Testbeds for Transition Metal Dichalcogenide Photonics: Efficacy of Light Emission Enhancement in Monomer vs Dimer Nanoscale Antennae

Abstract

Monolayer transition metal dichalcogenides are uniquely-qualified materials for photonics because they combine well-defined tunable direct band gaps and self-passivated surfaces without dangling bonds. However, the atomic thickness of these two-dimensional (2D) materials results in low photo absorption limiting the achievable photo luminescence intensity. Such emission can, in principle, be enhanced via nanoscale antennae resulting in; a) an increased absorption cross-section enhancing pump efficiency, b) an acceleration of the internal emission rate via the Purcell factor mainly by reducing the antenna's optical mode volume beyond the diffraction limit, and c) improved impedance matching of the emitter dipole to the free-space wavelength.Plasmonic dimer antennae show orders of magnitude hot-spot field enhancements when an emitter is positioned exactly at the mid-gap. However, a 2D material cannot be grown, or easily transferred, to reside in mid-gap of the metallic dimer cavity. In addition, a spacer layer between the cavity and the emissive material is required to avoid non-radiative recombination channels.Using both computational and experimental methods, in this work we show that the emission enhancement from a 2D emitter-monomer antenna cavity system rivals that of dimers at much reduced lithographic effort. We rationalize this finding by showing that the emission enhancement in dimer antennae does not specifically originate from the gap of the dimer cavity, but is an average effect originating from the effective cavity crosssection taken below each optical cavity where the emitting 2D film is located. In particular, we test an array of different dimer and monomer antenna geometries and observe a representative ~300% higher emission for both monomer and dimer cavities as compared to intrinsic emission of Chemical Vapor Deposition (CVD)-synthesized WS 2 flakes. Observed enhanced light emission from these 3 atomically thin flakes together with the lithographic control of plasmonic antennae on them opens opportunities for engineering light-matter interaction in 2D systems in a test-bed comparable fashion, enabling bright and large-scale 2D opto-electronics.

Enhanced light-matter interaction in an atomically thin semiconductor coupled with dielectric nano-antennas

“…By coupling semiconducting TMDs to such plasmonic structures, large photoluminescence (PL) enhancements 16–21 , strong light–matter coupling 22–24 , brightening of the dark excitonic states 25 , and modification of optical properties of quantum light emitters 26,27 have been observed. In some of these reports, special care had to be taken to overcome optical losses in metallic plasmonic structures by introducing a few nm dielectric spacer separating the TMD layer 20,21,29 . This turns out to be particularly important to suppress quenching for quantum light emitters 26,27 .…”

Engineering photonic environments for two-dimensional materials

“…dielectric-metal interface-thus, overcoming the diffraction limit. This extreme reduction in V eff compensates for the lower Q of plasmonic resonators and allows for Purcell enhancement over a broader range of wavelengths than typical dielectric resonators [113]. Enhancement of both absorption and PL emission was achieved, shown in Figure 2G, using a plasmonic nanopatch antenna design [45].…”

Surface plasmon polariton–enhanced photoluminescence of monolayer MoS2 on suspended periodic metallic structures

“…Especially, plasmonic nanocavities are regarded as ideal candidates for both PL enhancement and subwavelength integration of light sources because of the unique capability of pronounced local resonances and extreme field concentration at nanoscales [12][13][14][15]. Thus far, localized surface plasmons of metallic nanoparticles, gap surface plasmons, and surface plasmon polaritons (SPPs) along structured metallic films have been intensively used to enhance PL of the two-dimensional (2D) exciton systems [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. However, most of the TMD/metal hybrid structures are suffering from limited active material areas with enhanced radiative and nonradiative decay rate [32], which severely hinders the applications of plasmon enhanced PL of TMD monolayers.…”

“…Here we demonstrate the SPP-enhanced exciton emission of MoS 2 monolayer through a suspended periodic metallic (SPM) structure by visible laser excitements. Thanks to the coherent propagating mode of the SPP that is in resonance with the excitation lasers, two orders of magnitude amplification of PL signals are enabled in the experiment, overwhelming most of plasmon-enhanced PL systems thus far [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. The underlying physics of such pronounced PL amplification is theoretically explored, which can be ascribed to the excitation rate increment from the boosted electric field of SPP as well as quantum yield enhancement of monolayer MoS 2 modulated by the suspended metallic nanostructures.…”

“…To unravel the underlying mechanisms of the PL enhancement, the EF averaged over area of P 2 is used due to the position-dependent plasmonic effects, which is defined as [22,34,40] EF…”