## 4 Outlook  
Significant progress has been made in the understanding of 2D materials, and their unique optical properties promise to shape the future of photonic and optoelectronic technologies [10]. In this review, we have highlighted how wideband operation and ultrafast relaxation dynamics offered by a broad array of 2D materials could play a role in enabling practical, low-cost and flexible saturable absorber devices for integrated pulsed fibre lasers.  
Few-layer semiconducting TMDs are of particular interest due to their engineerable optical properties mediated by a layer-count dependent bandgap covering the visible region that transitions from indirect to direct gap behaviour for monolayers [11; 24]. After initial successes demonstrating the effectiveness of MoS\({}_{2}\)[30] as a saturable absorber in pulsed fibre lasers, recent studies have considered other members of the TMD family of 2D materials, including MoSe\({}_{2}\), WS\({}_{2}\)and WSe\({}_{2}\), revealing complementary optical properties and suitability as a high-potential photonic platform. Strong intensity-dependent absorption has been measured, with the magnitude of the nonlinear optical response being found to depend on flake size and thickness, suggesting that few-layer TMD-based devices could be engineered for specific applications [60; 62].  
Consequently, passively Q-switched and mode-locked fibre lasers realized using TMDs have been reported throughout the near-infrared region. To date, the majority of TMD-based short-pulse laser demonstrations have operated at near-infrared wavelengths, corresponding to photon energies lower than the material bandgap for few-layer TMDs. The observed wideband saturable absorption behaviour has been explained by considering the role of crystal defects, including edge states [32]. Sources have been developed generating pulses at repetition rates from kHz-GHz, with durations from fs-\(\upmu\)s. The maximum output powers to date have been limited to 150 mW, although this value and the parameter space of accessible pulse properties is expected to increase in the future by exploiting emerging fibre laser designs [1; 77; 78] and using TMD-based pulse sources to seed master-oscillator power fibre amplifier schemes [1].  
One particularly promising avenue for exploiting TMD-based SAs is for the development of visible fibre lasers, where emission wavelengths can coincide with the fundamental resonances of the material; compared to graphene, this approach could offer significantly reduced saturation intensities and improved system performance. Preliminary reports have emerged using MoSe\({}_{2}\)-based devices to Q-switch a Pr:ZBLAN fibre laser at 635 nm [79], but further work is needed to improve lasers at this wavelength and to explore the role of excitonic effects in the materials.  
Beyond TMDs, few-layer TIs, with metallic edge-states, also offer potential wideband operation for photonic systems [70; 71]. In addition, BP has been shown to exhibit a layer-dependent bandgap, tunable from 0.3 eV (bulk) to 2.0 eV (monolayer) [80], that could be widely exploited for both photonic and optoelectronic applications.  
With the broad and increasing catalogue of available 2D materials, further studies to critically compare their performance as SAs under controlled conditions are required. An additional exciting opportunity lies in the engineering of hybrid-systems by stacking multiple single-layers to form van der Waals heterostructures [19]. Very recently, a graphene-Bi\({}_{2}\)Te\({}_{3}\) heterostructure was demonstrated as a successful SA [81], prompting further work to understand the origin and scale of the nonlinearoptical response. The transfer of 2D material-based SA devices from the research laboratory to a commercially-viable technology will depend on long-term reliability and the large-scale manufacturing potential. If successful, this new platform could leverage benefits in short-pulse fibre laser technology.