Control of valley polarization in monolayer MoS2 by optical helicity

Abstract

Electronic and spintronic devices rely on the fact that free charge carriers in solids carry electric charge and spin, respectively. There are, however, other properties of charge carriers that might be exploited in new families of devices. In particular, if there are two or more conduction (or valence) band extrema in momentum space, then confining charge carriers in one of these valleys allows the possibility of valleytronic devices 1-4 . Such valley polarization has been demonstrated by using strain 5,6 and magnetic fields 7-10 , but neither of these approaches allow for dynamic control. Here we demonstrate that optical pumping with circularly-polarized light can achieve complete dynamic valley polarization in monolayer MoS 2 11,12 , a twodimensional (2D) non-centrosymmetric crystal with direct energy gaps at two valleys [13][14][15][16] . Moreover, this polarization is retained for longer than 1 ns. Our results, and similar results by Zeng et al. 17 , demonstrate the viability of optical valley control and valley-based electronic and optoelectronic applications in MoS 2 monolayers.Since optical photons do not carry significant momentum, they cannot selectively populate different valleys based on this attribute. For appropriate materials, however, carriers in different valleys are associated with well-defined, but different angular momenta. This suggests the possibility of addressing different valleys by control of the photon angular momentum, i.e., by the helicity (circular polarization state) of light. Indeed, just such valley-specific circular dichroism of interband transitions has been predicted in non-centrosymmetric materials 3,4,11,12 . Graphene with its two prominent K and K' valleys has been considered theoretically in this context 3,4 . For this approach to be applicable, however, the inherent inversion symmetry of the single-and bilayer graphene must be broken 3,4 , and the effect has not yet been realized experimentally. Monolayer MoS 2 , on the other hand, is a direct band gap semiconductor [13][14][15][16] that possesses a structure similar to graphene, but with explicitly broken inversion symmetry. It has recently been proposed as a suitable material for valleytronics 11,12 .Monolayer MoS 2 consists of a single layer of Mo atoms sandwiched between two layers of S atoms in a trigonal prismatic structure 18 (Fig. 1a). Inversion symmetry is broken since the two sublattices are occupied, respectively, by one Mo and two S atoms. At the K and K' valleys in momentum space, the highest energy valence bands (VB) and the lowest energy conduction bands (CB) are mainly of Mo d-orbital character 18 . Because of the broken inversion symmetry, spin-orbit (SO) interactions split the VBs by about 160 meV 11,19,20 (Fig. 1b). The spin projection along the c-axis of the crystal, S z , is well defined and the two bands are of spin down (! ↓ ) and spin up (! ↑ ) in character. This broken spin degeneracy, in combination with time reversal symmetry (! ↓ (!) = ! ↑ (−!), where ! is crystal momentum) implies tha...

Quantum emission from hexagonal boron nitride monolayers

“…These new atomically thin layers enabled realization of lasing from single monolayers 13 , generation of phonon polaritons [14][15][16] , super resolution imaging and spin valley transport [17][18][19] .…”

Exciton Radiative Lifetimes in Two-Dimensional Transition Metal Dichalcogenides

“…Time-reversal symmetry requires the VBM to be degenerate at K ± and the value of S z at K + to be opposite to the value at K − . 13,14 Figure 1 shows the absorption spectra of the TMD monolayers studied here (the corresponding bandstructures are shown in Figure S1 of the Supporting Information). Following a procedure from our recent work, 8 the BSE spectra are expressed in units of absorbance, here the percent fraction of absorbed light at each photon energy, thus enabling direct comparison with experiments.…”

Origin of Indirect Optical Transitions in Few-Layer MoS2, WS2, and WSe2

“…Electronic structure of two-dimensional (2D) crystals such as graphene and MoS 2 depends strongly on thickness and stacking sequence of individual layers [1][2][3][4][5][6][7] . Single layers are most distinct in that their thickness is in its ultimate limit.…”

“…In bilayers and thicker multilayers, interlayer interaction, geometrical confinement, and crystal symmetry play a collective role in defining their electronic structures. [1][2][3][4][5][6][7] Correspondingly, stark differences are observed in electrical and optical properties of these materials in the single-to few-layer thickness regime. 5,6,11 The band structure of MoS 2 and its isoelectronic compounds of the group 6 transition metal dichalcogenide (TMD) family, such as MoSe 2 , WS 2 , and WSe 2 , is distinctly different from that of graphene.…”