirror, showing uniform nonlinear optical characteristics (a modulation depth of ∼ 80% and a saturation intensity ∼ 2.5 MW/cm2 over the broad spectral range 720–812 nm.[88]

For pulsed laser applications, TMD-enabled pulsed laser was first demonstrated by Zhang and co-workers in early 2014, where few-layer MoS₂ obtained by hydrothermal method was used to mode-lock a Yb-doped fiber laser operating at 1054.3 nm, though a relatively long pulse duration of ∼ 800 ps was obtained.^[89] Soon after the first report, Luo and Xu et al. demonstrated femtosecond pulse generation (∼ 700 fs) using a MoS₂-PVA composite absorber in an Er-doped ring cavity.^[90] A number of reports followed, unambiguously verifying the capability of TMDs as effective saturable absorbers for fiber lasers operating across the visible to the near infrared range ∼ 2 μm.^[91–103] In particular, broadband operation is manifested in the demonstration of a wavelength-tunable Q-switched regime,^[91] a wavelength-tunable mode-locked regime^[92] and a single MoS₂ device-enabled, Q-switched, Yb-, Er-, and Tm-doped fiber laser across 1–2 μm.^[93] While the initial studies primarily employed MoS₂ as the SA material, other TMD materials such as WS₂^[98–101] and MoSe₂^[102,103] have also been demonstrated. The earliest study on the ultrafast nonlinear optical properties of TMDs revealed strong nonlinear absorption of MoS₂ in the visible and infrared range (see Fig. 8).^[89,104] As with the relaxation dynamics, nonlinear optical characteristics are also dependent on material quality. Combining with various methods available for repairing the defects in TMDs,^[105] it opens an exciting route of controlling the nonlinear optical properties. Luo and co-workers demonstrated the first Q-switched operation of visible ZBLAN fiber lasers, employing a range of LPE-processed TMD materials. The red-light passive Q-switching generated stable pulse trains with the pulse width of ∼ 200 ns and a wide range of repetition rates from 232.7 to 512.8 kHz (see Fig. 9).^[84] The demonstration of compact and efficient mode-locked visible lasers, especially for visible diode lasers widely found in the UV and visible band, is of potential broad interest and may mitigate the dependence of bulky and expensive harmonic generation setups based on Ti:sapphire lasers. On the other hand, the sub-bandgap optical absorption has attracted attention as a way to scale the operation of TMDs into the long-wavelength range. A theoretical model based on edge-state absorption that may account for the sub-band gap behaviour of TMDs has been recently proposed.^[106] Compared with graphene, TMDs are relatively less explored in solid-state lasers, probably due to the high insertion loss. Very recently, a bulk laser operating close to 3 μm has been demonstrated by using MoS₂’s long-wave absorption, and more experimental studies, particularly on the direct revealing of optical signatures of edge states, will help provide more physical foundation for the use of TMDs for long-wavelength operations.^[107]

Fig. 8.

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Fig. 8. (color online) (a) Schematic diagram of the Z-scan experimental setup.[89] (b) open-aperture Z-scan measurements of MoS2 dispersions in a 1-mm cuvette at 400 nm (left) and 800 nm (right) with different input fluences.[89] (c) Open-aperture Z-scan results of the MoS2 dispersions under the excitation of 100 fs pulses at 800 nm in different solvents, showing clear influence nonlinear optical characteristics from the environment (left), and comparison of nonlinear performance of MoS2 () and graphene () dispersions (right), where MoS2 shows a stronger modulation depth. Inset: normalized transmission as a function of fluence.[104]

Fig. 9.

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	Fig. 9. (color online) (a) Photograph of the passively Q-switched visible fibre laser. Inset: the output beam spot. (b) Schematic of the visible fiber laser. (c) Q-switched pulse trains under different pump powers.[84]

Most of the existing TMD-based pulsed lasers employ LPE for obtaining single- or few-layer TMD flakes. Li and co-workers demonstrated the first mode-locked laser based on a CVD-grown multilayer MoS₂ SA. The authors showed robust mode-locking in an erbium-doped fiber laser, illustrating the potential of using large and uniform, polymer-free material for pulsed laser operation, although the multilayer still has to be transferred onto the fiber ferrule end.^[108] Following similar CVD growth and transfer methods, Khazaeizhad and co-workers demonstrated an evanescent field interaction device, where both anomalous and normal dispersion cavities were successfully mode-locked for 1.55 μm operation.^[109] One advantage of TMD compared with graphene is the compatibility of large-area deposition directly on dielectric substrates, such as quartz and silica. Single crystal material with lateral sizes larger than several tens of microns can now be routinely grown. Devices that are free from transfer processes would be highly desirable, i.e., MoS₂ multilayers directly grown on optical substrate or fiber end, and will greatly facilitate highly repeatable device fabrication.

Since graphene was identified as a promising SA material, topological insulators (Tis), another mainstream category of Dirac materials that have drawn broad attention in the field of condensed-matter physics have been identified as potential saturable absorber materials (such as Bi₂Te₃,Bi₂Se₃, Sb₂Te₃, etc.).^[110–119] This review however will not summarize those results in detail as similar results were demonstrated following the same vein as with graphene absorbers, though the first report of TI-based SA preceded those of TMDs.^[91] In the following we focus on the recently emerging black phosphorus which has become the new focus of the field.

Black phosphorus (BP), the most thermodynamically stable allotrope of the element phosphorus, was recently rediscovered as an intriguing two-dimensional material as motivated by the demonstration of a BP-based field-effect transistor by Zhang and co-workers.^[120] For photonics applications, BP’s unique strength lies in its layer-dependent direct bandgap, i.e., from ∼ 0.3 eV (bulk form) to ∼ 2.0 eV (monolayer), which provides an effective bridge between the energy gap of the semi-metallic graphene and the relatively large band gap TMDs.^[121] In addition to its tunable bandgap and high carrier mobility, the structural anisotropy of BP leads to highly anisotropic mechanical, electrical, and optical properties.^[122,123] A number of electronic and optoelectronic applications based on BP and its heterostructures have been developed thus far.^[124,125] For example, a BP-based photodetector that can respond to mid-infrared light ***∼3.39 μm with a high responsivity of ∼ 82 A/W was recently demonstrated,^[126] showing great potential for chip-scale mid-infrared sensing and imaging applications based on the inherent advantages offered by this emerging 2D material.

Due to its relatively narrow direct bandgap, multilayer BP was considered a promising saturable absorber for the long wavelength range, where light–matter interactions in graphene and TMDs are not as strong. BP nanoplatelets dispersed in different solvents obtained by liquid phase exfoliation method^[127] were first investigated by Z-scan technique across a broad spectral range, spanning 400–1930 nm.^[128] Regardless of the characterization wavelength and pulse duration (femtosecond or picosecond), the BP-based saturable absorbers are found to exhibit quite large modulation depth (> 10%), though the saturation intensity deduced from the femtosecond Z-scan measurement is quite high, about several hundreds of GW/cm². On the other hand, a few studies on the photocarrier relaxation dynamics in BP have been reported.^[129–131] While angle-sensitive transient effects were identified in all three pump–probe studies, the present results do not converge to a unified physical picture by which photocarriers’ dynamics can be well accounted for. Further investigation of BP’s photocarrier excitation and relaxation would be important for understanding their detailed nonlinear optical behavior.

For short-pulsed laser applications, mechanically exfoliated BP was first used as a saturable absorber to enable both Q-switched and mode-locked operations for a telecom band Er-doped fiber laser ∼ 1.55 μm.^[132] Inspiringly, the nonlinear optical characteristics of the exfoliated sample can be effectively tuned through layer thickness control, i.e., the modulation depth in transmittance can be changed from 8% (thin sample) to 18.5% (thick sample). The operation wavelength of BP-enabled pulsed laser was soon extended to the thulium fiber band ∼ 1.9–2.0 μm, where a mechanically exfoliated thin film of BP (about 300 nm in thickness) was deposited directly onto the fiber ferrule end.^[133] The thulium laser generates 739 fs pulses, with a ∼ 36.8 MHz repetition rate and 1.5 mW output power, which corresponds to a pulse energy of 80 pJ. No damage or degradation was observed for the BP saturable absorber at this operation condition. To mitigate the potential damage caused by direct laser irradiation, evanescent field interaction was employed, where the BP is deposited either on micro-fiber or D-shape fibers. A number of Q-switched or mode-locked lasers operating in the 1–1.55 μm range were demonstrated using this scheme.^[134–137] By fabricating a BP saturable absorber mirror, Zhang and co-workers demonstrated the first mode-locked bulk laser operating at a wavelength of 1064 nm. The pulse duration is relatively long ∼ 6.1 ps, with an average output power of 460 mW.^[138] Through intra-cavity dispersion control, the pulse duration and output power were significantly optimized, with 272 fs pulses and a peak power approaching 24 MW achieved.^[139] Thus far, the longest wavelength bulk laser enabled by a BP absorber is a passively Q-switched Cr:ZnS_e laser operating at a wavelength around 2.4 μm, where pulses as short as 189 ns were achieved, corresponding to a pulse energy of 205 nJ.^[140]

Qin and co-workers were the first to achieve 3 μm operation by using LPE processed BP-NMP solution. A reflective device is fabricated by dropping the solution onto a gold-coated mirror and drying it in a cabinet (80% reflectance at 2.9 μm). An Er:ZBLAN fiber laser operating at 2.78 μm was successfully Q-switched with pulse width around 1.2 μs, and the BP SAM device was used as a cavity mirror (free-space coupled).^[141] By transferring a mechanically exfoliated BP onto the gold-coated mirror, mode-locked operation at 2.78 μm was subsequently obtained with a pulse duration of 42 ps, a repetition rate of 24 MHz, which are similar to those achieved by graphene-based absorbers.^[142] Through thickness characterization, the BP flake was found to be ∼143 nm, corresponding to ∼238 layers. According to the bandgap equation, a bandgap of 0.33 eV is inferred, which means an operating range up to 3.8 μm is possible.^[142] It should be noted that the measured modulation depth and saturation intensity is about 19% and 9 μJ/cm², respectively (Fig. 10). More recently, LPE-processed BP was used to enable a 2.97-μm mode-locked ZBLAN fiber laser with 8.6 ps pulses, which represents the longest mode-locking wavelength for BP-based pulsed lasers.^[143] The authors studied the long-term stability of the laser and found that due to the degradation of the BP (mainly through interaction with oxygen and water molecules^[144]), the laser can operate stably within about three days. This relatively poor environmental stability needs to be dramatically improved to match the level of standard semiconductor-based devices. Among others, the nonlinear optical properties of BP in the mid-infrared, particularly beyond 3 μm, has remained less well characterized and studies of this topic will help provide more solid evidence for its potential use as a mid-infrared saturable absorber material. Due to BP's exciting prospect for novel nanoelectronic and nanophotonic applications, increasing efforts are dedicated to improving the material's environmental stability.^[145–147] Large-area growth and advances in passivation and stability enhancement will undoubtedly strengthen the further application of BP in the area of lasers and photonics.

Fig. 10.

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	Fig. 10. (color online) (a) Schematic diagram of the BP mode-locked Er:ZBLAN fiber laser. (b) Saturable absorption measurement of BP SAM at 2.8 μm. Inset: experimental setup for the nonlinear absorption measurement. (c) Autocorrelation trace of the mode-locked pulses as well as the corresponding mode-locked optical spectrum.[142]