Resonant optical Stark effect in monolayer WS2

AbstractBreaking the valley degeneracy in monolayer transition metal dichalcogenides through the valley-selective optical Stark effect (OSE) can be exploited for classical and quantum valleytronic operations such as coherent manipulation of valley superposition states. The strong light-matter interactions responsible for the OSE have historically been described by a two-level dressed-atom model, which assumes noninteracting particles. Here we experimentally show that this model, which works well in semiconductors far from resonance, does not apply for excitation near the exciton resonance in monolayer WS2. Instead, we show that an excitonic model of the OSE, which includes many-body Coulomb interactions, is required. We confirm the prediction from this theory that many-body effects between virtual excitons produce a dominant blue-shift for photoexcitation detuned from resonance by less than the exciton binding energy. As such, we suggest that our findings are general to low-dimensional semiconductors that support bound excitons and other many-body Coulomb interactions.

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Ultrafast generation of pseudo-magnetic field for valley excitons in WSe2monolayers

Science ◽

10.1126/science.1258122 ◽

2014 ◽

Vol 346 (6214) ◽

pp. 1205-1208 ◽

Cited By ~ 156

Author(s):

Jonghwan Kim ◽

Xiaoping Hong ◽

Chenhao Jin ◽

Su-Fei Shi ◽

Chih-Yuan S. Chang ◽

...

Keyword(s):

Magnetic Field ◽

Stark Effect ◽

Transition Metal Dichalcogenides ◽

Degree Of Freedom ◽

Energy Splitting ◽

Pump Probe ◽

Valley Polarization ◽

Metal Dichalcogenides ◽

Optical Stark Effect ◽

Probe Spectroscopy

The valley pseudospin is a degree of freedom that emerges in atomically thin two-dimensional transition metal dichalcogenides (MX2). The capability to manipulate it, in analogy to the control of spin in spintronics, can open up exciting opportunities. Here, we demonstrate that an ultrafast and ultrahigh valley pseudo-magnetic field can be generated by using circularly polarized femtosecond pulses to selectively control the valley degree of freedom in monolayer MX2.Using ultrafast pump-probe spectroscopy, we observed a pure and valley-selective optical Stark effect in WSe2monolayers from the nonresonant pump, resulting in an energy splitting of more than 10 milli–electron volts between the K and K′ valley exciton transitions. Our study opens up the possibility to coherently manipulate the valley polarization for quantum information applications.

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Modulation of photocarrier relaxation dynamics in two-dimensional semiconductors

Light Science & Applications ◽

10.1038/s41377-020-00430-4 ◽

2020 ◽

Vol 9 (1) ◽

Author(s):

Yuhan Wang ◽

Zhonghui Nie ◽

Fengqiu Wang

Keyword(s):

Electric Fields ◽

Mechanical Strain ◽

Transition Metal Dichalcogenides ◽

Binding Energies ◽

Two Dimensional ◽

Coulomb Interactions ◽

Relaxation Dynamics ◽

Strong Light ◽

2D Semiconductors ◽

Metal Dichalcogenides

AbstractDue to strong Coulomb interactions, two-dimensional (2D) semiconductors can support excitons with large binding energies and complex many-particle states. Their strong light-matter coupling and emerging excitonic phenomena make them potential candidates for next-generation optoelectronic and valleytronic devices. The relaxation dynamics of optically excited states are a key ingredient of excitonic physics and directly impact the quantum efficiency and operating bandwidth of most photonic devices. Here, we summarize recent efforts in probing and modulating the photocarrier relaxation dynamics in 2D semiconductors. We classify these results according to the relaxation pathways or mechanisms they are associated with. The approaches discussed include both tailoring sample properties, such as the defect distribution and band structure, and applying external stimuli such as electric fields and mechanical strain. Particular emphasis is placed on discussing how the unique features of 2D semiconductors, including enhanced Coulomb interactions, sensitivity to the surrounding environment, flexible van der Waals (vdW) heterostructure construction, and non-degenerate valley/spin index of 2D transition metal dichalcogenides (TMDs), manifest themselves during photocarrier relaxation and how they can be manipulated. The extensive physical mechanisms that can be used to modulate photocarrier relaxation dynamics are instrumental for understanding and utilizing excitonic states in 2D semiconductors.

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Dynamic polaronic screening for anomalous exciton spin relaxation in two-dimensional lead halide perovskites

Science Advances ◽

10.1126/sciadv.abb7132 ◽

2020 ◽

Vol 6 (47) ◽

pp. eabb7132

Author(s):

Weijian Tao ◽

Qiaohui Zhou ◽

Haiming Zhu

Keyword(s):

Spin Dynamics ◽

Transition Metal Dichalcogenides ◽

Two Dimensional ◽

Coulomb Interactions ◽

Strong Light ◽

Exciton Interaction ◽

Metal Dichalcogenides ◽

Excitonic Interactions ◽

Halide Perovskites ◽

Lead Halide

Two-dimensional lead halide perovskites with confined excitons have shown exciting potentials in optoelectronic applications. It is intriguing but unclear how the soft and polar lattice redefines excitons in layered perovskites. Here, we reveal the intrinsic exciton properties by investigating exciton spin dynamics, which provides a sensitive probe to exciton coulomb interactions. Compared to transition metal dichalcogenides with comparable exciton binding energy, we observe orders of magnitude smaller exciton-exciton interaction and, counterintuitively, longer exciton spin lifetime at higher temperature. The anomalous spin dynamics implies that excitons exist as exciton polarons with substantially weakened inter- and intra-excitonic interactions by dynamic polaronic screening. The combination of strong light matter interaction from reduced dielectric screening and weakened inter-/intra-exciton interaction from dynamic polaronic screening explains their exceptional performance and provides new rules for quantum-confined optoelectronic and spintronic systems.

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MoS2 Based Photodetectors: A Review

Sensors ◽

10.3390/s21082758 ◽

2021 ◽

Vol 21 (8) ◽

pp. 2758

Author(s):

Alberto Taffelli ◽

Sandra Dirè ◽

Alberto Quaranta ◽

Lucio Pancheri

Keyword(s):

Electric Fields ◽

Spectral Range ◽

Performance Metrics ◽

Transition Metal Dichalcogenides ◽

Visible Spectral Range ◽

Detector Performance ◽

Strong Light ◽

Near Ultraviolet ◽

Passivation Layers ◽

Low Dimensional

Photodetectors based on transition metal dichalcogenides (TMDs) have been widely reported in the literature and molybdenum disulfide (MoS2) has been the most extensively explored for photodetection applications. The properties of MoS2, such as direct band gap transition in low dimensional structures, strong light–matter interaction and good carrier mobility, combined with the possibility of fabricating thin MoS2 films, have attracted interest for this material in the field of optoelectronics. In this work, MoS2-based photodetectors are reviewed in terms of their main performance metrics, namely responsivity, detectivity, response time and dark current. Although neat MoS2-based detectors already show remarkable characteristics in the visible spectral range, MoS2 can be advantageously coupled with other materials to further improve the detector performance Nanoparticles (NPs) and quantum dots (QDs) have been exploited in combination with MoS2 to boost the response of the devices in the near ultraviolet (NUV) and infrared (IR) spectral range. Moreover, heterostructures with different materials (e.g., other TMDs, Graphene) can speed up the response of the photodetectors through the creation of built-in electric fields and the faster transport of charge carriers. Finally, in order to enhance the stability of the devices, perovskites have been exploited both as passivation layers and as electron reservoirs.

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Correlation Between Corrugation-Induced Flexoelectric Polarization and Conductivity of Low-Dimensional Transition Metal Dichalcogenides

Physical Review Applied ◽

10.1103/physrevapplied.15.044051 ◽

2021 ◽

Vol 15 (4) ◽

Author(s):

Anna N. Morozovska ◽

Eugene A. Eliseev ◽

Hanna V. Shevliakova ◽

Yaroslava Yu. Lopatina ◽

Galina I. Dovbeshko ◽

...

Keyword(s):

Transition Metal ◽

Transition Metal Dichalcogenides ◽

Metal Dichalcogenides ◽

Low Dimensional

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Exciton-polaron Rydberg states in monolayer MoSe2 and WSe2

Nature Communications ◽

10.1038/s41467-021-26304-w ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Erfu Liu ◽

Jeremiah van Baren ◽

Zhengguang Lu ◽

Takashi Taniguchi ◽

Kenji Watanabe ◽

...

Keyword(s):

Boron Nitride ◽

Excited States ◽

Ground State ◽

Transition Metal Dichalcogenides ◽

Rydberg States ◽

Energy Shift ◽

Many Body ◽

Electron Hole ◽

Metal Dichalcogenides ◽

Excitonic States

AbstractExciton polaron is a hypothetical many-body quasiparticle that involves an exciton dressed with a polarized electron-hole cloud in the Fermi sea. It has been evoked to explain the excitonic spectra of charged monolayer transition metal dichalcogenides, but the studies were limited to the ground state. Here we measure the reflection and photoluminescence of monolayer MoSe2 and WSe2 gating devices encapsulated by boron nitride. We observe gate-tunable exciton polarons associated with the 1 s–3 s exciton Rydberg states. The ground and excited exciton polarons exhibit comparable energy redshift (15~30 meV) from their respective bare excitons. The robust excited states contradict the trion picture because the trions are expected to dissociate in the excited states. When the Fermi sea expands, we observe increasingly severe suppression and steep energy shift from low to high exciton-polaron Rydberg states. Their gate-dependent energy shifts go beyond the trion description but match our exciton-polaron theory. Our experiment and theory demonstrate the exciton-polaron nature of both the ground and excited excitonic states in charged monolayer MoSe2 and WSe2.

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Strong coupling in two-dimensional materials-based nanostructures: a review

Journal of Optics ◽

10.1088/2040-8986/ac47b3 ◽

2022 ◽

Author(s):

Ye Ming Qing ◽

Yongze Ren ◽

Dangyuan Lei ◽

Hui Feng Ma ◽

Tie Jun Cui

Keyword(s):

Strong Coupling ◽

Coupling Effect ◽

Transition Metal Dichalcogenides ◽

Hybrid Nanostructures ◽

Two Dimensional ◽

Strong Light ◽

Two Dimensional Materials ◽

Potential Applications ◽

Metal Dichalcogenides ◽

Emission Behavior

Abstract Strong interaction between electromagnetic radiation and matter leads to the formation of hybrid light-matter states, making the absorption and emission behavior different from those of the uncoupled states. Strong coupling effect results in the famous Rabi splitting and the emergence of new polaritonic eigenmodes, exhibiting spectral anticrossing behavior and unique energy-transfer properties. In recent years, there has been a rapidly increasing number of works focusing on strong coupling between nanostructures and two-dimensional materials (2DMs), because of the exceptional properties and applications they demonstrate. Here, we review the significant recent advances and important developments of strong light-matter interactions in 2DMs-based nanostructures. We adopt the coupled oscillator model to describe the strong coupling and give an overview of various hybrid nanostructures to realize this regime, including graphene-based nanostructures, black phosphorus-based nanostructures, transition-metal dichalcogenides-based nanostructures, etc. In addition, we discuss potential applications that can benefit from these effects and conclude our review with a perspective on the future of this rapidly emerging field.

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Strain-induced effects on the electronic properties of 2D materials

Nanomaterials and Nanotechnology ◽

10.1177/1847980420902569 ◽

2020 ◽

Vol 10 ◽

pp. 184798042090256 ◽

Cited By ~ 6

Author(s):

Sara Postorino ◽

Davide Grassano ◽

Marco D’Alessandro ◽

Andrea Pianetti ◽

Olivia Pulci ◽

...

Keyword(s):

Electronic Properties ◽

Density Functional ◽

2D Materials ◽

Transition Metal Dichalcogenides ◽

Many Body ◽

Functional Theory ◽

Metal Dichalcogenides ◽

Extreme Sensitivity ◽

Nonuniform Strain

Thanks to the ultrahigh flexibility of 2D materials and to their extreme sensitivity to applied strain, there is currently a strong interest in studying and understanding how their electronic properties can be modulated by applying a uniform or nonuniform strain. In this work, using density functional theory (DFT) calculations, we discuss how uniform biaxial strain affects the electronic properties, such as ionization potential, electron affinity, electronic gap, and work function, of different classes of 2D materials from X-enes to nitrides and transition metal dichalcogenides. The analysis of the states in terms of atomic orbitals allows to explain the observed trends and to highlight similarities and differences among the various materials. Moreover, the role of many-body effects on the predicted electronic properties is discussed in one of the studied systems. We show that the trends with strain, calculated at the GW level of approximation, are qualitatively similar to the DFT ones solely when there is no change in the character of the valence and conduction states near the gap.

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Monolayer MoS2 for nanoscale photonics

Nanophotonics ◽

10.1515/nanoph-2019-0533 ◽

2020 ◽

Vol 9 (7) ◽

pp. 1557-1577 ◽

Cited By ~ 5

Author(s):

Xianguang Yang ◽

Baojun Li

Keyword(s):

Near Infrared ◽

Transition Metal Dichalcogenides ◽

Single Layer ◽

Band Gaps ◽

Laser Source ◽

Two Dimensional ◽

Strong Light ◽

Metal Dichalcogenides ◽

Optoelectronic Applications ◽

Flexible Photodetector

AbstractTransition metal dichalcogenides are two-dimensional semiconductors with strong in-plane covalent and weak out-of-plane interactions, resulting in exfoliation into monolayers with atomically thin thickness. This creates a new era for the exploration of two-dimensional physics and device applications. Among them, MoS2 is stable in air and easily available from molybdenite, showing tunable band-gaps in the visible and near-infrared waveband and strong light-matter interactions due to the planar exciton confinement effect. In the single-layer limit, monolayer MoS2 exhibits direct band-gaps and bound excitons, which are fundamentally intriguing for achieving the nanophotonic and optoelectronic applications. In this review, we start from the characterization of monolayer MoS2 in our group and understand the exciton modes, then explore thermal excitons and band renormalization in monolayer MoS2. For nanophotonic applications, the recent progress of nanoscale laser source, exciton-plasmon coupling, photoluminescence manipulation, and the MoS2 integration with nanowires or metasurfaces are overviewed. Because of the benefits brought by the unique electronic and mechanical properties, we also introduce the state of the art of the optoelectronic applications, including photoelectric memory, excitonic transistor, flexible photodetector, and solar cell. The critical applications focused on in this review indicate that MoS2 is a promising material for nanophotonics and optoelectronics.

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