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Photonic-Crystal Exciton-Polaritons in Monolayer Semiconductors

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Photonic-Crystal Exciton-Polaritons in Monolayer Semiconductors ARTICLE DOI: 10.1038/s41467-018-03188-x OPEN Photonic-crystal exciton-polaritons in monolayer semiconductors Long Zhang 1, Rahul Gogna2, Will Burg3, Emanuel Tutuc3 & Hui Deng 1,2 Semiconductor microcavity polaritons, formed via strong exciton-photon coupling, provide a quantum many-body system on a chip, featuring rich physics phenomena for better photonic technology. However, conventional polariton cavities are bulky, difficult to integrate, and fl 1234567890():,; in exible for mode control, especially for room-temperature materials. Here we demonstrate sub-wavelength-thick, one-dimensional photonic crystals as a designable, compact, and practical platform for strong coupling with atomically thin van der Waals crystals. Polariton dispersions and mode anti-crossings are measured up to room temperature. Non-radiative decay to dark excitons is suppressed due to polariton enhancement of the radiative decay. Unusual features, including highly anisotropic dispersions and adjustable Fano resonances in reflectance, may facilitate high temperature polariton condensation in variable dimensions. Combining slab photonic crystals and van der Waals crystals in the strong coupling regime allows unprecedented engineering flexibility for exploring novel polariton phenomena and device concepts. 1 Physics Department, University of Michigan, 450 Church Street, Ann Arbor, MI 48109-2122, USA. 2 Applied Physics Program, University of Michigan, 450 Church Street, Ann Arbor, MI 48109-1040, USA. 3 Microelectronics Research Center, Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, TX 78758, USA. Correspondence and requests for materials should be addressed to H.D. (email: [email protected]) NATURE COMMUNICATIONS | (2018) 9:713 | DOI: 10.1038/s41467-018-03188-x | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-03188-x – ontrol of light-matter interactions is elementary to the implemented25 28. They are more compact and flexible, but suffer development of photonic devices. Existing photonic from intrinsically large absorption loss and poor dipole-overlap C fi 25,29 technologies are based on weakly coupled matter-light between the exciton and eld . systems, where the optical structure perturbatively modifies the Here we demonstrate sub-wavelength-thick, one-dimensional electronic properties of the active media. As the matter-light dielectric photonic crystals (PCs) as a readily designable platform interaction becomes stronger and no longer perturbative, light for strong coupling, which is also ultra-compact, practical, and and matter couple to form hybrid quasi-particles—polaritons. In especially well suited to the atomically thin vdWCs. Pristine particular, quantum-well (QW) microcavity exciton polaritons vdWCs can be directly laid on top of the PC without further feature simultaneously strong excitonic nonlinearity, robust processing. Properties of the optical modes, and in turn the photon-like coherence, and a meta-stable ground state, providing polariton modes, can be modified with different designs of the a fertile ground for quantum many-body physics phenomena1,2 PC. We confirm polariton modes up to room temperature by that promise new photonic technology3. Numerous novel types of measuring the polariton dispersions and mode anti-crossing in many-body quantum states with polaritons and polariton both reflectance and photoluminescence (PL) spectra. Strongly quantum technologies have been conceived, such as suppressed non-radiative decay to dark excitons due to the – topological polaritons4 6, polariton neurons7, non-classical state polaritonic enhancement is observed. We show that these – – generators8 10, and quantum simulators11 13. Their imple- polaritons have anisotropic polariton dispersions and adjustable mentation require confined and coupled polariton systems with reflectance, suggesting greater flexibility in controlling the exci- engineered properties, which, on one hand, can be created by tations in the system to reaching vdWC-polariton condensation engineering the optical component of the strongly coupled at lower densities in variable dimensions. Extension to more modes, on the other hand, is difficult experimentally using con- elaborate PC designs and 2D PCs will facilitate research on ventional polariton systems. polariton physics and devices beyond 2D condensates. Conventional polariton system are based on vertical Fabry Perot (FP) cavities made of thick stacks of planar, distributed Bragg reflectors (DBRs), which have no free design parameter for Results mode engineering and are relatively rigid and bulky against post- The system. We use two kinds of transition metal dichalcogen- processing. Different cavity structures have been challenging to ides (TMDs) as the active media: a monolayer of tungsten dis- implement for polariton systems as conventional materials are elenide (WSe2) or a monolayer of tungsten disulfide (WS2). The sensitive to free surfaces and lattice mismatch with embedding monolayers are placed over a PC made of a silicon-nitride (SiN) crystals. The recently emerged two-dimensional (2D) semi- grating, as illustrated in Fig. 1a. The total thickness of the grating conductor van der Waals crystals (vdWCs)14,15 are uniquely t is around 100 nm, much shorter than half a wavelength, making compatible with diverse substrate without lattice matching16. the structure an attractive candidate for compact, integrated However, most studies of vdWC polaritons so far continue to use polaritonics. In comparison, typical dielectric FP cavity structures – FP cavities17 23, which are even more limiting for vdWCs than are many tens of wavelengths in size. A schematic and scanning for conventional materials. This is because monolayer-thick electron microscopy images of the TMD-PC polariton device are vdWCs need to be sandwiched in between separately fabricated shown in Fig. 1. More details of the structure and its fabrication DBR stacks and positioned very close to the cavity-field max- are described in Methods. Since the grating is anisotropic in- imum. The process is complex, hard to control, and may change plane, its modes are sensitive to both the propagation and or degrade the optical properties of vdWCs22,24. Alternatively, polarization directions of the field. As illustrated in Fig. 1a, we metal mirrors and plasmonic structures have been define the direction along the grating bars as the x-direction, z ac y x ky TMD y kx Silicon nitride x TM Silicon dioxide TE Silicon t h d 500 nm 20.0 kV 8.9 mm L-× 220k b R (100%) d 0 1 0.75 200 0.5 (nm) z 400 0.25 μ 600 0 10 m 0 100 200 300 400 5.0 kV 9.9 mm L-× 15.0k y (nm) Fig. 1 The vsWC-PC structure. a Schematic of the 1D PC with a monolayer TMD placed on top. The SiN PC has multiple design parameters, including the period Λ, filling factor η, total thickness t, and the grating thickness h. The SiO2 capping layer has a thickness of d. b The TE-polarized electric-field profile of the PC in the y−z plane. The white lines mark the outline of the PC. The red line marks the position of the monolayer TMDs. c A side-view SEM image of the bare PC. d A top-view SEM image of the TMD laid on top of a PC 2 NATURE COMMUNICATIONS | (2018) 9:713 | DOI: 10.1038/s41467-018-03188-x | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-03188-x ARTICLE abcd Bare PC WSe2-PC WSe2-PC 0 0.25 0.5 0.75 1 0 0.25 0.5 0 0.25 0.5 0.75 1 2 2 R (100%) R (100%) PL (a.u.) A (100%) ⎪X⎪ ⎪C⎪ 1.8 1.78 Exp, R Sim, R Exp, R Sim, R Exp, PL Sim, A 1.76 1.74 Energy (eV) 1.72 1.7 –4 –2 024 –4 –2 0 24–4 –2 0 2 4 –4 –2 024 μ –1 μ –1 μ –1 μ –1 kx ( m ) kx ( m ) kx ( m ) kx ( m ) 0 0.25 0.5 0.75 1 0 0.25 0.5 0 0.25 0.5 0.75 1 2 2 R (100%) R (100%) PL (a.u.) A (100%) ⎪C⎪ ⎪X⎪ 1.8 1.78 Exp, R Sim, R Exp, R Sim, R Exp, PL Sim, A 1.76 1.74 Energy (eV) 1.72 1.7 –4 –2 024–4 –2 024 –4 –2 0 24 –4 –2 0 2 4 μ –1 μ –1 μ –1 μ –1 ky ( m ) ky ( m ) ky ( m ) ky ( m ) Fig. 2 Strong coupling between TE-polarized WSe2 exciton and PC modes measured by angle-resolved reflectance and PL at 10 K. The top/bottom row shows the along-bar/cross-bar directions, respectively. The left/right panels of a–c show the measured/simulated results, respectively. a Angle-resolved reflectance spectra of the bare PC, showing a sharp, dispersive PC mode. b Angle-resolved reflectance spectra of the WSe2-PC integrated device, showing split, anti-crossing upper and lower polariton modes. c Angle-resolved PL data (left) compared with the simulated absorption spectra of the WSe2-PC integrated device, showing the same anti-crossing polariton modes as in b. d The polariton energies ELP,UP vs. wavevector kx, ky obtained from the spectra in c. The lines are fits to the LP and UP dispersion with the coupled harmonic oscillator model, giving a vacuum Rabi splitting of 18.4 and 16.1 meV for the along-bar (top) and cross-bar (bottom) directions, respectively. The corresponding Hopfield coefficients |C2| and |X2|, representing the photon and exciton fractions in the LP modes, respectively, are shown in the top sub-plots across the grating bars as the y-direction, and perpendicular to Correspondingly, the effective mass and group velocity of the the grating plane as the z-direction. For the polarization, along polaritons are also highly anisotropic, which provide new degrees the grating corresponds to transverse-electric (TE), and across the of freedom to verify polariton condensation and to control its bar, transverse-magnetic (TM).
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