AbstractâOptical control on specific valley polarization in transition metal dichalcogenide (TMD) monolayers is highly desirable for applications in Valleytronics.
Valley selective optical emission of 2D excitons using chiral metasurface S. Guddala1, R. Bushati1,2, V. M. Menon1,2 1
Department of Physics, City College, City University of New York (CUNY), New York, USA 2 Department of Physics, Graduate Center, City University of New York (CUNY), New York, USA Abstract—Optical control on specific valley polarization in transition metal dichalcogenide (TMD) monolayers is highly desirable for applications in Valleytronics. We demonstrate specific valley polarization aided through the integration of TMD 2D materials with chiral metasurface at room temperature. Keywords— 2D Materials, interactions, valleytronics
metamaterials,
Mengyao Li2,3, A. B. Khanikaev2,3 3
Department of Electrical Engineering, City College, City University of New York (CUNY), New York, USA
way for applications in valleytronics and advanced information technology. II. DESIGN AND FABRICATION OF CHIRAL METASURFACE
light-mater
I. INTRODUCTION The atomic layer thick transition metal dichalcogenides (TMDs) have seen large scientific interest for the past few years, owing to their dramatic quantum confinement effects, such as indirect to direct band gap transition, broken inversion symmetry, large exciton binding energy and strong photoluminescence, etc. In addition, the inversion symmetry breaking combined with provided time reversal symmetry can make these TMDs show valley specific polarized emission due to the excitation of specific valleys in the momentum space i.e, the transitions at two valleys K and K' are allowed for σ+ (left) and σ- (right) circular polarized lights respectively, called valley polarization [1]. Although the generation and detection of this mechanism has been achieved through optical and electrical means, the control on the selective valley transition (K or K') has been a strong demand for its application in spintronics, valleytronics and information carriers in quantum computation. In recent years, there has been strong interest in achieving selective valley polarization by means of magnetic fields [2] and intense optical fields [3]. The directional coupling of specific valley emission has also been achieved through the integration of TMD materials with plasmonic nanostructures [4-6]. Here, we report a novel approach to address selective valley polarization and its directional emission at room temperature by engineering a dielectric chiral metasurface to work with alone incident linear polarization. This chiral metasurface with TMD monolayer generates left and right handed circular polarization depending on the in-plane orientation of the incident linear polarization with respect to the unit cell as shown in the schematic Fig. 1a. Thus, our metasurface design permits selective valley (K and K') excitation and its emission from the valley as respective σ+ (left) and σ- (right) circularly polarized light. So, we prove that the designed chiral metasurface has great potential in room temperature optical control of the valley emission in TMD material and paving the
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Fig. 1. a) Illustration of valley excitons emission with opposite helicity by a chiral metasurface with asymmetric Ge nanorods. b) SEM image of the fabricated Ge chiral metasurface on the TMD monolayer. A part of the TMD WS2 monolayer below the nanorods can also be noticed. The inset shows the single unit cell of the metasurface.
The dielectric chiral metasurface was designed with two Ge nanorods of asymmetric lengths arranged orthogonal to each other on a WS2 monolayer, which was mechanically exfoliated on to PDMS stamp and then transferred to thermal oxide coated silicon substrate. A schematic of the chiral metasurface design is shown in Fig. 1a. The structural dimensions were simulated by using finite element method based COMSOL Multiphysics software for plane wave incidence on Ge nanorod array. The unit cell of the structure (Fig. 1b inset) contains two Ge rods with two asymmetric lengths of 165 and 285 nm along horizontal and vertical directions, respectively, with a corner spacing of 20nm. The width and height of the rods are 60 and 40 nm respectively. The phase difference between two orthogonal nanorods reflected waves is tailored to be 900 at the desired wavelength of operation by controlling the length, width and thickness of the nanorods as well as the gap between them. At the resonance wavelength, the metasurface generates left-handed circular polarization (σ+) for the linear polarization normal incidence with electric field orientation is at +450 to the in-plane rods of the unit cell and similarly generates righthanded circular polarization (σ-) for -450 as shown in Fig.2a and 2b respectively. The electric field distribution shows hotspots with one handedness circular polarization generation. The electric field distribution corresponding to the resonance at 575 nm shows strong confinement of electric field at 20 nm gap between the rods. +450
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[1] K. F. Mak, K. He, J. Shan and T. F. Heinz, “Control of valley polarization in monolayer MoS2 by optical helicity” Nat. Nanotech. 7, 490-493 (2012). [2] D. MacNeill, C. Heikes, K. F. Mak, Z. Anderson, A. Kormányos, V. Zólyomi, J. Park, and D. C. Ralph, “Breaking of Valley Degeneracy by Magnetic Field in Monolayer MoSe2” Phy. Rev. Lett., 114, 037401-5 (2015).
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[3] E. J. Sie, J. W. McIver, Y. H. Lee, L. Fu, J. Kong and N. Gedik, “Valleyselective optical Stark effect in monolayer WS2” Nat. Mat 14, 290-294 (2015).
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Authors acknowledge financial support of Army Research Office through grant number W911NF-16-1-0256 and NSF through grant number ECCS-1509551. Author S. Guddala acknowledge the fellowship from IUSSTF and SERB India. .
REFERENCES
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ACKNOWLEDGMENT
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excitation wavelength of 625 nm and k-space spectroscopy. The laser line was filtered by the high band pass 600 nm filter. Valley polarization of the bare WS2 monolayer outside the metasurface was first studied with excitation wavelength of 625 nm, for both σ+ (LCP) and σ- (RCP) polarizations incidence. A combination of a quarter wave plate and a linear polarizer was used to resolve the emission from the two valleys of the monolayer. Now the metasurface with monolayer for the linearly polarized resonant excitation wavelength of 575 nm shows valley specific emission. For the linear polarization incidence of +450 the K valley (σ-) emission is found to be more than the K' valley (σ+) emission (Fig. 2c). Similarly, for the other angle of -450 incidence, in the reverse, the valley K' (σ+) emission is found to be more than the K valley (σ-) emission (Fig. 2d). It is evident that linear polarization +450 incidence produces σ- polarized light and thereby prefered the K valley emission of the monolayer and vice versa for the -450 incidences. This indicates that the polarization of the reflected light from the metasurface for the +450 or -450 determines the specific valley emission of the TMD monolayer. The results suggest that a stringent observation of preferential valley polarization aided by the novel design of chiral metasurface.
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Figure 2: Simulated Electric field distributions and arrow map shows generated circular polarization for c) +450 and d) -450 incidence of linear polarization. The inset shows the orientation of the incident linear polarization electric field with respect to Ge nanorods. PL spectra shows c) σ- (K valley) emission is dominant over σ+ (K' valley) emission for +450 and d) σ+ (K' valley) emission is dominant over σ- (K valley) emission for -450 for helicity resolved conditions.
III. VALLEY EMISSION CONTROL BY METASURFACE The optical control of valley polarization in the fabricated metasurface was investigated by performing photoluminescence measurements by using linearly polarized
[4] S. H. Gong, F. Alpeggiani, B. Sciacca, E.C. Garnett, and L. Kuipers, “Nanoscale chiral valley-photon interface through optical spin-orbit coupling” Science 359, 443-447 (2018). [5] T. Chervy, S. Azzini, E. Lorchat, S. Wang, Y. Gorodetski, J. A. Hutchison, S. Berciaud, T.W. Ebbesen, and C. Genet, “Room Temperature Chiral Coupling of Valley Excitons with Spin-Momentum Locked Surface Plasmons” ACS Photonics (2018). [6] L. Sun, C.Y. Wang, A. Krasnok, J. Choi, J. Shi, J.S. Gomez-Diaz, A. Zepeda, S. Gwo, C.K. Shih, A. Alu, and X. Li,. “Routing Valley Excitons in a Monolayer MoS2 with a Metasurface” arXiv preprint arXiv:1801.06543 (2018).
