Probing spin dynamics of 2D excitons with twisted light

Abstract

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.

Photoexcitation of planar Wannier excitons by twisted photons

“…Due to the fact that a twisted photon carries a definite projection of the total angular momentum that can be larger than one, it can be used to excite the rotational degrees of freedom of a material in the way inaccessible for dipole transitions and thereby to investigate quantum dynamics of these degrees of freedom. In particular, the twisted photons can produce the excitons in the states with large projection of the total angular momentum [15][16][17]25], for example, the dark excitons [9,12,14,26]. We shall show in the present paper that the transition probabilities of photoexcitation of the exciton that has been already created are independent of the Bloch wave functions of the electron and the hole and are determined by the matrix elements of the operator r m .…”

“…In the present paper, we did not take into account the spins of the electron and the hole in the exciton, nor was the spin-orbit interaction included [9,16,34,39]. In particular, we did not study the creation of dark excitons by twisted photons.…”

“…Furthermore, m 1 and m 2 are the effective masses of the electron and the hole. We neglect the exchange and spin contributions assuming that they are small [7,9,16,[32][33][34]. Without loss of generality, the constant in (3) can be set to zero.…”

“…When a low energy electron and a low energy hole are confined to a semiconducting thin film or a monoatomic layer, they form a planar exciton until they annihilate [1][2][3]. Recently, the investigations of properties of such two-dimensional excitons have attracted considerable efforts by many research groups for monolayers of transition metal dichalcogenides and phosphorene [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. These materials provide the examples of structures with the direct bandgap and the relatively small permittivity entering into the electron-hole interaction potential.…”

A Steady-State Approach for Studying Valley Relaxation Using an Optical Vortex Beam

“…Here, we have proposed an approach to probe the valley dynamics in TMDs by tuning the COM of the exciton, by using an optical vortex (OV) beam or twisted light beam. , OV beams are interesting because they carry an orbital angular momentum (OAM) per photon along with spin angular momentum , due to their helical phase profile ( ). They have found myriad applications, such as in optical trapping, super-resolution fluorescence microscopy, quantum communication, and radio communication. , Recently we have demonstrated tunable PL emission in WS 2 and WSe 2 monolayers by controlling the intravalley scattering of excitons and the longitudinal electric field component of light with excitation by twisted light …”