Tunable Broadband Nonlinear Optical Properties of Black ... - MDPI

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Tunable Broadband Nonlinear Optical Properties of Black Phosphorus Quantum Dots for Femtosecond Laser Pulses Xiao-Fang Jiang 1 , Zhikai Zeng 1 , Shuang Li 1 , Zhinan Guo 2 , Han Zhang 2 , Fei Huang 1, * and Qing-Hua Xu 2,3,4, * 1

2

3 4

*

State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, China; [email protected] (X.-F.J.); [email protected] (Z.Z.); [email protected] (S.L.) SZU-NUS Collaborative Innovation Center for Optoelectronic Science and Technology, and Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, Shenzhen University, Shenzhen 518060, China; [email protected] (Z.G.); [email protected] (H.Z.) National University of Singapore (Suzhou) Research Institute, Suzhou 215123, China Department of Chemistry, National University of Singapore, Singapore 117543, Singapore Correspondence: [email protected] (F.H.); [email protected] (Q.-H.X.)

Academic Editor: Irene D’Amico Received: 6 January 2017; Accepted: 16 February 2017; Published: 21 February 2017

Abstract: Broadband nonlinear optical properties from 500 to 1550 nm of ultrasmall black phosphorus quantum dots (BPQDs) have been extensively investigated by using the open-aperture Z-scan technique. Our results show that BPQDs exhibit significant nonlinear absorption in the visible range, but saturable absorption in the near-infrared range under femtosecond excitation. The calculated nonlinear absorption coefficients were found to be (7.49 ± 0.23) × 10−3 , (1.68 ± 0.078) × 10−3 and (0.81 ± 0.03) × 10−3 cm/GW for 500, 700 and 900 nm, respectively. Femtosecond pump-probe measurements performed on BPQDs revealed that two-photon absorption is responsible for the observed nonlinear absorption. The saturable absorption behaviors observed at 1050, 1350 and 1550 nm are due to ground-state bleaching induced by photo-excitation. Our results suggest that BPQDs have great potential in applications as broadband optical limiters in the visible range or saturable absorbers in the near-infrared range for ultrafast laser pulses. These ultrasmall BPQDs are potentially useful as broadband optical elements in ultrafast photonics devices. Keywords: black phosphorus; quantum dots; nonlinear optics; two-photon absorption; femtosecond laser

1. Introduction The upsurge of two-dimensional (2D) materials was triggered by the discovery of graphene in 2004 when it was isolated from its parent graphite [1]. Due to their outstanding physical and chemical properties and promising applications in various fields such as photovoltaics [2,3] and electronics [4,5], 2D materials became a new class of nanomaterials that have a groundbreaking impact on nanotechnology. Current studies on 2D materials mainly focus on graphene and wide-bandgap transitional metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2 ) [6]. Most importantly, there is significant interest in studying 2D materials’ optical properties and their applications as nonlinear optical materials (optical limiters and saturable absorbers). Graphene and graphene oxide have been reported to display broadband optical-limiting properties due to strong two-photon absorption [7–10]. Broadband nonlinear absorption was also discovered in TMDs [11,12] and their applications in ultrafast lasers have been investigated [13,14]. Besides 2D materials, ultrasmall Materials 2017, 10, 210; doi:10.3390/ma10020210

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quantum dots (QDs) are being also extensively studied due to their unique electronic and optical properties arising from the quantum confinement effect [15]. Black phosphorous (BP), also known as phosphorene, is the latest member of the family of 2D materials [16,17]. BP is the most thermodynamically stable allotrope of phosphorus [18] and its unique structure as well as fascinating optical and electronic properties have attracted a lot of research interests [19]. As bulk BP consists of phosphorene monolayers stacked together by van der Waals force, it can be mechanically [20] or chemically exfoliated [21–23] into few-layer or single-layer nanosheets, or quantum dots. Due to its attractive properties, BP has been applied to field-effect transistors (FET) [17,24]. Few-layer BP can be used in thin-film solar cells [25] and p-n diodes [26]. It is also theoretically predicted to be used in flexible ambipolar transistors [27], energy storage [28] and moveable vibratory devices [29]. In particular, it has been previously reported that few-layer BP can act as an effective saturable absorber for ultrashort pulse generation in solid-state and fiber mode–locked lasers operating in the 1000–2000 nm wavelength range [30–35]. Unlike graphene’s zero-bandgap nature which limits its electronic and photonic applications, it is noteworthy that the bandgap of BP depends on the number of layers, ranging from 0.3 (bulk) to 2.0 eV (single layer) [36,37]. It has also been found that the applied strain force [38,39], stacking order [25] and external electric field [40] can also modulate the bandgap of BP. The tunable bandgap of BP shows great potential in bridging the space between zero-bandgap semi-metallic graphene and wide-bandgap TMDs (1–2 eV). Hence, BP is considered to be suitable for extremely broadband nonlinear optical applications, including optical limiters and saturable absorbers [30–35]. The past decades have witnessed significant research efforts in developing broadband optical materials. Optical-limiting materials exhibit decreased transmittance at high-input laser intensity, which can be used to protect human eyes and sensitive instruments from damage by high-intensity laser beams [41]. In contrast, saturable absorbers show an increased transmittance at high-input laser intensity, which can be utilized in pulse compression, mode locking and Q-switching [42]. Herein, for the first time, black phosphorus quantum dots (BPQDs) are extensively investigated in broadband femtosecond nonlinear optical properties from visible to near-infrared (near-IR) range. Based on the simple solution exfoliation method, a suspension of BPQDs was prepared in N-Methylpyrrolidone solvent (NMP). The utrasmall BPQDs were experimentally demonstrated to display nonlinear optical responses in a broad wavelength range by femtosecond open-aperture Z-scan measurements. Under femtosecond laser excitation, BPQDs exhibited significant nonlinear absorption in the visible range, but saturable absorption in the near-infrared (near-IR) range. Femtosecond pump-probe measurements performed on BPQDs revealed that two-photon absorption is primarily responsible for the observed nonlinear absorption. The results suggest that BPQDs have great potential in applications as broadband optical limiters in the visible range or as saturable absorbers in the near-IR range for ultrafast laser pulses. These ultrasmall BPQDs are potentially useful as broadband optical elements in fiber lasers and other ultrafast photonics devices. 2. Results and Discussion 2.1. Sample Preparation and Characterizations Mechanical and liquid exfoliation methods are commonly used as simple and effective techniques to prepare 2D nanomaterials and QDs from bulk crystals, such as BP [21,22] and graphene [43]. In the simple liquid exfoliation technique, solvents with a suitable surface energy can serve as stable dispersions for layered materials. Here in this article, the solvent exfoliation combined with probe sonication and bath sonication were used to fabricate BPQDs dispersed in N-Methylpyrrolidone (NMP) solvent [44]. BP has been known to be sensitive to water and oxygen and can be oxidized under visible-light irradiation [45,46]. In our experiments, BPQDs were prepared in ambient conditions and dispersed in NMP solution to avoid any exposure to air as much as possible. The absorption and Raman spectra were measured and compared before and after nonlinear optical property

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3 of 11 for any possible degradation. More experimental details can be found in examine morphologyelectron of the as-prepared BPQDs.measurements The TEM image (Figure 1a) shows that the Section 3.the Transmission microscopy (TEM) were conducted to examine the examine the morphology of the as-prepared BPQDs. The TEM image (Figure 1a) shows that the ultrasmall BPQDs an average lateral of image about (Figure 2–3 nm,1a) which corresponds to the stacking morphology of thehave as-prepared BPQDs. Thesize TEM shows that the ultrasmall BPQDs ultrasmall BPQDs have an average lateral size of about 2–3 nm, which corresponds to the stacking number layerslateral of 2 ±size 1. The absorption 1b) show BPQDs have of a broad have an of average of about 2–3 nm,spectra which (Figure corresponds to thethat stacking number layers number of layers of 2 ± 1. The absorption spectra (Figure 1b) show that BPQDs have a broad absorption band, spanning from(Figure the UV toshow near-IR range. The absorption band at the UV-visible of 2absorption ± 1. The absorption spectra 1b) that BPQDs have a broad absorption band, spanning band, spanning from the UV to near-IR range. The absorption band at the UV-visible range is similar those of other 2D2D layered such graphene oxide, assimilar well as asto few-layer from the UV toto near-IR range. The absorption band at theas UV-visible those of range is similar to those of other layeredmaterials materials such as graphenerange oxide, is as well few-layer BP. Compared to BP, a huge absorption band in the near-IR range from 1250 to 1630 nm with two other 2D layered materials such as graphene oxide, as well as few-layer BP. Compared to BP, huge BP. Compared to BP, a huge absorption band in the near-IR range from 1250 to 1630 nm witha two absorption maxima at 1438 and 1550 nm was observed in the BPQD dispersion. This near-IR broad absorption band in theatnear-IR range 1250observed to 1630 nm with two absorption at 1438 and absorption maxima 1438 and 1550from nm was in the BPQD dispersion. maxima This near-IR broad absorption band might bethe due to to thedispersion. defect absorption induced by size the 1550absorption nm was observed in BPQD This near-IR broad absorption band might beofof due to band might be due the defectstate state absorption induced by the the ultrasmall ultrasmall size the QDs [47],[47], which indicates possible interesting nonlinear optical properties of BPQDs both in visible the QDs defect state absorption induced by the ultrasmall size of the QDs [47], which indicates possible which indicates possible interesting nonlinear optical properties of BPQDs both in visible and near-IR regions. interesting nonlinear optical properties of BPQDs both in visible and near-IR regions. and near-IR regions.

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Wavelength (nm) Figure 1. (a) TEM image of BPQDs; (b) absorption spectra of BP and BPQDs. Figure1.1.(a) (a)TEM TEMimage imageof ofBPQDs; BPQDs;(b) (b)absorption absorptionspectra spectraof of BP BP and and BPQDs. BPQDs. Figure

BPQDs were also characterized by Raman spectroscopy. Samples were prepared by BPQDs were also characterized by Raman spectroscopy. Samples by BPQDs were characterized Raman spectroscopy. Samples were on prepared byprepared spin-coating spin-coating thealso BPQD dispersion by onto the quartz substrates, drying them a were heating plate under spin-coating the BPQD dispersion onto the quartz substrates, drying them on a heating plate under the 348 BPQD dispersion onto thekeeping quartz them substrates, drying them6 on under2, 348 for 5 K for 5 min, and then in N2 for the next h. a Asheating shown plate in Figure the Kthree 1g)the −1, the 348 Kand for then 5 min, and keeping them N26for theshown next 6 in h.mode As shown Figure three min, keeping them in N2 for next h. As Figure three observed observed peaks canthen be attributed to the onein out-of-plane phonon (A2, atin361.16 cm2, and peaks two 1 ) at 361.16 −1g,)and 2g, at −1, respectively, observed peaks can be to one out-of-plane mode at 361.16 cm−1, with and two canin-plane be attributed towhich oneattributed out-of-plane mode (Aphonon cm(A two in-plane modes, modes, are B2g and Aphonon 438.22, and 465.65 cm consistent the g 2 − 1 2 −1 2 in-plane B2g and g, at 438.22, and B465.65 cm , respectively, consistent with by the reported Compared the bulk both g modes 2g and A of BPQDs are red-shifted which aremodes, B2gvalues andwhich A[15]. 438.22, andtoA 465.65 cmBP,, respectively, consistent with the reported values [15]. g , atare 1 , while −1, while reported values Compared to the bulk BP, Bcm g be 2g −1 and A2red-shifted modes of by BPQDs are−Raman red-shifted by the A1gboth mode is and red-shifted byboth 2.32 . Itare can explained that the shift 2.30 cm Compared to the[15]. bulk BP, B2g A2g modes of BPQDs 2.30 cm the is A1g 1g and A2g 1red-shifted −1 dependent thickness and dimensions. frequency A , whileonthe A mode by 2.32The cmthat . Itthe canRaman bedifference explained that thethe Raman shift is 2.30 cmis−1red-shifted mode by1g2.32 cmis−lateral . It can be explained shift isbetween dependent on thickness −1, which is larger than the reported 1 2 1 modes of BPQDs equals 104.3 cm value of bulk BP [48] and dependent thickness and dimensions. The frequency difference A g and A2g and lateralon dimensions. The lateral frequency difference between the A modes ofthe BPQDs equals g and Ag between − 1 −1 confirms that we reduced the thickness the BP BP.[48] modes of BPQDs equals 104.3 which isvalue larger than the reported value ofthat bulk [48] and 104.3 cm , which issuccessfully larger thancm the ,reported ofof bulk and confirms weBP successfully confirms that we successfully reduced the thickness of the BP. reduced the thickness of the BP.

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Figure 2. Raman spectra of BPQDs and bulk BP. Figure 2. Raman spectra of BPQDs and bulk BP.

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2.2. The Nonlinar Optical Properties of BPQDs in Visible Range Open-aperture femtosecond Z-scan measurements were used to characterize the nonlinear optical properties of BPQDs. The BPQD dispersion was placed in a 1 mm cuvette for Z-scan measurements. The detailed experimental setup is described in Section 3. Briefly, femtosecond laser pulses with a pulse duration of 120 fs and a repetition rate of 1 kHz were focused onto the cuvette with the samples, which were moved towards and away from the focus by using a motorized translational stage to study the excitation power intensity–dependent transmission of samples. Similar Z-scan measurements were performed with femtosecond laser pulses at 500, 700 and 900 nm, which were generated by an optical parametric amplifier (TOPAS-Prime) pumped by a mode-locked Ti:sapphire oscillator–seeded regenerative amplifier. The Z-scan results showed that a very weak saturable absorption effect was exhibited in the visible spectra range upon low excitation intensity due to ground-state bleaching. As the excitation intensity increased, reverse saturable absorption occurred and finally became dominant. The results under the high excitation peak power intensity of 147 GW/cm2 are shown in Figure 3. The subfigures (a–c) are corresponding to the Z-scan results at 500 nm (a), 700 nm (b) and 900 nm (c), respectively. NMP solvent in a 1 mm cuvette was measured and no nonlinear absorption was found, which excludes the effects of NMP and the 1 mm cuvette. For all wavelengths, the normalized transmittance gradually decreased as the sample move towards the focus point (Z = 0), indicating an optically induced reverse saturable absorption. The Z-scan curves can be fitted to achieve the nonlinear absorption coefficient β with the following approximate equation: T (Z) =

∑ ∞m=0 (− βI0 Le f f )m /(1 + Z2 /Z02 )

m

(m + 1)3/2 ≈ 1 − βI0 Le f f /23/2 (1 + Z2 /Z02 )

where Z is the distance between the sample and the focus; Z0 is the Rayleigh diffraction length; I0 is the

peak power density of the excitation laser pulses; Le f f = (1 − e −α0 L /α0 is the effective sample length; L is the sample length; α0 is the linear absorption coefficient. The fitting values of nonlinear absorption coefficient β at 500, 700 and 900 nm were found to be (7.49 ± 0.23) × 10−3 , (1.68 ± 0.078) × 10−3 and (0.81 ± 0.03) × 10−3 cm/GW for BPQDs, respectively. The obtained nonlinear absorption coefficients of these BPQDs are higher than those of other optical-limiting materials such as CdSe QDs with average size of around 2 nm ((1.1 ± 0.15) × 10−3 cm/GW) [49], CdO nanoflakes (0.69 × 10−3 cm/GW) [50] and Fe2 O3 hexagonal nanostructures (0.82 × 10−3 cm/GW) [51], obtained by using the similar femtosecond laser pulses. These results indicated that BPQDs can act as good broadband optical-limiting materials in the visible range. The results are similar to the previous studies on the nonlinear absorption properties of BP nanoplatelets at 800 nm [52]. These nonlinear absorption properties can be well analyzed based on the band structure of BP. The bandgap of bulk BP is ~0.3 eV, and the bandgap increases with the layer decreasing. With the size and layer of BPs decreasing, the quantum effects become increasingly important. Upon excitation by the photon with an energy of 1.3–2.48 eV (equivalent to 500–900 nm), electrons cannot be promoted from the top of the valence band (VB) through direct transition but can only jump by indirect transition or through the defects level, as the photon energy is much larger than the bandgap of BP. For the energy (2.6–4.96 eV) of two photons at 500–900 nm, when the excitation intensity is high, electrons can be directly promoted from the top of the valence band through the two-photon absorption (TPA) process, which deduced the observed significant optical-limiting properties.

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900 nm 700 nm 3. The open femtosecond 0.97 Figure0.88 3. results of BPQD dispersion in NMP measured by femtosecond Fitting Fitting 0.985 0.97 at 500-8 nm laser pulses central and 900-10nm-8 (c)-5 under peak -10 -8with -5 -3 0 3 wavelength 5 8 10 -10 -5 (a); -3 07003 nm 5 (b) 8 10 -3 0 the 3 5same 8 10 Z(mm) Z(mm) Z(mm) power intensity respectively. intensity of of 147 147 GW/cm GW/cm22, ,respectively.

Figure 3. The open aperture Z-scan results of BPQD dispersion in NMP measured by femtosecond

2.3. The The Ultrafast Ultrafast Carrier Dynamics of BPQDs BPQDs 2.3. Carrier of laser pulses withDynamics central wavelength at 500 nm (a); 700 nm (b) and 900 nm (c) under the same peak power intensity of 147 GW/cm2, respectively.

To better betterunderstand understandthe thepossible possiblemechanisms mechanisms behind nonlinear absorption properties in To behind thethe nonlinear absorption properties in the the BPQDs, absorption (TA) spectra wellsingle-wavelength as the single-wavelength were 2.3. Thetransient Ultrafast Carrier Dynamics of BPQDs BPQDs, transient absorption (TA) spectra as well as as the dynamics dynamics were measured 2. TA spectra of 2 . TA measured under excitation at 400 nm (3.1 eV) with a pump fluence of 16 μJ/cm under excitation at 400 nm (3.1 eV) with a pump fluence of 16 µJ/cm spectra of BPQDs in NMP To better understand the possible mechanisms behind the nonlinear absorption properties in BPQDs indelay NMPtimes at various delay times are shown Figure 4a. A broad negative transient at various are shown in Figure 4a. A negative transient differential transmission the BPQDs, transient absorption (TA) spectra asbroad well in as the single-wavelength dynamics were 2 differential transmission spanning from 450 toa pump 750 nm was offound. The observed measured under450 excitation at 400 nmfound. (3.1 eV)The with fluence 16 μJ/cm . TA spectra band spanning from toband 750 nm was observed negative transmission changeofnegative signal is BPQDs in NMP at various delay times are shown in Figure 4a. A broad negative transient transmission change signal is commonly assigned to the excited state absorption. This result is also commonly assigned to the excited state absorption. This result is also consistent with the previously differential transmission band spanning from 450 to 750free nm carrier was found. The observed negative consistent with the previously reported photon-induced participated interband transient reported photon-induced free carrier participated interband transient absorption in few-layer BP and transmission change signal is commonly assigned to the excited state absorption. This result is also absorption[53,54]. in few-layer BP and graphene [53,54]. As mentioned above, (3.1 the excitation photon energy graphene As mentioned above, the excitation photon energy eV) is much larger than consistent with the previously reported photon-induced free carrier participated interband transient (3.1 eV) is much larger than the bandgap of BP. Thus, the negative transmission change signal the bandgap of BP. Thus, the transmission change signal induced by photon interband transient absorption in few-layer BP negative and graphene [53,54]. As mentioned above, the excitation energy induced(3.1bycould interband transient absorption be attributed thepump-probe multiphoton absorption absorption attributed to the multiphoton absorption Our further eV) is be much larger than the bandgap ofcould BP. Thus, the process. negativeto transmission changeresults signal process.induced Ournonlinear pump-probe results further support absorption behaviors observed by by interband transient absorption couldthe be attributed to the multiphoton support the absorption behaviors observed bynonlinear the Z-scan measurement in absorption the visible range. process. Our pump-probe results further support the nonlinear absorption behaviors observed by the Z-scan measurement in the visible range. Figure 4b shows the single-wavelength dynamics Figure 4b shows the single-wavelength dynamics probed at the wavelengths of 520 and 700 nm. the measurementofin520 the and visible range. Figure 4b shows the single-wavelength dynamics probed at Z-scan the wavelengths 700 nm.at The photo-induced absorption decays probedand at The photo-induced absorption decays probed different wavelengths show no difference probed at the wavelengths of 520 and 700 nm. The photo-induced absorption decays probed at different wavelengths show no difference and could be well fit with a bi-exponential equation with could be well fit with a bi-exponential equation with timefitconstants of τ1 = 78equation ± 6 pswith (35%) and different wavelengths show no difference and could be well with a bi-exponential constants τ1 of= τ78 (35%) 612constants (65%). The two characteristic time τtime ± 30 psof(65%). characteristic on the of tens of picoseconds time constants 1 The = ±786two ±ps 6 ps (35%)and and ττ22 time == 612 ± ±3030psps (65%). Theorder two characteristic time 2 = 612 constants on the order of tens of of picoseconds and hundreds picoseconds could be attributed to constants the order of could tens picoseconds and of of picoseconds could be attributed to and hundreds ofon picoseconds be attributed to hundreds the electron-phonon and slower phonon-phonon the electron-phonon and slower phonon-phonon scattering during the interband transition. the electron-phonon and slower phonon-phonon scattering during the interband transition. scattering during the interband transition.

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450 500 550 600 650 700 750 0 200 400 600 800 1000 1200 1400 Figure 4. (a) Transient absorption spectra of BPQDs in NMP at the different delay times under Figure 4. (a) Transient absorption spectra of BPQDs in NMP at the different delay times under excitation (nm) (ps)by excitation at Wavelength 400 nm; (b) Single-wavelength dynamics at probed at 525 and Delay 700 nm,time measured at 400 nm; (b) Single-wavelength dynamics at probed at 525 and 700 nm, measured by femtosecond femtosecond pump-probe technique.

Figure 4. (a)technique. Transient absorption spectra of BPQDs in NMP at the different delay times under pump-probe excitation at 400 nm; (b) Properties Single-wavelength at probed at 525 and 700 nm, measured by 2.4. The Nonlinar Optical of BPQDs indynamics Near-IR Range femtosecond pump-probe technique. 2.4. The Nonlinar Optical Properties of BPQDs in Near-IR Range We also investigated the nonlinear absorption responses of BPQDs in NMP at 1050, 1350 nm, respectively. The open aperture Z-scan measurement results under different excitation peak power

WeNonlinar also investigated the nonlinear absorption responses 2.4. The Optical Properties of BPQDs in Near-IR Range of BPQDs in NMP at 1050, 1350 nm, respectively. The open aperture Z-scan measurement results under different excitation peak power We also investigated the nonlinear absorption responses of BPQDs in NMP at 1050, 1350 nm, respectively. The open aperture Z-scan measurement results under different excitation peak power

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intensities at the focal point are shown in Figure 5. The normalized transmittance gradually increased intensities at theoffocal point are shownwith in Figure 5. to The transmittance graduallythat the with theMaterials approaching the BPQD sample respect thenormalized focal point (Z = 0), indicating 2017, 10, 210 6 of 11 = 0), is well increased with the approaching of the BPQD sample with respect to the focal point ( Z This absorption of BPQDs becomes saturated with the increase of the incident pump intensity. indicating that the absorption of BPQDs becomes saturated with the increase of the incident pump at the focal point are shown inAs Figure 5. The normalized transmittance gradually known intensities as the saturable absorption behavior. shown in the absorption spectra, there is a strong intensity. This isthe well known as of thethe saturable behavior. shown the absorption = 0), to the increased with approaching BPQD absorption sample with respect As to the focalinpoint (Z absorption band in the near-IR range. The observed saturable absorption activity can be ascribed spectra, there a strong absorption band in the near-IR range. absorption indicating thatisthe absorption of BPQDs becomes saturated withThe the observed increase ofsaturable the incident pump ground-state induced byground-state photo-excitation. Figure 5 shows the power-dependent saturable activitybleaching can be is ascribed to the bleaching induced by photo-excitation. 5 shows intensity. This well known as the saturable absorption behavior. As shown in Figure the absorption absorption. It can be seen that the peaks of the open Z-scan curves increased with the increasing the power-dependent saturable absorption. It can be seen that the peaks of the open Z-scan curves spectra, there is a strong absorption band in the near-IR range. The observed saturable absorption input increased with theresults increasing power intensity. results further confirm that theindeed saturable power intensity. These confirm that theThese saturable absorption responses originate activity can be ascribed tofurther theinput ground-state bleaching induced by photo-excitation. Figure 5 shows absorption responses indeed originate from the intrinsic optical absorption effects in BPQDs otherdamage from thethe intrinsic optical absorption effects in BPQDs than suchZ-scan as sample power-dependent saturable absorption. It can be other seen that thefrom peaksartifacts of the open curves than fromwith artifacts such as input sample damage or contamination. Whenconfirm the excitation intensity increased the increasing power intensity. These results further that the or contamination. When the excitation intensity increased further until it reached thesaturable photo-damage increased further until it reached the photo-damage threshold, the reverse saturable absorption absorption responses indeed originate from the intrinsic optical absorption effects in BPQDs threshold, the reverse saturable absorption effect wastonot observed. These results can other be ascribed effect was not observed. results can be ascribed smaller two-photon absorption than from artifacts suchThese as sample damage or contamination. When the excitationcoefficients intensity to smaller two-photon absorption coefficients of BPQDs at the near-IR spectra range, which are not of BPQDsfurther at the until near-IR spectra the range, which are threshold, not enoughthetoreverse overcome the ground-state increased it reached photo-damage saturable absorption bleaching. These results are consistent with the wavelength dependence of the two-photon enougheffect to overcome the ground-state bleaching. These results are consistent with the wavelength was not observed. These results can be ascribed to smaller two-photon absorption coefficients absorption coefficient in thespectra visible range, spectracoefficient range: are the two-photon absorption coefficient decreased dependence of the two-photon absorption in the visible spectra range: the two-photon of BPQDs at the near-IR which not enough to overcome the ground-state with the increasing wavelength. bleaching. Thesedecreased results arewith consistent with the wavelength. wavelength dependence of the two-photon absorption coefficient the increasing

(a)

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absorption coefficient in the visible spectra range: the two-photon absorption coefficient decreased with(a) the increasing wavelength. 1.12 (b) 1.03 2 2 108 GW/cm 169 GW/cm2 220 GW/cm2 108 GW/cm2 2 1691350 GW/cm nm 220 GW/cm2

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-10 -5 nm 0 (b) under 5 10 15 -15The -10 -5 0 Z-scan 5 results 10 of 15 Figure 5. open aperture BPQDs at 1050-15 nm (a) and 1350 different

Figure 5. The open apertureZZ-scan of BPQDs at 1050 nm (a) and 1350 nm (b) under different Z (mm) (mm) results excitation peak power intensities at the focal point. excitation peak power intensities at the focal point. Figure 5. The open aperture Z-scan results of BPQDs at 1050 nm (a) and 1350 nm (b) under different Similar measurements have also been performed at 1550 nm. Figure 6 shows the Z-scan curve excitation peak power intensities at the focal point.

of BPQDs under the excitation peak power intensity of 38 GW/cm and the corresponding fitting Similar measurements have also been performed at 1550 nm. Figure 6 shows the Z-scan curve of curveSimilar of saturable absorption. Based on the relationatbetween the laser6beam size and the 2 and measurements also intensity been performed 1550 nm. Figure showsspot the Z-scan curve BPQDs under the excitation peakhave power of 38 GW/cm the corresponding fitting curve of relative the nonlinear absorption be corresponding derived (Figure 6b). 2 and the of BPQDsseparation, under the excitation peak saturable power intensity of 38 curve GW/cmcould fitting saturable absorption. Based on the relation between laser beam spot size anddepth the relative separation, 2 and The onset of saturation intensity was around 2.5 the GW/cm the wasand 8.2% at curve of saturable absorption. Based on the relation between the modulation laser beam spot size the the nonlinear saturable absorption curve could be derived (Figure 6b). The onset of saturation intensity 1550 nm. Therefore, the measured results indicate that the as-prepared BPQDs can be used as relative separation, the nonlinear saturable absorption curve could be derived (Figure 6b).a 2 and saturable absorber for ultrafast laser pulse generation in near-IR fiber lasers. was around 2.5 GW/cm the modulation depth was 8.2% at 1550 nm. Therefore, the measured 2 The onset of saturation intensity was around 2.5 GW/cm and the modulation depth was 8.2% at results indicate the as-prepared BPQDs can be used a saturable absorber laser 1550 nm.that Therefore, the measured results indicate thatasthe as-prepared BPQDs for can ultrafast be used as a pulse saturable absorber for ultrafast in near-IR fiber lasers. 1.10 fiber (b) generation in (a) near-IR lasers. laser pulse generation 2

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Ionset=2.5 GW/cm Ionset=2.5 GW/cm 1 10 Intensity (GW/cm2)

10 1 its corresponding 10 saturable Figure 6.-20 The open-10 aperture0Z-scan results of20 BPQDs at 1550 nm (a) and Z (mm) Intensity (GW/cm2) absorption curve (b).

Figure 6. The open aperture Z-scan results of BPQDs at 1550 nm (a) and its corresponding saturable

Figure 6. The open aperture Z-scan results of BPQDs at 1550 nm (a) and its corresponding saturable absorption curve (b). absorption curve (b).

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3. Materials and Methods 3.1. Synthesis of BPQDs NMP was used to isolate the BPQDs from the bulk BP crystal. First, the BP bulk crystal (99.998%, purchased from smart elements) was pulverized to prepare the BP powder. The dispersed suspension of BPQDs was then prepared by ultrasound probe sonication followed by ice bath sonication of bulk BP powder in 1-methyl-2-pyrro-lidone (NMP, 99.5% anhydrous, purchased from Aladdin Reagents). Then 25 mg of the BP powder was dispersed into 25 mL of NMP in a 50 mL sealed conical tube and sonicated with a sonic tip for 3 h at the power of 1200 W. The ultrasonic frequency ranged from 19 to 25 kHz and the ultrasound probe worked 2 s with an interval of 4 s. Afterwards, an ultrasonic bath was adopted to sonicate the dispersion consecutively for another 10 h at the power of 300 W. During the sonication, the temperature of the sample solution was kept and monitored below 277 K in an ice bath. The dispersion was then centrifuged at the speed of 7000 rpm for 20 min. The supernatant containing BPQDs was collected. To further increase the concentration of the BPQDs dispersion, the collected solution was centrifuged for 20 min at the speed of 12,000 rpm. The resulting supernatant was collected and was kept in a sealed tube for further tests. Transmission electron microscopy (TEM) measurements were conducted to examine the morphology of the as-prepared BPQDs. The TEM images were taken on the Tecnai G2 F20 S-Twin transmission electron microscope at an acceleration voltage of 200 kV. The absorption spectra were taken by a Shimadzu UV3600 spectroscopy with QS-grade quartz cuvette at room temperature. 3.2. Nonlinear Optical Property Measured by Z-Scan Technique The open aperture Z-scan measurements were performed by using an optical parametric amplifier (TOPAS-Prime) pumped by a mode-locked Ti:sapphire oscillator seeded regenerative amplifier (Spectra-Physics Spitfire Ace), which gives output laser pulses with tunable central wavelength from UV to near-IR range, pulse duration of ~120 fs and repetition rate of 1 kHz. The laser beam was focused onto the sample with a beam radius of ~23 µm. Samples were prepared in the quartz cuvette with the optical path length of 1 mm. The transmittance of the samples was measured as a function of input intensity, which was varied by moving the samples in and out of beam focus along the z-axis. At least five cycles were repeated to reduce the experimental error and the average was calculated to plot the Z-scan curves. 3.3. Pump-Probe and Transient Absorption Spectra Mansurement In our experiments, a Ti:sapphire oscillator seeded regenerative amplifier laser system (Spectra Physics Spitfire Ace) with output pulse energy of 2 mJ at 800 nm and a repetition rate of 1 kHz was used as the source of the pump and beam pulses. The detailed experimental setup is described in Figure 7. The 800 nm laser beam was split into two portions. One portion passes through a BBO crystal to generate the 400 nm pump beam by second harmonic generation. The other portion of the 800 nm beam was used to generate white light continuum in a 2.5 mm sapphire plate. The white light beam was split into two portions: one as the probe and another as the reference. The pump beam was focused onto the sample with a beam size with 300 µm in diameter and overlapped with the smaller probe beam (100 µm in diameter). The probe beam is collected after passing through the sample and its intensity is monitored by a photodiode as the detector. Both the detectors of probe and reference beam are connected to the lock-in amplifier to correct the pulse-to-pulse intensity fluctuations. The time delay between the pump and probe pulse was varied by a computer-controlled translation stage. The pump beam was modulated by an optical chopper at the frequency of 500 Hz. In a single-wavelength dynamics scan, a fixed probe wavelength is picked and the transmittance change of probe with pump and without pump (∆T/T) was monitored as a function of the various delay between pump and probe beam. The transient absorption spectra at different delay time were measured by passing the probe through a monochromator before detector.

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Figure setup of of pump-probe pump-probeand andtransient transientabsorption absorption measurements. Figure 7. 7. Experimental Experimental setup measurements.

4.4.Conclusions Conclusions Inconclusion, conclusion, BPQDs BPQDs were fabricated method. The nonlinear In fabricated by byaasimple simplesolvent solventexfoliation exfoliation method. The nonlinear absorptionproperties properties were investigated laser pulses absorption investigated with withZ-scan Z-scanmeasurements measurementsusing usingfemtosecond femtosecond laser pulses 500, 700, 700, 900, 900, 1050, 1350 and exhibited atat 500, and 1550 1550nm. nm. These These ultrasmall ultrasmallBPQS BPQSininNMP NMPsuspension suspension exhibited broadbandoptical-limiting optical-limiting activities activities in the was broadband the visible visiblerange. range.The Thenonlinear nonlinearabsorption absorptioncoefficient coefficient was − 3 − 3 − 3 found to to be and found be(7.49 (7.49±±0.23) 0.23)××10 10−3,, (1.68 (1.68±± 0.078) 0.078)××10 10−3 and and(0.81 (0.81±±0.03) 0.03)××10 10−3 cm/GW cm/GW for for500, 500,700 700 and 900 nm, respectively. Femtosecond pump-probe measurements performed on the BPQD suspension 900 nm, respectively. Femtosecond pump-probe measurements performed on the BPQD suspension furthersupport supportthat thatnonlinear nonlinearabsorption absorptiondue dueto totwo-photon two-photon absorption absorption played played an an important important role role in further in the observed optical-limiting activities of the BPQDs. Furthermore, saturable absorption due to to the observed optical-limiting activities of the BPQDs. Furthermore, saturable absorption due ground-state bleaching was found in BPQDs by photo-excitation at 1050, 1350 and 1550 nm. Our results ground-state bleaching was found in BPQDs by photo-excitation at 1050, 1350 and 1550 nm. Our will provide the basis applications of BPQDs broadband optoelectronic devices such as optical results will provide thefor basis for applications of in BPQDs in broadband optoelectronic devices such as limiters in the visible range or saturable absorbers in the near-IR range. optical limiters in the visible range or saturable absorbers in the near-IR range. Acknowledgments: We thank the financial support from the Guangdong Innovative Research Team Program of Acknowledgments: We thank the financial support from the Guangdong Innovative Team Program China (Grant No. 201101C0105067115), the Young Scientists Fund of the National NaturalResearch Science Foundation of (Grant No. 51603069), the National Natural Science Foundation of China 21673155), the Scientific ofChina China (Grant No. 201101C0105067115), the Young Scientists Fund (Grant of theNo. National Natural Science Research Staring Natural Science of Guangdong Province, ChinaFoundation (Grant No.of2016A030310432), Foundation of China (Grant No. Foundation 51603069), the National Natural Science China (Grant No. the Fundamental Research Funds for the Central Universities and the China Postdoctoral Science Foundation. 21673155), the Scientific Research Staring Natural Science Foundation of Guangdong Province, China (Grant ForFundamental research articles with several a short Universities paragraph specifying their individual Author Contributions:the No. 2016A030310432), Research Funds authors, for the Central and the China Postdoctoral contributions must be provided. The following statements should be used “X.F. Jiang, Z. Zeng, S. Li, F. Huang, Science Foundation. and Q.-H. Xu conceived and designed the experiments; X.F. Jiang and Z. Zeng performed the experiments, analyzed the data as well asresearch wrote thearticles paper; with Z. Guo and H. Zhang contributed the materials; X.F. Jiang, Z. Zeng Author Contributions: For several authors, a short paragraph specifying their individual and Q.-H. Xu wrote the paper. All the authors have contributed substantially to the work reported. contributions must be provided. The following statements should be used “X.F. Jiang, Z. Zeng, S. Li, F. Huang, Conflicts The and authors declarethe no conflict of interest. and Q.-H. of XuInterest: conceived designed experiments; X.F. Jiang and Z. Zeng performed the experiments, analyzed the data as well as wrote the paper; Z. Guo and H. Zhang contributed the materials; X.F. Jiang, Z.References Zeng and Q.-H. Xu wrote the paper. 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