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Article

Broken Symmetry Induced Strong Nonlinear Optical Effects in Spiral WS Nanosheets 2

Xiaopeng Fan, Ying Jiang, Xiujuan Zhuang, Hongjun Liu, Tao Xu, Weihao Zheng, Peng Fan, Honglai Li, Xueping Wu, Xiaoli Zhu, Qinglin Zhang, Hong Zhou, Wei Hu, Xiao Wang, Litao Sun, Xiangfeng Duan, and Anlian Pan ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b01457 • Publication Date (Web): 12 Apr 2017 Downloaded from http://pubs.acs.org on April 13, 2017

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Broken Symmetry Induced Strong Nonlinear Optical Effects in Spiral WS2 Nanosheets Xiaopeng Fan,†,# Ying Jiang,†,# Xiujuan Zhuang,† Hongjun Liu,†,* Tao Xu,‡ Weihao Zheng,† Peng Fan,† Honglai Li,† Xueping Wu,† Xiaoli Zhu,† Qinglin Zhang,† Hong Zhou,† Wei Hu,† Xiao Wang,† Litao Sun,‡ Xiangfeng Duan,§ Anlian Pan†,*

†

Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, School of Physics and Electronic Science, and State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, P. R. China. ‡ SEU-FEI Nano-Pico Center, Key Lab of MEMS of Ministry of Education, Southeast University, Nanjing 210096, P. R. China. § Department of Materials Science and Engineering, Department of Chemistry and Biochemistry, and California NanoSystems Institute, University of California, Los Angeles, California 90095, United States. *Corresponding authors. E-mail: [email protected], [email protected]

ABSTRACT. Transition metal dichalcogenides have provided a fundamental stage to study the light-matter interaction and optical application at atomic scale for their ultrathin thickness and their appropriate bandgap in the visible region. Here, we report the strong nonlinear optical effects, including second harmonic generation (SHG) and third harmonic generation (THG) in spiral WS2 structures. SHG intensity quadratically increases with layer numbers, other than diminishing oscillation of 2H stacking TMDs. The contrary SHG behavior is attributed to the broken symmetry from twisted screw structures, revealed by aberration corrected transmission electronic microscope (AC-TEM) observation. Furthermore, the twist angle of the screw structure (5 degrees) was obtained by high resolution transmission microscope 1

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(HRTEM) measurements, and confirmed by polarization test of SHG output. Moreover, we roughly estimate the effective second-order nonlinear susceptibility. The discovery and understanding of the accumulation of nonlinear susceptibility of spiral structures with increasing thickness will extend the nonlinear application of TMDs.

KEYWORDS: Tungsten disulfide, second harmonic generation (SHG), third harmonic generation (THG), nonlinear optics, polarization, spiral structure

Multilayer transition metal dichalcogenides (TMDs) have recently exhibited attractive layer-dependent optical and electronic properties since successful exfoliation from their bulk.1-9 Different from metallic graphene, TMDs have appropriate optical bandgap of 1.5-2.0 eV in a visible range, leading to great potential optical and optoelectronic application.1,5,6,10-12 Their optical properties, such as photoluminescence,13-15 valley selective excitation3,16-18 and the giant exciton binding energy,19-22 have been extensively studied on pristine, alloyed and heterojunctional TMDs. Due to the broken symmetry from a bulk to a monolayer, monolayer TMDs also shows unusual nonlinear optical response,23-28 which can extend their potential applications beyond linear optical effects. The atomic thickness of monolayer TMDs limits the conversion efficiency for nonlinear optical devices, which is proportional to the square of the effective interference length.29 Thus, thicker TMDs films with high conversion efficiency are demanded for their nonlinear optical application. However,

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second harmonic generation (SHG) effect of multilayer TMDs shows a diminishing oscillation with increasing numbers of layers.26 So it is difficult to obtain large SHG conversion efficiency by just increasing the layer numbers of TMDs. How to enhance the nonlinear conversion efficiency of TMDs is still a challenge to extend their nonlinear application. Two-dimensional layered structures provide a viable approach to realize their desired properties at atomic level by artificially stacking them in different ways.30-34 Specific structures without inversion symmetry center, such as heterostructures or twisted bilayers, have been prepared and SHG effects were observed on such kind of structures.35-37 Besides, the spiral structures can also be promising structures to produce larger effective conversion coefficient.38,39 Spiral MoS2 was suggested to be an AA stacking structure from TEM diffraction and theoretical calculation, rather than a normal centrosymmetric AB stacking model, showing stronger second-order optical nonlinearity with more layers.38 Up to now, the detailed atomic structures of spiral TMDs are still missing yet. In this work, the underneath nature of the strong nonlinearity of chemical vapor deposition (CVD)-grown spiral WS2 nanosheet are carefully studied. Their layer-dependent and power-dependent nonlinear properties, including SHG and third harmonic generation (THG), have been fully examined, and the results are interpreted based on the resolved atomic structures from aberration corrected transmission electron microscope (AC-TEM). A twisted AA stacking structure was proposed based on the TEM observation and confirmed by polarization test of SHG output. We believe that the twisted structures with broken symmetry lead 3


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to the large SHG effect of the WS2 nanosheet. Meanwhile, the effective nonlinear susceptibility was roughly estimated. Our findings will give important information for the development of nonlinear optical devices based on TMDs. RESULTS AND DISCUSSION

The optical image of CVD-grown spiral WS2 nanosheet in Figure 1a shows screw morphology, in which the brightness in central region is much higher than that at edges. A zoomed atomic force microscope (AFM) image of the central region in Figure 1b gives more detailed information, and blue dashed guide lines clearly show the screw structures. The line profile in Figure 1c, marked with a white dashed line in Figure 1b, reveals obvious plateaus with step height of about 0.7 nm, which agrees with the thickness of a single WS2 atomic layer. Thus, it is proposed that in a spiral nanosheet, a single WS2 layer spirally grows around the screw core which is normal to the bottom layer, schematically shown in Figure 1d. As-grown spiral nanosheets were then transferred onto a quartz substrate to test their nonlinear optical properties with transmission mode. Second-order optical nonlinearity of spiral WS2 nanosheets was tested on different layers and under various power densities, layer-dependent and power-dependent SHG intensities are listed in Figure 1e and f, respectively. As layer numbers increase, SHG intensity increases rapidly in Figure 1e, which is completely different from normal 2H stacking TMDs, whose SHG intensity shows the odevity to layer numbers.26 In the inset of Figure 1f, SHG intensity quadratically increases with increasing power density, agreeing well

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with the nonlinear optical principal, in which SHG intensity is proportion to the square of thickness and power density. Nonetheless, the plot of intensity vs layer number is not a parabola in the inset of Figure 1e. It is attributed to the large size of the incident laser beam, which may cover more than one layer during the measurements, except for that on the bottom layer. Meanwhile, large THG effect was observed on the same spiral WS2 nanosheet under the same incident laser, shown in Figure 2a. In a large wavelength range of an incident laser, from 1120 ~ 1330 nm, both SHG and THG output were obtained at the same time. Power-dependent third-order nonlinearity was also studied under various incident power densities, and the results are listed in Figure 2b (refer to Supporting Information Figure S1 for more examples). For comparison, SHG output with increasing power density is also listed in the same chart. The log plots of output intensity vs incident power density are listed in the inset in Figure 2b. The slopes for the plots of SHG and THG are 2 and 3 respectively, which is well identical with the nonlinear optical principle. Usually, THG effect can be from the third order nonlinear susceptibility χ(3) or the summary frequency generation (SFG) of the SHG output and incident laser. To identify the real origin for THG, the same measurements were performed on a normal 2H stacking WS2 nanosheet, shown in the inset in Figure 2c. The layer-dependent and incident power dependent outputs are shown in Figure 2c and 2d, respectively. Similar incident power dependent results as the spiral WS2 were obtained, and the slopes for the log plot are 2 for SHG and 3 for THG, shown in Figure 2d. However, for SHG in the normal 2H stacking sample, the SHG output is 5


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almost diminishing with increasing layer numbers, while the THG output still increases sharply in Figure 2c. Therefore, the THG effect is from the third order nonlinear susceptibility χ(3), rather than SFG effect since the diminishing SHG effect will heavily weaken the SFG effect. To understand the nature of the strong nonlinear optical effect with increasing thickness in a spiral WS2 nanosheet, atomic structures were studied on an as-grown spiral nanosheet. AC-TEM measurements were performed on the cross-section of a spiral WS2 nanosheet to resolve the screw structure at atomic scale. Figure 3a is a scanning electronic microscope (SEM) image of the cross-section specimen for AC-TEM observation, and the position for the fabrication of cross-section sample is shown with a red line in the inset. Figure 3c is the AC-TEM cross-section image of the spiral structure with the detailed stacking information. Red and green short lines show the distance between a W atom and two neighboring S atoms, corresponding to the red and green lines on the side-view atomic model of monolayer WS2 in Figure 3b. The distances between a W atom and two neighboring S atoms in side view are the same for each layer, indicating that stacking between two neighboring layers is AA stacking, agreeing with the results from TEM electron diffraction.38 The atomic model of monolayer is imposed onto the AC-TEM image with AA stacking, shown in Figure 3c. From the red dashed lines in Figure 3c, we may note that there is no period structure along the [0001] direction (here, [0001] direction is defined to the direction normal to the (0001) plane of monolayer WS2, neglecting the small distortion caused by the screw), which is different from the normal 2H stacking or 3R stacking in 6


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Figure 3d. The complete absence of period along [0001] direction indicates that the stacking of spiral WS2 nanosheet is a twisted structure with strain caused by screw growth.

High resolution transmission electron microscope (HRTEM) measurement was employed to check the twisted angle of the spiral structure. Three positions marked in Figure 4a in the spiral structure, in the bottom layer, in the middle layer and in the top layer, were selected for HRTEM measurements, and corresponding HRTEM images are shown in Figure 4b-d respectively. Dashed arrows with different colors indicate the direction of lattice sites, and the dashed arrows in Figure 4b and 4c were translated to the image in Figure 4d. By comparing the red dashed arrow and the yellow dashed one, we may get the total twisted angle of the screw structure, about 5 degree, as shown in Figure 1d. Meanwhile, the green dashed arrow is between the red and yellow dashed arrows, indicating the gradual change during the screw growth.

Furthermore, polarization of SHG and THG output was tested on this spiral structure, since the polarization direction of SHG can reveal the structural symmetry of materials.23,26,36,40-45 During this measurement, a sample was fixed on a measurement stage and the polarization direction of an incident laser was also fixed at the direction normal to the brim of the triangle bottom layer. The polaroid between the spectrograph and the collected lens was rotated for a circle to test the polarization of the output light. In the experiments, the initial direction is parallel to the polarization of the incident laser. (refer to the Supporting Information Figure S2 for the more

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examples of the measurements with different initial positions). For comparison, polarization tests were also performed on a monolayer WS2 nanosheet under the same condition. The test results for monolayer and spiral structure are listed in Figure 5a and b. For monolayer, the polarization direction for both SHG and THG output is identical with that of the incident laser (in Figure 5a). But for spiral structure, the polarization direction for the SHG output has a little shift and the azimuthal angle is about 15 degree, while the polarization direction for THG output is still the same as that of the incident laser in Figure 5b. Thus, the shift of the azimuthal angle is attributed to the twist between bottom layer and top layer. For SHG, the angle dependence can be described as I = I0sin2(3ϴ), where I and I0 are the observed intensity and the maximum intensity during the whole observation of SHG output, respectively. Our results in Supporting Information Figure S2 also experimentally proved the angular relation between incident laser and SHG output. Therefore, the angle shift between the SHG polarization direction and incident polarization direction is about 5 degrees, shown in Figure 1d, which agrees well with the observed results from HRTEM measurements. In case that there are some large system deviation for the above polarization test, angle-dependent polarization of SHG output was further tested in a different way. We fixed the direction of the incident laser and the polaroid between the spectrograph and the collected lens, while rotating the sample with the same initial direction as the above polarization test in Figure 5a and 5b. The collected results for a monolayer and a spiral nanosheet are listed in Figure 5c and 5d respectively. The polarization 8


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angle-dependent SHG intensities of a monolayer WS2 nanosheet show clear six-fold rotation symmetry with the polarization direction parallel to the armchair direction, agreeing well with the reported results on monolayer TMDs.24.25,27,37 Nonetheless, the SHG polarization of the spiral nanosheet has a little shift of about 5 degrees from the armchair direction of bottom layer although they also show six-fold rotation symmetry, which is the same as that from the above polarization test and TEM measurement. Acquisition of such a small atom shift, 5 degrees, by polarization test should be believable, and the measurement of 3 degrees shift by polarization test was reported recently by Xu. et al..46 The atomic structures from AC-TEM demonstrated the full absence of the period along [0001] direction besides AA stacking in Figure 3b, indicating the absence of the inversion symmetry center for the spiral WS2 nanosheet. Moreover, the gradual shift of the azimuthal angle from the HRTEM measurements in Figure 4d suggested that the screw structure should be a twisted structure, rather than a reported thermodynamically stable stacking structure,38 which is further affirmed by the polarization test. The broken symmetry alone [0001] direction from the twisted structures is believed to lead to their accumulated SHG effect with increasing layer numbers，rather than other effects, such as surface enhance SHG47 or plasmonic enhance SHG.48 Finally, the effective second-order nonlinear susceptibility of this spiral WS2 nanosheet was roughly estimated, using a commercial BBO crystal as a reference sample. Both the spiral WS2 nanosheet and BBO crystal were tested under the same 9


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conditions (such as the same beam size, wavelength, et al.) except for the incident laser power intensity, and the results are shown in Figure 6a. Since the conversion efficiency of the BBO crystal is too strong, much lower intensity was used during the measurement. For an o + o → e SHG process, the conversion efficiency can be described as the following:29

=

χ ∆ ∆

(1)

Here P2 and P1 are the SHG output power and the incident power for the measurement, χeff is the effective second-order nonlinear susceptibility, L is the length of the sample, ε0 is the vacuum dielectric constant, c is light speed in vacuum, λ1 and λ2 are wavelengths for the incident laser and SHG output respectively, n1, n2 are the refractive index of samples at λ1 and λ2 respectively, and A is the area of the laser beam. If we neglect the refractive index difference between BBO and WS2 and suppose that Δ k is zero for phase matched SHG process, we can roughly estimate the effective second-order susceptibility of the spiral WS2 nanosheet as the following: ()

() () ()

= ∗ , ∗ ",

,

,

(2)

In our experiments, the length of WS2 can be directly obtained from the AFM measurement. The inset in the upright in Figure 6b shows that the length for the experiment is about 20 nm. Since its length is much shorter than the wavelength of the incident laser, we can take the directly measured length as the effective interference 10


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length for SHG measurements. The calculated effective second-order nonlinear susceptibility of BBO for the SHG process of 880 nm → 440 nm is 2.04 pm/V.49 Then, we can roughly estimate the effective second-order susceptibility of the spiral WS2 nanosheet to be 0.68 nm/V. It may be noted that our obtained effective second-order nonlinear susceptibility is a rough estimated value since we maybe overestimate it by neglecting the difference of collection efficiency and the refraction index between BBO and spiral WS2 sheet (the nanostructures have a larger dielectric constant than the bulk50). Nonetheless, our result is the same order as the reported effective second-order susceptibility of monolayer TMDs, such as 0.5 nm/V for monolayer WS2,24 0.6 nm/V for monolayer MoS2,27 and 0.15 nm/V for stacking twisted bilayer.36

CONCLUSIONS

In conclusion, strong nonlinear optical effects, including SHG and THG, have been observed in CVD-grown WS2 spiral structures. SHG intensity quadratically increases with the layer numbers, other than the diminishing oscillation of normal 2H stacking TMDs. The contrary SHG behavior is attributed to the broken symmetry from the twisted screw structures revealed by AC-TEM observation. Furthermore, the twist angle of the screw structure, 5 degrees, was measured by HRTEM, and confirmed by the polarization test of SHG output. Moreover, we roughly estimate the effective second-order nonlinear susceptibility of spiral WS2 nanosheet. The investigation of light-matter interaction at atomic scale can help opticists gain insight

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into the origin of the radiation, and enhance the designation and preparation of optical devices. The discovery and the understanding of the accumulation of nonlinear susceptibility of the spiral structures with increasing thickness break the symmetric limitation of TMDs, which not only provides a fundamental stage to investigate the light-matter interaction at atomic scale on the structures with desired thickness, but also find a promising way to extend the nonlinear application of TMDs.

Methods

Sample preparation: An alumina boat loaded with WS2 powder was firstly placed at the heating center of a 1-inch quartz tube, and a piece of Si wafer (with 300nm SiO2) was placed at the downstream about 13 centimeters to the center of furnace. Argon gas flow was introduced into the system at a flow rate of 60~100 sccm for 60 minutes to exhaust the oxygen inside the tube before heating. The furnace was then rapidly heated to 1150°C and maintained at this temperature for about 30 min, and WS2 nanosheets with spiral structures were deposited on SiO2 surfaces. Optical measurements. SHG measurements on a spiral sample were performed using a confocal microscope (WITec, alpha-300). A mode-locked Ti:Sapphire laser (Tsunami) at 800 nm (pulse width 80 fs, repetition frequency 80 MHz) is amplified by a regenerative amplifier laser (Spitfire Ace 100, 1KHz) and then is introduced into an optical parameter amplifier (OPA, TOPAS Prime). The output laser from OPA can be continuously tuned from 1120 nm to 1330 nm used for the SHG measurements. The source light was focused by an infrared objective lens (10×, zeiss) from bottom to the sample. The SHG signal was collected by another objective lens (20×, zeiss) while the light source was filtered by an optical filter and SHG signal passed. TEM characterization. Cross-sectional specimens were fabricated using a dual-beam instrument (FEI Helios 600i), which combines scanning electron microscope imaging, focused ion beam deposition and milling. Fabrication process started with the 12


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deposition of a 1-µm-thick Pt strap layer on the surface at a chosen location, which protects the region of interest during milling and provides mechanical stability to the cross-sectional slice after its removal. 30 kV Ga+ beam with a current of 0.1-5 nA was adopted to mill the cross-section slice and reduced the thickness to ~1 µm. Then the milled slice was welded to a nanomanipulator needle using further Pt deposition, extracted and transferred to an Omni probe copper TEM grid. Finally, a gentle polish with Ga+ ions (at 5 kV and 48 pA) was used to remove side damage and reduce the specimen thickness to less than 100 nm before insert into TEM for imaging. TEM imaging was carried out using an image aberration-corrected TEM (FEI Titan 80-300 operated at 80 kV) equipped with a CCD camera (Gatan UltraScan 1000). The exposure time was 1 s. The third-order spherical aberration was set in the range 1-15 µm. Extra Fourier filter was applied to TEM images to improve signal-to-noise ratio.

ASSOCIATED CONTENT Supporting Information Available:

Including more examples for power-dependent SHG and THG results and polarization results. This material is available free of charge on the ACS Publication website at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Corresponding email: [email protected], [email protected] Notes

The authors declare no competing financial interest.

Author Contributions 13


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#

X.P.F. and Y.J. contributed equally to this work.

Acknowledgement The authors are grateful to the National Natural Science Foundation of China (No. 51672076, No. 11374092, No. 61474040, No.61574054, No.61505051), the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, the Hunan Provincial Science and Technology Department (No. 2014FJ2001, No. 2014GK3015, No. 2014TT1004), Joint Research Fund for Overseas Chinese, Hong Kong and Macau Scholars of the National Natural Science Foundation of China (No. 61528403), and The Foundation for Innovative Research Groups of NSFC (Grant 21521063).

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Grigorenko, A. N.; Geim, A. K.; Casiraghi, C.; Castro Neto, A. H.; Novoselov, K. S. Strong Light-Matter Interactions in Heterostructures of Atomically Thin Films. Science 2013, 340, 1311-1314. 31. Withers, F.; Del Pozo-Zamudio, O.; Mishchenko, A.; Rooney, A. P.; Gholinia, A.; Watanabe, K.; Taniguchi, T.; Haigh, S. J.; Geim, A. K.; Tartakovskii, A. I.; Novoselov, K. S. Light-emitting Diodes by Band-structure Engineering in van der Waals Heterostructures. Nat. Mater. 2015, 14, 301-306. 32. Meng, F.; Morin, S. A.; Forticaux, A.; Jin. S, Screw Dislocation Driven Growth of Nanomaterials. Acc. Chem. Res. 2013, 7, 1616-1626. 33. Chen, L.; Liu, B.; Abbas, A.-N.; Ma, Y. Q.; Fang, X.; Liu, Y. H.; Zhou, C. W, Screw-Dislocation-Driven Growthof Two-Dimensional Few-Layer and Pyramid-like WSe2 by Sulfur-Assisted Chemical Vapor Deposition. ACS Nano 2014, 8, 11543-11551. 34. Morin, S. A.; Forticaux, A.; Bierman, M. J.; Jin, S., Screw Dislocation-Driven Growth of Two-Dimensional Nanoplates. Nano Lett. 2011, 11, 4449-4455. 35. Hsu, W.-T.; Zhao, Z.-A.; Li, L.-J.; Chen, C.-H.; Chiu, M.-H.; Chang, P.-S.; Chou, Y.-C.; Chang, W.-H. Second Harmonic Generation from Artificially Stacked Transition Metal Dichalcogenide Twisted Bilayers. ACS Nano 2014, 8, 2951-2958. 36. Zhao, M.; Ye, Z.; Suzuki, R.; Ye, Y.; Zhu, H.; Xiao, J.; Wang, Y.; Iwasa, Y.; Zhang, X. Atomically Phase-Matched Second-Harmonic Generation in a 2D Crystal. Light-Sci Appl. 2016, 5, e16131. 19


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Plasmonic

Nanostructures:

From

Fundamental

Principles

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Confinement. Physica B 2012, 407, 4627-4634.

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Figure 1. Strong SHG effect of spiral WS2 nanosheet. (a,b) the optical image and the zoomed AFM image of spiral WS2 nanosheet. The black dashed lines show the zoomed area, and the blue dashed guide lines indicate the screw structures of the nanosheet. (c) The line profile marked with white dashed line in (b), giving the height of 0.7 nm for a single WS2 layer. d) is the corresponding schematic for the screw structure. The blue and red arrows show the angle difference between top layer and bottom

layer

along

the

armchair

direction.

(e,f)

Layer-dependent

and

power-dependent SHG of the nanosheet. The inset in (e) shows that SHG intensity increases with increasing layer numbers. The inset in (f) shows that parabolic increase of SHG intensity with increasing power density.

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Figure 2. THG of spiral WS2 nanosheet. (a) Strong SHG and THG effect in a large range from 1120 ~ 1300 nm. (b) Power-dependent SHG and THG effect under different power density. The inset shows the log plot of the intensity of SHG and THG with increasing power density. The slope is 2 for SHG, and 3 for THG, respectively. (c) Layer-dependent SHG and THG for a normal stacking structure. The inset shows the optical image of a normal stacking structure, in which the top layer is highlighted with green. (d) Power-dependent SHG effect and THG effect under different power density. The inset shows the log plot of the intensity of SHG and THG with increasing power density. The slope is 2 for SHG, and 3 for THG, respectively.

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ACS Nano

Figure 3. Atomic structures of spiral WS2 sample. (a) SEM image of a cross-sectional specimen for AC-TEM observation. The red line in the inset shows the position for the fabrication of the specimen. (b) Side view and top view of atomic structures of WS2. The red and green shorts lines in side-view model show the distance between a W atom and two neighboring S atom. The side-view models are imposed onto the AC-TEM images in (c). (c) The atomic AC-TEM image of the cross-sectional WS2 specimen, with the corresponding side-view atomic model imposed onto it. Red dashed lines show the broken symmetry along c direction. d) Atomic model for normal 2H and 3R stacking structures of WS2.

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Figure 4. HRTEM image of a spiral WS2 nanosheet. (a) A TEM image of a spiral structure on copper grid. Three positions were selected for further HRTEM measurements, and the corresponding results are listed in (b-d), respectively. Dashed arrows indicate the direction of lattice sites, and the dashed arrows in the images (b,c) are translated to the image (d) with the same color. These three arrows in (d) show that the lattice keeps rotation from the bottom layer to the top layer. The rotation angle between the top layer and the bottom layer is about 5 degrees, showing that the spiral structure is a twisted structure.

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ACS Nano

Figure 5. Polarization angle dependent SHG and THG intensity. (a,c) are for monolayer WS2 and (b,d) are for spiral structures. During the measurement, a polaroid was put between the spectrograph and the collected lens. For (a,b), the intensity of SHG and THG was collected by rotating the polaroid with the initial direction parallel to the polarization of the incident laser, while the polarization of the incident laser and sample position were fixed during the measurement. For (c,d), the intensity was collected by rotating the sample with the same initial direction as the measurement in (a,b), while the polarization of the incident laser and the polaroid were fixed. During the experiments, the polarization of the incident laser is always adjusted to be parallel to the armchair direction of the bottom layer, normal to the brim of the bottom layer.

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Figure 6. The comparison of SHG output between a spiral WS2 nanosheet and a commercial BBO crystal. (a) SHG outputs for a femtosecond laser under the same conditions (such as the same beam size, wavelength, et al.) except for the incident laser power intensity, shown with red for that on a WS2 nanosheet and green for one on a commercial BBO respectively. Since the output of the BBO crystal is too strong, much lower intensity was used during the measurement. (b) AFM image of the spiral WS2 nanosheet for the above SHG measurement, and line profile marked with black dashed line is shown in the inset.

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90x39mm (300 x 300 DPI)


Broken Symmetry Induced Strong Nonlinear ... - ACS Publications

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