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Rapid and Large-Area Characterization of Exfoliated Black Phosphorus Using Third-Harmonic Generation Microscopy Anton Autere, Christopher R Ryder, Antti Saynatjoki, Lasse Karvonen, Babak Amirsolaimani, Robert A. Norwood, Nasser Peyghambarian, Khanh Kieu, Harri Lipsanen, Mark C Hersam, and Zhipei Sun J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00140 • Publication Date (Web): 07 Mar 2017 Downloaded from http://pubs.acs.org on March 8, 2017

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The Journal of Physical Chemistry Letters

Rapid and Large-Area Characterization of Exfoliated Black Phosphorus Using Third-Harmonic Generation Microscopy Anton Autere,†,∥ Christopher R. Ryder,‡,∥ Antti Säynätjoki,†,¶ Lasse Karvonen,† Babak Amirsolaimani,§ Robert A. Norwood,§ Nasser Peyghambarian,§,¶,† Khanh Kieu,§ Harri Lipsanen,† Mark C. Hersam,‡ and Zhipei Sun∗,† †Department of Electronics and Nanoengineering, Aalto University, Tietotie 3, FI-02150 Espoo, Finland ‡Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States ¶Institute of Photonics, University of Eastern Finland, Yliopistokatu 7, FI-80101 Joensuu, Finland §College of Optical Sciences, University of Arizona, 1630 E. University Boulevard, Tucson, Arizona 85721, USA ∥Contributed equally to this work E-mail: [email protected]

1 ACS Paragon Plus Environment


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Abstract Black phosphorus (BP) is a layered semiconductor that recently has been the subject of intense research due to its novel electrical and optical properties, which compare favorably to those of graphene and the transition metal dichalcogenides. In particular, BP has a direct bandgap that is thickness-dependent and highly anisotropic, making BP an interesting material for nanoscale optical and optoelectronic applications. Here, we present a study of the anisotropic third-harmonic generation (THG) in exfoliated black phosphorus using a fast scanning multiphoton characterization method. We find that the anisotropic THG arises directly from the crystal structure of BP. We calculate the effective third-order susceptibility of BP to be ∼ 1.64 ×10−19 m2 V−2 . Further, we demonstrate that multiphoton microscopy can be used for rapid, large-area characterization indexing of the crystallographic orientations of many exfoliated BP flakes from one set of multiphoton images. This method is therefore beneficial for samples of areas ∼1 cm2 in future investigations of the properties and growth of BP.

Graphical TOC Entry

Keywords Black phosphorus, third-harmonic generation, multiphoton microscopy


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Recently, black phosphorus (BP) has attracted tremendous interest 1–3 as a layered semiconducting material due to its unique electrical and optical properties. 4–9 In contrast to semi-metallic graphene, BP has a thickness-tunable direct bandgap (∼1.7 to 0.3 eV), 3,10–13 spanning the visible, near-infrared, and mid-infrared spectral regimes. BP also compares favorably to the semiconducting transition metal dichalcogenides (TMDs), which generally have direct bandgaps only at a single layer, and have charge carrier mobilities far inferior to BP. These unique properties enable a large range of high-performance electronic and optoelectronic devices, 3 such as transistors, 4,6–9 photodetectors, 14–16 and ultrafast lasers. 17,18 Further, it has been demonstrated that various properties of BP (such as mobility, 5 linear 10,19 and nonlinear 17 absorption, Raman response, 20 luminescence, 21 and photoresponsivity 22,23 ) strongly depend on its crystalline orientation because of the anisotropic bonding in its crystal structure. Such anisotropy can be exploited for numerous photonic and optoelectronic applications, 17,19,21,23 to achieve functions that are not possible with isotropic materials. Emerging nanomaterials have exhibited interesting fundamental physical phenomena in nonlinear light-matter coupling at the nanoscale. These properties also may be utilized in technological applications, such as all-optical signal processing in telecommunications, for novel and high-performance nanoscale devices. 24 Recently, the nonlinear response of two-dimensional (2D) nanomaterials has been demonstrated as a rapid and reliable characterization method. Such methods are imperative for evaluating homogeneity in large scale materials synthesis and for identifying materials of interest in randomly oriented and polydisperse samples, such as those obtained from mechanical exfoliation and chemical vapor deposition. Indeed, the nonlinear optical properties of 2D nanomaterials have recently attracted increasing attention. Several reports demonstrated that TMDs 25–29 and hexagonal boron nitride (h-BN) 27 exhibit strong second harmonic generation (SHG), due to the noncentrosymmetric crystal structure. SHG has been utilized in these materials for detecting the crystallographic orientation 27 and grain boundaries. 25 Also third harmonic generation (THG) has been detected in graphene, 30,31 monolayer MoS2 32,33 and thin (∼ 5 nm) MoS2



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films. 34 Recently, even higher order multiphoton processes, such as fourth harmonic generation 32 and high-order harmonic generation (HHG) up to 13th order, 35 has been reported in MoS2 monolayers. However, thus far, most of the studies on BP have concentrated on the electrical and linear optical properties of BP, with only few studies about the nonlinear optical properties that are mainly related to nonlinear saturable absorption. 17,36,37 Two recent reports (refs. 38 and 39) detailed anisotropic THG emission from exfoliated BP. However, in ref. 38 a corroborating method for indexing the crystallographic directions was not used. In this work, we report that the THG from exfoliated BP flakes can be used as a rapid characterization method to determine crystallographic orientations. By correlating polarized THG with polarized Raman measurements, we show that the anisotropic THG arises from the crystal structure of BP and can therefore be used reliably for this purpose. We also derive the equations that relate the intensity of the generated TH signal to the polarization of the incident laser beam. The measured polarized THG emission is in good agreement with the derived model. We estimate the magnitude of the bulk-like effective third-order nonlinear susceptibility |χeff | of BP to be ∼ 1.64 × 10−19 m2 V−2 . Finally, we demonstrate that the (3)

fast scanning rate of this multiphoton technique, ∼20 µs/pixel, allows for rapid and detailed characterization of mechanically exfoliated BP samples by determining the crystallographic orientations of a large number of BP flakes in a single field of view. The BP samples were prepared via mechanical exfoliation and deposited onto amorphous glass or SiO2 /Si substrates. Since BP is chemically reactive, 40 precautions must be taken to passivate exfoliated BP 41–43 to mitigate ambient degradation and photooxidation 44 during optical characterization. For these purposes samples were immediately encapsulated with a AlOx coating grown by atomic layer deposition (ALD) at 50 ◦ C as described in reference 41. Polarized Raman measurements of exfoliated BP flakes ∼15—40 nm thick were acquired using a 532 nm excitation wavelength in a parallel excitation/detection configuration. The armchair direction of BP was assigned to the main axis of the fitted polarization dependence of the A2g mode (i.e. the axis that contained the maxima of the fits). Raman spectroscopy of


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BP has been well discussed, including polarized Raman measurements. 45–48 However, while this is a common method of indexing the crystallographic orientation of exfoliated BP, the assignment of the armchair and zigzag directions can be fraught with ambiguity due to the anisotropic nature of the optical absorption and electron-phonon interactions of BP. 48 For this reason, the crystallographic directions of BP were tentatively assigned before correlating the polarized Raman measurements to the polarized THG measurements. The THG measurements were acquired with a custom-made multiphoton microscope. A schematic illustration of the measurement setup is shown in Figure 1 (details in Methods). A variable attenuator was used to control the power of the excitation beam at a wavelength of ∼ 1560 nm. THG measurements of all samples were obtained by scanning the fundamental beam over the flakes and recording the generated light intensity at the wavelength of the generated TH signal with a photomultipliertube (PMT). A spectrum of the THG emission and measured power dependence of a representative BP flake are shown in Fig. 2a and b, respectively. All of the data in the main text, except the spectrum shown in Figure 2a, were obtained from flakes on glass substrates. Only the large-area THG image shown in Figure S4 in the supporting information and the flake on Figure 2a are from samples on an SiO2 /Si substrate. From the spectrum in Figure 2a, it can be seen that the observed peak is located at ∼ 521 nm, corresponding well with the expected value of one third of the fundamental wavelength. The n-th order nonlinear processes depend on the n-th power of the light intensity of fundamental beam. 49 The excitation power dependence of the THG signal was measured, confirming the cubic dependence on the optical power of the pump laser (Figure 2b), thus we assert that the detected light is THG emitted from the BP flake. The polarization of the fundamental beam was rotated with a half-wave plate for polarization dependent THG measurements. The measurements were done with a step of 9◦ . The measurements were performed over approximately 260◦ because the rotation angle of the half-wave plate in our setup is limited. However, this range is sufficient for obtaining the polarization dependence of THG, as can be seen from Figure 3. The measurements were performed with a



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Figure 1: A schematic illustration of the measurement setup. MLL : mode-locked fiber laser, VA : variable attenuator, HWP : half-wave plate, BD: beam dump. peak irradiance of 558 GW cm−2 (average power of 12 mW). The polarization dependence of the measurement system was obtained from reference measurements of a bare substrate and was accounted for in all the subsequent measurements. The measured polarization dependence of THG from a representative BP flake is shown in Figure 3a. The red and purple arrows indicate the armchair (AC) and zigzag (ZZ) crystal directions obtained from Raman measurements (see Methods for details). Corresponding THG images are shown in Figure 3c and d, displaying the clear difference between the generated TH signal when the incident light polarization is along the AC or ZZ direction.


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Figure 2: a) THG spectrum of a representative BP flake (thickness 30 nm). b) Power dependency of the THG from the same flake as in a).

Figure 3: (a) Polar plot of the measured angular dependence of the THG from a 26 nm BP flake on a glass substrate. The red and purple arrows indicate the AC and ZZ crystal directions obtained from polarized Raman measurements. (b) Optical image of exfoliated BP flake measured in a). The yellow arrows in (b) are the x- and y-axes corresponding to radial (lab) coordinates in a). c) and d) THG images obtained with incident polarization along AC (c) and ZZ (d). Scale bar in b)-d) is 10 µm.



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The generated TH intensity (I3ω ) of BP can be expressed by the following equation: ( )2 3 (3) 2 I3ω ∝ Iω3 [ χ(3) xx cos θ + 3χxy cos θ sin θ ( )2 3 (3) 2 + χ(3) yy sin θ + 3χyx sin θ cos θ ],

(1)

where subscripts 3ω and ω refer to the fields at TH and fundamental frequencies, respectively. Here θ is the angle between the polarization direction of the fundamental wave and the AC-axis of the BP crystal. The different components of the third-order susceptibility (3)

(3)

tensor are represented with χjj and χjk , where {j, k}={x, y}. The details for the derivation of Eq. (1) are presented in Supporting Information. The intensity of the THG signal can be fitted to Eq. (1) to obtain the AC-direction of the BP crystal. The fitted results are shown in Figure 3a. The black solid line is the fitted total THG intensity. The red and purple dashed lines correspond to the x- and y-components of the 3-order polarization vector, respectively. From Figure 3a, it can be seen that the measured data agrees very well with the theoretical model. Also, it is clear that the maximum intensity does not occur when the (3)

incident polarization is along AC-direction. This is due to the contribution form χyx compo(3)

nent. Based on the values obtained from the fit, the χxy component is almost zero but the (3)

(3)

χyx -component is comparable to χyy which results in the four-fold polarization dependence pattern. However, care must be taken when evaluating the relative magnitudes of the χ(3) components that are obtained from the fit, since slightly different values can be obtained with fairly good overall fit. However, the crystallographic orientation can be obtained from the fit with good confidence even though the individual tensor components may vary. The results are summarized in Table 1. As can be seen, the AC directions (θAC ) obtained from the fits agree well with the directions determined by polarized Raman measurements, (θAC,raman ). The polarized Raman measurements are provided in the Supporting Information Figure S1. Also the thickness of the flakes d, obtained from atomic force microscopy (AFM), and the number of layers (N ) are shown in Table 1.


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Table 1: Comparison of crystal orientations obtained from Raman measurements and THG images of four different samples.

θAC [deg] θAC,raman [deg] d [nm] N

F1 359 4 26.0±0.2 ∼43

F2 13 26 24.4±1.0 ∼41

Sample F3 103 110 13.7±1.0 ∼23

F4 82 82 28.4±1.0 ∼47

Further, we have measured THG from BP flakes with various thicknesses. The emitted TH power as a function of flake thickness is shown in Figure 4a. We observe that the THG first increases as the flake thickness increases. At thicknesses around 25 nm, the THG power starts to decrease so that at 40 nm the THG power has decreased by a factor of ∼ 2.5. The observed thickness dependence can be explained by the differences in absorption at the TH and fundamental wavelengths. As shown in an earlier study, the absorption coefficient of BP is substantially smaller at 1550 nm than at 520 nm. 17 Thus, part of the pump is absorbed in the flake and THG is generated throughout the flake. Because the measured THG is backward generated, it has to pass through the flake and thus it experiences significant absorption for thicker flakes. On the other hand, the intensity of the generated TH signal increases as more material takes part in the frequency conversion process. Thus, the thickness dependence of THG signal is a combination of three processes: weak absorption of the pump beam, absorption of the THG signal and layer dependent increase in the detected THG power (P3ω ). This can be described with a simplified equation P3ω ∝ N a × exp(−α × d),

(2)

where d is the flake thickness, N = d/0.53 is the number of layers, a is a factor describing the exponential increase in the THG intenisty and α is an absorption coefficient that includes both absorption of the pump and THG signal. In reference 39 similar thickness dependence was reported, the authors derived an analytical solution for thickness-dependent THG that



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accounted for the absorption of THG signal and the phase mismatch between backward generated THG and the pump. The coherence length of backward propagating THG in BP with pump at 1560 nm is ≈ 40 nm, 39 which is in the same order as the thickness of the thickest flakes examined here. Thus, for accurate determination of the thickness dependence of the THG, the phase-mismatch should be considered. Despite this, it is found that Equation (2) sufficiently describes the thickness dependence of the THG power, and is shown as a dashed line in Figure 4a. Best fit to experimental data was obtained with a = 2.2, which is in agreement with the previously reported quadratic dependence with the number of layers for graphene, 31 MoS2 32 and GaSe, 50 when the flake thickness is considerably smaller than the coherence length. It should be noted that specifically in the case of BP the exponential term in Equation (2) can deviate, since the band gap of BP depends on the number of layers. For example, the THG power is expected to increase substantially when either the BP thickness or the wavelength of the fundamental is tuned so that the photon energy at the fundamental or THG wavelength are in resonance 38 with excitons or band-to-band transitions. Thus, Equation (2) is valid when the energies of fundamental and THG are far from resonances. We obtained a value of α ∼ 70 µm−1 , which is ∼ 7× higher than what was reported in ref. 17 for BP at 520 nm. According to Equation (2), in case of negligible absorption of the fundamental beam, the THG power would diminish and approach zero as the flake thickness increases. However, THG will be emitted from a surface layer with thickness approximately equal to absoprtion length regardless of the actual flake thickness. The absorption length of BP at 520 nm is ∼ 100 nm 17 and thus our simplified model is reasonably accurate for the samples examined in this work and for BP flakes with thicknesses below the absorption length. More accurate results could be obtained by taking also the effects caused by the multilayer structure (Air-AlOx -BP-Glass) into account by incorporating the Fresnel transmission and reflection coefficients of this structure into the analysis. This could potentially enable the determination of the thicknesses of the BP flakes from the same multiphoton images that


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are used for crystallographic indexing. Furthermore, the exciton resonances are expected to have a significant effect on the nonlinear processes when the energy of pump photons is closer to the resonance. In a recent study, a laser-thinning method was used to measure the THG emitted from a BP as the flake thickness decreases. 38 Far greater THG intensity was observed in very thin (few-layer) regions, which was attributed to increased effect of the exciton resonance. This effect peaked when excitons were on resonance with the pump (3)

photon energy (∼ 0.8 eV). In ref. 38 the authors measured the |χeff | of BP to be ∼ 10–20 (3)

× that of graphene, close to the exciton resonance. Here we measure |χeff | of BP to be less than half of that of graphene, far from the resonances. Thus, the results obtained in ref. 38 and the measurements reported here strongly imply that exciton resonance effects play a significant role in the nonlinear frequency conversion process of BP. The band gap of the flakes examined here ranged from ∼0.40-0.47 eV (calculated from the thicknesses according to Ref. 10), requiring a pump laser with a wavelength of > 2.5 µm to fully probe the effect of the exciton resonance. (3)

We calculate the effective bulk-like third-order nonlinear susceptibility |χeff | of BP by

(3)

Figure 4: a) THG power as a function of flake thickness b) Measured |χeff | at different thicknesses. The dashed line is an exponential decay fit to the data. comparing the detected TH signal to the results from monolayer graphene on a similar 11 ACS Paragon Plus Environment


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substrate. The calculation is done with the following equation 51 √ (3) |χeff |

dgr = dBP

T HGBP (3) |χ |, T HGgr gr

(3)

where dgr = 0.33 nm and dBP are the thicknesses of monolayer graphene and BP, respectively. THGgr is the measured average power of THG from graphene monolayer and THGBP is (3)

the THG power from the measured BP flake. Using graphene reference |χgr | = 3.25 × 10−19 m2 V−2 (from Ref. 52), we obtain value of |χBP | ≈ 1.64 ×10−19 m2 V−2 for a 9.5 nm (3)

thick flake. For graphene, 31 MoS2 32 and GaSe, 50 at thicknesses of fewer than ∼ 15 layers, the THG power was found to depend quadratically with the layer number, therefore producing a (3)

constant χeff from Equation (3). However, in ref. 30 it was calculated that greater absorption can lead to decay in the THG power. Because the absorption of pump and THG signals and the effect of the coherence length is not taken into account in Equation (3), the calculated (3)

values in Figure 4b decrease as the flake thickness increases. The measured |χeff | of BP is ∼ 2× lower than |χ(3) | of graphene and comparable to other well known 3D crystals that exhibit strong third-order nonlinearity, such as CdS and ZnSe. 49 In ref. 39 a similar value, 1.4 ×10−19 m2 V−2 , was reported for a 14.5 nm thick BP flake, at similar pump wavelength as here. It must be noted however that the model does not account for absorption of the fundamental or the third-harmonic signal in the BP flake. Thus the actual value of |χ(3) | of BP may be even higher. Additionally, we expect a strong TH response of BP when the pump light is resonant to the exciton energy, as mentioned earlier. Strong third-order nonlinear processes can potentially be used for various nonlinear optical applications in the mid-infrared spectral region, such as an ultrafast optical switch. 53,54 Emerging 2D nanomaterials are typically studied initially via the mechanical exfoliation of bulk layered crystals and deposition of thin layers onto substrates. While this technique produces high-quality materials, which is critical for their evaluation for existing and future technological applications, it also produces samples with high polydispersity in thickness


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and random crystallographic orientations. For an anisotropic material such as BP, the latter factor can bear significant variations in observed device metrics due to anisotropy in the optical and electrical conductivities. Fast, large scale characterization techniques to determine the crystallographic orientations of flakes are therefore highly important. In Figure 5, the THG emission from eight BP flakes in a ∼ 260×260 µm2 area is shown. This image has a resolution of 1000 × 1000 pixels and was acquired at a rate of ∼20 µs/pixel, yielding total measurement time of 20 s. The fast scan rate allows for this region to be characterized at many different excitation polarizations, making it possible to determine the crystallographic orientations of all exfoliated BP flakes (of reasonable lateral size) in this field of view in minutes. The generated THG signal from several BP flakes with changing polarization of the incident laser is shown in Figure 5a-d. Here it can be seen that the THG signal clearly increases and decreases on flakes with different crystallographic orientation. The AC directions of 8 different flakes are obtained from a set of measurements with varying polarization and are displayed in Figure 5e. Based on this, the flakes that can be seen on the image can be classified to three different groups that originated from different crystallites. These groups are displayed in Figure 5a with yellow dashed boxes. Flakes F5—F7 were previously indexed with polarized Raman measurements (see Supporting Information Figure S2). The assigned AC directions, by polarized THG(Raman), are 58(61)◦ , 63(68)◦ and 67(72)◦ for flakes F5, F6 and F7, respectively, showing that the crystallographic assignments between THG and Raman measurements are in good agreement. Additionally, the high scan rate used in this THG technique makes it practical to scan large area on typical substrates (∼1 cm2 ). An example of such a measurement is shown in the supporting information, Figure S4. In Figure S4, the THG emission from exfoliated BP on an entire substrate is measured by producing a composite image of 225 individual images each consisting of 512 × 512 pixels. The size of the sample in Figure S2 is ∼ 0.5 cm × 0.5 cm and total measurement time was ∼ 2.7 h. By utilizing polarized THG measurements and forming a composite image, this technique makes it possible to index the crystallographic



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Figure 5: Determination of crystal orientations from 8 different flakes. Total image size 260×260 µm. a)-d) THG images recorded by rotating the polarization of the incident laser. e) THG image with overlayed AC crystal directions of the 8 different flakes. directions of exfoliated BP flakes on entire sample substrates. With further understanding of the relationship between THG emission intensity and BP thickness, this technique may be used to characterize both the crystallographic orientation and thickness, the two most important factors dictating the nanomaterial properties of exfoliated BP, across entire substrates. In summary, we have experimentally characterized the third-order nonlinear optical properties of exfoliated BP with multiphoton microscopy. We determined that the anisotropic nature of the THG is a consequence of the crystal structure of BP and derived the equations for the polarization dependence of THG for BP. We also studied the thickness dependence of THG and determined that the emitted THG power is highly layer dependent due to (3)

the depletion of the THG signal. In addition, we calculated the |χeff |-value of BP to be 1.64 ×10−19 m2 V−2 , which is in the same order as for monolayer graphene. Lastly, we demonstrated that multiphoton microscopy can be used as a rapid and large-area characterization method for determining the crystallographic orientations of exfoliated BP. With further understanding of how THG emission is affected by the substrate, phase matching, 14 ACS Paragon Plus Environment

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pump wavelength and absorption in BP, this technique may also be used to determine flake thickness. Overall, our results provide new understanding of the anisotropic nonlinear optical properties of BP, which may be used for polarized optical applications. Furthermore, as a rapid and reliable characterization method, multiphoton microscopy should be beneficial for future studies of the materials properties of BP and future efforts in the synthesis of large scale, thin BP.

Experimental Methods Exfoliation and passivation of BP samples. The exfoliated BP samples were prepared by mechanical exfoliation of bulk BP (HQ Graphene) with Scotch Magic Tape onto glass (Fisher) or 300 nm SiO2 /Si (SQI) substrates. The substrates were sonicated in acetone and isopropanol (both Fisher) prior to BP exfoliation. Immediately following exfoliation, the samples were encapsulated with a ∼2.5 nm AlOx coating grown at 50 ◦ C. 41 The overall time unprotected exfoliated BP was exposed to ambient conditions was minimized (less than 30 seconds). Raman characterization. Homogeneous exfoliated BP flakes of 15—40 nm thickness and ∼10 µm lateral size were identified with optical microscopy. The flake thickness and morphology was then measured with AFM in non-contact mode on an Asylum Research Cypher microscope. The polarized Raman measurements were made on a Horiba XploRA microscope using a 532 nm excitation wavelength and a 100× objective. The measurements were acquired in a parallel excitation/detection configuration while rotating the sample. A low laser power of ∼0.2 mW was used and the laser spot size was estimated to be ∼2 µm. All spectra were fit to Gaussian functions. The armchair direction of BP was assigned to the dominant axis of the polarization dependence of the A2g mode, i.e. the axis containing the maxima, which was determined by fitting to the pertinent derived equations in references 20 and 48.



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THG measurement setup. The THG measurements were done with a multiphoton microscope. A schematic illustration of the measurement setup is shown in Figure 1. A mode-locked erbium-doped fiber laser with a carbon nanotube saturable absorber was used as an excitation laser. The center wavelength of excitation light was ∼ 1560 nm, repetition rate 8 MHz and pulse duration 100 fs. A galvo scanner was used to steer the pump beam, and the beam was then focused on the samples with a 20× microscope objective. The beam size on the sample, measured with the nonlinear razorblade method, is ∼ 1.85 µm. The backward generated TH signal was collected with the same objective and separated from the excitation beam with a dichroic mirror. Another dichroic mirror is used to separate THG from the generated signal. A bandpass filter (520 nm) was placed in front of the PMT to ensure that only THG is detected. The polarization of incident light on the sample plane was controlled with a half-wave plate placed before the galvo scanner.

Acknowledgement The authors acknowledge funding from the European Union’s Seventh Framework Programme (REA grant agreement No. 631610), Graphene Flagship grant (GrapheneCore1 No. 696656), Academy of Finland (No. 276376, 284548, 295777, 304666), and Nokia foundation. A.A. acknowledges funding from Tekniikan edistämissäätiö (TES) and the Foundation for Aalto University Science and Technology. C.R.R. and M.C.H. acknowledge support from the Office for Naval Research (N00014-14-1-0669). R.A.N., N.P., and K.K. acknowledge support from the Office of Naval Research through the NECom MURI program. We also acknowledge the provision of technical facilities of the Micronova, Nanofabrication Centre of Aalto University.


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Supporting Information Available Derivation of equations relating the incident polarization to the detected THG intensity. Characterization data of 12 different BP flakes. Large-area THG image.

This material is

available free of charge via the Internet at http://pubs.acs.org/.

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