Broadband nonlinear optical response in multilayer black phosphorus: an emerging infrared and mid-infrared optical material S. B. Lu,1 L. L. Miao,2 Z. N. Guo,1 X. Qi,3 C. J. Zhao,1,2 H. Zhang,1,* S. C. Wen,2 D. Y. Tang,4 and D. Y. Fan1 1 SZU-NUS Collaborative Innovation Centre for Optoelectronic Science & Technology, and Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China 2 Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, College of Physics and Microelectronic Science, Hunan University, Changsha 410082, China 3 Laboratory for Quantum Engineering and Micro-Nano Energy Technology and Faculty of Materials and Optoelectronic Physics, Xiangtan University, Hunan 411105, China 4 School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore * [email protected]
Abstract: Black phosphorous (BP), the most thermodynamically stable allotrope of phosphorus, is a high-mobility layered semiconductor with direct band-gap determined by the number of layers from 0.3 eV (bulk) to 2.0 eV (single layer). Therefore, BP is considered as a natural candidate for broadband optical applications, particularly in the infrared (IR) and mid-IR part of the spectrum. The strong light-matter interaction, narrow direct band-gap, and wide range of tunable optical response make BP as a promising nonlinear optical material, particularly with great potentials for infrared and mid-infrared opto-electronics. Herein, we experimentally verified its broadband and enhanced saturable absorption of multi-layer BP (with a thickness of ~10 nm) by wide-band Z-scan measurement technique, and anticipated that multi-layer BPs could be developed as another new type of two-dimensional saturable absorber with operation bandwidth ranging from the visible (400 nm) towards mid-IR (at least 1930 nm). Our results might suggest that ultra-thin multi-layer BP films could be potentially developed as broadband ultra-fast photonics devices, such as passive Q-switcher, mode-locker, optical switcher etc. ©2015 Optical Society of America OCIS codes: (160.4330) Nonlinear optical materials; (160.4236) Nanomaterials; (190.7110) Ultrafast nonlinear optics.
References and Links 1. 2. 3. 4. 5. 6. 7. 8. 9.
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1. Introduction The emergence of graphene and graphene-like two-dimensional (2D) materials have highlighted a new family of nano-materials with outstanding physical and chemical properties, and then refreshed our traditional viewpoints on nanotechnology, and even opened up a new play-ground with unprecedented chances for testing and realizing conceptually new electronic and optoelectronic devices [1,2]. Current research in 2D materials primarily focuses on graphene, the insulating hexagonal boron nitride (hBN), the topological insulator (Bi2Te3, Bi2Se3 etc.), and members from the wide-bandgap transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS2) and molybdenum diselenide (MoSe2) [3–6]. Each type of 2D materials possesses some intrinsic advantages for particular applications. However, graphene suffers from intrinsic limitations or dis-advantages for electronic and photonic applications because of its zero-bandgap nature, delimiting its on-off ratio in electronic and photonics devices. Although various solutions such as graphene hybrid with metallic nano-particles [7], graphene plasmons [8] and micro-cavities [9,10] have been applied in order to open the band gap and boost light-matter interaction in graphene, some other unexpected limitations (such as reduced operation bandwidth, lower response speed, larger footprint or weaker figure of merit) were also accompanied together with these treatments, making them less attractive for specific applications. Recently, Black phosphorous (BP), the most thermodynamically stable allotrope of phosphorus, has recently joined in the family of 2D materials [11]. The same with another famous layered-material MoS2, which was extensively investigated in nonlinear optical response [12–15] and applied in ultra-fast lasers for saturable absorber [16–21], BP is a highmobility layered semiconductor with band-gap sensitively dependent on the number of layers from 0.3 eV (bulk) to 2.0 eV (single layer) [22,23]. However, different from the MoS2, which owns indirect band-gap at multilayer format, BP has the direct transition for all thickness, which shows much more absorption and should be more suitable for optoelectronic applications particularly at the long wavelength range where strong motivation on optical communications and military purposes provoke. The high electronic mobility has already been confirmed in few-layer BPs and applied to the applications of field-effect transistors (FET) [24–26]. Due to its narrow band-gap and anisotropic feature, BP FET was experimentally verified to exhibit broadband and polarized photocurrent response [27–29]. Few-layer BP is also a p-type semiconductor, by combining with few layer MoS2 sheet, it can be developed as solar cell [30] and p-n diode [31]. Besides the dependence on the thickness, the researches suggest that the strain can also modulate the band-gap of the BP [32–34]. The #234572 - $15.00 USD © 2015 OSA
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band-gap of BP sits between that of graphene and TMDs semiconductor; therefore, BP is considered as a natural candidate that might fill the “gap” between the semi-metallic graphene and wide band-gap MoS2, making it capable for broadband optical applications, particularly at the infrared and mid-infrared (IR) part of the spectrum where other 2D layer materials are difficult to fit. In principle, the inter-band optical absorption in direct-band semiconducting BP could become readily saturated under strong illumination due to a combined consequence of the valence band depletion, conduction band filling and ultra-fast intra-band carrier thermalization. The strong light-matter interaction, narrow direct band-gap, and wide range of tunable optical response make BP as a promising nonlinear optical material, particularly with great potentials for IR and mid-IR optoelectronics. Given that bulk BP will show quite low optical transmittance with strong absorption and scattering loss, in order to both characterize the nonlinear optical response of BP and completely take advantage of its nonlinear optical property of BP for optics applications, we need to chemically exfoliate bulk BP in order to get thinner BP flakes so that efficient light-matter interaction could occur inside BPs with low transmission loss. Very recently, Coleman et al. have prepared the few-layer BP by liquid exfoliation method and its saturable absorption was verified at the near infrared band [35]. In this contribution, we firstly experimentally verified the ultrafast nonlinear optical response of multi-layer BP nanoplatelets (NPs) by Z-scan measurement technique. Based on the liquid exfoliation method, a series of dispersions with BP nanoplatelets were prepared in isopropyl alcohol (IPA), N-Methylpyrrolidone solvents (NMP) and ethyl alcohol (EA), respectively. The BP NPs exhibit significant saturable absorption (SA) under femto-second excitation at both 400 nm and 800 nm band. Besides, the SA behavior had also been observed under the excitation of picosecond pulse at 1562 nm and 1930 nm band. The results suggest that multi-layer BPs might be further developed as a novel two-dimensional saturable absorber for the ultra-short pulse generation and that ultra-thin BP films are potentially useful as broadband optical elements in fiber lasers and other BP-related ultra-fast photonics devices, which will be our future target. 2. Synthesis and characterization of BP nanoplatelets The liquid phase exfoliation (LPE) is widely used as a simple and effective technique to prepare 2D nano-material from the layered bulk crystals towards the multiple layered structures, including graphene, topological insulator and MoS2 [36,37]. The solvent with proper surface energy can provide stable dispersions of layered materials. Here, three different sets of organic solvents (IPA, NMP, and EA) were respectively chosen to disperse the as-fabricated multi-layer BP-flakes. The entire preparation procedure is systematically shown in Fig. 1. Firstly, the BP powder was prepared from the BP bulk crystal (99.998%, purchased from smart elements) by grinding. Then the powder was dispersed into IPA for 1 mg/mL and ultra-sonicated for 2 hours. In the following, the dispersions were settled for more than 24 hours in order to remove the large size sedimentations with a centrifugation at a speed of 1500 rpm for 20 minutes. The top one third of the dispersions was collected and put in 1 mm cuvette as the sample for experiment testing.
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Fig. 1. The preparation process of BP nanoplatelets.
Figure 2 shows the characterization of the BP samples. The SEM image of BP crystal is shown in Fig. 2(a), from which we can clearly see the typical smooth and flat layered surface of bulk BPs. Figure 2(b) shows the typical XRD pattern of the as-prepared BP powder. Several distinct diffraction peaks at 16.9°, 26.4°, 34.5°, 40.0° and 52.4° are observed, corresponding to the (020), (021), (040), (041) and (060) crystal planes of orthorhombic black phosphorus (JCPDS File no. 73-1358). In order to further investigate the morphologies of the as-prepared BP nanoplatelets, the characterization on the TEM and high-resolution TEM (HRTEM) images of the BP NPs were also performed. BP NPs were experimentally observed to show platelet-like structure with an average size of several-hundred nanometers, as shown in Fig. 2(c). HRTEM image with clear lattice fringes and its corresponding fast Fourier transformation (FFT) with expected hexagonal spot pattern confirm the high quality of prepared BP NPs, as shown in Fig. 2(d).
Fig. 2. (a) SEM image of BP crystal; (b) XRD pattern of BP powder; (c) TEM and (d) Highresolution TEM images of as-prepared BP nanoplatelets, the inset image is the FFT of the image d.
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To determine the size and thickness of the BP NPs, the atomic force microscopy (AFM) measurement was also conducted. The sample was prepared by drop-casting the BP dispersion on the surface of the quartz substrate and then drying in vacuum drying oven. Figure 3(a) shows the 2D topographical images of the as-prepared BP NPs sample, many BP NPs could be clearly observed. For precisely analyzing the thickness of BP NPs, four different sections were chosen for the measurements, as marked in Fig. 3(a). Figure 3(b) shows the height difference between the substrate and target sections. The thicknesses of the section A to section D are measured to be about 7.6 nm, 13.9 nm, 4.9 nm and 6.8 nm, respectively. We can find that the thickness of separated and smaller section is smaller than that of the agminated and larger size one, which should due to the unavoidable aggregation in the process of sample preparation. Figures 3(c) and 3(d) illustrate the histograms of thickness and widths of the deposited BP NPs, respectively. More than 51% of the nanoplatelets out of the 52 objects have an averaged thickness between 15 nm and 20 nm. As discussed in Fig. 3(b), aggregation may largely affect the results of the thickness measurements. We believed the essential thickness of the samples might be much smaller than the value in the histogram. From the value of the separated sample, we estimated that the thickness of BP NPs in the solvents is at the range of 5~10 nm. Given that single layer BP has a thickness of ~0.6 nm [38], the thickness of prepared sample is estimated to be more than ten layers. More than ~62% of the widths of BP NPs sample are between the 150 nm and 350 nm, in good agreement with the results of the above TEM images.
Fig. 3. (a) 2D topographical AFM image of BP NPs; (b) Height Profiles of the sections marked in (a); (c) and (d) are the histograms of thickness and width of the BP NPs, respectively.
Raman measurement of BP powder on silicon wafer was taken by a Horiba Jobin-Yvon LabRam HR VIS high-resolution confocal Raman microscope equipped with a 633 nm laser. There are three peaks can be seen in the Raman result, which are three characteristic peaks corresponding to one out-of-plane vibration mode A1g and two in-plane vibration mode B2 g
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and Ag2 . As shown in Fig. 4(a), active modes of multilayer BP observed at 361.7, 438.4 and 466.4 cm–1, respectively [39,40]. Similar to MoS2, Raman spectra is dependent with the thickness [38,41]. When the thickness is increased, the A1g and Ag2 mode will shift toward each other. Compared with the Raman image of the Bulk BP, we find the wavenumber between A1g and Ag2 mode become larger in prepared multilayer BP NPs. It confirms that we successfully decrease the thickness of the BP. The linear absorption spectrum of the BP NPs dispersions was measured with a spectrophotometer and shown in Fig. 4(b). It clearly shows that BP NPs has a smooth absorption curve in the UV-Vis wavelength band, suggesting that multi-layer BP NPs might be a promising broadband optical material.
Fig. 4. (a) Raman spectrum of the multi-layer BP NPs; (b) The linear absorption spectrum of the multi-layer BP NPs.
3. Nonlinear optics results and discussion 3.1. Saturable absorption of BP at 400 nm and 800 nm band In order to investigate the ultrafast nonlinear optical response of BP nanoplatelets, Z-scan technique was used in the experiments [42]. The experimental setup, which had been well described in our previous work [43], was calibrated with CS2 solvent by using a Coherent femto-second lase (center wavelength: 800 nm, pulse duration: 100 fs, 3 dB spectral width:15 nm and repetition rate: 1 kHz). According to the CS2 measurement, the incident beam waist was fitted to be about 30 μm. The open aperture Z-scan measurements of BP samples at 400 nm and 800 nm are shown in Figs. 5(a) and 5(b), respectively. The normalized transmittance gradually increases with the approaching of the BP sample with respect to the focus point (Z = 0), indicating that the absorption of multi-layer BP NPs becomes saturated with the increase of the incident pump intensity. This process is also well-known as the typical saturable absorption (SA) behavior. We also performed the power dependent Z-scan measurements, which clearly show that with the increase of the input peak intensity from 175 GW/cm2 to 515 GW/cm2, the peaks of the open Z-scan curves correspondingly increased. It further confirms that the SA response indeed originates from the intrinsic optical absorption effect in multilayer BPs other than artifices, such as sample damage. By taking the advantage of the saturable absorption property, BP NPs could be developed as passive mode-locker or Qswitcher device for ultra-short pulsed lasers in the NIR region.
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Received 13 Feb 2015; revised 11 Apr 2015; accepted 11 Apr 2015; published 21 Apr 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.011183 | OPTICS EXPRESS 11189
Fig. 5. (a) and (b) are the open aperture Z-scan measurements of BP NPs dispersions under different intensities at 400 nm and 800 nm, respectively; (c) Relation between normalized transmittance and input intensity for BP NPs dispersions at 800 nm; (d) The open aperture Zscan measurements of BP NPs dispersions in IPA, NMP and EA at intensities of 515 GW/cm2.
Black phosphorus is a semiconductor with direct band-gap determined by the number of layers. The bandgap of bulk BP is 0.3 eV at the Z-point while that of monolayer is 2.0 eV at the Γ-point of the first Brillouin zone. Given that multi-layer BP possesses similar electronic band property as bulk BP, its band-gap is presumably to be 0.3 eV. Therefore, under the excitation of 400 nm (3.1 eV) and 800 nm (1.55 eV) femto-second laser, one photon is enough to excite an electron from valence band to conduct band in multi-layer BPs. Differently, concerning the optical absorption in monolayer BP, the excitation requires the participation of two photons simultaneously, as illustrated in the inset of Fig. 5(c). Therefore, in theory, monolayer BP will show significant revered saturable absorption effect induced by the two-photon absorption (TPA). On the contrary to SA, TPA effect will cause the decrease of transmittance with increasing the input intensity. On the contrary, due to the existence of smaller direct band-gap in multi-layer BPs, saturable absorption instead of two-photon absorption (TPA) occurs if under high intensity excitation. Hence, the SA curve in the Fig. 5(c) may origin from the multilayer BP in the dispersions. In the multi-layer BP NPs dispersions, the absorption coefficients α(I) consists of two parts: α(I) = α0 + αNL(I)I, where α0 is the linear absorption coefficient and αNL is the nonlinear absorption coefficient. The nonlinear absorption αNL mainly origins from the BP NPs, because there is no nonlinear effect found in the dispersant solvents. According to the nonlinear theory [42], we fitted the Z-scan curves in Figs. 5(a) and 5(b) with the following approximate equation ∞
(−α NL I 0 Leff ) m
α NL I 0 Leff
1 (1) 2 2 2 2 (1 + z / z 0 ) m = 0 (1 + z / z ) (m + 1) Here, T(z) is the normalized transmittance, I0 is peak on-axis intensity at focus, z is the position of sample with respect to the focal position, z0 is the Rayleigh range, Leff = (1–e– α0L )/α0 is the effective length, L is the length of the sample. For example of SA curves at 800 nm band, the linear absorption of the BP NPs in IPA is about 85.6%, and the corresponding T (z) =
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2
2 m 0
3/ 2
≈ 1−
Received 13 Feb 2015; revised 11 Apr 2015; accepted 11 Apr 2015; published 21 Apr 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.011183 | OPTICS EXPRESS 11190
linear absorption coefficient α0≈1.6 cm–1. We fitted the average value of αNL which is found to be about –(6.17 ± 0.38) × 10−3 cm/GW at different peak intensities. In Fig. 5 (c), we fitted the data with the most common SA model for one photon absorption. The transmittance T has a relation with the input intensity I as [44] T = 1−
αs 1 + I / Is
− α ns
(2)
where αs is the saturable absorption component, also termed as the modulation depth, αns is the non-saturable absorption components, Is is the saturable intensity, defined as the optical intensity when the optical absorbance is reduced to half of its unbleached value. The experimental data matches quite well with the Eq. (2). By fitting the experimental data, the saturable intensity, the modulation depth, and the non-saturable absorption are found to be 334.6 ± 43 GW/cm2, 12.4%, and 1.9%, respectively. LPE method is selective to the dispersion liquid, and different solvent has different dispersibility and behave different nonlinear property [45]. Therefore, we investigated the nonlinear optical absorption of multi-layer BP NPs dispersed in three different solvents (IPA, NMP and EA) at 800 nm, as shown in Fig. 5(d). The obvious SA response was observed in all three dispersions. At the same incident intensity of 515 GW/cm2, IPA and NMP dispersions showed better SA response than that in EA dispersions. All the fitted linear and nonlinear parameters of BP NPs dispersions are summarized in Table 1. Because different solvents have different dispersibility, they exhibit different linear absorbance and concentrations. The BPIPA dispersions show larger nonlinear coefficient than other two dispersions. Owe to high absorption, the BP-NMP dispersions have the largest saturable intensity and modulation depth. Because of low concentration, the BP-EA dispersions show highest transmittance and lowest nonlinearity coefficient. All of three dispersions have similar non-saturable component. By comparing all three experimental results, we conclude that IPA and NMP are ideal dispersion liquid for BP. Table 1. Linear and Nonlinear Optical Parameters of the BP dispersions αNL (cm/GW, × 10−3)
Is (GW/cm2)
αs (%)
αns (%)
–(16.2 ± 2.8)
455.3 ± 55
27.6
2.1
1.55
–(6.17 ± 0.19)
334.6 ± 43
12.4
1.9
83.1
1.85
–(4.08 ± 0.11)
647.7 ± 60
13.3
2.1
89.3
1.13
–(2.87 ± 0.18)
261.3 ± 36
7.4
1.9
Material
λ (nm)
T (%)
α0 (cm−1)
BP/IPA
400
70.5
3.49
BP/IPA
800
85.6
BP/NMP
800
BP/EA
800
3.2. Saturable absorption of BP at near- and mid-infrared band We also investigate the nonlinear absorption of BP NPs at 1563 nm and 1930 nm band. Because IPA solvent has obvious nonlinear absorption at these band, we mixed the BP NPs with polymethyl-methacrylate (PMMA) and spin-coated the sample on the 1 mm quartz substrate. After putting in the drying oven for 8 hours, the BP-PMMA sheet sample was fabricated. Under the excitation of 1563 nm fiber laser (pulse duration: 1.5 ps, repetition: 20.8 MHz), the BP-PMMA sample exhibits strong SA behavior, as shown in Figs. 6(a) and 6(b). Due to PMMA protection, the BP NPs could be prevented from high incident intensity and chemical reaction with water vapor and oxygen in the air. To make sure the BP-PMMA is not been damaged by the thermal effect in the experiments, we have taken multiple measurements to check and we got the similar results. The Z-scan measurement of pure PMMA sheet show non-nonlinear absorption, which tells that the BP-PMMA should be undamaged and the nonlinear absorption comes from the BPs sample. Fitting the experimental data with Eq. (1), #234572 - $15.00 USD © 2015 OSA
Received 13 Feb 2015; revised 11 Apr 2015; accepted 11 Apr 2015; published 21 Apr 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.011183 | OPTICS EXPRESS 11191
the spot radius of the light at the focus point is inferred to be 35.3 μm. The saturable intensity and the modulation depth of the prepared sample are fitted to be about 18.54 MW/cm2 and 19.5% with Eq. (2), respectively. The ultrafast saturable absorption of BP NPs sample indicates BP could be used as a mode locker and Q-switcher at optical communication band.
Fig. 6. (a) The open aperture Z-scan measurements of BP-PMMA on quartz at peak intensity of 4.1 MW/cm2 and (b) relation between normalized transmittance and intensity at 1563 nm band.
Using a commercial two micron high power fiber laser (AdValue Photonics, AP-ML 1), which has a wavelength of 1930 nm, pulse width of 2.8 ps and repetition of 32.3 MHz, we also observed the saturable absorption behavior of BP-PMMA sample by Z-scan measurement. The measurement results are shown in Fig. 7, from which we can see the typical SA-type Z-scan curves. The fitted beam waist radius is about 103 μm, and the saturable intensity and modulation depth of BP sample are respective 4.56 MW/cm2 and 16.1%. The SA behavior of BP sample at 1930 nm (0.6 eV) band also coincides with bandgap of multilayer BP (bandgap≈0.3 eV) we prepared form sides. From all of these measurements, we can conclude that multilayer BP indeed behaves a broadband saturable absorption.
Fig. 7. (a) The open aperture Z-scan measurement of BP-PMMA on quartz at peak intensity of 0.66 MW/cm2 and (b) relation between normalized transmittance and intensity at 1930 nm band.
3.3. Basic energy-band principle of saturable absorption in BP Following the vein of absorption principle of two-dimensional materials, such as graphene and topological insulators [46,47], excitation processes on linear and nonlinear light absorption of BP could be schematically shown in Fig. 8. Under an incident light with photon energy of E = ћω, electrons from the valence band (red) are excited into the conduction band (yellow), as shown Fig. 8(a). After photo-excitation, hot electrons quickly thermalize to #234572 - $15.00 USD © 2015 OSA
Received 13 Feb 2015; revised 11 Apr 2015; accepted 11 Apr 2015; published 21 Apr 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.011183 | OPTICS EXPRESS 11192
establish a hot Fermi-Dirac distribution. Correspondingly, the originally possible inter-band optical transitions in a range of kBTe (kB is the Boltzmann constant) around the energy of − E / 2 at the valance band could be partially blocked by the newly created electron-hole pairs, and therefore optical absorbance of photons at ћω ~kBTe becomes decreased. In the following, the thermalized carriers further cool down through intra-band scattering. After that, electron-hole recombination then dominates until the relaxation of the equilibrium electron and hole distributions (Fig. 8(b)). However, the above process only accounts for the description of the linear optical transition under relatively weak excitation. If the excitation is sufficiently strong, the population of the photo-generated carriers increases significantly in concentration (much larger than the intrinsic electron and hole carrier densities in BP at room temperature), it can cause the states near half of the photon energy to be filled, blocking further absorption (Fig. 8(c)). Due to this Pauli blocking process, it is impossible to have two identical electrons filling the same state, rendering the occurrence of the bleach of light absorption. The same as MoS2, the band gap (ΔE) of BP can be tuned by reducing the thickness. The band gap of bulk one is about 0.3 eV, while that of monolayer is 2.0 eV. It suggests that BP could possess broadband SA response from visible to mid-infrared band, and it provides a large potential for BP to be applied in ultra-fast photonics. Different from the MoS2, BP can maintain the direct transition of electron at different states of thickness, which could absorb more photons at light excitation and afford larger modulation depth in saturable absorption than MoS2.
Fig. 8. The schematic diagram of the saturable absorption in multi-layer BP NPs.
4. Conclusion In conclusion, multi-layer BPs had been fabricated by using the liquid exfoliation method, and its broadband saturable absorption property from the visible to the mid-IR band had been well characterized by the wide-band open-aperture Z-scan measurement technique. The saturation intensity and normalized modulation depth were experimentally found to be 455.3 ± 55 GW/cm2 and 27.6% at 400-nm, 334.6 ± 43 GW/cm2 and 12.4% at 800-nm if under femtosecond laser excitation, and that of BP-PMMA flakes are 18.54 MW/cm2 and 19.5% at 1563-nm, and 4.56 MW/cm2 and 16.1% at 1930-nm if under pico-second laser excitation, respectively. Our systematic investigation clearly evidence that BP could be a kind of promising nonlinear optical material with broadband operation, particularly at the long wavelength range. In the future, it is anticipated that BP-based IR or mid-IR devices such as
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Received 13 Feb 2015; revised 11 Apr 2015; accepted 11 Apr 2015; published 21 Apr 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.011183 | OPTICS EXPRESS 11193
passive Q-switcher, mode-locker, optical switcher or light modulator might emerge, encouraged by the broadband and strong nonlinear optical response in BP. However, the light matter interaction of BP is far from understanding and there remain plenty of unexplored BP-related properties/applications that deserve our strong attentions. First of all, long term stability is a critical problem that might delimit its large scale applications. Currently, according to our preliminary results, water-stable multi-layer BPs remains challenging. Therefore, researchers need to incorporate multi-layer BPs with other structures, such as polymer, optical wave-guides, in order to protect BPs from oxidization or reactions with water. Secondly, to increase the optical damage threshold of multi-layer BPs is another key issue that we need to address. Thirdly, layer dependent nonlinear optical response in multi-layer BPs might be another interesting research topics since that the direct band-gap of few-layer BPs (from monolayer up to 5-layer) sensitively depends on the layer number. Large size, ultra-thin few-layer BPs rather than multi-layer BPs are strongly called for further optics studies. Unfortunately, the current liquid exfoliation methods cannot fit this requirement. Therefore, further advancement on the fabrication procedure is needed for the optics studies. Lastly, the internal carrier dynamics of multi-layer BPs remains completely unknown, and it might be fundamentally interesting to understand the role of carrier dynamics in the ultra-fast and slow time scale. Overall, there will be plenty of opportunities on the optics studies of BP-related materials, which represents another type of 2D materials with direct and tunable band-gap. Acknowledgment This work is partially supported by the National 973 Program of China (Grant No. 2012CB315701), and the National Natural Science Fund (Grant Nos. 61222505, 61435010 and 61475102).
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Received 13 Feb 2015; revised 11 Apr 2015; accepted 11 Apr 2015; published 21 Apr 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.011183 | OPTICS EXPRESS 11194
Abstract: Black phosphorous (BP), the most thermodynamically stable allotrope of phosphorus, is a high-mobility layered semiconductor with direct band-gap determined by the number of layers from 0.3 eV (bulk) to 2.0 eV (single layer). Therefore, BP is considered as a natural candidate for broadband optical applications, particularly in the infrared (IR) and mid-IR part of the spectrum. The strong light-matter interaction, narrow direct band-gap, and wide range of tunable optical response make BP as a promising nonlinear optical material, particularly with great potentials for infrared and mid-infrared opto-electronics. Herein, we experimentally verified its broadband and enhanced saturable absorption of multi-layer BP (with a thickness of ~10 nm) by wide-band Z-scan measurement technique, and anticipated that multi-layer BPs could be developed as another new type of two-dimensional saturable absorber with operation bandwidth ranging from the visible (400 nm) towards mid-IR (at least 1930 nm). Our results might suggest that ultra-thin multi-layer BP films could be potentially developed as broadband ultra-fast photonics devices, such as passive Q-switcher, mode-locker, optical switcher etc. ©2015 Optical Society of America OCIS codes: (160.4330) Nonlinear optical materials; (160.4236) Nanomaterials; (190.7110) Ultrafast nonlinear optics.
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1. Introduction The emergence of graphene and graphene-like two-dimensional (2D) materials have highlighted a new family of nano-materials with outstanding physical and chemical properties, and then refreshed our traditional viewpoints on nanotechnology, and even opened up a new play-ground with unprecedented chances for testing and realizing conceptually new electronic and optoelectronic devices [1,2]. Current research in 2D materials primarily focuses on graphene, the insulating hexagonal boron nitride (hBN), the topological insulator (Bi2Te3, Bi2Se3 etc.), and members from the wide-bandgap transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS2) and molybdenum diselenide (MoSe2) [3–6]. Each type of 2D materials possesses some intrinsic advantages for particular applications. However, graphene suffers from intrinsic limitations or dis-advantages for electronic and photonic applications because of its zero-bandgap nature, delimiting its on-off ratio in electronic and photonics devices. Although various solutions such as graphene hybrid with metallic nano-particles [7], graphene plasmons [8] and micro-cavities [9,10] have been applied in order to open the band gap and boost light-matter interaction in graphene, some other unexpected limitations (such as reduced operation bandwidth, lower response speed, larger footprint or weaker figure of merit) were also accompanied together with these treatments, making them less attractive for specific applications. Recently, Black phosphorous (BP), the most thermodynamically stable allotrope of phosphorus, has recently joined in the family of 2D materials [11]. The same with another famous layered-material MoS2, which was extensively investigated in nonlinear optical response [12–15] and applied in ultra-fast lasers for saturable absorber [16–21], BP is a highmobility layered semiconductor with band-gap sensitively dependent on the number of layers from 0.3 eV (bulk) to 2.0 eV (single layer) [22,23]. However, different from the MoS2, which owns indirect band-gap at multilayer format, BP has the direct transition for all thickness, which shows much more absorption and should be more suitable for optoelectronic applications particularly at the long wavelength range where strong motivation on optical communications and military purposes provoke. The high electronic mobility has already been confirmed in few-layer BPs and applied to the applications of field-effect transistors (FET) [24–26]. Due to its narrow band-gap and anisotropic feature, BP FET was experimentally verified to exhibit broadband and polarized photocurrent response [27–29]. Few-layer BP is also a p-type semiconductor, by combining with few layer MoS2 sheet, it can be developed as solar cell [30] and p-n diode [31]. Besides the dependence on the thickness, the researches suggest that the strain can also modulate the band-gap of the BP [32–34]. The #234572 - $15.00 USD © 2015 OSA
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band-gap of BP sits between that of graphene and TMDs semiconductor; therefore, BP is considered as a natural candidate that might fill the “gap” between the semi-metallic graphene and wide band-gap MoS2, making it capable for broadband optical applications, particularly at the infrared and mid-infrared (IR) part of the spectrum where other 2D layer materials are difficult to fit. In principle, the inter-band optical absorption in direct-band semiconducting BP could become readily saturated under strong illumination due to a combined consequence of the valence band depletion, conduction band filling and ultra-fast intra-band carrier thermalization. The strong light-matter interaction, narrow direct band-gap, and wide range of tunable optical response make BP as a promising nonlinear optical material, particularly with great potentials for IR and mid-IR optoelectronics. Given that bulk BP will show quite low optical transmittance with strong absorption and scattering loss, in order to both characterize the nonlinear optical response of BP and completely take advantage of its nonlinear optical property of BP for optics applications, we need to chemically exfoliate bulk BP in order to get thinner BP flakes so that efficient light-matter interaction could occur inside BPs with low transmission loss. Very recently, Coleman et al. have prepared the few-layer BP by liquid exfoliation method and its saturable absorption was verified at the near infrared band [35]. In this contribution, we firstly experimentally verified the ultrafast nonlinear optical response of multi-layer BP nanoplatelets (NPs) by Z-scan measurement technique. Based on the liquid exfoliation method, a series of dispersions with BP nanoplatelets were prepared in isopropyl alcohol (IPA), N-Methylpyrrolidone solvents (NMP) and ethyl alcohol (EA), respectively. The BP NPs exhibit significant saturable absorption (SA) under femto-second excitation at both 400 nm and 800 nm band. Besides, the SA behavior had also been observed under the excitation of picosecond pulse at 1562 nm and 1930 nm band. The results suggest that multi-layer BPs might be further developed as a novel two-dimensional saturable absorber for the ultra-short pulse generation and that ultra-thin BP films are potentially useful as broadband optical elements in fiber lasers and other BP-related ultra-fast photonics devices, which will be our future target. 2. Synthesis and characterization of BP nanoplatelets The liquid phase exfoliation (LPE) is widely used as a simple and effective technique to prepare 2D nano-material from the layered bulk crystals towards the multiple layered structures, including graphene, topological insulator and MoS2 [36,37]. The solvent with proper surface energy can provide stable dispersions of layered materials. Here, three different sets of organic solvents (IPA, NMP, and EA) were respectively chosen to disperse the as-fabricated multi-layer BP-flakes. The entire preparation procedure is systematically shown in Fig. 1. Firstly, the BP powder was prepared from the BP bulk crystal (99.998%, purchased from smart elements) by grinding. Then the powder was dispersed into IPA for 1 mg/mL and ultra-sonicated for 2 hours. In the following, the dispersions were settled for more than 24 hours in order to remove the large size sedimentations with a centrifugation at a speed of 1500 rpm for 20 minutes. The top one third of the dispersions was collected and put in 1 mm cuvette as the sample for experiment testing.
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Received 13 Feb 2015; revised 11 Apr 2015; accepted 11 Apr 2015; published 21 Apr 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.011183 | OPTICS EXPRESS 11186
Fig. 1. The preparation process of BP nanoplatelets.
Figure 2 shows the characterization of the BP samples. The SEM image of BP crystal is shown in Fig. 2(a), from which we can clearly see the typical smooth and flat layered surface of bulk BPs. Figure 2(b) shows the typical XRD pattern of the as-prepared BP powder. Several distinct diffraction peaks at 16.9°, 26.4°, 34.5°, 40.0° and 52.4° are observed, corresponding to the (020), (021), (040), (041) and (060) crystal planes of orthorhombic black phosphorus (JCPDS File no. 73-1358). In order to further investigate the morphologies of the as-prepared BP nanoplatelets, the characterization on the TEM and high-resolution TEM (HRTEM) images of the BP NPs were also performed. BP NPs were experimentally observed to show platelet-like structure with an average size of several-hundred nanometers, as shown in Fig. 2(c). HRTEM image with clear lattice fringes and its corresponding fast Fourier transformation (FFT) with expected hexagonal spot pattern confirm the high quality of prepared BP NPs, as shown in Fig. 2(d).
Fig. 2. (a) SEM image of BP crystal; (b) XRD pattern of BP powder; (c) TEM and (d) Highresolution TEM images of as-prepared BP nanoplatelets, the inset image is the FFT of the image d.
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Received 13 Feb 2015; revised 11 Apr 2015; accepted 11 Apr 2015; published 21 Apr 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.011183 | OPTICS EXPRESS 11187
To determine the size and thickness of the BP NPs, the atomic force microscopy (AFM) measurement was also conducted. The sample was prepared by drop-casting the BP dispersion on the surface of the quartz substrate and then drying in vacuum drying oven. Figure 3(a) shows the 2D topographical images of the as-prepared BP NPs sample, many BP NPs could be clearly observed. For precisely analyzing the thickness of BP NPs, four different sections were chosen for the measurements, as marked in Fig. 3(a). Figure 3(b) shows the height difference between the substrate and target sections. The thicknesses of the section A to section D are measured to be about 7.6 nm, 13.9 nm, 4.9 nm and 6.8 nm, respectively. We can find that the thickness of separated and smaller section is smaller than that of the agminated and larger size one, which should due to the unavoidable aggregation in the process of sample preparation. Figures 3(c) and 3(d) illustrate the histograms of thickness and widths of the deposited BP NPs, respectively. More than 51% of the nanoplatelets out of the 52 objects have an averaged thickness between 15 nm and 20 nm. As discussed in Fig. 3(b), aggregation may largely affect the results of the thickness measurements. We believed the essential thickness of the samples might be much smaller than the value in the histogram. From the value of the separated sample, we estimated that the thickness of BP NPs in the solvents is at the range of 5~10 nm. Given that single layer BP has a thickness of ~0.6 nm [38], the thickness of prepared sample is estimated to be more than ten layers. More than ~62% of the widths of BP NPs sample are between the 150 nm and 350 nm, in good agreement with the results of the above TEM images.
Fig. 3. (a) 2D topographical AFM image of BP NPs; (b) Height Profiles of the sections marked in (a); (c) and (d) are the histograms of thickness and width of the BP NPs, respectively.
Raman measurement of BP powder on silicon wafer was taken by a Horiba Jobin-Yvon LabRam HR VIS high-resolution confocal Raman microscope equipped with a 633 nm laser. There are three peaks can be seen in the Raman result, which are three characteristic peaks corresponding to one out-of-plane vibration mode A1g and two in-plane vibration mode B2 g
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Received 13 Feb 2015; revised 11 Apr 2015; accepted 11 Apr 2015; published 21 Apr 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.011183 | OPTICS EXPRESS 11188
and Ag2 . As shown in Fig. 4(a), active modes of multilayer BP observed at 361.7, 438.4 and 466.4 cm–1, respectively [39,40]. Similar to MoS2, Raman spectra is dependent with the thickness [38,41]. When the thickness is increased, the A1g and Ag2 mode will shift toward each other. Compared with the Raman image of the Bulk BP, we find the wavenumber between A1g and Ag2 mode become larger in prepared multilayer BP NPs. It confirms that we successfully decrease the thickness of the BP. The linear absorption spectrum of the BP NPs dispersions was measured with a spectrophotometer and shown in Fig. 4(b). It clearly shows that BP NPs has a smooth absorption curve in the UV-Vis wavelength band, suggesting that multi-layer BP NPs might be a promising broadband optical material.
Fig. 4. (a) Raman spectrum of the multi-layer BP NPs; (b) The linear absorption spectrum of the multi-layer BP NPs.
3. Nonlinear optics results and discussion 3.1. Saturable absorption of BP at 400 nm and 800 nm band In order to investigate the ultrafast nonlinear optical response of BP nanoplatelets, Z-scan technique was used in the experiments [42]. The experimental setup, which had been well described in our previous work [43], was calibrated with CS2 solvent by using a Coherent femto-second lase (center wavelength: 800 nm, pulse duration: 100 fs, 3 dB spectral width:15 nm and repetition rate: 1 kHz). According to the CS2 measurement, the incident beam waist was fitted to be about 30 μm. The open aperture Z-scan measurements of BP samples at 400 nm and 800 nm are shown in Figs. 5(a) and 5(b), respectively. The normalized transmittance gradually increases with the approaching of the BP sample with respect to the focus point (Z = 0), indicating that the absorption of multi-layer BP NPs becomes saturated with the increase of the incident pump intensity. This process is also well-known as the typical saturable absorption (SA) behavior. We also performed the power dependent Z-scan measurements, which clearly show that with the increase of the input peak intensity from 175 GW/cm2 to 515 GW/cm2, the peaks of the open Z-scan curves correspondingly increased. It further confirms that the SA response indeed originates from the intrinsic optical absorption effect in multilayer BPs other than artifices, such as sample damage. By taking the advantage of the saturable absorption property, BP NPs could be developed as passive mode-locker or Qswitcher device for ultra-short pulsed lasers in the NIR region.
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Received 13 Feb 2015; revised 11 Apr 2015; accepted 11 Apr 2015; published 21 Apr 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.011183 | OPTICS EXPRESS 11189
Fig. 5. (a) and (b) are the open aperture Z-scan measurements of BP NPs dispersions under different intensities at 400 nm and 800 nm, respectively; (c) Relation between normalized transmittance and input intensity for BP NPs dispersions at 800 nm; (d) The open aperture Zscan measurements of BP NPs dispersions in IPA, NMP and EA at intensities of 515 GW/cm2.
Black phosphorus is a semiconductor with direct band-gap determined by the number of layers. The bandgap of bulk BP is 0.3 eV at the Z-point while that of monolayer is 2.0 eV at the Γ-point of the first Brillouin zone. Given that multi-layer BP possesses similar electronic band property as bulk BP, its band-gap is presumably to be 0.3 eV. Therefore, under the excitation of 400 nm (3.1 eV) and 800 nm (1.55 eV) femto-second laser, one photon is enough to excite an electron from valence band to conduct band in multi-layer BPs. Differently, concerning the optical absorption in monolayer BP, the excitation requires the participation of two photons simultaneously, as illustrated in the inset of Fig. 5(c). Therefore, in theory, monolayer BP will show significant revered saturable absorption effect induced by the two-photon absorption (TPA). On the contrary to SA, TPA effect will cause the decrease of transmittance with increasing the input intensity. On the contrary, due to the existence of smaller direct band-gap in multi-layer BPs, saturable absorption instead of two-photon absorption (TPA) occurs if under high intensity excitation. Hence, the SA curve in the Fig. 5(c) may origin from the multilayer BP in the dispersions. In the multi-layer BP NPs dispersions, the absorption coefficients α(I) consists of two parts: α(I) = α0 + αNL(I)I, where α0 is the linear absorption coefficient and αNL is the nonlinear absorption coefficient. The nonlinear absorption αNL mainly origins from the BP NPs, because there is no nonlinear effect found in the dispersant solvents. According to the nonlinear theory [42], we fitted the Z-scan curves in Figs. 5(a) and 5(b) with the following approximate equation ∞
(−α NL I 0 Leff ) m
α NL I 0 Leff
1 (1) 2 2 2 2 (1 + z / z 0 ) m = 0 (1 + z / z ) (m + 1) Here, T(z) is the normalized transmittance, I0 is peak on-axis intensity at focus, z is the position of sample with respect to the focal position, z0 is the Rayleigh range, Leff = (1–e– α0L )/α0 is the effective length, L is the length of the sample. For example of SA curves at 800 nm band, the linear absorption of the BP NPs in IPA is about 85.6%, and the corresponding T (z) =
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2
2 m 0
3/ 2
≈ 1−
Received 13 Feb 2015; revised 11 Apr 2015; accepted 11 Apr 2015; published 21 Apr 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.011183 | OPTICS EXPRESS 11190
linear absorption coefficient α0≈1.6 cm–1. We fitted the average value of αNL which is found to be about –(6.17 ± 0.38) × 10−3 cm/GW at different peak intensities. In Fig. 5 (c), we fitted the data with the most common SA model for one photon absorption. The transmittance T has a relation with the input intensity I as [44] T = 1−
αs 1 + I / Is
− α ns
(2)
where αs is the saturable absorption component, also termed as the modulation depth, αns is the non-saturable absorption components, Is is the saturable intensity, defined as the optical intensity when the optical absorbance is reduced to half of its unbleached value. The experimental data matches quite well with the Eq. (2). By fitting the experimental data, the saturable intensity, the modulation depth, and the non-saturable absorption are found to be 334.6 ± 43 GW/cm2, 12.4%, and 1.9%, respectively. LPE method is selective to the dispersion liquid, and different solvent has different dispersibility and behave different nonlinear property [45]. Therefore, we investigated the nonlinear optical absorption of multi-layer BP NPs dispersed in three different solvents (IPA, NMP and EA) at 800 nm, as shown in Fig. 5(d). The obvious SA response was observed in all three dispersions. At the same incident intensity of 515 GW/cm2, IPA and NMP dispersions showed better SA response than that in EA dispersions. All the fitted linear and nonlinear parameters of BP NPs dispersions are summarized in Table 1. Because different solvents have different dispersibility, they exhibit different linear absorbance and concentrations. The BPIPA dispersions show larger nonlinear coefficient than other two dispersions. Owe to high absorption, the BP-NMP dispersions have the largest saturable intensity and modulation depth. Because of low concentration, the BP-EA dispersions show highest transmittance and lowest nonlinearity coefficient. All of three dispersions have similar non-saturable component. By comparing all three experimental results, we conclude that IPA and NMP are ideal dispersion liquid for BP. Table 1. Linear and Nonlinear Optical Parameters of the BP dispersions αNL (cm/GW, × 10−3)
Is (GW/cm2)
αs (%)
αns (%)
–(16.2 ± 2.8)
455.3 ± 55
27.6
2.1
1.55
–(6.17 ± 0.19)
334.6 ± 43
12.4
1.9
83.1
1.85
–(4.08 ± 0.11)
647.7 ± 60
13.3
2.1
89.3
1.13
–(2.87 ± 0.18)
261.3 ± 36
7.4
1.9
Material
λ (nm)
T (%)
α0 (cm−1)
BP/IPA
400
70.5
3.49
BP/IPA
800
85.6
BP/NMP
800
BP/EA
800
3.2. Saturable absorption of BP at near- and mid-infrared band We also investigate the nonlinear absorption of BP NPs at 1563 nm and 1930 nm band. Because IPA solvent has obvious nonlinear absorption at these band, we mixed the BP NPs with polymethyl-methacrylate (PMMA) and spin-coated the sample on the 1 mm quartz substrate. After putting in the drying oven for 8 hours, the BP-PMMA sheet sample was fabricated. Under the excitation of 1563 nm fiber laser (pulse duration: 1.5 ps, repetition: 20.8 MHz), the BP-PMMA sample exhibits strong SA behavior, as shown in Figs. 6(a) and 6(b). Due to PMMA protection, the BP NPs could be prevented from high incident intensity and chemical reaction with water vapor and oxygen in the air. To make sure the BP-PMMA is not been damaged by the thermal effect in the experiments, we have taken multiple measurements to check and we got the similar results. The Z-scan measurement of pure PMMA sheet show non-nonlinear absorption, which tells that the BP-PMMA should be undamaged and the nonlinear absorption comes from the BPs sample. Fitting the experimental data with Eq. (1), #234572 - $15.00 USD © 2015 OSA
Received 13 Feb 2015; revised 11 Apr 2015; accepted 11 Apr 2015; published 21 Apr 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.011183 | OPTICS EXPRESS 11191
the spot radius of the light at the focus point is inferred to be 35.3 μm. The saturable intensity and the modulation depth of the prepared sample are fitted to be about 18.54 MW/cm2 and 19.5% with Eq. (2), respectively. The ultrafast saturable absorption of BP NPs sample indicates BP could be used as a mode locker and Q-switcher at optical communication band.
Fig. 6. (a) The open aperture Z-scan measurements of BP-PMMA on quartz at peak intensity of 4.1 MW/cm2 and (b) relation between normalized transmittance and intensity at 1563 nm band.
Using a commercial two micron high power fiber laser (AdValue Photonics, AP-ML 1), which has a wavelength of 1930 nm, pulse width of 2.8 ps and repetition of 32.3 MHz, we also observed the saturable absorption behavior of BP-PMMA sample by Z-scan measurement. The measurement results are shown in Fig. 7, from which we can see the typical SA-type Z-scan curves. The fitted beam waist radius is about 103 μm, and the saturable intensity and modulation depth of BP sample are respective 4.56 MW/cm2 and 16.1%. The SA behavior of BP sample at 1930 nm (0.6 eV) band also coincides with bandgap of multilayer BP (bandgap≈0.3 eV) we prepared form sides. From all of these measurements, we can conclude that multilayer BP indeed behaves a broadband saturable absorption.
Fig. 7. (a) The open aperture Z-scan measurement of BP-PMMA on quartz at peak intensity of 0.66 MW/cm2 and (b) relation between normalized transmittance and intensity at 1930 nm band.
3.3. Basic energy-band principle of saturable absorption in BP Following the vein of absorption principle of two-dimensional materials, such as graphene and topological insulators [46,47], excitation processes on linear and nonlinear light absorption of BP could be schematically shown in Fig. 8. Under an incident light with photon energy of E = ћω, electrons from the valence band (red) are excited into the conduction band (yellow), as shown Fig. 8(a). After photo-excitation, hot electrons quickly thermalize to #234572 - $15.00 USD © 2015 OSA
Received 13 Feb 2015; revised 11 Apr 2015; accepted 11 Apr 2015; published 21 Apr 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.011183 | OPTICS EXPRESS 11192
establish a hot Fermi-Dirac distribution. Correspondingly, the originally possible inter-band optical transitions in a range of kBTe (kB is the Boltzmann constant) around the energy of − E / 2 at the valance band could be partially blocked by the newly created electron-hole pairs, and therefore optical absorbance of photons at ћω ~kBTe becomes decreased. In the following, the thermalized carriers further cool down through intra-band scattering. After that, electron-hole recombination then dominates until the relaxation of the equilibrium electron and hole distributions (Fig. 8(b)). However, the above process only accounts for the description of the linear optical transition under relatively weak excitation. If the excitation is sufficiently strong, the population of the photo-generated carriers increases significantly in concentration (much larger than the intrinsic electron and hole carrier densities in BP at room temperature), it can cause the states near half of the photon energy to be filled, blocking further absorption (Fig. 8(c)). Due to this Pauli blocking process, it is impossible to have two identical electrons filling the same state, rendering the occurrence of the bleach of light absorption. The same as MoS2, the band gap (ΔE) of BP can be tuned by reducing the thickness. The band gap of bulk one is about 0.3 eV, while that of monolayer is 2.0 eV. It suggests that BP could possess broadband SA response from visible to mid-infrared band, and it provides a large potential for BP to be applied in ultra-fast photonics. Different from the MoS2, BP can maintain the direct transition of electron at different states of thickness, which could absorb more photons at light excitation and afford larger modulation depth in saturable absorption than MoS2.
Fig. 8. The schematic diagram of the saturable absorption in multi-layer BP NPs.
4. Conclusion In conclusion, multi-layer BPs had been fabricated by using the liquid exfoliation method, and its broadband saturable absorption property from the visible to the mid-IR band had been well characterized by the wide-band open-aperture Z-scan measurement technique. The saturation intensity and normalized modulation depth were experimentally found to be 455.3 ± 55 GW/cm2 and 27.6% at 400-nm, 334.6 ± 43 GW/cm2 and 12.4% at 800-nm if under femtosecond laser excitation, and that of BP-PMMA flakes are 18.54 MW/cm2 and 19.5% at 1563-nm, and 4.56 MW/cm2 and 16.1% at 1930-nm if under pico-second laser excitation, respectively. Our systematic investigation clearly evidence that BP could be a kind of promising nonlinear optical material with broadband operation, particularly at the long wavelength range. In the future, it is anticipated that BP-based IR or mid-IR devices such as
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Received 13 Feb 2015; revised 11 Apr 2015; accepted 11 Apr 2015; published 21 Apr 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.011183 | OPTICS EXPRESS 11193
passive Q-switcher, mode-locker, optical switcher or light modulator might emerge, encouraged by the broadband and strong nonlinear optical response in BP. However, the light matter interaction of BP is far from understanding and there remain plenty of unexplored BP-related properties/applications that deserve our strong attentions. First of all, long term stability is a critical problem that might delimit its large scale applications. Currently, according to our preliminary results, water-stable multi-layer BPs remains challenging. Therefore, researchers need to incorporate multi-layer BPs with other structures, such as polymer, optical wave-guides, in order to protect BPs from oxidization or reactions with water. Secondly, to increase the optical damage threshold of multi-layer BPs is another key issue that we need to address. Thirdly, layer dependent nonlinear optical response in multi-layer BPs might be another interesting research topics since that the direct band-gap of few-layer BPs (from monolayer up to 5-layer) sensitively depends on the layer number. Large size, ultra-thin few-layer BPs rather than multi-layer BPs are strongly called for further optics studies. Unfortunately, the current liquid exfoliation methods cannot fit this requirement. Therefore, further advancement on the fabrication procedure is needed for the optics studies. Lastly, the internal carrier dynamics of multi-layer BPs remains completely unknown, and it might be fundamentally interesting to understand the role of carrier dynamics in the ultra-fast and slow time scale. Overall, there will be plenty of opportunities on the optics studies of BP-related materials, which represents another type of 2D materials with direct and tunable band-gap. Acknowledgment This work is partially supported by the National 973 Program of China (Grant No. 2012CB315701), and the National Natural Science Fund (Grant Nos. 61222505, 61435010 and 61475102).
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Received 13 Feb 2015; revised 11 Apr 2015; accepted 11 Apr 2015; published 21 Apr 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.011183 | OPTICS EXPRESS 11194
