Nonlinear Absorption Induced Transparency and Optical Limiting of


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Nonlinear Absorption Induced Transparency and Optical Limiting of Black Phosphorus Nanosheets Jiawei Huang, Ningning Dong, Saifeng Zhang, Zhenyu Sun, Wanhong Zhang, and Jun Wang ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00598 • Publication Date (Web): 27 Jul 2017 Downloaded from http://pubs.acs.org on July 28, 2017

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Nonlinear Absorption Induced Transparency and Optical Limiting of Black Phosphorus Nanosheets Jiawei Huang, †,‡ Ningning Dong, † Saifeng Zhang, † Zhenyu Sun, §,* Wanhong Zhang, ǁ and Jun Wang†,‡,#* †

Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China. ‡ §

University of Chinese Academy of Sciences, Beijing 100049, China.

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China ǁ

Shaanxi Institute of Electronic Technology, Xi’an 710032, China

#

State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China

*Corresponding authors: E-mail: [email protected] and [email protected] Keywords: nonlinear absorption, optically induced transparency, optical limiting, nonlinear scattering, black phosphorus Abstract This work reports the wavelength and pulse duration dependent nonlinear optical properties of exfoliated black phosphorus (BP). We found that BP nanosheets exhibit better saturable absorption response in the visible range than that in near-infrared range, and better response under 6 ns pulse excitation than that under 340 fs excitation. In addition, we propose that the optically induced transparency and optical limiting properties of BP dispersions mainly originate from the competition between saturable absorption and nonlinear scattering. Intriguing optical properties can emerge in diverse two-dimensional (2D) nanomaterials when thinning down to their layer limits. Graphene, as a pioneer, has opened up a new research field for 2D materials since it was firstly isolated through micromechanical exfoliation in 2004.1-3 In comparison to gapless graphene, black phosphorus (BP) has an intrinsic direct bandgap which depends on its layer thickness from ~ 0.3 eV for the bulk to ~ 1.5 eV for monolayer.4-6 Few-layered BP exhibits unique properties, such as high charge-carrier mobility, large current on/off ratios, strong anisotropy in heat or electric conducting and a broadband nonlinear optical (NLO) response,7, 8 making it have potential applications in field-effect transistors, optical limiters, mode-locked and Q-switched lasers, and optical modulators.9, 10 Recently, optical modulators which can control absorption and/or refraction of 2D materials through another light beam have drawn heightened attention due to the increasing demand of advanced photonic and optoelectronic devices. However, the optical modulation behavior of BP have been rarely reported to date. To this end, the first thing is to investigate the NLO properties of BP and understand the nonlinear mechanisms clearly. In this work, BP nanosheet dispersions in ethanol and N-methyl pyrrolidone (NMP) were prepared by using a liquid-phase exfoliation technique. Techniques of scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), optical absorption spectroscopy and Raman spectroscopy were performed to characterize the structure

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and quality of the obtained BP nanosheets. Saturable absorption (SA) was observed under the excitation of both fs pulses at 1030/515 nm and ns pulses at 1064/532 nm through a typical open-aperture Z-scan setup. Subsequently, fast and slow SA models were employed to estimate the excited-state absorption (ESA) and the ground-state absorption (GSA) cross section of BP which could disclose why BP nanosheets have better NLO response, namely, larger nonlinear absorption coefficient αNL, imaginary part of the third order nonlinear optical susceptibility Imχ(3), and figure of merit (FOM) under longer ns pulses pumping. In addition, SA induced optical transparency and nonlinear scattering (NLS) induced optical limiting in the ns regime were investigated through a pump-probe technique. Results and discussion

Figure 1. (a) Typical SEM image of BP nanosheets. (b) HRTEM image of BP nanosheets. The inset is a detailed view of HRTEM. (c) Optical absorption spectra of the BP dispersion in ethanol. The inset is a photograph of the BP dispersion. (d) Raman spectrum of the BP nanosheets. The BP dispersions used in this work were prepared througth a liquid exfoliation technique, which appears to be an effective and practical method to exfoliate diverse layered-materials with interlayer cohesive energies less than 200 meV per atom yielding single- and few-layer nanosheets, as shown in the inset of Figure 1c.11-14 Figure 1a,b exhibit SEM and HRTEM images of BP nanosheets. Characterization of SEM exhibits the surface morphological structure of exfoliated free BP flakes in our experiment. Uniform and regular structure of exfoliated BP nanosheets in HRTEM reveals no oxidation and collapse. The detailed view of HRTEM in Figure 1b

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measurement shows that the lattice fringes are about 0.30 nm and 0.44 nm which well match the monolayer structure and good crystallinity. Figure 1c exhibits that the absorbance of BP nanosheet dispersions gradually decreases from 350 nm to 800 nm. And there is no distinct absorption peak for BP nanosheet dispersions. Raman spectrum of BP nanosheets was excited by 532 nm cw laser. As shown in Figure 1d, three distinct peaks could be found in the Raman spectrum, which represents the out-of-plane phonon mode A1g at 361.9 cm-1 and two in plane mode B2g at 437.9 cm-1 and A2g at 465.4 cm-1. This is in a good agreement with the previous reported results.15 Z-scan is a reliable, rapid and sensitive technique and has been widely used to characterize the NLO properties of materials, such as nonlinear optical absorption, refraction and scattering.16, 17 In this work, a modified open-aperture Z-scan setup was employed to measure the transmittance and scattering of BP nanosheet dispersion, which has been applied in our previous work.18 The experiments were performed through 340 fs pulses at 1030 nm and its second harmonic 515 nm with the pulse repetition rate of 100 Hz, and 6 ns pulses at 1064 nm and its second harmonic 532 nm with the repetition rate of 10 Hz. The BP dispersions were placed in 10 mm × 1 mm quartz cuvettes, and were adjusted to have comparable linear transmittances of ~ 79.7% at 532 nm, 81.8% at 1064 nm, 86.2% at 515 nm and 80.3% at 1030 nm.

Figure 2. The open-aperture Z-scan results of the BP dispersion in ethanol for ns pulses excitation at (a) 532 nm and (b) 1064 nm, and fs pulses excitation at (c) 515 nm and (d) 1030 nm. The solid lines represent the corresponding fitting results. The normalized transmissions of BP nanosheets in ethanol under different excitation sources and pulse energies have been presented in Figure 2. It can be clearly seen that SA responses increase gradually with the incident pulse energy enhancing both at ns and fs pulses excitation.

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The level of SA is about 7% when the excitation pulse energy is 10 µJ at 532 nm, and the value enlarges to 18% at an excitation pulse energy of 40 µJ. It need to point out that we did not observe any obvious signal in ethanol solvent in the experiment, which means that the significant SA mainly originate from the BP nanosheets. Furthermore, the acoustic shock waves induced optical bleaching under nanosecond laser excitation at the repetition rate of 10 Hz 19, 20 is quite weak comparing to the strong SA response, and therefore we ignore this effect in our experiment. The Z-scan results were fitted through a nonlinear beam propagation model,17 () 

= − ( )

(1)

where I is the incident intensity, z’ is the propagation distance within the sample. We have obtained the fitting parameters in the following table by employing αNL model α( ) = +  , where α0 and αNL indicate the linear and nonlinear absorption coefficient, respectively. By employing SA model α( ) = ⁄(1 + ⁄  ), we have also attained the saturation intensity Isat (Table 1). It should be noted that the SA model above could be employed only when the two-level system is in steady-state condition.21 In other words, the duration of the excited pulse should be far longer than the population relaxation time. The interband relaxation time of BP is in the order of tens picosecond as reported in Ref. 22. Therefore, it is not suitable to figure out Isat through this model under slow SA situation in our excitation of 340 fs pulsed laser. In addition, the imaginary part of the third order nonlinear optical susceptibility Imχ(3) and the figure of merit (FOM) have also been summarized in the Table 1. Imχ(3) is in proportion to αNL, that is Imχ() = 10  ! ⁄96$ ! %α , where c is the velocity of light, λ is wavelength of incident light, and n is the refractive index. FOM is defined as &Imχ() / &, which could remove the discrepancy caused by the different linear absorption coefficient α0. Table 1. Linear and NLO parameters of BP dispersion in Ethanol. Pulse width 6 ns 340 fs

Wavelength (nm)

T0 (%)

α0 (cm-1)

αNL (cm/GW)

Imχ(3) (esu)

FOM (esu cm)

Isat (GW/cm2)

532

79.7

2.27

-(2.85±0.39)×10+1

-(9.02±1.24)×10-12

(4.00±0.55)×10-12

(4.85±0.39)×10-2

1064

81.9

2.00

-(9.17±0.94)×10+0

-(5.80±0.60)×10-12

(2.90±0.30)×10-12

(1.37±0.26)×10-1

515

86.2

1.48

-(1.59±0.17)×10-2

-(4.85±0.52)×10-15

(3.30±0.35)×10-15

N/A

2.19

-3

-15

-15

N/A

1030

80.3

-(8.62±1.78)×10

-(5.28±1.08)×10

(2.40±0.49)×10

As shown in Table 1, the nonlinear absorption coefficient αNL is larger at 532/515 nm than that at 1064/1030 nm, and Isat is smaller at 532 nm than that at 1064/1030 nm, which means that BP nanosheets possess lower saturation intensity for saturable absorber devices in the visible range than that in the near-infrared range. On the other hand, the αNL, Imχ(3) and FOM values at 532/1064 nm ns pulses excitation are about 3 orders larger than that at 515/1030 nm fs pulses excitation, suggesting that BP exhibits better SA performance in ns pulses excitation. This pulse duration dependence of SA ability could be explained through the fast and slow SA models.23, 24 Two important parameters, the excited-state absorption (ESA) cross section σes and ground-state absorption (GSA) cross section σgs, could be estimated by employing the fast and slow SA models. The fast SA model is applied in the case when the pulse duration of incident light is quite longer than the interband relaxation time. By solving following equation,23 one could acquire the transmission T of a fast absorber.

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()

*+,∙( ) / *+( )

1

∆3

. = ( ; S = 23 ; D = 3 ; 45

85

(2)

where T0 is the linear transmission, ( = 9 345  . N is the absorber density and L is the thickness of sample. τ denotes the interband relaxation time. I(0) is the input beam intensity in unit of photons per unit area per unit time, and ∆σ ≡