**4.2. Transition metal dichalcogenides**

Transition metal dichalcogenides are in general characterized by the chemical formula MX2, where M is a transition metal, e.g., molybdenum (Mo) or tungsten (W), and X is a group VI element: sulfur (S), selenium (Se), or tellurium (Te). Those materials were already investigated in the late 1960s of the twentieth century [109]. The general interest in TMDCs was renewed after the great success of graphene. Materials as such as MoS2, MoSe2, WS2, WTe2, MoTe2, and WSe2 are currently extensively investigated, since they allow applications as transistors, photodetectors, and optoelectronic devices [110]. In contrast to graphene, TMDCs are charac‐ terized by a band gap, which varies significantly with the material thickness [111].

The first TMDC material used as SA in lasers was molybdenum disulfide. The saturable absorption effect in MoS2 nanosheets was already investigated in 2013 [112]. Less than one year after this discovery, Zhang et al. [34] demonstrated the first laser mode-locked with the use of MoS2. The Yb-doped fiber laser generated stable pulses centered at 1054.3 nm, with a 3 dB spectral bandwidth of 2.7 nm and duration of 800 ps. Later, the same group has demon‐ strated ultrashort pulse generation from an Er-doped fiber laser mode-locked with MoS2-based saturable absorber [35]. The laser generated 710 fs pulses centered at 1569.5 nm wavelength with a repetition rate of 12.09 MHz. Wavelength-tunable operation of a MoS2-based fiber laser in a very broad spectral range was reported by Zhang et al. [113]. The demonstrated laser utilized a PVA-MoS2 saturable absorber and enabled continuous tuning from 1535 to 1565 nm. Molybdenum disulfide can be also used in combination with tapered fibers. For example, Du et al. [46] demonstrated an Yb-doped fiber laser which generated dissipative solitons at 1042.6 nm with pulse duration of 656 ps and a repetition rate of 6.74 MHz. Also a harmonically modelocked Er-doped laser incorporating a microfiber-based MoS2 SA was reported [114]. Very recently, Wu et al. [115] demonstrated a reflective MoS2 saturable absorber for a short-cavity Er-doped fiber laser. They have achieved 606 fs pulses at 1556.3 nm with 463 MHz repetition rate.

The saturable absorption effect was also confirmed for other TMDCs [116], but up to date, probably not all of them were used as mode-lockers in lasers. Recently WS2 has found attention of the ultrafast laser community. Lasers operating at 1.03 μm [117], 1.55 μm [118, 119], and 1.94 μm [41] were already reported with pulse durations down to 595 fs [119].

### **4.3. Black phosphorus**

Similarly to TMDCs, black phosphorus is a material which was once on interest of the physics community (in the 1980s and 1990s of the twentieth century [120, 121]), and was "rediscovered" in the recent years after the great success of other 2D materials.

It has a graphene-like layered structure, in which the layers are bound with van der Walls forces [122, 123]. Mechanical exfoliation of black phosphorus leads to obtaining a single-layer 2D material called phosphorene [121–124] (analogously to "graphene"). Similarly to TMDCs, the energy band gap of BP scales with the number of layers. It might be tuned from approx. 1.5 eV for monolayer phosphorene up to ∼0.3 eV for bulk black phosphorus [125]. Up till now, the broadband nonlinear optical response of BP has been confirmed at wavelengths ranging from of 400 to 1930 nm [126, 127].

However, the mode locking was not truly SA based—the cavity contained an additional polarizer and wave-plates. Thus, the mode locking was a result of a hybrid combination of saturable absorption and NPR. The shortest pulses achieved directly from an oscillator mode-

Transition metal dichalcogenides are in general characterized by the chemical formula MX2, where M is a transition metal, e.g., molybdenum (Mo) or tungsten (W), and X is a group VI element: sulfur (S), selenium (Se), or tellurium (Te). Those materials were already investigated in the late 1960s of the twentieth century [109]. The general interest in TMDCs was renewed after the great success of graphene. Materials as such as MoS2, MoSe2, WS2, WTe2, MoTe2, and WSe2 are currently extensively investigated, since they allow applications as transistors, photodetectors, and optoelectronic devices [110]. In contrast to graphene, TMDCs are charac‐

The first TMDC material used as SA in lasers was molybdenum disulfide. The saturable absorption effect in MoS2 nanosheets was already investigated in 2013 [112]. Less than one year after this discovery, Zhang et al. [34] demonstrated the first laser mode-locked with the use of MoS2. The Yb-doped fiber laser generated stable pulses centered at 1054.3 nm, with a 3 dB spectral bandwidth of 2.7 nm and duration of 800 ps. Later, the same group has demon‐ strated ultrashort pulse generation from an Er-doped fiber laser mode-locked with MoS2-based saturable absorber [35]. The laser generated 710 fs pulses centered at 1569.5 nm wavelength with a repetition rate of 12.09 MHz. Wavelength-tunable operation of a MoS2-based fiber laser in a very broad spectral range was reported by Zhang et al. [113]. The demonstrated laser utilized a PVA-MoS2 saturable absorber and enabled continuous tuning from 1535 to 1565 nm. Molybdenum disulfide can be also used in combination with tapered fibers. For example, Du et al. [46] demonstrated an Yb-doped fiber laser which generated dissipative solitons at 1042.6 nm with pulse duration of 656 ps and a repetition rate of 6.74 MHz. Also a harmonically modelocked Er-doped laser incorporating a microfiber-based MoS2 SA was reported [114]. Very recently, Wu et al. [115] demonstrated a reflective MoS2 saturable absorber for a short-cavity Er-doped fiber laser. They have achieved 606 fs pulses at 1556.3 nm with 463 MHz repetition

The saturable absorption effect was also confirmed for other TMDCs [116], but up to date, probably not all of them were used as mode-lockers in lasers. Recently WS2 has found attention of the ultrafast laser community. Lasers operating at 1.03 μm [117], 1.55 μm [118, 119], and 1.94

Similarly to TMDCs, black phosphorus is a material which was once on interest of the physics community (in the 1980s and 1990s of the twentieth century [120, 121]), and was "rediscovered"

μm [41] were already reported with pulse durations down to 595 fs [119].

in the recent years after the great success of other 2D materials.

terized by a band gap, which varies significantly with the material thickness [111].

locked only by a TI-based SA were 128 fs [48].

140 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

**4.2. Transition metal dichalcogenides**

rate.

**4.3. Black phosphorus**

**Figure 14.** Characterization of the mechanically exfoliated BP layers transferred onto an optical fiber: (a) SEM image with marked fiber clad and core, (b) EDX spectroscopy data, (c) AFM image of the core area, and (d) cross section through the fiber core area indicating approx. 200–300 nm height of the BP flake on the core.

The first report on the usage of BP as a saturable absorber in a laser was posted on arXiv in 2015 [128]. Mode locking at both 1.55 and 1.9 μm wavelengths was reported. In both lasers, the BP layers were exfoliated mechanically from bulk material using an adhesive tape. Afterward, a ~300 nm thick layer was transferred onto a fiber connector, and connected with another one, just like shown in **Figure 14**. A scanning electron microscope (SEM) image of the fiber tip (with marked core and cladding of the fiber) with deposited BP layer is shown in **Figure 14(a)** [36]. The composition of the transferred layer onto the fiber core area was confirmed by energy-dispersive X-ray spectroscopy (EDX). The analysis of the spectroscopy data is shown in the inset of **Figure 14**. It confirms that the transferred material is black phosphorus (the Si and O peaks originate from the optical fiber). The atomic force microscope (AFM) measurement confirmed the average thickness of the layer at the level of 250–300 nm (**Figure 14b**).

The performance of the Tm-doped fiber laser mode-locked with the described BP-based saturable absorber is depicted in **Figure 15**. The laser generated soliton-shaped optical spectra centered at 1910 nm with 5.8 nm of FWHM bandwidth (a), which corresponded to a 739 fs pulse (b). It is worth mentioning that the authors claimed a high damage threshold of the BP layers. The laser was pumped with relatively high power (up to 400 mW) and the SA was not damaged or degraded during any of the performed experiments [37].

**Figure 15.** Optical spectrum generated by the BP mode-locked Tm-doped laser (a) and the autocorrelation trace of the emitted 739 fs pulse (b).

Black phosphorus in form of nanopalettes (NPs) was also used in combination with microfibers for evanescent field interaction. In their work, Yu et al. [129] reported that the SA had a modulation depth of 9.8% measured at 1.93 μm. A stable mode-locking operation at 1898 nm was achieved with a pulse width of 1.58 ps and a fundamental repetition rate of 19.2 MHz.

Similarly to graphene, BP is also suitable for operation in the midinfrared. In 2016, Wang et al. [130] demonstrated the first Cr:ZnSe laser incorporating BP as saturable absorber. However, the laser was not mode-locked but Q-switched. Generation of 189 ns pulses with average output power of 36 mW was obtained at 2.4 μm wavelength.
