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It attracts wide interest to seek universe saturable absorber covering wavelengths from near infrared to midinfrared band. Multilayer black phosphorus, with variable direct bandgap (0.3-2 eV) depending on the layer number, becomes a good alternative as a universe saturable absorber for pulsed lasers. In this contribution, we first experimentally demonstrated broadband saturable absorption of multilayer black phosphorus from 1 μm to 2.7 μm wavelength. With the as-fabricated black phosphorus nanoflakes as saturable absorber, stable Qswitching operation of bulk lasers at 1.03 μm, 1.93 μm, 2.72 μm were realized, respectively. In contrast with large-bandgap semiconducting transition metal dichalcogenides, such as MoS 2 , MoSe 2 , multilayer black phosphorus shows particular advantage at the long wavelength regime thanks to its narrow direct bandgap.This work will open promising optoelectronic applications of black phosphorus in mid-infrared spectral region and further demonstrate that BP may fill the gap of between zero-bandgap graphene and large-bandgap TMDs.Keywords: multilayer black phosphorus; saturable absorber; mid-infrared pulsed laser. IntroductionBlack phosphorus (BP), a layered allotrope of phosphorus, owns its unique properties. Due to puckered layer or zigzag direction of intralayer atoms, the BP crystal exhibits highly anisotropic mechanical properties [1].Moreover, few-layered BP presents highly anisotropic electric conductance and strain-controlled anisotropic electric mobility [2]. By use of its high carrier mobility, few-layer BP has been manufactured into fast fieldeffect transistors (FETs) and opens its electronic applications [3][4]. Similar to MoS 2 , BP also possesses thickness-depended energy bandgap with 1.5-2.0 eV for single layer [5][6][7], 0.59 eV for five layers [8] and 0.3-0.33 eV for bulk sample [1,3]. However, MoS 2 shows a transition from direct bandgap to indirect bandgap once it becomes multilayer while BPs always show direct bandgap property [5]. Such difference would endow multilayer BPs with intrinsically stronger light-matter interaction than multilayer MoS 2 . This might lead to

Liquid-phase exfoliated 2D material multilayer MoS 2 is transferred onto a gold mirror and its saturable absorption at the 2 µm wavelength region is experimentally observed. This transferred MoS 2 saturable absorber has a modulation depth of 13.6% and a saturation intensity of 23.1 MW cm −2 . This saturable absorber is integrated into a linear Tm 3+ fiber laser cavity, and stable fundamental-frequency mode-locking operation is realized at 2 µm with pulse energy of 15.5 nJ, pulse width of ~843 ps, and a repetition rate of 9.67 MHz. The laser spectral width is ~17.3 nm with a center wavelength of 1905 nm. This first presence of modelocking with multilayer MoS 2 sheets in the 2 µm wavelength region verifies that multilayer MoS 2 is a good candidate for broadband mode-locking comparable to graphene, as well as a good mode-locker for achieving high pulse energy.

With MoS 2 as saturable absorber, passive Q-switching and Q-switched mode-locking operations of a Tm-doped calcium lithium niobium gallium garnet (Tm:CLNGG) laser were experimentally demonstrated. The Q-switched laser emitted a maximum average output power of 62 mW and highest pulse energy of 0.72 μJ. Q-switched mode locking was also obtained in the experiment. The research results will open up applications of MoS 2 at the mid-infrared wavelength.

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