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Nanophotonics with 2D transition metal dichalcogenides [Invited]

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Abstract

Two-dimensional semiconducting transition metal dichalcogenides (TMDCs) have recently become attractive materials for several optoelectronic applications, such as photodetection, light harvesting, phototransistors, light-emitting diodes, and lasers. Their bandgap lies in the visible and near-IR range, and they possess strong excitonic resonances, high oscillator strengths, and valley-selective response. Coupling these materials to optical nanocavities enhances the quantum yield of exciton emission, enabling advanced quantum optics and nanophotonics devices. Here, we review the state-of-the-art advances of hybrid exciton-polariton structures based on monolayer TMDCs coupled to plasmonic and dielectric nanocavities. We discuss the optical properties of 2D WS2, WSe2, MoS2 and MoSe2 materials, paying special attention to their energy bands, photoluminescence/absorption spectra, excitonic fine structure, and to the dynamics of exciton formation and valley depolarization. We also discuss light-matter interactions in such hybrid exciton-polariton structures. Finally, we focus on weak and strong coupling regimes in monolayer TMDCs-based exciton-polariton systems, envisioning research directions and future opportunities for this material platform.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Figures (6)

Fig. 1
Fig. 1 Optical properties of 1L-WS2. (a) Energy structure of 1L-WS2, blue and red curves demonstrate the valence and conduction bands. (b) Absorption spectra (orange curve) and photoluminescence (blue curve) of 1L-WS2 at room temperature. (c) Photoluminescence spectra at the back-gate voltages between −40 and + 40 V at room temperature. (d) Log-log plots of integrated intensities of four emission components versus excitation power at 4.2K. Solid lines are linear fit curves, and the corresponding α values in the legend are the slopes. Adapted with permission from: (a),(b) – [97], (c),(d) – [18].
Fig. 2
Fig. 2 Optical properties of 1L-WSe2. (a) Energy structure; blue and red curves demonstrate the valence and conduction bands. (b) Spectra of absorption (orange curve) and photoluminescence (blue curve) of 1L-WSe2 at room temperature. (c) Absolute electron-hole pair densities for 1s excitons (black spheres), unbound electron−hole pairs (red spheres), and total density of the two contributions (blue diamonds) as a function of the delay time after nonresonant excitation (3.04 eV). (d) The valley polarization degree as a function of delay time for several temperatures. The theory lines are compared with experiments from [122] (dots). The shadow gray region represents the pump pulse. Adapted with permission from: (a) – [108], (b) – [75], (c) – [108], (d) – [123].
Fig. 3
Fig. 3 Optical properties of 1L-MoS2 and 1L-MoSe2 (a), (c) Energy structure of 1L-MoS2 and 1L-MoSe2; blue and red curves demonstrate the valence and conduction bands. (b), (d) Spectra of absorption (orange curve) and photoluminescence (blue curve) of 1L-MoS2 and 1L-MoSe2. Adapted with permission from: (a) – [125], (b) – [60], (c) – [126], (d) – [59].
Fig. 4
Fig. 4 Optical properties of dielectric (c-Si) and metallic (Ag) spherical nanoparticles. (a) Normalized total scattering cross-section (black curve) with partial contributions of ED (red solid curve), MD (blue solid curve), EQ (brawn dashed curve) and MQ (blue dashed curve) of a Si nanoparticle of R = 80 nm in air ε h =1. Red dashed curve shows scattering efficiency. Inset: electric field distribution at MD resonance. (b) The same but for R = 100 nm. (c) Normalized total scattering cross-section of the Ag nanoparticle of R = 10 nm (black solid curve) and R = 30 nm (black dashed curve) in air. Red curves show scattering efficiency. (d) Normalized total scattering cross-section of the Ag nanoparticle of R = 30 nm (black solid curve) with partial contributions of ED (red solid curve) and EQ (brawn dashed curve) hosted in a dielectric media with ε h =1.73. Red dashed curve shows scattering efficiency.
Fig. 5
Fig. 5 1L-TMDCs-based nanostructures in the weak coupling regime. PL intensity of CVD-grown 1L-WS2 before and after fabrication of four different optical antennae. Insets are SEM images of each type of optical cavity; the scale bar is 400 nm. (b) PL spectra from a 1L-MoS2 on a SiO2/Si substrate (blue) and in the nanocavity (red) obtained using a diffraction-limited excitation spot. The intensity is measured per unit of excitation power and per unit of integration time. (c) PL spectra of the 1L-MoS2 with and without spiral structures, under the excitation of different circular polarized light at 633 nm. Laser power was controlled at 2.1 mW. Inset: schematic of spiral ring structure on SiO2/Au/SiO2/Si substrate with circularly polarized light excitation. (d) Probing radiative emission of dark excitons of 1L-WSe2 through polarization dependence of tip-enhanced photoluminescence. Excitation polarization dependent TEPL spectra of 1L-WSe2 on an Au substrate at ~1 nm tip-sample distance with tip selective XD emission (orange curve). Inset: Finite-difference time domain (FDTD) simulation of the in-plane (right) and out-of-plane (left) optical field intensity and confinement. Adapted with permission from: (a) – [61], (b) – [60], (c) – [65], (d) – [24].
Fig. 6
Fig. 6 1L-TMDCs-based nanostructures in the strong coupling regime. (a) Schematic showing the heterostructure composed of an individual Au nanorod coupled to the WS2. (b) Dark-field scattering spectra from different individual gold nanorods coupled to the same 1L-WS2 flake (different curves correspond to various aspect ratios of the gold nanorods). (c) Colored coded normalized scattering spectra from the heterostructures with different detunings between the plasmon resonances and exciton. The two white lines represent the fittings using the coupled harmonic oscillator model. The dashed white line indicates the exciton transition energy. The Rabi splitting energy is determined as 106 meV. (d) Artist view of the hybrid system: high density of photonic states (hot-spots) is shown at the corners of the Ag nanoprism, which overlaps with the 1L-WS2 for efficient plasmon-exciton interaction. Inset shows the SEM image of such particle (scale bar 100 nm) and a magnified view of the dark-field image. (e) Dark field spectra for an individual Ag nanoprism – 1L-WS2 hybrid as a function of temperature. Semi-transparent red curves show coupled oscillator model fits of the data. (f) Temperature dependence of the coupling constants for exciton and trion extracted via coupled oscillator model. Adapted with permission from: (a) – [64], (b) – [17].

Tables (1)

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Table 1 Quantitative comparison of different structures supporting the strong coupling regime.

Equations (3)

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S sct = 2 (kR) 2 l=1 (2l+1) ( | a l | 2 +| b l | 2 ), S ext = 2 (kR) 2 l=1 (2l+1) Re( a l + b l ), S abs = S ext S sct ,
V eff = ε(r)|E(r) | 2 dV max( ε(r)|E(r) | 2 ) ,
F p γ ex γ ex 0 = g 2 γ cav γ ex 0 = 3 4 π 2 ( λ n ) 3 Q V eff ,
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