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Black phosphorus a new saturable absorber material for ultrashort pulse generation

Low-dimensional materials, due to their versatile properties are very interesting for numerous electronics and optoelectronics applications. Recently rediscovered black phosphorus, with a graphite-like structure can be exfoliated up to the single atomic layer. In contrary to graphene it possesses a direct band gap controllable by the number of stacked atomic layers. For those reasons, it is now intensively investigated. Here we demonstrate, that black phosphorus can serve as a broadband saturable absorber and can be used for ultrashort optical pulse generation. The mechanically exfoliated ~300 nm thick layers of black phosphorus were transferred onto the fiber core and under pulsed excitation at 1560 nm wavelength its transmission increases by 4.4%. It was used to generate 272 fs-short pulses at 1550 nm and 739 fs at 1910 nm. The obtained results shows that black phosphorus can be effectively used for ultrashort pulse generation and proves its great potential to future applications.

Recently the research community is focused on black phosphorous which is considered as a new promising material for optoelectronic applications 47 . Its structure is similar to graphite, where the following monolayers are bound with van der Walls forces 48,49 . Mechanical exfoliation of black phosphorus 50 allows to obtain a two-dimensional material called phosphorene 51 , analogously to graphene. In terms of electronic applications, the most important feature is that black phosphorus is a direct bandgap semiconductor in the mono-, few and bulk form. Its band gap is scaled with the number of layer form ~1.5 eV for phosphorene to ~0.3 eV for bulk black phosphorus [52][53][54][55] . Up till now, the black phosphorus has been studied as a promising material for transistors 47,51,56,57 , photodetectors 58 , solar cells 59 . It is also considered as future material for thermoelectric devices 60,61 , gas sensing 62 and electrodes in lithium ion batteries 63 . Moreover, the optical absorption of a few layer black phosphorus can be saturated by high intensity laser light.
Up till now, the effect of saturable absorption in black phosphorus was shown at 532 nm and 1030 nm 64 . Taking into account the band-gap of black phosphorus and the fact that its absorption can be saturated it might serve as a broadband SA in the spectral range up to ~4 µm.
In this paper we experimentally demonstrate, that mechanically exfoliated ~300 nm-thick layers of black phosphorus deposited on the end-facet of a fiber connector might serve as a saturable absorber for ultrashort pulse generation in fiber lasers. We have characterized the

Results and discussion
The black phosphorus-based SA was prepared by mechanical exfoliation method extensively developed for graphene and other two-dimensional materials 15,23,24 . The obtained layers were transferred onto the fiber ferrule, so that they entirely cover the fiber core, what is clearly seen in the scanning electron microscope (SEM) image presented in the Figure.1a. Thanks to the visible boundary of the 125 µm fiber cladding, we could precisely locate the position of the core under the black phosphorous layer. The composition of the transferred layer was confirmed by the energy dispersive X-ray spectroscopy (EDX). The analysis of the spectroscopy data presented in Figure. 1b confirms that the transferred material is black phosphorus. The images taken from the fiber core area show only the presence of phosphorus, oxygen and silicone. The last two elements originate from the substrate which is the optical fiber made from high purity silicone dioxide (SiO2). The morphology of the deposited layers was investigated by atomic force microscopy (AFM). The 50x50 µm 2 AFM image comprising the 81 µm 2 fiber core area (the diameter of the SMF-28 fiber core is 9 µm) is depicted in the Figure 1c. The highest thickness of the transferred layer in the investigated area was ~700 nm. However, the maximum thickness in the area which covers the core was around 300 nm. The cross section of the AFM image made through the fiber core shows that the sample was relatively uniform in this range.
The fiber connector with deposited black phosphorus layer was then connected with a clean one via a fiber adapter, forming a fiber-integrated SA device. Its transmittance in the 900 -2200 nm spectral range measured using a low intensity broadband white light source is depicted in the Figure 2a. it changes from around 30% at 900 nm to 55% at 2200 nm. The oscillatory character of the measured transmittance correlates very well with the previously calculated optical conductivity for 20 nm thick black phosphorus and can be related to the underlying electronic subbands structure 65 . The power-dependent transmittance of the SA shown in the Figure 2b was measured using a high-intensity mode-locked laser operating at 1560 nm. Changing the incident fluence in the 10 -550 µJ/cm 2 range (limited by the available pump source) the transmittance of black phosphorus was increased by maximum of 4.4%. Due to the fact, that black phosphorus is an anisotropic crystal, its linear absorption is sensitive to the incident light polarization state 55,65 .
Hence, in order to investigate the impact of the polarization on the nonlinear parameters of black phosphorus, the polarization azimuth of the measurement beam incident on the SA was changed by a polarization controller. Similar to T.Low et al. 65 the linear absorption is polarizationdependent and can be maximally tuned by about ~11%. For all tested azimuths the black phosphorus layer exhibits saturable absorption effect (Figure 2b), but the modulation depth is polarization-dependent. The effective modulation depths (limited by the available excitation fluence) varies in the 0.6 -4.4% range. The saturation fluence increases with losses, was also observed for graphene 28 . Comparing our results with that reported previously 65 the highest transmittances is obtained when the polarization is parallel to the y-axis of the black phosphorus lattice structure 55,65. In that case full saturation of the SA was observed. Further increase of the input fluence leads to a roll-off in the transmittance curve, which is connected mainly with twophoton absorption (TPA) in the SA [66][67][68] . The measurement data are in very good agreement with the theoretical calculations done according to the saturable absorber model 67,68 (the olive lines in the Figure 2b). Based on the model and taking into account the measurement data where the rolloff effect was clearly observed (the two curves with higher transmittance) the TPA coefficient (βTPA) of ~500 cm/MW was calculated. It is at the same level as bi-layer graphene βTPA 69 . Taking into account the broadband absorption of black phosphorus and its nonlinear optical response leading to absorption saturation at relatively low fluence, it meets basic criteria of a passive SA for fiber lasers.
The as-prepared and characterized black phosphorus-based SA was used to generate ultrashort optical pulses in two ring cavities based on Er-and Tm-doped fibers. The general concept of the laser setup is presented in Figure 3. In order to obtain pulsed operation in modelocked regime the lasers were pumped above the threshold and the intracavity polarization state was optimized by the polarization controller (PC). The mode-locked operation in the Er-doped  (Figure 5b). The S/N ratio of the generated pulses was better than 65 dB (Figure 5c). The RF spectrum and the corresponding oscilloscope trace (Figure 5d) indicate that the laser operation was free of parasitic spectral modulations. The pulse repetition rate was 36.8 MHz. The output power measured at 395 mW was 1.5 mW. The characteristic dips observed on the optical spectrum are related to the water absorption lines, which are densely located in the 1.9 μm region. In order to confirm it, we have simulated the absorption of light over 1 m path in air with 1% water content using HITRAN database 70 . The simulated absorption lines are plotted (with blue line) in the Figure 5a. It can be seen, that the location and amplitude of the absorption peaks on the spectrum ideally match the simulated water lines.
It is worth to emphasize the high optical damage threshold of the black phosphorus layer.
During the performed experiments both lasers were pumped up to the 0.5 W and no signs of optical damage of the SA were observed. Moreover this first demonstration of ultrashort pulse generation using mechanically exfoliated black phosphorus presents significantly better results than the first reports on mode-locking with the use of carbon nanotubes 18 and graphene 12,13 . This demonstrates the grate potential of black phosphorus not only in electronics but also in photonics.
In conclusion, we have shown that the mechanically exfoliated layers of black phosphorus

Methods
As a base material for SA preparation a commercially available high purity piece of black phosphorus (Smart Elements, purity 99.998%) was used. In the first step, black phosphorus flakes were obtained by precise scraping the lumps with a sharp blade. Then, the flakes were mechanically exfoliated using scotch tape. Afterwards, the thin layers were pressed with a fiber ferule (the FC/APC connector), previously ultrasonically cleaned in the presence of isopropyl alcohol and transferred onto the fiber core.
The fiber with deposited black phosphorus layer was investigated with AFM in tapping mode (Veeco diDimension V SPM with NanoScope V controller). The fiber was placed in a holder, in order to adjust the measured surface parallel to the microscope scan area. The measurement was done using Bruker MPP-11100-10 AFM cantilever with 0.1 Hz scan rate. The amplitude of the lever was set to 500 mV.
The position and quality of the black phosphorus layer transferred onto the fiber ferrule were visualized using scanning electron microscope (SEM, Hitachi SU 6600). To confirm unambiguously correct location of black phosphorus layer we utilize the SEM equipped with an energy dispersive X-ray Spectrometer (EDX) -NORAN System 7 X-ray Microanalysis System.
Better visualization was obtain by using an advanced variable pressure technology in the SEM microscope. Despite the measured sample is non-conductive, it was not covered with metal coating. Nonetheless, the quality of obtained images is good enough to visualize the exfoliated layer placed on the fiber core. The composition of the exfoliated layer was confirmed to be pure phosphorus by EDS analysis.
Optical characterization of the black phosphorus-based SA was performed in the linear and nonlinear regime using a broadband white light source and a pulsed fiber laser, respectively. The linear transmittance of the SA was measured in the range from 900 nm to 2200 nm using white light source (Yokogawa AQ4305) and optical spectrum analyzers (Yokogawa AQ6370B, Yokogawa AQ6375). The measurement were performed with 1 nm spectral resolution and the highest available sensitivity. The nonlinear power-dependent transmission was investigated using an all-fiber setup (Figure 6), similar to that reported before 13,31 with femtosecond laser operating at 1560 nm used as an excitation source (Menlo Systems T-Light, 100 MHz, 100 mW). The power from the reference arm and passing through the measured sample was simultaneously measured by the optical power meter (Newport 2936-R with 918D-IG-OD3R heads). The average power incident on the sample was changed by the electrically controlled variable optical attenuator (VOA) with about 3 dB insertion losses. The laser beam was divided into reference and measurement arm by a 20/80 coupler. The pulse duration in the SA plane was 900 fs. The measurement data were fitted with a commonly used two-level model function of SA 67,68 including the TPA effect: where αns -non-saturable losses: ∆α -modulation depth, Fsat -saturation fluence, F2 -inverse slope of the induced absorption effect 68  In order to investigate the influence of polarization azimuth on the power-dependent transmittance the polarization in the measurement setup depicted in Figure 6 was changed via PC.
The polarization azimuth and polarization state in the plane of the SA was monitored using the polarimeter (Thorlabs PAX 5710) equipped with the measurement head (PAN5710IR3).
The general setup of the mode-locked fiber laser is depicted in Fig. 4. In both cases the same laser cavity configuration was used. As active fiber, a 40 cm long piece of LIEKKI™ Er80-   Fig. 1. Characterization of the exfoliated black phosphorus layers, transferred onto the fiber core: a) SEM image with marked fiber clad and core. It is clearly confirmed that exfoliated layer cover the fiber core, b) EDX spectroscopy data, c) AFM image including the cross section through the fiber core area Fig. 2. The transmittance of the exfoliated black phosphorus layer measured using: a) low intensity (broadband white light), b) high intensity (mode-locked laser) light source. The power-dependent transmittance was measured for polarization azimuths changing form E ǁ x-axis to E ǁ y-axis