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

Abstract

Twisted light carries a well-defined orbital angular momentum (OAM) of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>l</mml:mi><mml:mi>ℏ</mml:mi></mml:math> per photon. The quantum number <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>l</mml:mi></mml:math> 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 <i>via</i> experiments. In this work, photoluminescence (PL) spectroscopy studies of monolayer molybdenum disulfide (MoS₂), 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>l</mml:mi></mml:math> of the incident light. The nonlinear <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>l</mml:mi></mml:math>-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 <i>via</i> this work's experimental setup.