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

Monolayer transition metal dichalcogenides (TMDs) are materials with unique potential for photonic and optoelectronic applications. They offer well-defined tunable direct band gaps in a broad electromagnetic spectral range. The small optical path across them naturally limits the light matter interactions of these two-dimensional (2-D) materials, due to their atomic thinness. Nanoscale plasmonic antennae offer a substantial increase of field strength over very short distances, comparable to the native thickness of the TMD. For instance, it has been demonstrated that plasmonic dimer antennae generate hot-spot field enhancements by orders of magnitude when an emitter is positioned exactly over the middle of their gap. However, 2-D materials cannot be grown or easily transferred, to reside midgap of the metallic dimer cavity. Hence, it is not plausible to simply take the peak intensity as the emission enhancement factor. Here we show that the emission enhancement generated in a 2-D TMD film by a monomer antenna cavity rivals that of dimer cavities at a reduced lithographic effort. We rationalize this finding by showing that the emission enhancement in dimer antennae depends not on the peak of the field enhancement at the center of the cavity but rather from the average field enhancement across a plane located beneath the optical cavity where the emitting 2-D film is present. We test multiple dimer and monomer antenna geometries and observe a representative 3-fold emission enhancement for both monomer and dimer cavities as compared to the intrinsic emission of chemical vapor deposition (CVD)-synthesized WS2 flakes. This finding suggests facile control and enhancement of the photoluminescence yield of 2-D materials based on engineering of light matter interactions that can serve as a testbed for their rapid and detailed optical characterization.