Subcategory: Physics (not Nanoscience)
Zheng Sun - City University of New York
Co-Author(s): Jie Gu, Zav Shotan, Christopher R. Considine, Michael Dollar, and Vinod Menon, City University of New York - Graduate Center & City College of New York, NY
Two dimensional (2D) atomic crystal of transition-metal dichalcogenides (TMDs) have large exciton binding energy (~0.3-0.7 eV) and has been used to demonstrate strong coupling of the 2D excitons with photons in a micro-cavity at 300K with near resonance excitation . 2D TMDs also provide a platform for investigating the valley polarized polaritons . Here we demonstrate helicity-dependent micro-cavity polariton emission from 2D atomic crystal of WS2 at room temperature via near resonant pump. A helicity of 27% is observed based on the strong coupling regime with Rabi splitting of 90meV. The observed effect is in contrast to conventional spin polarized polariton emission which only occurs above the condensation threshold . We designed and fabricated a low quality factor (Q) metallic FP micro-cavity with the detuning of -55meV. A large size of ~20 m× 20 m of monolayer WS2 was exfoliated mechanically and properly incorporated in the cavity. Raman and PL spectrum were carried out for demonstrating the high quality of the samples. Clear anti-crossing was observed in both angle resolved reflectivity and photoluminescence at 300K indicating the strong coupling between cavity mode and exA of WS2 with a large Rabi splitting of 94meV. Power dependence of angle-resolved spectrum were carried out and the corresponding helicity was extracted and analyzed, it shows stable helicity of 10% range from -30° to 30°. The results indicate the helicity-dependent micro-cavity polariton emission from 2D atomic crystal at room temperature via near-resonant pump. The optical helicity reaches about 27% for the whole lower polariton branch. The angle dependent polarization resolved helicity of exciton- polariton indicates this comes from the excitonic component in the strongly coupled exciton-polariton state which reduces the loss of helicity due to phonon scattering. The simple WS2-based metallic optical micro-cavity as the platform opens up the prospect for exploiting the valley polarization in 2D materials and the possibility of realizing practical polariton- based valleytronic devices.
References: Liu, X.; Galfsky, T.; Sun, Z.; Xia, F.; Lin, E.-c.; Lee, Y.-H.; Kéna-Cohen, S.; Menon, V. M. Nature Photonics 2015, 9, 30–34. Zeng, H., Dai, J., Yao, W., Xiao, D. & Cui, X. Valley polarization in MoS2 monolayers by optical pumping. Nature Nanotech. 7, 490–493 (2012). Mak, K. F., He, K., Shan, J. & Heinz, T. F. Control of valley polarization in monolayer MoS2 by optical helicity. Nature Nanotech. 7, 494–498 (2012). A. Amo, T. C. H. Liew, C. Adrados, R. Houdré, E. Giacobino, A. V. Kavokin & A. Bramati. Exciton–polariton spin switches. Nature Photon. 4, 361–366 (2010)
Funder Acknowledgement(s): We acknowledge the support of NSF through the EFRI program-grant # EFMA-1542863 and the ECCS Division through grant # ECCS-1509551.
Faculty Advisor: Vinod Menon, firstname.lastname@example.org
Role: Z.S., and V.M. conceived and designed the experiments, Z.S. fabricated the Micro-cavity, Z.S. performed reflectivity and polarizzation resolved PL measurements, Z.S. conducted simulations for the design.