We propose a mechanism of dark to bright exciton transitions in spin-valley coupled twodimensional systems by transferring momentum of light into exciton center of mass using optical vortex (OV) beams. By varying the dispersion of light using the topological charge of OV beam, we demonstrate a unique approach to control the intra-valley spin-flip scattering rate of excitons. From our photoluminescence measurements, we demonstrate that the intravalley scattering rate in W-based TMDs can be tuned externally by OV beams. Variation of photoluminescence intensity with topological charges shows a crossover temperature (> 150 K), indicating competitions among time scales involving radiative recombination, spin-flip scattering, and thermal relaxations. Our proposed technique utilizing a structured light beam can open up a new approach to explore the physics of excitons in 2D systems.
Monolayer (ML) transition-metal dichalcogenides (TMDCs) represent a novel class of materials for investigating excitonic quasiparticles in two dimensions and designing novel nanoscale optoelectronic devices. Practical application of the TMDC MLs requires their integration with appropriate substrates. In this context, dielectric substrates are of particular interest. Here, we report on the impact of an epitaxial high-Κ dielectric substrate on the excitonic quasiparticles in TMDC MLs. Investigations were performed by comparing the photoluminescence (PL) response of exfoliated MoSe 2 MLs directly transferred onto an epitaxial Gd 2 O 3 layer grown on Si(001) and for comparison on the same Gd 2 O 3 layer covered with a few monolayers of hexagonal boron nitride (hBN) (hBN/Gd 2 O 3 ). We demonstrate that in reference to hBN, the epitaxial Gd 2 O 3 substrate does not induce any significant biaxial strain to the MoSe 2 MLs. Epitaxial Gd 2 O 3 led to a strong reduction in the inhomogeneous broadening of the emission peaks of MoSe 2 MLs and only a marginal red shift in the A exciton and X − trion peak positions in comparison to the hBN/Gd 2 O 3 substrate. The PL response of MoSe 2 MLs on epitaxial Gd 2 O 3 is dominated by X − trion resonance over a large range of temperatures, revealing strong charge transfer doping by the substrate. Our work illustrates the effect of the epitaxial high-κ dielectric substrate on the optical properties of MoSe 2 monolayers and paves the way for realizing high-quality emission and modulation of excitonic quasiparticles through substrate engineering.
Light–matter coupling in van der Waal’s materials holds significant promise in realizing bosonic condensation and superfluidity. The underlying semiconductor’s crystal asymmetry, if any, can be utilized to form anisotropic half-light half-matter quasiparticles. We demonstrate generation of such highly anisotropic exciton-polaritons at the interface of a biaxial layered semiconductor, stacked on top of a distributed Bragg reflector. The spatially confined photonic mode in this geometry couples with polarized excitons and their Rydberg states, creating a system of highly anisotropic polariton manifolds, displaying Rabi splitting of up to 68 meV. Rotation of the incident beam polarization is used to tune coupling strength and smoothly switch regimes from weak to strong coupling, while also enabling transition from one three-body coupled oscillator system to another. Light–matter coupling is further tunable by varying the number of weakly coupled optically active layers. Our work provides a versatile method of engineering devices for applications in polarization-controlled polaritonics and optoelectronics.
We propose a mechanism of intravalley spin–flip scattering in spin–valley-coupled two-dimensional (2D) systems by transferring the momentum of light into the exciton center of mass using optical vortex (OV) beams. By varying the dispersion of light using the topological charge of the OV beam, we demonstrate a unique approach to control the intravalley spin–flip scattering rate of excitons. From our photoluminescence measurements, we demonstrate that the intravalley scattering rate in W-based TMDs can be tuned externally by OV beams. Variation of photoluminescence intensity with topological charges shows a crossover temperature (>150 K), indicating competition among time scales involving radiative recombination, spin–flip scattering, and thermal relaxations. Our proposed technique utilizing a structured light beam can open up a new approach to exploring the physics of excitons in 2D systems.
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