Optical Coherence in Atomic-Monolayer Transition-Metal Dichalcogenides Limited by Electron-Phonon Interactions

Dey, Prasenjit; Paul, Jeffrey; Wang, Zefang; Stevens, Christopher E.; Liu, C.; Romero, Aldo H.; Shan, Jie; Hilton, David; Karaiskaj, D.

doi:10.1103/physrevlett.116.127402

Cited by 125 publications

(141 citation statements)

References 49 publications

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“…The two independent parameters are the intervalley decoherence time T 2 , and the optical Stark shift. We obtain a match to experiment for T 2 = (350 ± 50) fs, a value comparable to the decoherence time measured by recent photonecho experiments [26][27][28] . Moreover, the fitted Stark shift also agrees with the value determined by our frequency-domain measurements (Supplementary Sections III and IV).…”

supporting

confidence: 59%

Optical manipulation of valley pseudospin

Ye¹,

Sun²,

Heinz³

2016

Nature Phys

229

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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supporting

confidence: 59%

Optical manipulation of valley pseudospin

Ye¹,

Sun²,

Heinz³

2016

Nature Phys

229

223

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“…The linewidth of 2 meV is even smaller than a previously reported value of the homogeneous linewidth in ML MoS 2 on SiO 2 substrates (4.5 meV) measured with four-wave mixing [36]. In temperature-dependent experiments we measure the broadening of the optical transition linewidth due to interactions with acoustic and optical phonons.…”

Section: Introductionmentioning

confidence: 68%

“…This transition usually attributed to the neutral exciton is possibly merged with a broad charged exciton and/or defect contribution, as discussed recently [40]. The PL and reflectivity FWHM measured here is for certain samples smaller than the homogeneous linewidth extracted by means of four-wave mixing techniques for MLs on SiO 2 [36]. This represents a major breakthrough since MoS 2 is the most abundant TMDC found in nature.…”

Section: Neutral Exciton Transition In Pl and Reflectivitymentioning

confidence: 88%

“…Also, all timeresolved experiments using pulsed laser, such as pumpprobe spectroscopy, time-resolved PL, and four-wave mixing [12,31,36,90,91], will benefit from these samples with much higher threshold for optical damage as compared to uncapped samples. Note added in proof.-Narrow linewidth emission in hBN encapsulated samples is also reported in [92].…”

Section: Discussionmentioning

confidence: 99%

“…1(d), and it can be seen that it decreases by more than 1 order of magnitude when passing from room to cryogenic temperatures, which is a signature of an efficient coupling with phonons. The evolution of the linewidth can be phenomenologically approximated by a phonon-induced broadening [36,72]: [37][38][39]. In order to exclude any possible problems due to localization effects on the linewidth evolution at low temperatures, we perform reflectivity measurements in addition to PL as a function of temperature in MoS 2 MLs.…”

Section: B Temperature-induced Line Broadeningmentioning

confidence: 99%

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Excitonic Linewidth Approaching the Homogeneous Limit in MoS2 -Based van der Waals Heterostructures

et al. 2017

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The strong light-matter interaction and the valley selective optical selection rules make monolayer (ML) MoS 2 an exciting 2D material for fundamental physics and optoelectronics applications. But, so far, optical transition linewidths even at low temperature are typically as large as a few tens of meV and contain homogeneous and inhomogeneous contributions. This prevented in-depth studies, in contrast to the bettercharacterized ML materials MoSe 2 and WSe 2 . In this work, we show that encapsulation of ML MoS 2 in hexagonal boron nitride can efficiently suppress the inhomogeneous contribution to the exciton linewidth, as we measure in photoluminescence and reflectivity a FWHM down to 2 meV at T ¼ 4 K. Narrow optical transition linewidths are also observed in encapsulated WS 2 , WSe 2 , and MoSe 2 MLs. This indicates that surface protection and substrate flatness are key ingredients for obtaining stable, high-quality samples. Among the new possibilities offered by the well-defined optical transitions, we measure the homogeneous broadening induced by the interaction with phonons in temperature-dependent experiments. We uncover new information on spin and valley physics and present the rotation of valley coherence in applied magnetic fields perpendicular to the ML.

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Ultrafast Laser Spectroscopy of Two‐Dimensional Materials Beyond Graphene

Ceballos

Zhao

2016

Adv Funct Materials

134

125

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Starting with the discovery of graphene in 2004, the interest in two-dimensional materials since then has been exponentially growing. Across many disciplines, their exceptional electrical, chemical, thermal, and optical properties have drawn considerable attention that has created an entire field within a decade of their discovery. Driven by the mechanical exfoliation technique that allows for the quick exploration of these two-dimensional materials and their novel devices, joint efforts have been made in order to understand and exploit their potential, consequently leading to the development of their large-scale growth. This review focuses on recent studies using ultrafast laser spectroscopy that have revealed the photocarrier dynamics in two-dimensional materials and laid the foundation of their behavior. We provide a brief introduction on ultrafast laser spectroscopy, discuss several aspects of the photocarrier dynamics, and conclude with our perspective on future developments.

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Optical Coherence in Atomic-Monolayer Transition-Metal Dichalcogenides Limited by Electron-Phonon Interactions

Cited by 125 publications

References 49 publications

Optical manipulation of valley pseudospin

Optical manipulation of valley pseudospin

Excitonic Linewidth Approaching the Homogeneous Limit in MoS2 -Based van der Waals Heterostructures

Ultrafast Laser Spectroscopy of Two‐Dimensional Materials Beyond Graphene

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