Optical generation of excitonic valley coherence in monolayer WSe2 (original) (raw)

Direct measurement of exciton valley coherence in monolayer WSe2

Nature Physics, 2016

In crystals, energy band extrema in momentum space can be identified by a valley index. The internal quantum degree of freedom associated with valley pseudospin indices can act as a useful information carrier, analogous to electronic charge or spin 1-4. Interest in valleytronics has been revived in recent years following the discovery of atomically thin materials such as graphene and transition metal dichalcogenides 5-7. However, the valley coherence time-a crucial quantity for valley pseudospin manipulation-is di cult to directly probe. In this work, we use two-dimensional coherent spectroscopy to resonantly generate and detect valley coherence of excitons (Coulomb-bound electron-hole pairs) in monolayer WSe 2 (refs 8,9). The imposed valley coherence persists for approximately one hundred femtoseconds. We propose that the electron-hole exchange interaction provides an important decoherence mechanism in addition to exciton population recombination. This work provides critical insight into the requirements and strategies for optical manipulation of the valley pseudospin for future valleytronics applications. Group-VI transition metal dichalcogenides (TMDs) with 2H structure (for example, MX 2 , M = Mo, W; X = S, Se) are a particularly intriguing class of semiconductors when thinned down to monolayers 6,7. The valence and conduction band extrema are located at both K and K points at the corners of the hexagonal Brillouin zone, as illustrated in Fig. 1a. The degenerate K and K points are related to each other by time reversal symmetry and give rise to the valley degree of freedom (DoF) of the band-edge electrons and holes. Strong Coulomb interactions lead to the formation of excitons with remarkably large binding energies due to the heavy effective mass and reduced dielectric screening in monolayer TMDs (refs 10-12). An exciton as a bound electron-hole pair inherits the valley DoF. Because of valley-dependent optical selection rules, they can be excited only by σ + (σ −) circularly polarized light at the K(K) valley. Owing to its close analogy to spin 4 , the valley DoF can be considered as a pseudospin represented by a vector S on the Bloch sphere (Fig. 1b). The out-of-plane component S z and in-plane component S x,y describe the valley polarization and the coherent superposition of exciton valley states, respectively. After optical initialization, valley depolarization and decoherence are manifested by a reduction in the magnitudes of S z and S x,y , respectively. The ability to coherently manipulate spins and pseudospins is at the heart of spintronics and valleytronics; however, previous investigations have focused mainly on the creation and relaxation of valley polarization using non-resonant photoluminescence (PL)

Superior Valley Polarization and Coherence of 2s Excitons in Monolayer WSe_{2}

Physical review letters, 2018

We report the experimental observation of 2s exciton radiative emission from monolayer tungsten diselenide, enabled by hexagonal boron nitride protected high-quality samples. The 2s luminescence is highly robust and persists up to 150 K, offering a new quantum entity for manipulating the valley degree of freedom. Remarkably, the 2s exciton displays superior valley polarization and coherence than 1s under similar experimental conditions. This observation provides evidence that the Coulomb-exchange-interaction-driven valley-depolarization process, the Maialle-Silva-Sham mechanism, plays an important role in valley excitons of monolayer transition metal dichalcogenides.

Valley dynamics probed through charged and neutral exciton emission in monolayerWSe2

Physical Review B, 2014

Optical interband transitions in monolayer transition metal dichalcogenides such as WSe 2 and MoS 2 are governed by chiral selection rules. This allows efficient optical initialization of an electron in a specific K-valley in momentum space. Here we probe the valley dynamics in monolayer WSe 2 by monitoring the emission and polarization dynamics of the well separated neutral excitons (bound electron hole pairs) and charged excitons (trions) in photoluminescence. The neutral exciton photoluminescence intensity decay time is about 4ps, whereas the trion emission occurs over several tens of ps. The trion polarization dynamics shows a partial, fast initial decay within tens of ps before reaching a stable polarization of ≈ 20%, for which a typical valley polarization decay time larger than 1ns can be inferred. This is a clear signature of stable, optically initialized valley polarization.

Observation of biexcitons in monolayer WSe2

Nature Physics, 2015

Transition metal dichalcogenide (TMDC) crystals exhibit new emergent properties at monolayer thickness 1,2 , notably strong many-body e ects mediated by Coulomb interactions 3-6. A manifestation of these many-body interactions is the formation of excitons, bound electron-hole pairs, but higher-order excitonic states are also possible. Here we demonstrate the existence of four-body, biexciton states in monolayer WSe 2. The biexciton is identified as a sharply defined state in photoluminescence at high exciton density. Its binding energy of 52 meV is more than an order of magnitude greater than that found in conventional quantum-well structures 7. A variational calculation of the biexciton state reveals that the high binding energy arises not only from strong carrier confinement, but also from reduced and non-local dielectric screening. These results open the way for the creation of new correlated excitonic states linking the degenerate valleys in TMDC crystals, as well as more complex many-body states such as exciton condensates or the recently reported dropletons 8. TMDC crystals, including MoS 2 , MoSe 2 , WS 2 and WSe 2 , are semiconductors that form layered structures with a plane of hexagonal metal atoms surrounded by two planes of chalcogen atoms in trigonal prismatic coordination. At monolayer thickness, these crystals exhibit direct band gaps at the K and K points in the Brillouin zone 1,2 , and recent studies have revealed the possibility of selectively accessing the K or K valley through the use of circularly polarized light 9-12 , as well as the existence of an associated valley Hall effect 13. Importantly, many-body Coulomb interactions in these monolayer TMDC crystals have been found to be particularly strong. This leads to excitonic optical transitions in the materials, with exciton binding energies of several hundred meV (refs 3-6). In the presence of free charges, stable charged excitons (trions) have also been identified and exhibit binding energies of tens of meV (refs 12,14-16). In view of the prominence of these two-and three-body excitonic states, it is natural to ask whether two-dimensional (2D) TMDC materials, just as for the much-studied zero-and one-dimensional nanostructures 7,17-21 , also support the formation of stable biexcitons 22,23. Here we demonstrate the presence of biexcitons in monolayer WSe 2 through the discovery of a sharp new emission peak under pulsed laser excitation. We further probe the properties of the biexciton state through measurements of its ultrafast dynamics, valley polarization and thermal stability. We establish a biexciton binding energy of 52 meV. This unusually high binding energy is compatible with results of a variational analysis of biexcitonic states performed using a non-locally screened Coulomb potential to describe the interactions of charges in the atomically thin 2D material.

Excitons and trions in WSSe monolayers

arXiv (Cornell University), 2022

The possibility of almost linear tuning of the band gap and of the electrical and optical properties in monolayers (MLs) of semiconducting transition metal dichalcogenide (S-TMD) alloys opens up the way to fabricate materials with on-demand characteristics. By making use of photoluminescence spectroscopy, we investigate optical properties of WSSe MLs with a S/Se ratio of 57/43 deposited on SiO2/Si substrate and encapsulated in hexagonal BN flakes. Similarly to the "parent" WS2 and WSe2 MLs, we assign the WSSe MLs to the ML family with the dark ground exciton state. We find that, in addition to the neutral bright A exciton line, three observed emission lines are associated with negatively charged excitons. The application of in-plane and out-of-plane magnetic fields allows us to assign undeniably the bright and dark (spin-and momentum-forbidden) negative trions as well as the phonon replica of the dark spin-forbidden complex. Furthermore, the existence of the single photon emitters in the WSSe ML is also demonstrated, thus prompting the opportunity to enlarge the wavelength range for potential future quantum applications of S-TMDs.

Spin-layer locking effects in optical orientation of exciton spin in bilayer WSe 2

Coupling degrees of freedom of distinct nature plays a critical role in numerous physical phenomena 1-10. The recent emergence of layered materials 11-13 provides a laboratory for studying the interplay between internal quantum degrees of freedom of electrons 14,15. Here we report new coupling phenomena connecting real spin with layer pseudospins in bilayer WSe 2. In polarization-resolved photoluminescence measurements, we observe large spin orientation of neutral and charged excitons by both circularly and linearly polarized excitation, with the trion spectrum splitting into a doublet at large vertical electrical field. These observations can be explained as a locking of spin and layer pseudospin in a given valley 15 , where the doublet implies an electrically induced spin splitting. The observed distinctive behaviour of the trion doublet under polarized excitation further provides spectroscopic evidence of interlayer and intralayer trion species, a promising step towards optical manipulation in van der Waals heterostructures 16 through interlayer excitons. Exploring the consequences of the interplay between distinct quantum degrees of freedom has been an active theme in modern physics. A salient example is spin-orbit coupling (SOC), which is essential in renowned condensed matter phenomena such as the spin Hall effect 1,2 , topological insulators 3,4 and Majorana fermions 5,6 ; in cold atom physics in the search for new condensate structures 7 ; and in technological applications such as magnetoelectric coupling in multiferroics 8 as well as optical and electrical control of spins for spintronics 9,10. All of these phenomena arise from the coupling of the motional degree of freedom of a particle with its real spin. A pseudospin describes another discrete internal degree of freedom of electrons, and in most systems has an orbital origin and can therefore couple to the real spin by SOC as well. An excellent example is found in monolayer transition-metal dichalcogenides (TMDCs), which have attracted a significant amount of interest recently 12,13,17-21. The inversion symmetry breaking allows for an effective coupling between the real spin and valley pseudospin 14 (the latter indexes the degenerate extrema of the electron energy dispersion in momentum space). In the presence of mirror and time-reversal symmetry, SOC can be manifested as an out-of-plane spin splitting with a valley-dependent sign (Fig. 1a). Bilayer two-dimensional materials (for example, bilayer graphene 22-26 and bilayer TMDCs; ref. 15) possess another distinct degree of freedom known as the layer pseudospin. An electronic state localized to the upper or lower layer can be labelled with pseudospin up or down, respectively, which corresponds to electrical polarization. In a layered material with spin-valley coupling and AB stacking, such as bilayer TMDCs, both spin and valley are coupled to layer pseudospin 15. As shown in Fig. 1a, because the lower layer is a 180 • in-plane rotation of the upper layer, the out-of-plane spin splitting has a sign that depends on both valley and layer pseu-dospins. Interlayer hopping thus has an energy cost equal to twice the SOC strength λ. When 2λ is larger than the hopping amplitude t ⊥ , a carrier is localized in either the upper or lower layer depending on its valley and spin state. In other words, in a given valley, the spin configuration is locked to the layer index. This is schematically illustrated in Fig. 1b,c. This spin-layer locking permits electrical manipulation of spins through gate control of layer polarization, which may lead to new magnetoelectric effects and quantum logic 15. Here, we report experimental signatures of coupling between this layer pseudospin and the spin and valley degrees of freedom in bilayers of WSe 2 (bi-WSe 2). In contrast to MoS 2 (ref. 19), the high quality of WSe 2 , in addition to much larger SOC, provides an excellent system for observing spin-layer locking. Although bi-WSe 2 is an indirect-bandgap semiconductor, the near degeneracy between indirect and direct transitions permits us to efficiently monitor direct-gap photoluminescence from the K valleys 12,13,27. Electrons in the conduction bands near the K valleys exhibit a spin splitting 2λ ∼ 30-40 meV, which is two orders of magnitude larger than the interlayer hopping amplitude t ⊥ at the ±K points (see Supplementary Information 1). For holes, 2λ ∼ 450 meV and 2λ/t ⊥ ∼ 7 (ref. 15). Large 2λ to t ⊥ ratios ensure that interlayer hopping for both electrons and holes is suppressed, achieving a spin-layer locking effect. We first identify the exciton states in bi-WSe 2 through gate-dependent photoluminescence measurements (see Methods) 28,29. Figure 2a shows the photoluminescence intensity from direct-gap exciton emission as a function of backgate voltage (V g) and photon energy. By comparing the gate-dependent patterns and emission energies of monolayer (Supplementary Section 2) 29 and bilayer WSe 2 , we can identify the weak feature near 1.74 eV and V g = 0 as neutral exciton (X o) emission, whereas the peak near 1.71 eV at positive or negative V g corresponds to negative (X −) or positive (X +) trions, respectively. The photoluminescence peak near 1.63 eV arises from impurity-bound excitons. Figure 2c shows photoluminescence spectra at three selected values of V g. The peaks here coincide with the lowest energy absorption feature shown in Fig. 2b, the gate-dependent differential reflectivity obtained by white-light reflection (Supplementary Section 3).