Giant Enhancement of the Optical Second-Harmonic Emission of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>WSe</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>Monolayers by Laser Excitation at Exciton Resonances

Wang, G; Marie, X.; Gerber, Iann C.; Amand, T.; Lagarde, Delphine; Bouet, L.; Vidal, M.; Balocchi, Andréa; Urbaszek, B.

doi:10.1103/physrevlett.114.097403

Cited by 525 publications

(509 citation statements)

References 61 publications

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“…The control pulses at a photon energy of 1.57 eV were obtained from a NOPA, as described above, with the desired polarization state defined by a Babinet-Soleil compensator. Excitation pulses were obtained from filtered supercontinuum radiation with a photon energy of 1.81 eV, 85 meV blue-detuned from the A exciton resonance, to maximize the linearly polarized coherent emission 37 . The fluence of the excitation pulses was limited to 1 µJ cm −2 to avoid saturation, while the fluence of the control pulses did not exceed 1 mJ cm −2 .…”

Section: Nature Physics Doi: 101038/nphys3891mentioning

confidence: 99%

Optical manipulation of valley pseudospin

Ye¹,

Sun²,

Heinz³

2016

Nature Phys

220

223

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The coherent manipulation of spin and pseudospin underlies existing and emerging quantum technologies, including quantum communication and quantum computation 1,2 . Valley polarization, associated with the occupancy of degenerate, but quantum mechanically distinct valleys in momentum space, closely resembles spin polarization and has been proposed as a pseudospin carrier for the future quantum electronics 3,4 . Valley exciton polarization has been created in the transition metal dichalcogenide monolayers using excitation by circularly polarized light and has been detected both optically 5-7 and electrically 8 . In addition, the existence of coherence in the valley pseudospin has been identified experimentally 9 . The manipulation of such valley coherence has, however, remained out of reach. Here we demonstrate all-optical control of the valley coherence by means of the pseudomagnetic field associated with the optical Stark e ect. Using below-bandgap circularly polarized light, we rotate the valley exciton pseudospin in monolayer WSe 2 on the femtosecond timescale. Both the direction and speed of the rotation can be manipulated optically by tuning the dynamic phase of excitons in opposite valleys. This study unveils the possibility of generation, manipulation, and detection of the valley pseudospin by coupling to photons.The broken inversion symmetry in monolayer transition metal dichalcogenide (TMDC) crystals in the semiconducting MX 2 (M = Mo, W; X = S, Se) family gives rise to a nontrivial Berry phase at the K and K valleys in momentum space 10 , where the optically accessible direct bandgap occurs in these materials 11,12 . This Berry phase, together with the angular momentum of the atomic orbitals, leads to different optical selection rules for the two valleys: excitons in the K valley couple to left-circularly polarized photons, while excitons in the K valley couple to right-circularly polarized photons, thus permitting the optical generation of valley polarization through control of the helicity of light [5][6][7] . Moreover, when a TMDC monolayer is excited with linearly polarized light, a coherent superposition state is established between the K and K valley excitons 9 . Such a coherent state can, in principle, be manipulated by lifting the energy degeneracy between valleys through the breaking of time-reversal symmetry in the material. In this regard, d.c. magnetic fields have been applied to achieve a valley splitting of a few meV [13][14][15][16] . Circularly polarized light also breaks time-reversal symmetry, and researchers have used it to create larger valley splittings, corresponding to pseudomagnetic fields up to 60 T (refs 17,18). In addition, this optical approach permits us to access the ultrafast timescale: in this work, we demonstrate the coherent manipulation of the valley pseudospin on the femtosecond timescale.The optical Stark effect has been applied to manipulate the spin/pseudospin in quantum systems, such as atomic quantum gases 19 , quantum dots 20,21 , and III-V quantum wells 22 . H...

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Section: Nature Physics Doi: 101038/nphys3891mentioning

confidence: 99%

Optical manipulation of valley pseudospin

Ye¹,

Sun²,

Heinz³

2016

Nature Phys

220

223

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“…They correspond to the optical transitions from the two spin-split valence bands to the corresponding conduction bands 1,3,20 [ Fig. 1(a)], modified by the strong e-h interactions [41][42][43][44][45] . Near !…”

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Probing the Spin-Polarized Electronic Band Structure in Monolayer Transition Metal Dichalcogenides by Optical Spectroscopy

et al. 2017

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We study the electronic band structure in the K/K' valleys of the Brillouin zone of monolayer WSe 2 and MoSe 2 by optical reflection and photoluminescence spectroscopy on dual-gated field-effect devices. Our experiment reveals the distinct spin polarization in the conduction bands of these compounds by a systematic study of the doping dependence of the A and B excitonic resonances. Electrons in the highest-energy valence band and the lowest-energy conduction band have antiparallel spins in monolayer WSe 2 , and parallel spins in monolayer MoSe 2 . The spin splitting is determined to be hundreds of meV for the valence bands and tens of meV for the conduction bands, which are in good agreement with first principles calculations. These values also suggest that both n-and ptype WSe 2 and MoSe 2 can be relevant for spin-and valley-based applications.

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“…[18][19][20][21][22] Applications require a scalable fabrication platform providing high-quality large-area monolayer TMDC. Unfortunately, the most promising approach today, namely chemical vapor deposition (CVD) growth [23][24][25][26][27] struggles to compete with exfoliated TMDC in terms of sample quality.…”

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Defect Healing and Charge Transfer-Mediated Valley Polarization in MoS₂/MoSe₂/MoS₂ Trilayer van der Waals Heterostructures

et al. 2017

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Monolayer transition metal dichalcogenides (TMDC) grown by chemical vapor deposition (CVD) are plagued by a significantly lower optical quality compared to exfoliated TMDC. In this work we show that the optical quality of CVD-grown MoSe 2 is completely recovered if the material is sandwiched in MoS 2 /MoSe 2 /MoS 2 trilayer van der Waals heterostructures. We show by means of density-functional theory that this 1 arXiv:1703.00806v2 [cond-mat.mes-hall] 14 Jun 2017 remarkable and unexpected result is due to defect healing: S atoms of the more reactive MoS 2 layers are donated to heal Se vacancy defects in the middle MoSe 2 layer. In addition, the trilayer structure exhibits a considerable charge-transfer mediated valley polarization of MoSe 2 without the need for resonant excitation. Our fabrication approach, relying solely on simple flake transfer technique, paves the way for the scalable production of large-area TMDC materials with excellent optical quality. KeywordsChemical Vapor Deposition, transition metal dichalcogenides, van der Waals heterostructures, defect healing, charge transfer mediated valley polarization Monolayer transition metal dichalcogenides (TMDC) have a direct bandgap situated in the visible range, which makes them ideal building blocks for novel electronic and optoelectronic devices. [1][2][3][4][5][6][7][8][9][10] The bandgap of monolayer TMDCs occurs at the inequivalent (but degenerate) K and K' points of the hexagonal Brillouin zone. The broken inversion symmetry of a TMDC monolayer combined with the time reversal symmetry imposes opposite magnetic moments at the K and K' valleys. This in turn determines the characteristic circular dichroism exhibited by these materials, wherein each valley can be addressed separately with circularly polarized light of a given helicity. 11-13 Additionally, optical spectra are influenced by the strong spin-orbit coupling, which lifts the degeneracy of band states at the valence band edges, resulting in well-resolved A and B resonances, as observed in reflectivity or absorption spectra. 2,14-16 The interplay of spin-orbit coupling with broken inversion symmetry and time reversal symmetry locks the valley and spin degrees of freedom, making TMDC attractive candidates for valleytronics. 17 The spin-valley index locking along with the large 2 distance in the momentum space between K and K' valleys preserves the valley polarization observed in the degree of circular polarization (DCP) in helicity resolved photoluminescence emission. [18][19][20][21][22] Applications require a scalable fabrication platform providing high-quality large-area monolayer TMDC. Unfortunately, the most promising approach today, namely chemical vapor deposition (CVD) growth 23-27 struggles to compete with exfoliated TMDC in terms of sample quality. Low temperature PL spectroscopy of CVD-grown MoS 2 and MoSe 2 reveals broad emission from defect bound excitons, which is significantly more intense than the free exciton peak [28][29][30] and is related to chalcogen vacancies induced...

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Giant Enhancement of the Optical Second-Harmonic Emission ofWSe2Monolayers by Laser Excitation at Exciton Resonances

Cited by 525 publications

References 61 publications

Optical manipulation of valley pseudospin

Optical manipulation of valley pseudospin

Probing the Spin-Polarized Electronic Band Structure in Monolayer Transition Metal Dichalcogenides by Optical Spectroscopy

Defect Healing and Charge Transfer-Mediated Valley Polarization in MoS₂/MoSe₂/MoS₂ Trilayer van der Waals Heterostructures

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Giant Enhancement of the Optical Second-Harmonic Emission ofWSe2Monolayers by Laser Excitation at Exciton Resonances

Cited by 525 publications

References 61 publications

Optical manipulation of valley pseudospin

Optical manipulation of valley pseudospin

Probing the Spin-Polarized Electronic Band Structure in Monolayer Transition Metal Dichalcogenides by Optical Spectroscopy

Defect Healing and Charge Transfer-Mediated Valley Polarization in MoS2/MoSe2/MoS2 Trilayer van der Waals Heterostructures

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Defect Healing and Charge Transfer-Mediated Valley Polarization in MoS₂/MoSe₂/MoS₂ Trilayer van der Waals Heterostructures