Ultrafast generation of pseudo-magnetic field for valley excitons in WSe
            <sub>2</sub>
            monolayers

Kim, Jonghwan; Hong, Xiaoping; Choi, Jin; Shi, Su‐Fei; Chang, Chih Yuan S.; Chiu, Ming Hui; Li, Lain‐Jong; Wang, Feng

doi:10.1126/science.1258122

Cited by 306 publications

(303 citation statements)

References 34 publications

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“…Fig 1d shows a pair of ∆α spectra measured using broadband probe pulses with helicities σ − (black) and σ + (red) at ∆t = 0.3 ps. This time delay was chosen to avoid contaminating effects of coherent light-matter interaction 7,8 . In contrast to what is expected, we observed strong bleaching peaks not only at A but also at B exciton transitions, and strikingly, these two peaks were present in both spectra measured using different helicities 22 .…”

Section: A Intervalley Biexcitonsmentioning

confidence: 99%

“…These include valley-selective photoexcitation [2][3][4] , valley Hall effect 5 , valley-tunable magnetic moment 6 , and valley-selective optical Stark effect 7,8 . Unlike traditional semiconductors, the Coulomb interaction in monolayer TMDs is unusually strong because screening is greatly suppressed and spatial overlap of the interaction is much larger.…”

Section: Introductionmentioning

confidence: 99%

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Intervalley biexcitons and many-body effects in monolayerMoS2

et al. 2015

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Interactions between two excitons can result in the formation of bound quasiparticles, known as biexcitons. Their properties are determined by the constituent excitons, with orbital and spin states resembling those of atoms. Monolayer transition metal dichalcogenides (TMDs) present a unique system where excitons acquire a new degree of freedom, the valley pseudospin, from which a novel intervalley biexciton can be created. These biexcitons comprise two excitons from different valleys, which are distinct from biexcitons in conventional semiconductors and have no direct analogue in atomic and molecular systems. However, their valley properties are not accessible to traditional transport and optical measurements. Here, we report the observation of intervalley biexcitons in the monolayer TMD MoS2 using ultrafast pump-probe spectroscopy. By applying broadband probe pulses with different helicities, we identify two species of intervalley biexcitons with large binding energies of 60 meV and 40 meV. In addition, we also reveal effects beyond biexcitonic pairwise interactions in which the exciton energy redshifts at increasing exciton densities, indicating the presence of many-body interactions among them.

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Section: A Intervalley Biexcitonsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Intervalley biexcitons and many-body effects in monolayerMoS2

et al. 2015

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“…Fig 3.2b shows a pair of Aa spectra measured using broadband probe pulses with helicities a-(black) and a' (red) at At = 0.3 ps. This time delay was chosen to avoid contaminating effects of coherent light-matter interaction [29,30]. In contrast to what is expected, we observed strong bleaching peaks not only at A but also at B exciton transitions, and strikingly, these two peaks were present in both spectra measured using different helicities [37].…”

Section: Resultsmentioning

confidence: 80%

“…Unlike in the red-detuned experiments [29,30], the incoherent signals in our bluedetuned experiments are unavoidable and quite significant, because a large population of the absorption peak is Pauli blocked. Here we assume that the electron population has not rearranged themselves to occupy the shifted absorption peak, so that the bleaching profile is simply derived from the original absorption peak.…”

Section: Coherent and Incoherent Optical Signalsmentioning

confidence: 91%

Coherent Light-Matter Interactions in Monolayer Transition-Metal Dichalcogenides

Sie

2018

Springer Theses

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Semiconductors that are thinned down to a few atomic layers can exhibit novel properties beyond those encountered in bulk forms. Transition-metal dichalcogenides (TMDs) such as MoS 2 , WS 2 and WSe 2 are prime examples of such semiconductors. They appear in layered structure that can be reduced to a stable single layer where remarkable electronic properties can emerge. Monolayer TMDs have a pair of electronic valleys which have been proposed as a new way to carry information in next generation devices, called valleytronics. However, these valleys are normally locked in the same energy level, which limits their potential use for applications.This dissertation presents the optical methods to split their energy levels by means of coherent light-matter interactions. Experiments were performed in a pump-probe technique using a transient absorption spectroscopy on MoS 2 and WS 2 , and a newly developed XUV light source for time and angle-resolved photoemission spectroscopy (TR-ARPES) on WSe 2 and WTe 2 . Hybridizing the electronic valleys with light allows us to optically tune their energy levels in a controllable valley-selective manner. In particular, by using off-resonance circularly polarized light at small detuning, we can tune the energy level of one valley through the optical Stark effect. At larger detuning, we observe a separate contribution from the so-called Bloch-Siegert effect, a delicate phenomenon that has eluded direct observation in solids. The two effects obey opposite selection rules, which enables us to separate the two effects at two different valleys.Monolayer TMDs also possess strong Coulomb interaction that enhances many-body interactions between excitons, both bonding and non-bonding interactions. In the former, bound excitonic quasiparticles such as biexcitons play a unique role in coherent light-matter interactions where they couple the two valleys to induce opposite energy shifts. In the latter, non-bonding interactions between excitons are found to exhibit energy shifts that effectively mimics the Lennard-Jones interactions between atoms. Through these works, we demonstrate new methods to optically tune the energy levels of electronic valleys in monolayer TMDs. AcknowledgmentsFirst and foremost, I would like to thank my advisor Prof. Nuh Gedik for his mentorship and support. He has been a constant source of inspiration since I first started my graduate studies. Actually it was even before that. I remember my first meeting at a conference in Boston where he presented a real-time image of crystalline lattice motion. The idea of watching Michael Jackson performing Billie Jean flashed through my head, because he was that cool and inspiring, and he continues to be so. He has been a tremendous support since day one, where I was given the opportunity to design and build the XUV beamline for ARPES in the lab as well as the freedom to build the white-light setup next to it. His vigor to make new innovations in timeresolved experiments is always inspiring. Throughout my graduate studies, he...

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“…The energy levels of these excitons can be tuned optically in a valley-selective manner by means of the optical Stark effect [5,6]. Prior research has demonstrated that monolayer TMDs driven by below-resonance (red-detuned) circularly polarized light can exhibit an upshifted exciton level, either at the K or K valleys depending on the helicity, while keeping the opposite valley unchanged.…”

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confidence: 99%

Observation of Intervalley Biexcitonic Optical Stark Effect in Monolayer WS₂

et al. 2016

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Coherent optical dressing of quantum materials offers technological advantages to control their electronic properties, such as the electronic valley degree of freedom in monolayer transition metal dichalcogenides (TMDs). Here, we observe a new type of optical Stark effect in monolayer WS2, one that is mediated by intervalley biexcitons under the blue-detuned driving with circularly polarized light. We found that such helical optical driving not only induces an exciton energy downshift at the excitation valley, but also causes an anomalous energy upshift at the opposite valley, which is normally forbidden by the exciton selection rules but now made accessible through the intervalley biexcitons. These findings reveal the critical, but hitherto neglected, role of biexcitons to couple the two seemingly independent valleys, and to enhance the optical control in valleytronics.Keywords: WS2, valley, biexciton, blue detuned, optical Stark effect, ultrafast opticsMonolayer TMDs host tightly-bound excitons in two degenerate but inequivalent valleys (K and K ), which can be selectively photoexcited using left (σ − ) or right (σ + ) circularly polarized light (Fig 1a) [1-4]. The energy levels of these excitons can be tuned optically in a valley-selective manner by means of the optical Stark effect [5,6]. Prior research has demonstrated that monolayer TMDs driven by below-resonance (red-detuned) circularly polarized light can exhibit an upshifted exciton level, either at the K or K valleys depending on the helicity, while keeping the opposite valley unchanged. This valley-specific phenomenon arises from the exciton state repulsion by the photon-dressed state in the same valley, a mechanism consistent with other optical Stark effects in solids [7,8].Despite much recent progress, a complete understanding of the optical Stark effect in monolayer TMDs is still lacking. First, the anticipated complementary effect of using above-resonance (blue-detuned) light to downshift the exciton level has not been demonstrated. This is challenging because the blue-detuned light excites real exciton population, which can easily obscure the optical Stark effect. Secondly, when the detuning is sufficiently small and comparable to the biexciton binding energy, the effect may involve a coherent formation of the recently identified intervalley biexcitons [9,10]. These biexcitons are expected to have profound contributions to the optical Stark effect, as indicated by earlier studies in semiconductor quantum wells [11,12]. Elucidating these processes is therefore crucial to investigate the role of intervalley biexcitons in monolayer TMDs in order to obtain a thorough understanding of the coherent lightmatter interactions in this system.In this letter, we explore the optical Stark effect using blue-detuned optical driving in monolayer TMD WS 2 . * gedik@mit.edu We found that by driving the system using intense laser pulses with blue-detuned and left circular polarization, we can lower the exciton energy at the K valley. In addition, as the driving pho...

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Ultrafast generation of pseudo-magnetic field for valley excitons in WSe ₂ monolayers

Cited by 306 publications

References 34 publications

Intervalley biexcitons and many-body effects in monolayerMoS2

Intervalley biexcitons and many-body effects in monolayerMoS2

Coherent Light-Matter Interactions in Monolayer Transition-Metal Dichalcogenides

Observation of Intervalley Biexcitonic Optical Stark Effect in Monolayer WS₂

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Ultrafast generation of pseudo-magnetic field for valley excitons in WSe 2 monolayers

Cited by 306 publications

References 34 publications

Intervalley biexcitons and many-body effects in monolayerMoS2

Intervalley biexcitons and many-body effects in monolayerMoS2

Coherent Light-Matter Interactions in Monolayer Transition-Metal Dichalcogenides

Observation of Intervalley Biexcitonic Optical Stark Effect in Monolayer WS2

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Ultrafast generation of pseudo-magnetic field for valley excitons in WSe ₂ monolayers

Observation of Intervalley Biexcitonic Optical Stark Effect in Monolayer WS₂