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Visualizing Optical Phase Anisotropy in Black Phosphorus Shoufeng Lan, Sean Rodrigues, Lei Kang, and Wenshan Cai ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00320 • Publication Date (Web): 28 Jun 2016 Downloaded from http://pubs.acs.org on July 3, 2016

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Visualizing Optical Phase Anisotropy in Black Phosphorus Shoufeng Lan,1 Sean Rodrigues,1,2 Lei Kang,2 Wenshan Cai1,2* 1 2

School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332

* Correspondence should be addressed to W.C. ([email protected])

KEYWORDS: black phosphorus, two-dimensional materials, optical anisotropy, birefringence, interferometry

ABSTRACT: Layered black phosphorus has triggered enormous interest since its recent emergence. Compared to most other two-dimensional materials, black phosphorus features a moderate band gap and pronounced in-plane anisotropy, which stems from the unique atomicpuckering crystal structure. The future potential of black phosphorus in optoelectronics demands a deeper understanding of its unique anisotropic behavior. In particular, the phase information of light when interacting with the material is imperative for many applications in the optical regime. In this work we have comprehensively studied a wide range of optical anisotropic properties of black phosphorus, including the Raman scattering, extinction spectra, and phase retardance by utilizing conventional spectral measurements and a uniquely developed interferometric spectroscopy and imaging technique. The phase retardance of light passed through black phosphorus is exploited in conjunction with polarization interferometric techniques to demonstrate an optical contrast an order of magnitude higher than a purely polarization-based measurement could offer.

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The past decade has witnessed the explosive development of two-dimensional (2D) materials, whose ever-expanding family now represents one of the most exciting frontiers in physics and materials science. The intriguing physical properties of 2D materials, primarily derived from their out of plane quantum confinement and the strong in-plane bonding, have enabled diverse applications in electronics, mechanics, as well as optics and photonics.1-7 Among this new class of material, graphene has laid the foundation for the 2D frontier of nanoscale optical applications in integrated nano-photonics,8, 9 ultrafast light detection,10, 11 and plasmonics.12, 13 Now, this field is quickly being populated with other materials such as transition metal dichalcogenide (TMDC) which exhibit exotic properties of their own. For instance, TMDC has demonstrated optical helicity controlled valley polarization which opens pathways for valleytronics.14-17 Recently, layered black phosphorus (BP) has been reintroduced to the family from its dormancy a century ago.18-21 As a new member of the 2D material class, BP bridges the semi-metallic graphene and semiconducting TMDC with a moderate band gap.22-26 Compared to most other 2D materials, whose electronic and photonic properties are isotropic in-plane, the most distinguishable property of BP is its in-plane anisotropy stemming from the unique atomic-puckering crystal structure. Previous studies have revealed a range of intriguing anisotropic behaviors of BP in terms of its optical spectrum, Raman scattering, light absorption, photo-detection, and electrical conductivity.27-34 While these properties reported in the literature have formed a valuable basis, many applications in the optical domain demand more information regarding the wave characteristics of the interacting light with the BP medium, specifically its phase. Moreover, the manipulation of the phase retardance of light in conjunction with interferometric techniques allows us to achieve optical contrast much stronger than an intensity measurement alone could 2 ACS Paragon Plus Environment

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offer, as has been demonstrated extensively in optical metrology. For this reason, we set to experimentally investigate the optical retardance of light passing through BP thin films. To comprehensively study the optical anisotropy of BP, a new method or tool is required to obtain a complete set of parameters that describes the anisotropic, thickness-dependent optical behavior. In this letter, we introduce a phase anisotropy measurement using polarization interferometry on BP flakes to uncover the anisotropic properties of this 2D material. Beyond conventional characterization techniques, we propose an interferometric measurement of phase anisotropy to not only precisely identify the crystalline axes with ultra-high sensitivity, but also unambiguously determine the optical principle axes (slow and fast) of the BP thin film. To verify our results, we independently collect Raman spectroscopy curves, as well as optical transmission, reflection and absorption spectra to identify the two crystalline orientations (a- and b- axis). Using this technique, we are able to distinguish a thickness difference of 5 nm between BP films. Furthermore, we perform phase imaging to visualize the optical anisotropy of the black phosphorus films. The background light transmitted from the isotropic substrate can be completely eliminated leaving a totally dark background and a distinct image from the destructive interference occurring on the substrate, hence demonstrating a highly valuable technique for surface characterization. The intensity contrast of the transmittances between the BP and the substrate using phase imaging can be an order of magnitude larger than that with regular polarized light microscopy.

Results and Discussion Black phosphorus comprises single element sharing of a honeycomb structure with graphene but in a puckering way. The atomic corrugations in the crystal structure of BP flakes 3 ACS Paragon Plus Environment

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(Figure 1a) induce anisotropic in-plane properties distinguishing it from other 2D materials like graphene and TMDCs. The anisotropy of the structure causes the in-plane photon-matter and electron-matter interactions to differ along the atomic puckering direction (a-axis) and the direction perpendicular to it (b-axis) as defined in Figure 1a. The microscopic images in Figure 1b indicate the two perpendicular principle axes of BP flakes with respect to the lab coordinates at representative thicknesses of 45 nm and 70 nm. The thicknesses are confirmed by AFM linecuts inset along the white dashed lines. In order to identify the BP samples and conduct preliminary anisotropic characterizations, Raman scattering spectroscopy is performed. An excitation source with a wavelength of 532 nm impinges on the BP flake along the z-direction. The Raman scattering peaks at around 365, 440 and 470 cm−1 show no spectral shift when input polarizations are manipulated along the two crystalline axes, however, the intensity differs dramatically. Specifically, the mode at 470 cm−1, which corresponds to the mode of the Γ-point in the band structure of BP and involves atomic motion primarily along the a-axis,35 has twice the intensity with the input polarization along the a-axis (Figure 1c, top) compared to the polarization along the b-axis (bottom). Electric effects induced by anisotropy are measured by I-V characterization of BP as depicted in Figure 1d. A voltage range from −1 V to 1 V is applied upon the electrode pairs separated by 22 µm. The 0° (90°) corresponds to the scenario where the electrode pairs are along the a- (b-) axis. The slope of the I-V curve, which relates to the electrical conductivity, with the electrodes along the a-axis is ~1.8 times as large as that along the b-axis. The anisotropic Raman signals and electrical conductance of our samples shown in Figure 1 are in good consistency with similar measurements previously reported.20,21


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To investigate the optical anisotropy of BP, we collect and analyze the transmission, reflection and absorption spectra of linearly polarized inputs at varied input angles. The polarization dependent transmission spectra for a representative flake (70 nm thick) illustrated in Figure 2a are normalized by that of the transparent substrate. The polarization direction of the incident light is varied at steps of 15° corresponding to the colored arrow inset in Figure 2a. A resonant transmission peak is observed at roughly 610 nm for an input polarization along the baxis. We note that the thickness of the BP used in the work is not enough to support Fabry-Perot resonance of even the lowest order. Moreover, the resonance phenomenon remains largely unchanged when we purposely alter the surrounding materials in a series of control experiments by changing the thickness of the cover film. In addition, BP is well known to be highly susceptible to oxygen with the bandgap of phosphorene oxides ranging from 1.76 eV to 2.13 eV.36-39 We believe the observed resonance behavior stems from the intrinsic properties of phosphorene oxides. The observation is also consistent with earlier results reported in [40]. Reflection spectra (Figure 2b, top) for the same sample, normalized to a perfect silver mirror, are collected with the same set of input polarization angles as seen in Figure 2a. The absorption () spectra (Figure 2b, bottom) are obtained from the transmission () and reflection () spectra by the relationship = 1 − − . A much larger absorption along the a-axis (red) than along the b-axis (purple) indicates a stronger photon-matter interaction. As governed by the optical selection rules, increased absorption occurs when the incident wave is polarized along the direction of atomic buckling (i.e., the a-axis).40 The spectroscopic characterizations described above provide valuable information about the anisotropic nature of the optical extinction of the specimen, but it still lacks a direct manifestation of how light waves are retarded along different directions in the crystal. In the 5 ACS Paragon Plus Environment

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subsequent study, we introduce an interferometric technique for measuring optical phase anisotropy not only to independently identify the crystalline directions, but also to unambiguously determine the optical fast and slow axes of black phosphorus. The two crystalline axes (a- and b-axis) of BP can be identified by placing the sample in between a pair of crossed polarizers. The anisotropic nature of BP gives rise to a notable contrast of the refractive index and light absorption between the two crystalline axes, as typically described in terms of the birefringence and the linear dichroism. As a result, any misalignment between the crystalline axis of the BP sample and the input polarization will result in a non-zero light output from the analyzer, because the light passing through the BP flake will become elliptically polarized in the general case. While the two principal axes can be determined with relative ease, identification of the optical fast and slow axis is a more daunting task. In order to determine the optical fast and slow axes and to quantify the phase anisotropy of BP, a voltage-controlled liquid crystal phase retarder is employed to calibrate the phase features, as illustrated in Figure 3a. A sensitivity of 0.1° of the phase delay from the liquid crystal, ∆ , can be obtained in the system. Light passing through the BP flake will experience destructive interference when ∆ + ∆ = , where the magnitude and polarity of the phase retardance from BP between the two optical axes (∆ ) can thereby be precisely determined. Detailed descriptions of the interferometry setup and how it functions for precise phase measurement are available in the Supporting Information. The transmitted light from the isotropic substrate is exactly zero when destructive interference occurs at the substrate (∆ = 0, ∆ = ), which provides us with the black background seen in Figure 3b, left. In this scenario, the imaging contrast between the sample and the substrate ( ⁄ ) verges on infinity. This demonstrates unique utility in the detection of single layer BP without direct mechanical contact and holds promising applications for future 6 ACS Paragon Plus Environment

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nanoscale device characterization. In the right image of Figure 3b, the destructive interference that occurs within the BP sample has a phase delay of ∆ =4.2º for an input wavelength of 635 nm. In this image, the interfered output from the sample is no longer precisely zero, but minimized, where the magnitude of the contrast equals the difference in transmission between the two polarizations along the optical axes of the BP. Moreover, the a-axis of the sample, which is along the atomic buckling of BP, is determined as the optical slow axis by the phase anisotropy measurement. Thickness dependence is another important property of the optical anisotropy of few-layered BP. We measure the polarization-dependent transmission for different samples with thicknesses ranging from 20 to 70 nm, as shown in Figure 3c (top). The transmission contrast at λ = 635 nm between the two optical principle axes, ∆ = −

!

, is shown in Figure 3c (middle) for the same set of thicknesses. The quasi-quadratic

relation between the transmission contrast and the thickness is confirmed by the phase anisotropy (∆ = ∆

!

− ∆ ) measurements for all thicknesses (Figure 3c, bottom), where the

retardance is defined as the absolute phase delay of light passing through the sample with respect to that through a substrate region. The thickness-dependent transmission contrast ∆Tab and phase anisotropy ∆φab are linear optical responses, and exhibit similar trends as observed in Fig. 3c. This is because the two linear optical responses, ∆Tab and ∆φab, are related to the contrasts in the real and imaginary parts, respectively, of the refractive index along the two crystalline axes. Therefore, the birefringence and the linear absorption dichroism are connected to each other thanks to the Kramers–Kronig relations. Expanding on our interferometric measurement of phase anisotropy we develop phase imaging of our BP sample. The polarization-dependent imaging contrast ( ⁄ ) for transmitted light is shown in the polar diagram in Figure 4a, where no phase interferometry is 7 ACS Paragon Plus Environment

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applied. The 0° (90°) direction corresponds to the scenario where the slow axis a (fast axis b) of the BP crystal is lined up with the polarization of the incident light. A relative contrast ", ⁄, # of 1.8 in transmitted intensity is observed between the two extreme cases. The two pictures at the top of Figure 4a show the transmission images of the 70-nm BP flake with input polarization along the a- (0º) and b- (90º) axis, respectively. In contrast, full-field phase anisotropic images are collected using low intensity laser light (λ = 635 nm) on the same BP flake without scanning the sample. The image contrast at the output of the interferometry is strongly sensitive to the orientation of the sample, as illustrated in Figure 4b. With a 70 nm thick BP flake, an intensity contrast ",$ ⁄,%& # of ∼17 is obtained for this imaging setup, which is an order of magnitude higher than imaging with a standard linear polarized input as seen in Figure 4a. To visualize the phase anisotropy, four interferometry measured, phase anisotropy images are provided in the left panel of Figure 4b, which correspond to the case when the slow (a-) axis is along 0°, 15°, 30° and 45°, respectively. Such a novel imaging technique provides a powerful tool for identifying the area, location, and crystal orientations of anisotropic 2D materials.

Conclusion The comprehensive study of the anisotropic optical properties of BP presented in this research is expected to form a solid foundation for the application of this novel 2D medium in photonics and optoelectronics. As few-layer crystals often exhibit distinct behaviors compared to their bulk counterparts, characterizations of their thickness-dependent, anisotropic responses in terms of the extinction spectra, phase retardance, and Raman scattering provide valuable information for a complete understanding of BP in the optical regime. In addition, the


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interferometric spectroscopy and imaging technique developed in this work offers a unique capability to uncover the phase information of light when interacting with BP flakes and is readily adaptable for profilometric and crystallographic studies of other anisotropic materials.

Methods

Sample Preparation. Black phosphorus thin films were produced by mechanical exfoliation of BP bulk crystals (Smart Element Inc.) on a silicon wafer covered by a 90-nm-thick thermally grown SiO2 layer. The thickness of the BP flakes was identified by atomic force microscopy (AFM, Bruker Dimension Fast Scan). Afterwards, a thin film of PMMA (polymethyl methacrylate) was spin-coated as a supportive layer and soaked in hot KOH solution (1 mol/L, 90 °C) for about 5 minutes until the PMMA layer with BP flakes was released from the wafer. After being rinsed in deionized water the PMMA/BP film was transferred onto a glass substrate and an additional PMMA layer, 150 nm thick, is spun onto the sample for protection.

Optical Characterizations. The anisotropy of BP films was first characterized by Raman and electrical measurements. Polarization resolved Raman spectroscopy was carried out using a micro Raman system (Horiba HR Lab Raman 300) equipped with a linearly polarized laser at an excitation wavelength of 532 nm. The transmission and reflection spectra of the BP flakes were measured using a home-made experimental setup dedicated to spectral measurements of samples with microscopic dimensions. Broadband light from a fiber coupled tungsten halogen source (B&W Tek BPS120) was collimated and used to illuminate the optical path. A set of linear polarizers and waveplates were employed to control the polarization state of the incident light and the light impinged upon the sample at near-normal incidence. The transmitted or reflected


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signal from the sample was collected by an objective lens (Mitutoyo, 100× Plan Apo NIR infinity-corrected) and transformed by another lens to form a magnified image of the sample. Then, light from a desired area within the beam profile was selected by an iris diaphragms placed at the image plane and delivered to a spectrograph system (Princeton Instruments Acton SP 2300i and PIXIS 400BR camera) for spectroscopy analyses. Light from the sample was calibrated to a bare substrate and a silver mirror, respectively, for the transmission and reflection modes. The interferometric spectroscopy and imaging were conducted using a voltage-controlled liquid crystal phase retarder, as described in the main text. A sensitivity of 0.1° in the phase anisotropy can be achieved in the system. All measurements were performed at ambient temperature.

ACKNOWLEDGMENTS This work was performed in part at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant ECCS-1542174). W.C. acknowledges the start-up fund from the Georgia Institute of Technology and the generous gift by OPE LLC in support of the scientific research in the Cai Lab. S.P.R. acknowledges the support of the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1148903.


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34. Mao, N. N.; Tang, J. Y.; Xie, L. M.; Wu, J. X.; Han, B. W.; Lin, J. J.; Deng, S. B.; Ji, W.; Xu, H.; Liu, K. H.; Tong, L. M.; Zhang, J. Optical Anisotropy of Black Phosphorus in the Visible Regime. J. Am. Chem. Soc. 2016, 138, 300-305. 35. Sugai, S.; Shirotani, I. Raman and Infrared Reflection Spectroscopy in Black Phosphorus. Solid State Commun. 1985, 53, 753-755. 36. Lu, W. L.; Nan, H. Y.; Hong, J. H.; Chen, Y. M.; Zhu, C.; Liang, Z.; Ma, X. Y.; Ni, Z. H.; Jin, C. H.; Zhang, Z. Plasma-Assisted Fabrication of Monolayer Phosphorene and Its Raman Characterization. Nano Res 2014, 7, 853-859. 37. Ziletti, A.; Carvalho, A.; Trevisanutto, P. E.; Campbell, D. K.; Coker, D. F.; Neto, A. H. C. Phosphorene Oxides: Bandgap Engineering of Phosphorene by Oxidation. Phys. Rev. B 2015, 91, 085407. 38. Favron, A.; Gaufres, E.; Fossard, F.; Phaneuf-L'Heureux, A. L.; Tang, N. Y. W.; Levesque, P. L.; Loiseau, A.; Leonelli, R.; Francoeur, S.; Martel, R. Photooxidation and Quantum Confinement Effects in Exfoliated Black Phosphorus. Nat. Mater. 2015, 14, 826-832. 39. Pei, J. J.; Gai, X.; Yang, J.; Wang, X. B.; Yu, Z. F.; Choi, D. Y.; Luther-Davies, B.; Lu, Y. R. Producing Air-Stable Monolayers of Phosphorene and Their Defect Engineering. Nat. Commun. 2016, 7, 10450. 40. Yuan, H. T.; Liu, X. G.; Afshinmanesh, F.; Li, W.; Xu, G.; Sun, J.; Lian, B.; Curto, A. G.; Ye, G. J.; Hikita, Y.; Shen, Z. X.; Zhang, S. C.; Chen, X. H.; Brongersma, M.; Hwang, H. Y.; Cui, Y. Polarization-Sensitive Broadband Photodetector Using a Black Phosphorus Vertical P-N Junction. Nat. Nanotechnol. 2015, 10, 707-713.


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Figure 1

Figure 1. Principal axes and anisotropic behavior of black phosphorus flakes. (a) Schematic of the crystal structure of black phosphorus (BP). The z-axis corresponds to the thickness of the layered BP. The a (armchair) and b (zigzag) axes are directed along and perpendicular to the atomic buckling of BP, respectively. (b) Microscopic images of representative samples of 45 nm and 70 nm thick. The thickness is confirmed with AFM measurement along the cutting lines. The principal axes a and b, determined by later experiments, are shown against the lab coordinates. (c) Anisotropic Raman spectra of a representative BP sample with a thickness of 60 nm. Linearly polarized 532-nm light is illuminated along the z-direction. No spectral shift for the peaks occurs with input polarizations along a- and b- axes, but the relative intensity changes significantly. (d) Angle-resolved I-V characteristics of BP flakes. The 0° (90°) direction corresponds to the a- (b-) axis of the BP sample. 15 ACS Paragon Plus Environment

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Figure 2

Figure 2. Optical anisotropy of BP flakes. (a) Polarization-dependent transmission spectra for the 70 nm thick BP flake. The polarization direction of the incident light for each curve is shown in the inset by arrow of the same color. Light interacts more strongly with the material along the atomic buckling (a-axis) direction. (b) Reflection (top) and absorption (bottom) spectra of the 70 nm BP flake with the same set of input polarization angles. 16 ACS Paragon Plus Environment

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Figure 3

Figure 3. Imaging and spectroscopy of phase anisotropy in black phosphorus. (a) Optical setup for the phase anisotropy measurement. The relative phase retardance between the principle axes of the BP is calibrated against a voltage-controlled liquid crystal retarder. The BP sample rotates around the optical path to locate the crystal axes and quantify the phase anisotropy. (b) Transmission images for a representative BP sample (45 nm thick) when destructive interference occurs at the substrate and the BP flake, respectively. The light source is monochromatic at 635 nm. (c) Thickness-dependent transmission contrast and phase anisotropy in black phosphorus. (Top) Microscopic images of the BP samples with a series of thicknesses, ranging from 20 nm to 70 nm. (Middle) Transmission contrast at λ = 635 nm for different thicknesses of the BP samples. Transmittance is lower when the incident light is polarized along the slow axis, a, of the BP structure. (Bottom) Phase anisotropy, ∆ = ∆

!

− ∆ , of layered black phosphorus

for various sample thicknesses determined using polarization interferometry. The retardation is defined as the absolute phase delay of light through the sample with respect to that through a substrate region.


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Figure 4

Figure 4. Phase anisotropic imaging provides enhanced intensity contrast. (a) Contrast of optical intensity in the transmitted light as a function of the incident polarization angle. The 0° (90°) direction of the polar diagram corresponds to the scenario where the slow axis a (fast axis b) of BP crystal is lined up with the polarization of the incident light. An intensity contrast of 1.8 is observed between the two extreme cases. The pictures on top show the transmission images of the 70-nm BP flake with input polarization along a- and b- axis, respectively. The two blue circles represent the sampling areas for BP and the substrate. (b) Intensity contrast of light through the BP flake based on phase anisotropy imaging. The same definition of the 0° (90°) direction as in Figure 5a is applied. A fixed phase delay of − ∆ is introduced by the liquid crystal phase retarder. An intensity contrast of ~17 is obtained in the phase-based images, an order of magnitude higher than that from regular, polarization-based anisotropy measurement. The four transmission images on the left correspond to the case when the slow (a-) axis is along 0°, 15°, 30° and 45°, respectively.


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