Recent studies of transition metal dichalcogenides (TMDs) have revealed exciting optical properties, such as stable excitons and chiral light–matter interactions. Chemical vapor deposition techniques provide a platform for the fabrication of nanostructures with diverse geometries, ranging from horizontal flakes to flower-like structures. Raman spectroscopy is commonly used to characterize TMDs and their properties. Here, we use polarization-resolved Raman spectroscopy to probe the nanogeometry and orientation of WS 2 nanoflower petals.Exciting the nanoflowers with linearly polarized light, we observe an enhanced Raman response from flower petals oriented along the excitation polarization direction. Furthermore, the helicity-resolved Raman response of vertically oriented wall-like flower petals exhibits clear differences with horizontally oriented flakes. Although the photoluminescence from the nanoflowers is strongly reduced, the Raman response upon excitation in resonance with the WS 2 excitonic transition does reveal the presence of the exciton, which results in a distinct temperature dependence of the Raman response.@en

Currently, the nonlinear optical properties of 2D materials are attracting the attention of an ever-increasing number of research groups due to their large potential for applications in a broad range of scientific disciplines. Here, we investigate the interplay between nonlinear photoluminescence (PL) and several degenerate and nondegenerate nonlinear optical processes of a WS2 monolayer at room temperature. We illuminate the sample using two femtosecond laser pulses at frequencies ω1 and ω2 with photon energies below the optical bandgap. As a result, the sample emits light that shows characteristic spectral peaks of the second-harmonic generation, sum-frequency generation, and four-wave mixing. In addition, we find that both resonant and off-resonant nonlinear excitation via frequency mixing contributes to the (nonlinear) PL emission at the A-exciton frequency. The PL exhibits a clear correlation with the observed nonlinear effects, which we attribute to the generation of excitons via degenerate and nondegenerate multiphoton absorption. Our work illustrates a further step toward understanding the fundamental relation between parametric and nonparametric nondegenerate optical mechanisms in transition-metal dichalcogenides. In turn, such understanding has great potential to expand the range of applicability of nonlinear optical processes of 2D materials in different fields of science and technology, where nonlinear mechanisms are typically limited to degenerate processes.

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Due to their intriguing optical properties, including stable and chiral excitons, two-dimensional transition metal dichalcogenides (2D-TMDs) hold the promise of applications in nanophotonics. Chemical vapor deposition (CVD) techniques offer a platform to fabricate and design nanostructures with diverse geometries. However, the more exotic the grown nanogeometry, the less is known about its optical response. WS₂nanostructures with geometries ranging from monolayers to hollow pyramids have been created. The hollow pyramids exhibit a strongly reduced photoluminescence with respect to horizontally layered tungsten disulphide, facilitating the study of their clear Raman signal in more detail. Excited resonantly, the hollow pyramids exhibit a great number of higher-order phononic resonances. In contrast to monolayers, the spectral features of the optical response of the pyramids are position dependent. Differences in peak intensity, peak ratio and spectral peak positions reveal local variations in the atomic arrangement of the hollow pyramid crater and sides. The position-dependent optical response of hollow WS₂pyramids is characterized and attributed to growth-induced nanogeometry. Thereby the first steps are taken towards producing tunable nanophotonic devices with applications ranging from opto-electronics to non-linear optics.

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Two-dimensional Transition Metal Dichalcogenites (2D TMDs) have recently attracted enormous scientific attention for their unique optical properties. 2D TMDs are semiconductors with a direct bandgap in the visible wavelength range. In their valleys, stable excitons are formed even at room-temperature. A valley pseudospin can be attributed to each valley, making it possible to address each valley individually with circularly polarized light.

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We characterize the polarization properties of the photoluminescence from CVD-grown WS2 monolayer flakes. We find an inverse relationship between the non-uniform WS2 photoluminescence intensity, the valley polarization and the valley coherence.@en

In this thesis we describe three results of the interaction between WS2 and light: photoluminescence, the formation of exciton-polaritons and Raman scattering. The chirality of the photoluminescence interaction between WS2 and light opens the way for applictions in nanophotonics and specifically valleytronics, the field of interacting with and manipulating the valley pseudospin. In this work we propose a way to optically address and read out the valley pseudospin using silver and ZnO nanowires. Subsequently we confirm the existence of coherence between the WS2 valleys. Furthermore, exciton-polaritons in WS2 hold the promise of applications in nanophotonics that make use of the enormous light-matter interaction. Raman spectroscopy is commonly used as a characterization tool to confirm the nature of a material and its properties. In this work we go one step further, determining how structural and morphological variations in WS2 pyramids manifest themselves in Raman spectra. In addition we describe how Raman spectroscopy can be used to probe the orientation of WS2 nanoflowers.@en

We characterize the optical Raman response of CVD-grown WS2 pyramids and nanoflowers. Studying the dependence of the Raman features on position, temperature and polarization, we find how the geometry of the nanostructures induces spectral modifications.@en

Valley pseudospin has emerged as a good quantum number to encode information, analogous to spin in spintronics. Two-dimensional transition metal dichalcogenides (2D TMDCs) recently attracted enormous attention for their easy access to the valley pseudospin through valley-dependent optical transitions. Different ways have been reported to read out the valley pseudospin state. For practical applications, on-chip access to and manipulation of valley pseudospins is paramount, not only to read out but especially to initiate the valley pseudospin state. Here, we experimentally demonstrate the selective on-chip, optical near-field initiation of valley pseudospins at room temperature. We exploit a nanowire optical waveguide, such that the local transverse optical spin of its guided modes selectively excites a specific valley pseudospin. Furthermore, spin-momentum locking of the transverse optical spin enables us to flip valley pseudospins with the opposite propagation direction. Thus, we open up ways to realize integrated hybrid opto-valleytronic devices.

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