Enhanced Nonlinear Light-matter Interactions in Transition-Metal-Dichalcogenide Metasurfaces

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2022

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Nauman, Mudassar

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Nonlinear light sources are central to a myriad of applications, driving a quest for their miniaturisation down to the nanoscale. In this quest, nonlinear metasurfaces hold a great promise, as they enhance nonlinear effects through their resonant photonic environment and high refractive index, such as in high-index dielectric metasurfaces. However, there is no empirical evidence of any dielectric metasurface to date, that may operate as an ultrathin nonlinear mirror to underpin the novel nonlinear light sources. To address the aforementioned challenge, this thesis primarily focuses on the study of nonlinear elastic light-matter interactions in bulk high-index Transition-metal-dichalcogenide (TMDC) metasurfaces via theoretical modelling and validation by experimental techniques. Firstly, by employing the electron beam lithography (EBL) method, we developed our fabrication recipe to realize arrays of TMDC metaatoms on low index transparent substrate. Leveraging the high index of TMDCs (Mo- and W- based sulphides and selenides), we have modelled and fabricated (by employing EBL technique) subwavelength arrays of TMDCs metaatoms on low index transparent Sapphire substrate. The proposed metasurfaces comprise different periodicity and shapes of metaatoms, that support electric and magnetic type Mie-resonances and high-Q resonances bound state in continuum (BIC), in the visible and near-infrared spectrum, respectively. Secondly, all the dielectric metasurfaces reported to date, operate in the diffractive regime for nonlinear emissions. Therefore, the ability to funnel nonlinear emissions into the zeroth-order beam to trigger the control to tune the directionality of second harmonic emissions from forward to backward direction remained as elusive as ever. To address this challenge, here we demonstrate a single-crystalline high index subwavelength TMDC MoS2 metasurfaces that exhibit enhanced single-beam second-and third-harmonic generation in the visible to the near-infrared regime. The highest refractive index > 4 of the proposed arrays of metaatoms allows us to induce multipolar resonances at the second harmonic (SH) wavelengths. These resonances can be tailored all optically (either by excitation wavelength or incident polarisation) to trigger the tuning of the unidirectional emission of SH light in a forward or backward direction. Moreover, the interference of these resonances with the SH light enables us to modulate the polrisation resolved SH response. Thirdly, in the quest of tunable nonlinear metasurfaces to underpin next-gen photonic devices, we proposed tunable enhanced second harmonic generation (SHG), enabled by exploiting the quantum interference effects between pronounced optical resonances such as the quasi-bound state in continuum (q-BIC) and thermally tunable excitons in high index subwavelength WS2 metasurfaces. Moreover, theoretical modelling and experimental studies were performed to engineer the novel combined effect of harmonic of q-BIC and exciton that enable us to observe the remarkable SHG enhancement and efficiency in the visible spectrum (600-650 nm), where WS2 is opaque. Finally, an additional study has been performed on inelastic light-matter interactions in MXene Quantum Dots-Monolayer WS2 Heterostructure. Highly enhanced photoluminescence (PL) is observed, in atomically thin WS2 sitting over the QDs arrays of MXene, by increasing the laser power at room temperature. In sum, this thesis aimed at engineering the nonlinear elastic light-matter interactions in high index TMDC metasurfaces. The intriguing results of the proposed metasurfaces may open new opportunities for metasurface-based nonlinear light sources, including ultrathin nonlinear mirrors and entangled-photon generation.

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Thesis (PhD)

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