Selective Photoexcitation of Finite-Momentum Excitons in Monolayer MoS₂ by Twisted Light

Abstract

Twisted light carries a well-defined orbital angular momentum (OAM) of ℏ per photon. The quantum number of its OAM can be arbitrarily set, making it an excellent light source to realize high-dimensional quantum entanglement and ultrawide bandwidth optical communication structures. In spite of its interesting properties, twisted light interaction with solid state materials, particularly two-dimensional materials, is yet to be extensively studied via experiments. In this work, photoluminescence (PL) spectroscopy studies of monolayer molybdenum disulfide (MoS 2 ), a material with ultrastrong light−matter interaction due to reduced dimensionality, are carried out under photoexcitation of twisted light. It is observed that the measured spectral peak energy increases for every increment of of the incident light. The nonlinear -dependence of the spectral blue shifts is well accounted for by the analysis and computational simulation of this work. More excitingly, the twisted light excitation revealed the unusual lightlike exciton band dispersion of valley excitons in monolayer transition metal dichalcogenides. This linear exciton band dispersion is predicted by previous theoretical studies and evidenced via this work's experimental setup.

Two-dimensional excitons from twisted light and the fate of the photon's orbital angular momentum

“…Our result agrees with a recent experiment in 2D transition metal dichalcogenides (TMDs) where twisted light has been used to reveal light-like exciton dispersion 25 . Using non-resonant Laguerre-Gaussian beams, they observed a blue shift of the exciton energy that increased with ℓ; this indicates that the OAM was transferred preferentially to the COM of the exciton during its creation.…”

“…In this context, we also note that small shifts of the spectrum are possible if the COM dispersion of the exciton is taken into account. This indeed has been observed in a recent experiment with excitons in a Dirac material, which found a blueshift of the exciton lines for sufficiently large values of photon OAM 25 .…”

Universal Scalings in Two-Dimensional Anisotropic Dipolar Excitonic Systems

“…This can be understood in terms of coherent summation of the dipole vectors resulting in constructive interference in the direction parallel to the dipole and destructive interference in the perpendicular direction, elaborated in the next section. We note that similar linear dispersion has been recently reported in monolayer MoS 2 and is attributed to a different type of physics associated with the inter-and intravalley exchange interactions [30][31][32]. The quadratic term is similar to the standard dispersion of NN-coupled models and serves as a background that is more isotropic in comparison [33].…”

“…S7. Spectroscopic signatures II: Absorption peak splittings.-The dispersion relation can be probed by scattering experiments such as the electron energy-loss spectroscopy that has become readily available for excitonic systems [49,50], and more recently using twisted light with designated orbital angular momenta exciton dispersions in the optical regime have also been experimentally measured [32]. On the other hand, a direct application of Eq.…”

Optical Emission from Light-like and Particle-like Excitons in Monolayer Transition Metal Dichalcogenides

“…That is, K and K states couple to produce light-like and particle-like bands that are approximately linear and parabolic, respectively [16,17]. While this unusual dispersion has been firmly established in previous theoretical [16,17] and experimental [18][19][20] work, we demonstrate in the present paper that equally striking consequences are expected for light emission from TMD excitons. To this end, we apply ab initio modeling to describe effects of center of mass motion on exciton en-ergies, optical matrix elements and light emission.…”