Efficient manipulation of the valley degree of freedom in Transition Metal Dichalcogenide (TMD) monolayers at the nanoscale becomes very desirable for future developments in valleytronics. Resonant optical nanostructures are considered as potential tools in this endeavor; however, it is still unclear how they affect polarization properties of valley-specific monolayer emission. Here, we present a systematic experimental and numerical study that is aimed to bridge this gap. As a simple model, we consider a hybrid system where valley-polarized photoluminescence [1] or second harmonic from MoS2 - monolayer is coupled with a plasmonic nanosphere. Through this study, we are not only aimed to refine the exciting simulation approaches for valleytronic devices, but also contribute to the deeper understanding of the rich physics of light-matter interactions at the nanoscale.
Bound states in the continuum (BICs) are a category of localized states that exist within the continuum of radiating modes. The high Q-factor exhibited by these states makes quasi-BICs interesting for enhancing the emission from quantum emitters. Quasi-BICs have been experimentally realized in silicon for applications in the infrared wavelength range. Instead of silicon, hydrogenated amorphous silicon (a-Si:H) has been used for achieving quasi-BIC resonance in parts of visible spectra. Titanium dioxide (TiO2) has emerged as an alternate material for fabricating dielectric metasurfaces with high Q-factor in the visible spectral range due to its lower absorptive losses and high refractive index. However, the fabrication process for TiO2 nanostructures presents challenges compared to the well-established fabrication processes in silicon. Our work focuses on the design and fabrication of TiO2 metasurfaces supporting a quasi-BIC mode around 795 nm, with a theoretical Q-factor of 353. Experimental results reveal a maximum Q-factor of 258 at 791 nm. We discuss encountered fabrication constraints and explore possibilities for improvement in both design and fabrication processes. This study contributes to the understanding of quasi-BIC resonance in TiO2 metasurfaces, and opens avenues for further exploration in the utilization of TiO2 for high-Q dielectric metasurfaces, offering insights into the design and optimization of these structures.
The study of strong field-driven nonlinear responses from high harmonic generation (HHG) in two dimension (2D) materials has been recognized as a valuable tool for probing ultrafast electron dynamics in the condensed matter systems. The laser polarization dependence of HHG in solids provides significant scientific insights into the crystal and band structure of the generation medium. In this work we experimentally investigate the impact of HHG in MoS2 monolayer by changing the laser polarization from linear to circular for different orientations relative to the symmetry axes of the crystal. The polarization analysis of harmonic emission reveals ellipticity and helicity dependent enhancement of even order harmonics which is strongly asymmetric near the symmetry directions of the crystal.
We report the first time nanoscale, tip enhanced Raman scattering (TERS) imaging of the SeMoS Janus monolayers crystals both as-grown on gold foil and transferred from the growth substrate to the gold-coated silicon wafers. Due to the preferential enhancement of the out-of-plane modes in the gap-mode of TERS, the TERS spectra of SeMoS differ from the conventional Raman spectra reported earlier [1 , 2]. The A11and A12 out-of-plane modes are shown to be the first and the second strongest Raman peaks in TERS, while in conventional Raman spectroscopy the A12 mode is extremely weak. Interestingly, the red shift of the spectral position of A12 mode correlates with a decrease of the contact potential difference in Kelvin probe force microscopy (KPFM) images. While the TERS maps mostly show the Raman spectra characteristic to the high quality SeMoS Janus monolayers, we observed in some cases narrow, below 20-30 nm, areas that featured a peak at 406 cm-1 which has been proved to be the A’ band of MoS2. The ability to detect the nanoscale imperfections in Janus monolayer crystals is a mandatory condition for optimizing their synthetic routes. TERS imaging cross-correlated with KPFM measurements demonstrate the applicability for the nanoscale assessment of the structural homogeneity of both the as-grown and transferred SeMoS Janus monolayer crystals.
References
1. Z. Gan, I. Paradisanos, A. Estrada-Real et.al. ADVANCED MATERIALS 2022 34, 2205226
2. Marko M. Petrić, Malte Kremser , Matteo Barbone et.al. PHYSICAL REVIEW B 103, 035414 (2021)
Two-dimensional semiconductors such as monolayer transition metal dichalcogenides (TMDs) exhibits remarkable optical properties such as robust valley polarization, making them ideal for optoelectronic and valleytronic devices. Manipulating the valley polarization by optical method is the key to realize valleytronic devices. Here, we demonstrate a resonant plasmonic nanostructure designed to spatially separate the emissions from different valleys of the WSe2 monolayer at cryogenic temperature. By changing the helicity of excitation, we show the directionality control of valley-based emission. Our hybrid nanostructure exhibits the possibility to realise the valleytronic devices.
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