Optical valley Hall effect for highly valley-coherent exciton-polaritons in an atomically thin semiconductor

Abstract

Spin-orbit coupling is a fundamental mechanism that connects the spin of a charge carrier with its momentum 1 . Likewise, in the optical domain, a synthetic spin-orbit coupling is accessible, for instance, by engineering optical anisotropies in photonic materials 2 . Both, akin, yield the possibility to create devices directly harnessing spin-and polarization as information carriers 3 . Atomically thin layers of transition metal dichalcogenides provide a new material platform which promises intrinsic spin-valley Hall features both for free carriers, two-particle excitations (excitons), as well as for photons 4 . In such materials, the spin of an exciton is closely linked to the high-symmetry point in reciprocal space it emerges from (K and K' valleys) 5,6 . Here, we demonstrate, that spin, and hence valley selective propagation is accessible in an atomically thin layer of MoSe2, which is strongly coupled to a microcavity photon mode. We engineer a wire-like device, where we can clearly trace the flow, and the helicity of exciton-polaritons expanding along a channel. By exciting a coherent superposition of K and K' tagged polaritons, we observe valley selective expansion of the polariton cloud without neither any applied external magnetic fields nor coherent Rayleigh scattering. Unlike the valley Hall effect for TMDC excitons 7 , the observed optical valley Hall effect (OVHE) 8 strikingly occurs on a macroscopic scale, and clearly reveals the potential for applications in spin-valley locked photonic devices.Spin-valley locking is a striking feature of free charge carriers and excitons emerging in monolayers of transition metal dichalcogenides (TMDCs) 6,9 . It originates form the strong spin-orbit interaction, which arises from the heavy transition metals in TMDCs and the broken inversion symmetry of the crystal lattice. This leads to inverted spin orientations at opposite K points at the corners of the hexagonal Brillouin zone, for both conduction band electrons and valence band holes. As a result, the K and K' valleys can be selectively addressed by σ + and σcircular polarized light 10,11 , which is referred to as valley-polarization. Likewise, coherent superpositions of both valleys can be excited by linear polarized light, which is referred to as valley coherence. The outstanding control of the valley pseudospin has attracted great interest in exploiting this degree of freedom to encode and process information by manipulating free charge carriers 12 and excitons 7,13,14 , which has led to the emerging field of valleytronics 4 . However, exciton spin-valley applications are strongly limited by the depolarization mechanisms due to the strong Coulomb exchange interaction of electrons and holes, as well as by the limited exciton diffusion and propagation lengths.

Routing valley exciton emission of a WS2 monolayer via delocalized Bloch modes of in-plane inversion-symmetry-broken photonic crystal slabs

“…1a. Interband transitions at valleys, which are excitonic transitions in nature for TMDCs, show highly valley-dependent optical selection rules [4][5][6][13][14][15][16][17][18] . This controllable selective population of certain valleys, called valley polarization, offers a new valley degree of freedom, spawning the emergent field of valleytronics.…”

“…To develop valleytronic devices based on TMDCs, effective approaches to separate valleys in the near or far field are indispensable. One feasible way is to selectively excite valleys by utilizing different external stimuli such as optical and electric fields [14][15][16][17][18] , while the usually required low-temperature environment makes it difficult for practical applications. Due to the powerful ability of manipulating light, nanostructures [19][20][21] are also proposed to separate valleys [22][23][24][25][26][27][28][29][30][31][32] .…”

Local photodoping in monolayer MoS2

“…Two-dimensional (2D) materials are strategically important for optoelectronic applications due to their ultra-thin nature and tunable electrostatic and optical properties [1][2][3][4][5][6][7][8][9][10][11][12].…”

Light-field and spin-orbit-driven currents in van der Waals materials

“…For the neutral excitons, an anomalous transversal velocity cannot result from an external electric field, but rather from the temperature or density gradient of excitons [156]. For exciton-polaritons strongly coupled to a microcavity photon mode, an optical valley Hall effect was observed in monolayer MoSe 2 , enabled by the long-range propagation of exciton-polaritons excited at a finite wave vector [157].…”