Ultrafast Nonlinear Excitation Dynamics of Black Phosphorus

Ultrathin 2D Transition Metal Carbides for Ultrafast Pulsed Fiber Lasers. .... Properties of Black Phosphorus and Recent Photonic and Optoelectronic A...
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Ultrafast Nonlinear Excitation Dynamics of Black Phosphorus Nanosheets from Visible to Mid-Infrared Kangpeng Wang,*,† Beata M. Szydłowska,† Gaozhong Wang,† Xiaoyan Zhang,‡ Jing Jing Wang,† John J. Magan,† Long Zhang,‡ Jonathan N. Coleman,† Jun Wang,‡,§ and Werner J. Blau*,† †

School of Physics and CRANN, Trinity College Dublin, Dublin 2, Ireland Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China § State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China ‡

S Supporting Information *

ABSTRACT: The recent progress on black phosphorus makes it a promising candidate material for broadband nanophotonic devices, especially operating in the mid-infrared spectral region. Here, the excited carrier dynamics and nonlinear optical response of unoxidized black phosphorus nanosheets and their wavelength dependence were systematically studied from 800 nm to 2.1 μm. The wavelength-dependent relaxation times of black phosphorus nanosheets are determined to be 360 fs to 1.36 ps with photon energies from 1.55 to 0.61 eV. In a comparative study with graphene, we found that black phosphorus has a faster carrier relaxation in near- and mid-infrared region. With regard to nonlinear optical absorption, the response of black phosphorus significantly increases from near- to mid-infrared, and black phosphorus is also confirmed to be better as saturable absorber to MoS2 in infrared region. KEYWORDS: 2D nanomaterials, black phosphorus, nonlinear optics, ultrafast spectroscopy, z-scan, pump−probe, saturable absorption

A

bulk) to visible (∼600 nm for monolayer) in BP has attracted particular attention from researchers who are working on optoelectronic and photonic devices operating in the infrared wavelength region.3,4,9,10 In order to exploit the potential of BP as a basic material for high-performance photonic devices, it is of great importance to determine its linear/nonlinear optical and photonic properties. Recently, some investigations of the BP’s nonlinear optical (NLO) response revealed BP’s promising properties for optical modulators.11−13 With the help of liquid-phase exfoliation

s a new member of 2D materials family, layered black phosphorus (BP) has emerged as a material with huge potential for nanoelectronic, nanooptics, and nanophotonic applications.1−5 Its unique properties include large direct bandgap transition from 0.3 to ∼2 eV, anisotropic properties, and high carrier mobility (104 cm2 V−1 s−1).2,6−8 As such, BP is believed to bridge the gap between graphene and transition-metal dichalcogenides (TMDs) where the former suffers the drawback of zero bandgap and the latter possess limited response in the near- to mid-infrared region because of their relatively larger bandgaps (e.g., 1.29−1.8 eV for MoS2) and problems associated with their indirect bandgap in multilayer nanosheets.3,4 Thus, the very broadband tunable direct bandgap which spans from mid-infrared (∼4 μm for © 2016 American Chemical Society

Received: April 25, 2016 Accepted: June 9, 2016 Published: June 9, 2016 6923

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Figure 1. (a) Representative AFM image of LPE prepared BP nanosheets. (b and c) The height profile across BP nanosheets is marked by the white dashed line; inset is the magnification of the dashed circle area in AFM image (a). (d) Absorbance (inset) and Raman spectra of the BP nanosheet dispersions, with Ag1/Ag2 measured as ∼0.6. (e) and (f) PL map of BP nanosheet dispersion over visible to near-infrared region.

of solvent protection by 1-cyclohexyl-pyrrolidinone (CHP), as we described in our recent work,11 there is a significant advantage in obtaining pristine BP nanosheets in CHP-based dispersions over mechanical exfoliation where the material may suffer from oxidization with ambient air and moisture.28,29

(LPE) technique, we prepared high-quality BP’s nanosheets and confirmed their saturable absorption (SA) by z-scan with 515 and 1030 nm femtosecond lasers.11 Following some early NLO investigations on graphene, H. Zhang et al. also reported BP’s wide band SA from 400 to 1930 nm based on isopropyl alcohol dispersions and PMMA.12,14−16 The realization of mode-locked pulse generation by BP was also reported by several groups with pulse duration down to ∼786 fs for a 1550 nm erbium fiber laser.17−19 The carrier dynamic in bulk and monolayer BP was also investigated by both theoretical and experimental methods.20−23 R. Long et al. predicted the defect effect in the recombination of carriers in BP.24 S. Ge et al. and J. He et al. reported the anisotropic carrier dynamic with nondegenerate pump−probe in mechanically exfoliated BP nanosheets,20,21 which are focused on the hot carrier cooling and recombination regime with time scale of hundreds of ps. Nevertheless, systematic and comparative investigations including both NLO and degenerate carrier dynamics over near- to mid-infrared in nonthermal time-scale are still needed as are the fundamentals for BP-based nanophotonic devices like modelocking. In this paper, we report a systematic comparative investigation of the NLO properties of BP nanosheets and other 2D nanomaterials across the near- to mid-infrared region. We prepared BP, graphene, and MoS2 nanosheets by the same LPE process reported previously by our group.11,25−27 The power and wavelength dependence of SA and carrier dynamics were investigated by open-aperture z-scan and pump−probe techniques with excitation from a femtosecond laser between 800 and 2100 nm. The studied wavelengths including 1.33, 1.42, and 2.1 μm were chosen to be the emission wavelengths of Nd-, Er-, and Tm/Ho-doped fiber lasers. Carrier relaxation parameters and NLO properties were obtained by utilizing nonlinear least-squares from experiments. Because of the effect

MATERIALS AND CHARACTERIZATION The BP dispersions were prepared by LPE. The BP crystal (Smart Elements, purity 99.998%) was immersed in CHP (Sigma-Aldrich) with a concentration of 1 mg/mL and sealed in an argon prefilled glass vial to avoid moisture from air. After 6 h of sonication (VibraCell CVX, 750 W, 30% amplitude), the obtained dispersions were centrifuged at 2000 rpm for 3 h. Sediment containing large and thick nanosheets or unexfoliated material was discarded. The supernatant, free from large nanosheets, was collected for experiments. Graphene and MoS2 dispersions intended for comparisons were produced in the same way, and all the materials were purchased from SigmaAldrich. In order to confirm effective exfoliation, atomic force microscopy (AFM) was performed to determine nanosheet profiles and their thickness distribution.11 The representative AFM images of BP are presented in Figure 1a, where small and large flakes are indicated by two dashed circles. Height profile plotting allows the determination of individual flake thickness and further the number of layers, as a BP monolayer is known to be ∼2 nm thick in AFM measurement, although the real thickness is ∼0.5−0.7 nm.11 In Figure 1b inset, one BP flake, marked before in Figure 1a, was determined to be monolayer by the height profile curve of BP flakes. The typical thick flake and its profile are shown in Figure 1c. Information on the distribution of BP nanosheet thickness and the number of layers is shown in Figure S1, where it can be seen that the most often occurring flakes are 8−9 layers thick with an average 6924

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Figure 2. (a−c) Pump−probe results of BP dispersions with 800, 1600, and 2026 nm femtosecond pulses with different pulse energy. (d−f) The transient absorption comparison of BP with the graphene dispersions. Insets: the same data as (d−f) but presents in the log y axis. All scatters are experimental data, while solid lines are showing the fitting results. Purple shades indicate the autocorrelation range between pump and probe pulses when the autocorrelation amplitude decreased to 1/e.

carrier cooling, the weaker probe pulses with a certain delayed time incident into the sample and then were collected by the detector. The carrier relaxation can be revealed by analyzing the differential transmission traces as a function of delayed time. Figure 2a−c plots the transient absorptive signals of BP nanoflakes from 800 to 2026 nm. The purple background indicates the autocorrelation scale of the pump and probe pulses. For comparative studies, we also measured the graphene dispersion with the same experimental conditions, as shown in Figure 2d−f. In general, to obtain the interaction time from the pump−probe experiments, a two exponential model,

number of 7.1 layers. Thus, although there are mono- and bilayer nanosheets in our dispersion, the whole material can be considered as bulk. Raman spectra of the BP nanoflakes with 633 nm laser excitation are shown in Figure 1d. The position of Ag1, Bg2, and Ag2 modes are observed to have a blue shift to ∼362, ∼438, and ∼467 cm−1 after exfoliated, corresponding with the previous reports.5,30 The Ag1/Ag2 ratio was measured to be ∼0.6, and the absence of a broad component under the Bg2 and Ag2 implies negligible oxidation of nanosheets.11,28 The inset of Figure 1d presents a typical UV−vis absorption spectrum.11 The photoluminescence (PL) excitation−emission map for the BP dispersion is displayed in Figure 1e,f, exhibiting strong emission lines at ∼620 and ∼910 nm, corresponding to mono- and bilayered BP nanosheets, respectively.11 Emissions from ∼1150 and ∼1260 nm can be also found in Figure S2, associating with 3- and 4-layers.

( ) + A exp(− ), can well fit the transient t

g (t ) = A1 exp − τ

1

2

t τ2

absorption curves, where A1 and A2 are the amplitude of each component, t is the time delay, and τ1 and τ2 are the lifetimes of the sample.31 This requires an instantaneous response (see Figure S4a for one example curve), i.e., the carrier relaxation should be faster than the temporal resolution of the experiment, and the signal rise time can be neglect.31 However, from Figure 2, it can be clearly seen that the rise of the transient transmission around zero is not instantaneous. In this case, the autocorrelation of pump and probe pulse should be taken into

RESULTS AND DISCUSSION Excited Carrier Dynamics. Degenerate pump−probe experiments were carried out to characterize the excited-state dynamics of BP nanosheets. The carriers are first thermalized in a few tens of femtoseconds by the pump pulses. During the 6925

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Figure 3. Open-aperture z-scan traces of BP dispersions. All scatters represent experimental data, whereas solid lines are from fitting. (a) Overview of typical z-scan from 800 to 2100 nm. (b−e) BP’s responses with 1330, 1420, 1550, and 1972 nm femtosecond pulses at variant pulse energies. (f) NLO response of BP presented in the normalized differential absorptivity as a function of pulse fluence; inset is an enlargement for better view of the stacked curves; arrow indicates the trend that the threshold of saturable absorption decrease with the wavelength.

account in results fitting, and the pump−probe signal can be expressed as31 ⎧ ⎛ σ ⎛ t⎞ − g (t ) = ⎨D1 exp⎜ − ⎟ erfc⎜ ⎝ 2 τ1 ⎝ τ1 ⎠ ⎩ ⎪

⎪

⎛ σ ⎛ t ⎞ + D2 exp⎜ − ⎟ erfc⎜ − ⎝ 2 τ2 ⎝ τ2 ⎠

in the early delayed time regime.32−34 After that, the relaxation process redistributes the hot carriers and governs the changes of ΔT/T. The broad band SA response of BP across 800−2026 nm agrees with our z-scan results in Figures 3 and 4. The maximum of ΔT/T increased roughly linearly with the pump pulse energy. For 800 and 2026 nm pulses, BP exhibited an obvious two-component response for pump−probe traces, i.e., two relaxation parts τ1 and τ2. For 1600 nm, BP showed just one component τ2 (see inset of Figure 2e, where blue scatters for BP is almost a straight line after zero delayed time). With eq 1, we obtained two transient absorption components from each pump−probe traces by fitting as showed by the solid lines in Figure 2. It should be noted that the above results reveal the average properties of a large amount BP with all orientations, due to the nature of liquid exfoliated dispersions. All parameters for fitting are summarized in Table 1. The average fast part of BP, τ1, was measured to be 16 and 32 fs for 800 and 2026 nm pulses. Both of the τ1 are in the coherent regime and below the pulse duration ∼100 fs. Thus, we may attribute them to a carrier−carrier scattering

t ⎞ ⎟ 2σ ⎠ t ⎞⎫ ⎟⎬ 2 σ ⎠⎭ ⎪

⎪

(1)

where g(t) is the pump−probe signal, D1 and D2 are the relative amplitudes, “erfc” represents the integral error function, and σ is the laser pulse duration (see Figure S4b for the simulated curve of this model). Employing a nonlinear least-squares algorithm to eq 1, we can fit the pump−probe traces with the results shown in Figure 2. From Figure 2a−c, it can easily be seen that the optical transmission increases after excitation, implying a SA response in BP dispersions. The SA comes from Pauli-blocking, when the photongenerated carriers occupy the corresponding interband transition-state and then cause a decrease of photon absorption 6926

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Figure 4. (a−e) The normalized differential absorptivity as a function of incident pulse fluence, showing the NLO responses of BP, MoS2, and graphene dispersions from 800 to 2100 nm. All scatters represent experimental data (see Figure S7), and all solid lines are from fitting using a slow absorber model. (f) NLO coefficient of BP as a function of incident wavelength. Dashed lines are for visual guide.

Table 1. Parameters Obtained from the Fitting of Pump−Probe Tracesa

a

The overall average value of τ1 and τ2 including all energies at each wavelength are marked by “AVG”. 6927

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ACS Nano process23,34 or the coherence spike within the autocorrelation regime.35,36 It is also noticed that some D1 errors in Table 1 are quite large, which may come from the small amplitude of the fast component and/or the resolution limitation of setup. As already mentioned, no observable fast component τ1 at 1600 nm was found. The reason may also be the fast component at 1600 nm is so short (reported to be ∼24 fs at 1550 nm)23 that it is beyond the time resolution and/or the weak amplitude of the fast component, i.e., the carrier−carrier scattering is much weaker comparing to carrier−phonon process. The slow part τ2 was measured to be 360 fs, 928 fs, and 1.358 ps for 800, 1600, and 2026 nm pulses. These relaxations are in the nonthermal regime, and we attributed them to carrier−phonon scattering.34 The τ2 at 800 nm implied ultrafast carrier relaxation in BP on the femtosecond scale, as Figure 2a shows. As the wavelength grew to 1600 and 2026 nm in Figure 2b−c, the pump−probe signal remained at SA, but the τ2 of excited states also increased. That variation of relaxation time with photon energy in BP can be explained by the faster cooling rate of hotter electrons, which was observed and explained by the reference in graphite through pump−probe experiments at different wavelengths.33 It is also noticed that the BP’s response seemed to be laser power dependent at 2026 nm, i.e., 0.92 ps for 21 nJ and 1.65 ps for 44 nJ. While the hot carrier cooling was reported by several studies to be ∼10 ps at 1550 nm and ∼100 ps at 1940 nm, we did not observe similar relaxation in this work, which may due to the less carrier thermalization by lower laser repetition rate.20−22 As one of the most promising photonic 2D materials, graphene is well investigated for its NLO properties and wide nanophotonic applications.13,15,37 For comparative purposes, we also measured the graphene dispersion with the same linear optical transmission as BP. The results were plotted together with BP in Figure 2d−f. For 800 nm, the maximum ΔT/T of BP is larger than graphene in Figure 2d, implying BP’s larger NLO coefficient at this wavelength. This larger NLO response of BP over graphene at 800 nm was also confirmed by our zscan results shown in Figure 4a. However, for 1600 and 2026 nm excitations in Figure 2e,f, the ΔT/T peak heights of graphene become less than those of BP, indicating the stronger SA response. These results also comply with our z-scans presented in Figure 4d,e. For clarity, the logarithm of ΔT/T about time delay is plotted in the insets of Figure 2d−f, where the larger slope directly indicates faster relaxation speed. For 800 and 1600 nm pulse excitation (Figure 2d−e), the slopes of BP are obviously steeper those of graphene, implying BP is a much faster saturable absorber than graphene at this wavelength. For 2026 nm excitation, it is clear that the slope of BP and G are close to each other, indicating comparable fast saturable absorption of BP and graphene. The more detailed results are summarized in Table 1. As for the fast component τ1 of graphene, we found it to have an average value of 47, 241, and 225 fs for 800, 1600, and 2026 nm wavelengths. All of the τ1 of graphene were larger than those of BP. The first τ1 at 800 nm may originate from carrier− carrier scattering or a coherence interference effect. However, we attributed the 241 and 225 fs relaxation for 1600 and 2026 nm to intraband carrier−carrier scattering process33,38 because they were out of the autocorrelation regime (±153 fs). As for the slow component τ2 of graphene, we determined them to be 0.434, 2.05, and 1.46 ps for the three wavelengths on average and associated them with carrier−phonon scattering.33,39,40 It is obvious that few-layer BP has a much shorter τ2 (360 and 928

fs) than graphene (434 fs and 2.05 ps) at 800 and 1600 nm, indicating faster carrier relaxation of BP. When the wavelength was further increased to 2026 nm, the BP still exhibited a faster response (1.36 ps) but also comparable to that of graphene (1.46 ps). Nonlinear Optical Absorption. We performed multiwavelength open-aperture z-scans on BP dispersions to investigate its NLO properties under ∼100 fs laser pulses from 800 to 2100 nm. In order to obtain the NLO absorption coefficients, a standard model for open-aperture z-scan was applied to fit the results.25,26,41 Normalized optical transmittance as a function of sample position can be described by TNorm(z) = ln(1 + q0)/q0

(2)

where q0(z) = q00/[1 + (z/z0) ], q00 = αNLI0Leff, Leff = [1 − exp(−α0L)]/α0, α0 is the linear optical absorptive coefficient, I0 is the intensity on the focus, and αNL is the effective NLO coefficient. A nonlinear least-squares algorithm was applied to determine NLO coefficients, which were utilized to fit the zscan curves. The imaginary part of the third order NLO susceptibility, Im χ (3) , is directly related to α NL by ⎡ 10−7cλn2 ⎤ α , where c is the speed of light, λ is the Im χ (3) = ⎣ 96π 2 ⎦ NL wavelength of the incident light, and n is the refractive index. Moreover, according to NLO theory, another model which describes saturable absorption in an optical medium is also utilized for curve fitting as described by following equation: 2

dI /dz = − α0I /(1 + I /Isat)

(3)

One can solve this propagating equation to obtain the saturated intensity Isat by the similar method to that used in our previous works, which is one of the key parameters for modelocking application.11,25,26 Our pump−probe experiments in Table 1 revealed that the exited-state lifetime of BP nanosheets is on the scale of 360 fs to 1.36 ps, which is obviously much longer than the laser pulse duration ∼100 fs. In other words, the BP response is much slower than incident laser pulse in our z-scans. Thus, a slow absorber model, modified Frantz−Nodvik equations, can be safely utilized to further analyze our z-scan results.42 The optical transmission is given by the following equation: T (L) = T0 +

TFN(L) − T0 (Tmax − T0) 1 − T0

−NσgsL

−NσesL

(4) σgsE(0)

where T0 = e , Tmax = e , and TFN = ln[1 + T0(e − 1)], N is the total density of ground state, L is the light travel distance in medium, E(0) is the laser flux presented by the number of photons per unit area, and σgs and σes are the absorptive cross section of ground and excited states, respectively. To apply the Frantz−Nodvik equations for fitting, the experimental z-scan results were converted into a function of change in the ratio of absorptive coefficient, Δα/α0 with laser pulse fluence. By employing Δα/α0, the ideal saturable absorber will have a value from 0 to 1 when the laser energy increases from linear optics to the NLO region, which is beneficial for comparison between different materials. Similar to z-scan fitting, the nonlinear least-squares algorithm was also applied to obtain the best fitting from experimental data. Figure 3a presents typical open-aperture z-scan curves of BP dispersions, where normalized transmission was plotted as a function of sample position along the z-axis from 800 to 2100 6928

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Table 2. Linear and NLO Coefficients of BP Dispersions in Comparison with Those of Graphene from the Fitting of OpenAperture z-Scan Traces

Figure 5. NLO parameters about BP and graphene dispersions as a function of wavelength: (a) saturated intensity Isat; (b) imaginary part of third-order NLO susceptibility Im χ(3); and (c,d) ground and excited-state absorptive cross sections.

nm. The peaks around z = 0 mm imply the obvious saturable responses of BP nanosheets spread over a very board range up to 2.1 μm. Further studies with different incident energy were conducted, and results at 1330, 1420, 1550, and 1972 nm are shown in Figure 3b−e. As the pulse energy increases, the peak of z-scan trace also rises. The standard open-z-scan model eq 2 exhibited very good fitting for all the z-scan curves in Figure 3a−e. The SA model eq 3 also works very well for all z-scan traces. All the NLO parameters from open aperture z-scan are presented in Table 2. The broadband saturable absorption of BP can be explained by Pauli-blocking due to the very small bandgap ∼0.3 eV (∼4.1 μm) of BP multilayer nanosheets.43 Although the mono- and bilayered BP nanosheets (∼2 eV and ∼1.3 eV bandgap respectively),1,11,43 whose presence was confirmed by AFM and PL results in Figure 1, may introduce the two-photon absorption (TPA), it is not surprising that

saturable absorption overwhelmed other effects because of the domain of multilayered BP nanosheets, just as we observed from MoS2 nanosheets.25,26 The NLO absorptive coefficient, αNL of BP dispersions, was estimated to be −0.014, −0.019, −0.018, and −0.057 cm/GW for 800, 1330, 1550, and 2100 nm wavelength respectively and is plotted in Figure 4f. There is a clear rise of αNL with wavelength increase. A more direct show of this trend is presented in Figure 3f, where the normalized differential absorptivity Δα/α0 are plotted as a function of pulse fluence. The arrow depicts that the BP nanosheets require lower energy density to induce NLO effect when wavelength increases. The similar trend can also be found in saturated intensity in Figure 5a, where Isat was 459, 382, 398, and 71.3 GW/cm2 at 800, 1330, 1550, and 2100 nm. The wavelengthdependent NLO coefficient may be explained by the relationship Isat ∝ hc/λ.44 Our Isat at 800 nm (459 GW/cm2) well 6929

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Table 3. NLO Coefficients of BP and Graphene Dispersions from Open-Aperture z-Scan Results Fitted by a Slow-Absorber Modela

a

CFG 1 represents free parameters of N, T0 and Tmax for all the wavelengths, while CFG 2 is the same except Tmax is fixed to be 1.

dropped from 764 to 25.4 GW/cm2. The similar relationship can be found in Im χ(3), where that of BP was estimated to be −7.85 × 10−15, −1.83 × 10−14, −1.98 × 10−14, and −8.49 × 10−14 esu at 800, 1330, 1550, and 2100 nm. The Im χ(3) of graphene is also fitted as −4.72 × 10−15, −7.11 × 10−14, −6.1 × 10−14, and −2.72 × 10−13 esu at these wavelengths, implying an increased SA response as the wavelength goes into the infrared. All the detail parameters of both BP and graphene are listed in Table 2. We used two configurations (CFG) for the curve fitting by the slower absorber model eq 4, including those shown in Figure 4a−e. CFG 1 treats three free parameters, i.e., N, T0, and Tmax, while CFG 2 is just about N and T0 and fixes Tmax ≡ 1. Both configurations well fitted the experimental traces in this work. We obtained the excited-state absorptive cross-section σes with CFG 1 and ground-state absorptive cross-section σgs with both configurations. The σgs of BP were fitted to be 17.1 × 10−18, 18.9 × 10−18, 27.7 × 10−18, and 86.5 × 10−18 cm2 for 800, 1330, 1550, and 2100 nm, while the σes of same wavelengths were estimated to be 5.92 × 10−18, 9.7 × 10−18, 19.8 × 10−18, and 52.8 × 10−18 cm2, respectively. The estimated σgs and σes as a function of wavelength are also plotted in Figure 5c,d, respectively. In another aspect of wavelength, both cross sections seemed to increase when the wavelength grows. All fitting parameters about N, T0 and Tmax are summarized in Table 3 for references.

agrees with the reported value which is from 334 to 774.4 GW/ cm2 in IPA/NMP dispersion.12,30 Although there are reports that revealed the Isat of BP/PMMA composite to be 18.5 MW/ cm2 at 1563 nm for fs laser and 4.56 MW/cm2 at 1930 nm for ps laser, they may be little comparative to ours as well as each other because of different laser and material.12 In order to carry out a comparative study, graphene and MoS2 dispersions were also characterized by z-scan under the same conditions. The linear optical transmission of graphene and MoS2 dispersions were adjusted to that of BP by concentration. In Figure 4a−e, we plotted the normalized differential absorptivity Δα/α0 as a function of pulse energy density from 800 to 2100 nm for BP and graphene dispersions. MoS2 is also introduced for comparison from 1330 to 2100 nm. These curves are converted from original z-scan traces (see Figure S7) and then fitted by eq 4, the slow absorber model. In Figure 4a at 800 nm, the BP dispersion behaved somewhat better as a saturable absorber than graphene.11 As the wavelength changed to the mid-infrared from 1330 to 2100 nm in Figure 4b−e, graphene exhibited a stronger SA response than BP. One possible reason may explain the extraordinary NLO response of BP at 800 nm may be the existence of different layered nanosheets in LPE samples. At 800 nm, the photon energy (1.55 eV) is larger enough to excite all nanosheets from bi- to multilayer (1.36 to 0.3 eV). At 1600 and 2026 nm (0.78 and 0.63 eV), only nanosheets larger than ∼5 layers can contribute to the saturable absorption because of the less photon energy.4 The saturable absorption of MoS2 at 800 nm has been reported, and the Imχ(3) was measured to be −(1.38 ± 0.45) × 10−14 esu in our previous work,25 which is greater than both of BP and graphene. Nevertheless, while both BP and graphene show broadband SA effects over 800−2100 nm, the MoS2 seemed to have no detectable NLO response over the tested intensity wavelengths from 1330 to 2100 nm in Figure 4b−e. The reason may be attributed to the large bandgap of bulk MoS2 nanosheets (1.29 eV, ∼961 nm), while BP has a much smaller bandgap to be 0.3 eV and that of graphene is known to have zero bandgap. Although some groups reported the SA response of MoS2 at infrared region above 1.3 μm, this SA may come from the Mo and S defect states introduced during the preparation process, which are expected to be much less prominent in LPE methods.45,46 The αNL, Isat, and Im χ(3) of BP and graphene as a function of wavelength are shown in Figures 4f and 5a,b. The Isat of BP decreased from 459 to 71.3 GW/cm2 as the wavelength was increased from 800 to 2100 nm, while that of graphene

CONCLUSIONS In this work, we systematically investigated the excited carrier dynamics and NLO saturable absorption of liquid-exfoliated BP nanosheets over a broad wavelength range from 800 to 2100 nm. Characterizations including AFM, Raman, and PL confirmed the effective exfoliation of BP crystal into a few even monolayered nanosheets without observable oxidation. The degenerate pump−probe revealed BP’s carrier−phonon scattering time to be 0.360, 0.928, 1.358 ps at 800, 1600, and 2026 nm, deceasing with increasing wavelength. Comparative studies indicate much faster relaxation of excited carriers of BP than that of graphene (0.434 and 2.05 ps) at 800 and 1600 nm and a bit faster lifetime at 2026 nm. The open aperture z-scans directly confirmed the wide-band saturable absorption of BP nanosheets at 800, 1330, 1420, 1550, and 2100 nm, where the last three measured wavelengths were chosen for the wavelengths of Nd, Er, and Tm/Ho-doped fiber lasers. Both of the saturable absorptions of BP and graphene were found to be stronger as the wavelength increased. The saturated intensity 6930

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we obtained the curves as a function of probe delay with the change in transmission. The polarization of both the pump and probe beams was kept parallel throughout all the experiments.

Isat of BP was estimated to be 459, 382, 464, 398, and 71.3 GW/cm2 for each wavelength in our BP samples. BP was exhibited to be less NLO response than graphene in infrared region from 1330 nm, but a much better saturable absorber than MoS2. With slow-absorber model, the ground-state density and cross-section at each wavelength were also calculated. Our results revealed faster carrier dynamics and broad-band SA of BP’s pristine nanosheets, which is required for device design such as optical modulators and switches in this wavelength region and especially mode-lockers for the corresponding rareearth-doped fiber lasers.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b02770. Additional figures about characterizations, setups and zscans (PDF)

AUTHOR INFORMATION

METHODS

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Characterizations. The AFM imaging was carried on a Veeco Nanoscope-IIIa (Digital Instruments) with an E-head in tapping mode. To get rid of the side effects of high boiling point solvent (CHP), the BP nanosheets were first transferred into anhydrous isopropanol (IPA)11 and then drop casted on preheated silicon wafer with oxide layer of 300 nm, which was used as a substrate. Raman Spectroscopy was performed using a Horiba Jobin Yvon LabRAM HR800 with 633 nm excitation laser in air under ambient conditions. PL was acquired on a Horiba Scientific Fluorog-3 system equipped with a liquid nitrogen cooled InGaS diode array detector with relevant filters. Laser Source. An optical parametric amplifier (Coherent OPA9800) was employed as laser source in this work to get a tunable wavelength. The 1330−2030 nm femtosecond pulses were from output of OPA9800, while ultrafast 800 nm pulses were directly introduced from the pumping Ti:sapphire laser (Coherent Reg9000A). The lasers were operated at 50 kHz, except 10 kHz for 800 nm to reduce the disturbance of the laser-induced thermal effect. All the measurements were carried out in dispersions at room temperature at 296 K. Open-Aperture Z-Scan. Infrared laser was first focused by a convex lens ( f = 10 cm) to generate another Gaussian beam with much shorter diffractive length (see Figure S3). While a motorized translation stage brings the sample along the focusing beam, the change of irradiance intensity generates the variance of sample transmission, and finally a z-scan curve about stage position vs transmission can be obtained. To reduce the electrical noise caused by different photodiode, we used a single photodiode scheme, i.e., measured the reference and signal beam using just one photodiode. Two type photodiodes were employed for a different wavelength. A large area Ge photodiode was operated at 800−1080 nm and another extended InGaAs photodiode was for 1330−2030 nm. Two optical choppers were employed to module the reference and signal beam at 475 and 733 Hz, respectively. With a lock-in amplifier (Signal Recovery, SR7270) in dual-reference mode, the intensities of reference and signal can be perfectly separated and high noise-signal ratio can be achieved. For all the measurements, a cuvette filled with pure CHP was also tested as references. No noticeable response from reference sample was detected (see Figures S5 and S6). Degenerate Pump−Probe. The laser pulses from source were first split into two beams by cellulose membrane (see Figure S3). The higher intensity beam was used as pump and keeps its power around one order larger than the probe beam. A small portion of pump beam was further split as reference and then chopped at 733 Hz. The rest of the main pump pulses were chopped at 475 Hz. After motorized delay line, the probe beam was parallel combined with the pump by Dshaped mirrors before incident into a focusing convex lens (f = 10 cm). Due to the nature of convex lens, the pump and probe beam met each other at the focal point, where the sample was placed. The beam spot radii of pump pulses at the focus were estimated to be 21, 35, 55 μm for 800, 1600, 2026 nm, respectively. After the sample, an aperture stopped the pump beam and let the probe beam pass to the Ge/ InGaAs photodiode. The lock-in amplifier was locked at pump frequency, 475 Hz, to obtain the pump modulated sample transmission. By changing delay of the probe pulses by the motorized stage,

Author Contributions

K.P.W. and W.J.B. conceived the idea. K.P.W., B.M.S., J.J.M., and W.J.B. wrote the paper. K.P.W. built the pump−probe and z-scan setup for this work and performed the optical experiments. K.P.W. and G.Z.W. analyzed the NLO data. B.M.S. and J.N.C. prepared the samples. B.M.S. performed and analyzed characterizations. J.J.W., X.Y.Z, and J.W. helped on the optical measurements. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully thank the financial supports from Science Foundation Ireland (SFI, 12/IA/1306) and European Commission under the Seventh Framework programme (ISLA project no. 287732). This work is also partially supported by NSFC (no. 61522510, no. 51302285), the External Cooperation Program of BIC, CAS (no. 181231KYSB20130007) and the Strategic Priority Research Program of CAS (no. XDB160307). The generous support from J.J.W. are greatly appreciated. REFERENCES (1) Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X.; Tománek, D.; Ye, P. D. Phosphorene: an Unexplored 2D Semiconductor with a High Hole Mobility. ACS Nano 2014, 8, 4033−4041. (2) Xia, F.; Wang, H.; Jia, Y. Rediscovering Black Phosphorus as an Anisotropic Layered Material for Optoelectronics and Electronics. Nat. Commun. 2014, 5, 4458. (3) Castellanos-Gomez, A. Black Phosphorus: Narrow Gap, Wide Applications. J. Phys. Chem. Lett. 2015, 6, 4280−4291. (4) Ling, X.; Wang, H.; Huang, S.; Xia, F.; Dresselhaus, M. S. The Renaissance of Black Phosphorus. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 4523−4530. (5) Kang, J.; Wood, J. D.; Wells, S. A.; Lee, J.-H.; Liu, X.; Chen, K.-S.; Hersam, M. C. Solvent Exfoliation of Electronic-grade, Twodimensional Black Phosphorus. ACS Nano 2015, 9, 3596−3604. (6) Wang, X.; Jones, A. M.; Seyler, K. L.; Tran, V.; Jia, Y.; Zhao, H.; Wang, H.; Yang, L.; Xu, X.; Xia, F. Highly Anisotropic and Robust Excitons in Monolayer Black Phosphorus. Nat. Nanotechnol. 2015, 10, 517−521. (7) Tran, V.; Soklaski, R.; Liang, Y.; Yang, L. Layer-controlled Band Gap and Anisotropic Excitons in Few-layer Black Phosphorus. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 235319. (8) Qiao, J.; Kong, X.; Hu, Z.-X.; Yang, F.; Ji, W. High-mobility Transport Anisotropy and Linear Dichroism in Few-layer Black Phosphorus. Nat. Commun. 2014, 5, 4475. (9) Bhimanapati, G. R.; Lin, Z.; Meunier, V.; Jung, Y.; Cha, J.; Das, S.; Xiao, D.; Son, Y.; Strano, M. S.; Cooper, V. R. Recent Advances in 6931

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NOTE ADDED AFTER ASAP PUBLICATION This paper published ASAP on 6/16/2016. Figure 5 was replaced and the revised version was reposted on 6/21/2016.

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DOI: 10.1021/acsnano.6b02770 ACS Nano 2016, 10, 6923−6932