## 1 Introduction  
Fibre lasers are a mature technology that has become an essential tool facilitating a wide range of scientific, medical and industrial applications. The flexibility, reliability and compact nature of such light sources enables them to outperform bulk lasers in many areas: offering alignment-free, turn-key operation for end-users [1]. Pulsed operation of fibre lasers is of particular importance, leading to a significant enhancement of the instantaneous power, suitable for driving nonlinear optical processes while enabling high-resolution, time-resolved applications. While many schemes for generating pulsed laser emission exist, passive mode-locking (_i.e._, phase-locking of many longitudinal cavity modes) or Q-switching (_i.e._, modulation of the laser cavity Q-factor) using a saturable absorber (SA) (a material that exhibits an intensity-dependent transmission) are often preferred, as they enable a wide space of pulse parameters to be accessed without employing costly and complex electrically-driven modulators that ultimately impose a lower limit on the pulse durations achievable directly from the laser source [1,2]. Saturable absorbers can be broadly divided into two categories: real SAs, materials that exhibit an intrinsic nonlinear decrease in absorption with increasing light intensity; and artificial SAs, devices that exploit nonlinear effects to mimic the action of a real saturable absorber by inducing an intensity-dependent transmission. Here, we restrict the focus of our review to real SAs, considering the role of emergent 2D nanomaterials for this function and highlighting the benefits that they offer in terms of wideband operation, switching speed and engineerable properties. Advances in saturable absorber technologies are almost synonymous with the evolution of the laser itself: the first demonstrations of SA-based pulse generation in 1964 using both a \"reversibly bleachable\" dye [3] and a coloured glass filter [4] to Q-switch a ruby laser were reported just four years after [PERSON]'s successful demonstration of laser operation [5]. In Figure 1, we highlight the historical evolution of the salient SA technologies. After these initial demonstrations, reversibly bleachable (or saturably absorbing) dyes were widely applied to mode-lock lasers, where the gain medium was also a dye, leading to the first demonstration of continuous-wave (CW) mode-locking [6]. With continued development in low-loss optical fibre, mode-locked lasers based on actively-doped fibre amplifiers emerged, including an early 1983 report of unstable mode-locking of a Nd:fibre laser using a dye SA [7]. However, the passive generation of stable mode-locked pulses using an SA in fibre systems remained challenging until the semiconductor saturable absorber mirror (SESAM) was proposed in the early 1990s [8,9]. SESAMs quickly became, and remain to be, a highly successful technology for generating ultrafast mode-locked pulses and high-energy Q-switched emission from fibre lasers. However, they offer only a narrow operating bandwidth, require costly fabrication and packing, and the relaxation speed is limited to picosecond time scales (unless expensive post-processing techniques are employed) [2]. These limitations are driving research into novel materials for SA applications; of particular interest are nanomaterials, where reduced dimensionality leads to strong quantum confinement, new physical phenomena and remarkable optoelectronic properties [10,11]. While it could be argued that early reports of saturable absorption using coloured glass filters exploited nanomaterials, since the glasses were doped with semiconductor nanocrystals (_i.e._, zero-dimensional quantum dots (QDs)), such as cadmium selenide [4], to modify their colour, it was not until 1997 that QDs were explicitly engineered for the purpose of pulse generation [12]. After  
Figure 1: The evolution of real saturable absorber technologies. Orange dots denote the first reported application of each technology in a pulsed laser.