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Title: Enhanced light-matter interaction in atomically thin semiconductors coupled with dielectric nano-antennas
Author: Sortino, Luca
ISNI:       0000 0004 8506 3085
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2019
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By confining strong electromagnetic fields at their surfaces, nano-structures with subwavelength dimensions can enhance the light-matter interaction of closely coupled emitters. While these phenomena have been extensively studied for plasmons in metals, recently high-refractive-index dielectrics have gained attention as they exhibit optical Mie resonances offering a low-loss platform to overcome plasmonic quenching mechanisms. Between atomically thin semiconductors, the family of transition metal dichalcogenides (TMDs) exhibits promising optical properties, such as tightly bound excitons and a transition to direct bandgap in the single layer form, with favourable integration with nano-photonic structures. Most of the efforts so far have been given to plasmonic structures, while coupling 2D-TMDs with dielectric nano-antennas is mostly unexplored. In this work, we transferred single and double layer TMD WSe2 onto an array of highindex gallium phosphide (GaP) dimer nano-antennas, leading to a strong enhancement of WSe2 photoluminescence (PL) at room temperature. We show that this is a result of an increased absorption and enhanced spontaneous emission rate, provided by the strongly confined optical mode of the nano-antennas, as well as an emission redirection. Further manifestation of the strong photonic confinement is observed in the enhanced Raman scattering signal and polarization dependent PL. Moreover, the nano-structures displace the 2D semiconducting layers in the out-of-plane direction, allowing the strain-tuning of the local WSe2 band structure. We model the strain topography in a mechanical picture and show a co-location of largest tensile strain and of the maximum photonic enhancement. This concurrence let us directly probe the strain-induced WSe2 band structure renormalization from the distinct spectral signatures. In monolayers we observe a tuning of the excitonic resonance above 50 meV. For bilayers, an indirect bandgap semiconductor, high level of strain results in the transition to direct bandgap. By studying the cryogenic PL emission at liquid helium temperatures, we confirm that the strain-induced potential acts as a trap for photo-generated excitons. Finally, we show that this approach can be exploited for the deterministic positioning of strain-induced bright single-photon emitters at the nano-antenna location, opening to the coupling between single 2D quantum emitters and dielectric nano-resonators. These results highlight dielectric nano-structures as a platform to improve light-matter interaction in 2D semiconductors and for nano-scale positioning of 2D quantum emitters.
Supervisor: Tartakovskii, A. I. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available