Dark excitons and the elusive valley polarization in transition metal dichalcogenides

Baranowski, Michał; Surrente, Alessandro; Maude, D. K.; Ballottin, Mariana V.; Mitioglu, Anatolie; Christianen, Peter C. M.; Kung, Yi; Dumcenco, Dumitru; Kis, András; Plochocka, P.

doi:10.1088/2053-1583/aa58a0

Cited by 80 publications

(131 citation statements)

References 59 publications

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“…It is remarkable that this mechanism yield such high degree even in MoSe2 monolayers, which is known exhibit a rather poor DOCP as bare monolayer 24 . It has been argued that the presence of a low-lying dark exciton state in the tungsten based TMDC materials, which is not subject to the MSS mechanism, is beneficial for the observation of large degrees of valley polarization and coherence 48 . Thus, we suspect that similar experiments, with even more pronounced degrees of polarization can be conceived based on high quality, tungsten based TMDC layers, potentially also a In d, the measurement is overlaid with a two-coupled oscillator model.…”

Section: Resultsmentioning

confidence: 99%

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

Lundt¹,

Dusanowski²,

Sedov

et al. 2019

Nat. Nanotechnol.

119

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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.

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Section: Resultsmentioning

confidence: 99%

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

Lundt¹,

Dusanowski²,

Sedov

et al. 2019

Nat. Nanotechnol.

119

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“…The coupling to the bright state is strongly enhanced by the cavity effect. Thus, we yield a second reservoir at large k||, which is robust against valley depolarization since dark excitons are not subject to strong exchange interactions 25 . See supplementary information S4 for further details.…”

Section: Polariton Valley Polarizationmentioning

confidence: 87%

“…While the relaxation into the bright exciton/exciton-polariton states is almost completely depolarizing, the dark exciton state is considered to maintain the excitation polarization 25,26 as illustrated in figure 4c.…”

Section: Polariton Valley Polarizationmentioning

confidence: 99%

Observation of macroscopic valley-polarized monolayer exciton-polaritons at room temperature

et al. 2017

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In this letter, we address the chiral properties of valley exciton-polaritons in a monolayer of WS2 in the regime of strong light-matter coupling with a Tamm-Plasmon resonance. We observe that the valley polarization, which manifests in the circular polarization of the emitted photoluminescence, is strongly enhanced in comparison to bare WS2 monolayers, and can even be observed under nonresonant excitation at ambient conditions. We study the relaxation and decay dynamics of excitonpolaritons in our device, and present a microscopic model to explain the wave vector-dependent valley depolarization as an interplay of bright and dark states, electron-hole exchange interaction and the linear polarization splitting inherent to the microcavity.

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“…Dark excitons consisting of electron–hole pairs with parallel spins have been revealed in 1L TMDs, which are strongly related to the splitting electronic structures around both valance and conduction band edges resulted from strong spin–orbit coupling ( Figure a) . The lowest transition of the intravalley A exciton in an ideal 1L WS 2 is optically dark according to electron and hole spin configurations .…”

Section: Luminescence Beyond Fundamental Excitonic Emission Of 2d Ws2mentioning

confidence: 98%

Optical Properties of 2D Semiconductor WS₂

Cong

Shang

Wang

et al. 2017

Advanced Optical Materials

301

182

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2D semiconductor tungsten disulfide (WS2) attracts significant interest in both fundamental physics and many promising applications such as light emitters, photodetectors/sensors, valleytronics, and flexible nanoelectronics, due to its fascinating optical, electronic, and mechanical properties. Herein, basic exciton properties of monolayer WS2 are reviewed including neutral excitons, charged excitons, bounded excitons, biexcitons, and the effects of electrostatic gating, chemical doping, strain, magnetic field, circular polarized light, and substrate on these excitonic structures. Besides basic excitonic emission, single‐photon emission, exciton–polaritons, and stimulated emission in monolayer WS2 are also discussed. The understanding of these optical phenomena is critical for the development of potential optical applications in electronic and optoelectronic devices. Finally, a summary and future prospective of the challengers and developments regarding 2D semiconductor WS2 is presented.

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Dark excitons and the elusive valley polarization in transition metal dichalcogenides

Abstract: Raman scattering of few-layers MoTe2 M Grzeszczyk, K Goasa, M Zinkiewicz et al.

Cited by 80 publications

References 59 publications

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

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

Observation of macroscopic valley-polarized monolayer exciton-polaritons at room temperature

Optical Properties of 2D Semiconductor WS₂

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Dark excitons and the elusive valley polarization in transition metal dichalcogenides

Abstract: Raman scattering of few-layers MoTe2 M Grzeszczyk, K Goasa, M Zinkiewicz et al.

Cited by 80 publications

References 59 publications

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

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

Observation of macroscopic valley-polarized monolayer exciton-polaritons at room temperature

Optical Properties of 2D Semiconductor WS2

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Product

Resources

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Optical Properties of 2D Semiconductor WS₂