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Optically dark states play an important role in the electronic and optical properties of monolayers (MLs) of semiconducting transition metal dichalcogenides. The effect of temperature on the in-plane-field activation of the neutral and charged dark excitons is investigated in a WSe2 ML encapsulated in hexagonal BN flakes. The brightening rates of the neutral dark (X D) and grey (X G) excitons and the negative dark trion (T D) differ substantially at particular temperature. More importantly, they weaken considerably by about 3–4 orders of magnitude with temperature increased from 4.2 K to 100 K. The quenching of the dark-related emissions is accompanied by the two-order-of-magnitude increase in the emissions of their neutral bright counterparts, i.e. neutral bright exciton (X B) and spin-singlet (T S) and spin-triplet (T T) negative trions, due to the thermal activations of dark states. Furthermore, the energy splittings between the dark X D and T D complexes and the corresponding bright X B, T S, and T T ones vary with temperature rises from 4.2 K to 100 K. This is explained in terms of the different exciton–phonon coupling for the bright and dark excitons stemming from their distinct symmetry properties.

We present femtosecond pump-probe measurements of neutral and charged exciton optical response in monolayer MoSe2 to resonant photoexcitation of a given exciton state in the presence of 2D electron gas. We show that creation of charged exciton (X−) population in a given K+, K− valley requires the capture of available free carriers in the opposite valley and reduces the interaction of neutral exciton (X) with the electron Fermi sea. We also observe spectral broadening of the X transition line with the increasing X− population caused by efficient scattering and excitation induced dephasing. From the valley-resolved analysis of the observed effects we are able to extract the spin-valley relaxation times of free carriers as a function of carrier density. Moreover, we analyze the oscillator strength and energy shift of X in the regime of interaction with electron Fermi sea under resonant excitation. From this we can observe the process of X decay by radiative recombination paired with trion formation. We demonstrate an increase of neutral exciton relaxation rate with the introduction of Fermi sea of electrons. We ascribe the observed effect to the increased efficiency of the trion formation, as well as the radiative decay caused by the screening of disorder by the free carriers.

Optically dark states play an important role in the electronic and optical properties of monolayers (MLs) of semiconducting transition metal dichalcogenides. The effect of temperature on the in-plane-field activation of the neutral and charged dark excitons is investigated in a WSe2 ML encapsulated in hexagonal BN flakes. The brightening rates of the neutral dark (XD) and grey (XG) excitons and the negative dark trion (TD) differ substantially at particular temperature. More importantly, they weaken considerably by about 3–4 orders of magnitude with temperature increased from 4.2 K to 100 K. The quenching of the dark-related emissions is accompanied by the two-order-of-magnitude increase in the emissions of their neutral bright counterparts, i.e. neutral bright exciton (XB) and spin-singlet (TS) and spin-triplet (TT) negative trions, due to the thermal activations of dark states. Furthermore, the energy splittings between the dark XD and TD complexes and the corresponding bright XB, TS, and TT ones vary with temperature rises from 4.2 K to 100 K. This is explained in terms of the different exciton–phonon coupling for the bright and dark excitons stemming from their distinct symmetry properties. [ABSTRACT FROM AUTHOR]

Using ultrafast resonant pump-probe spectroscopy in an unconventional experimental setup we investigate the spectral shape and dynamics of absorption features related to the A exciton in an hexagonal boron nitride (hBN)/MoSe2/hBN van der Waals heterostructure. While in a pure two-level system a pump-probe experiment measures the occupation or the polarization dynamics, depending on the time ordering of the pulse pair, in the transition metal dichalcogenide (TMD) system both quantities get thoroughly mixed by strong exciton–exciton interaction. We find that for short positive delays the spectral lines experience pronounced changes in their shape and energy and they relax to the original situation on a picosecond time scale. For negative delays, distinctive spectral oscillations appear indicating the first-time observation of perturbed free induction decay for a TMD system. The comparison between co-circular and cross-circular excitation schemes further allows us to investigate the rapid inter-valley scattering. By considering a three-level system as a minimal model including the local field effect, excitation-induced dephasing (EID), and scattering between the excited states we explain all phenomena observed in the experiment with excellent consistency. Our handy model can be even further reduced to two levels in the case of a co-circular excitation, for which we derive analytic expressions to describe the detected signals. This allows us to trace back the spectral shapes and shifts to the impact of local field effect and EID thus fully reproducing the complex behavior of the observed effects.

We experimentally demonstrate the creation of defects in monolayer WSe2 via nanopillar imprinting and helium ion irradiation. Based on the first method, we realize atomically thin vertical tunneling light-emitting diodes based on WSe2 monolayers hosting quantum emitters at deterministically specified locations. We characterize these emitters by investigating the evolution of their emission spectra in external electric and magnetic fields, as well as by inducing electroluminescence at low temperatures. We identify qualitatively different types of quantum emitters and classify them according to the dominant electron-hole recombination paths, determined by the mechanisms of intervalley mixing occurring in fundamental conduction and/or valence subbands. [ABSTRACT FROM AUTHOR]

Recent results on the optical properties of monolayer and few layers of semiconducting transition metal dichalcogenides are reviewed. Experimental observations are presented and discussed in the frame of existing models, highlighting the limits of our understanding in this emerging field of research. We first introduce the representative band structure of these systems and their interband optical transitions. The effect of an external magnetic field is then considered to discuss Zeeman spectroscopy and optical pumping experiments, both revealing phenomena related to the valley degree of freedom. Finally, we discuss the observation of single photon emitters in different types of layered materials, including wide band gap hexagonal boron nitride. While going through these topics, we try to focus on open questions and on experimental observations, which do not yet have a clear explanation.

Temperature-dependent (5 K–300 K) Raman scattering study of A1g/A′1 phonon modes in mono-layer (1L), bilayer (2L), trilayer (3L), and tetralayer (4L) MoTe2 is reported. The temperature evolution of the modes’ intensity critically depends on the flake thickness. In particular with λ=632.8-nm light excitation, a strongly non-monotonic dependence of the A1g mode intensity is observed in 2L MoTe2. The intensity decreases with decreasing temperature down to 220 K, and the A1g mode almost completely vanishes from the Stokes scattering spectrum in the temperature range between 160 K and 220 K. The peak recovers at lower temperatures, and at T=5 K, it becomes three times more intense that at room temperature. Similar non-monotonic intensity evolution is observed for the out-of-plane mode in 3L MoTe2 in which tellurium atoms in all three layers vibrate in-phase. The intensity of the other out-of-plane Raman-active mode (with vibrations of tellurium atoms in the central layer shifted by 180° with respect to the vibrations in outer layers) only weakly depends on temperature. The observed quenching of the Raman scattering in 2L and 3L MoTe2 is attributed to a destructive interference between the resonant and non-resonant contributions to the Raman scattering amplitude. The observed “antiresonance” is related to the electronic excitation at the M point of the Brillouin zone in few-layer MoTe2.

Optical response of crystalline solids is to a large extent driven by excitations that promote electrons among individual bands. This allows one to apply optical and magneto‐optical methods to determine experimentally the energy band gap —a fundamental property crucial to our understanding of any solid—with a great precision. Here it is shown that such conventional methods, applied with great success to many materials in the past, do not work in topological Dirac semimetals with a dispersive nodal line. There, the optically deduced band gap depends on how the magnetic field is oriented with respect to the crystal axes. Such highly unusual behavior is explained in terms of band‐gap renormalization driven by Lorentz boosts which results from the Lorentz‐covariant form of the Dirac Hamiltonian relevant for the nodal line at low energies. [ABSTRACT FROM AUTHOR]

Monolayers of transition metal dichalcogenides display a strong excitonic optical response. Additionally encapsulating the monolayer with hexagonal boron nitride allows to reach the limit of a purely homogeneously broadened exciton system. On such a MoSe2‐based system, ultrafast six‐wave mixing spectroscopy is performed and a novel destructive photon echo effect is found. This process manifests as a characteristic depression of the nonlinear signal dynamics when scanning the delay between the applied laser pulses. By theoretically describing the process within a local field model, an excellent agreement with the experiment is reached. An effective Bloch vector representation is developed and thereby it is demonstrated that the destructive photon echo stems from a destructive interference of successive repetitions of the heterodyning experiment. [ABSTRACT FROM AUTHOR]

Using ultrafast resonant pump-probe spectroscopy in an unconventional experimental setup we investigate the spectral shape and dynamics of absorption features related to the A exciton in an hexagonal boron nitride (hBN)/MoSe2/hBN van der Waals heterostructure. While in a pure two-level system a pump-probe experiment measures the occupation or the polarization dynamics, depending on the time ordering of the pulse pair, in the transition metal dichalcogenide (TMD) system both quantities get thoroughly mixed by strong exciton–exciton interaction. We find that for short positive delays the spectral lines experience pronounced changes in their shape and energy and they relax to the original situation on a picosecond time scale. For negative delays, distinctive spectral oscillations appear indicating the first-time observation of perturbed free induction decay for a TMD system. The comparison between co-circular and cross-circular excitation schemes further allows us to investigate the rapid inter-valley scattering. By considering a three-level system as a minimal model including the local field effect, excitation-induced dephasing (EID), and scattering between the excited states we explain all phenomena observed in the experiment with excellent consistency. Our handy model can be even further reduced to two levels in the case of a co-circular excitation, for which we derive analytic expressions to describe the detected signals. This allows us to trace back the spectral shapes and shifts to the impact of local field effect and EID thus fully reproducing the complex behavior of the observed effects. [ABSTRACT FROM AUTHOR]

Strong Coulomb correlations together with multi-valley electronic bands in the presence of spin-orbit interaction are at the heart of studies of the rich physics of excitons in monolayers of transition metal dichalcogenides (TMD). Those archetypes of two-dimensional systems promise a design of new optoelectronic devices. In intrinsic TMD monolayers the basic, intravalley excitons, are formed by a hole from the top of the valence band and an electron either from the lower or upper spin-orbit-split conduction band subbands: one of these excitons is optically active, the second one is dark, although possibly observed under special conditions. Here we demonstrate the s-series of Rydberg dark exciton states in tungsten diselenide monolayer, which appears in addition to a conventional bright exciton series in photoluminescence spectra measured in high in-plane magnetic fields. The comparison of energy ladders of bright and dark Rydberg excitons is shown to be a method to experimentally evaluate one of the missing band parameters in TMD monolayers: the amplitude of the spin-orbit splitting of the conduction band. Excitonic physics dominates the optical response of semiconductor monolayers but single particle band structure parameters are hard to probe experimentally. Here, spin-orbit splitting in the conduction band of monolayer WSe2 is revealed by the identification of the Rydberg series of dark excitons. [ABSTRACT FROM AUTHOR]

Micro‐magneto‐Raman scattering spectroscopy is made use of to probe magneto‐phonon resonances (MPR) in suspended mono‐ to pentalayer graphene. MPR correspond to avoided crossings between zone‐center optical phonons (G‐mode) and optically active inter‐Landau level (LL) transitions and provide a tool to perform LL spectroscopy at a fixed energy (≈197 meV) set by the G‐mode phonon. Using a single‐particle effective bilayer model, the velocity parameter associated with each MPR is readily extracted. A single velocity parameter slightly above the bulk graphite value suffices to fit all MPR for N ≥ 2 layer systems. In contrast, in monolayer graphene, it is found that the velocity parameter increases significantly from (1.23 ± 0.01) × 106 m s−1 up to (1.45 ± 0.02) × 106 m s−1 as the first to third optically active inter‐LL transitions couple to the G‐mode phonon. This result is understood as a signature of enhanced many‐body effects in unscreened graphene. [ABSTRACT FROM AUTHOR]

The spectral signatures associated with different negatively charged exciton complexes (trions) in a WS2 monolayer encapsulated in hBN are analyzed from low temperature and polarization resolved reflectance contrast (RC) and photoluminescence (PL) experiments, with an applied magnetic field. Based on results obtained from the RC experiment, we show that the valley Zeeman effect affects the optical response of both the singlet and the triplet trion species through the evolution of their energy and of their relative intensity, when applying an external magnetic field. Our analysis allows us to estimate a free electron concentration of ∼1.3 × 1011 cm−2. The observed evolutions based on PL experiments on the same sample are different and can hardly be understood within the same simple frame, highlighting the complexity of relaxation processes involved in the PL response. [ABSTRACT FROM AUTHOR]

We review the different results obtained in the last decade in the field of Raman scattering of graphene‐based systems, with an applied magnetic field. Electronic properties of graphene‐based systems with an applied magnetic field are first described. The phonon response in magnetic field, the magneto‐phono resonance, is then introduced and described for graphene monolayer and multilayer, as well as for bulk graphite. Electronic Raman scattering are then discussed in the context of Landau level spectroscopy, of electron–phonon interaction and of electron–electron interaction. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Temperature-dependent (5 K-300 K) Raman scattering study of A1g/A'1 phonon modes in mono-layer (1L), bilayer (2L), trilayer (3L), and tetralayer (4L) MoTe2 is reported. The temperature evolution of the modes' intensity critically depends on the flake thickness. In particular with λ = 632.8-nm light excitation, a strongly non-monotonic dependence of the A1g mode intensity is observed in 2L MoTe2. The intensity decreases with decreasing temperature down to 220 K, and the A1g mode almost completely vanishes from the Stokes scattering spectrum in the temperature range between 160 K and 220 K. The peak recovers at lower temperatures, and at T = 5 K, it becomes three times more intense that at room temperature. Similar non-monotonic intensity evolution is observed for the out-of-plane mode in 3L MoTe2 in which tellurium atoms in all three layers vibrate in-phase. The intensity of the other out-of-plane Raman-active mode (with vibrations of tellurium atoms in the central layer shifted by 180° with respect to the vibrations in outer layers) only weakly depends on temperature. The observed quenching of the Raman scattering in 2L and 3L MoTe2 is attributed to a destructive interference between the resonant and non-resonant contributions to the Raman scattering amplitude. The observed "antiresonance" is related to the electronic excitation at the M point of the Brillouin zone in few-layer MoTe2. [ABSTRACT FROM AUTHOR]

Bound electron-hole pairs called excitons govern the electronic and optical response of many organic and inorganic semiconductors. Excitons with spatially displaced wave functions of electrons and holes (interlayer excitons) are important for Bose-Einstein condensation, superfluidity, dissipationless current flow, and the light-induced exciton spin Hall effect. Here we report on the discovery of interlayer excitons in a bulk van der Waals semiconductor. They form due to strong localization and spin-valley coupling of charge carriers. By combining high-field magneto-reflectance experiments and ab initio calculations for 2H-MoTe2, we explain their salient features: the positive sign of the g-factor and the large diamagnetic shift. Our investigations solve the long-standing puzzle of positive g-factors in transition metal dichalcogenides, and pave the way for studying collective phenomena in these materials at elevated temperatures. [ABSTRACT FROM AUTHOR]

The energy spectrum of common two-dimensional electron gases consists of a harmonic (that is, equidistant) ladder of Landau levels, thus preventing the possibility of optically addressing individual transitions. In graphene, however, owing to its non-harmonic spectrum, individual levels can be addressed selectively. Here, we report a time-resolved experiment directly pumping discrete Landau levels in graphene. Energetically degenerate Landau-level transitions from n = −1 to n = 0 and from n = 0 to n = 1 are distinguished by applying circularly polarized THz light. An analysis based on a microscopic theory shows that the zeroth Landau level is actually depleted by strong Auger scattering, even though it is optically pumped at the same time. The surprisingly strong electron-electron interaction responsible for this effect is directly evidenced through a sign reversal of the pump-probe signal. [ABSTRACT FROM AUTHOR]

We probe electronic excitations betweenLandau levels in freestanding N-layer graphene overa broad energy range, with unprecedentedspectral and spatial resolution, using micro magneto-Raman scatteringspectroscopy. A characteristic evolution of electronic bands in upto five Bernal-stacked graphene layers is evidenced and shown to remarkablyfollow a simple theoretical approach, based on an effective bilayermodel. (N> 3)-layer graphenes appear as appealingcandidates in the quest for novel phenomena, particularly in the quantumHall effect regime. Our work paves the way toward minimally invasiveinvestigations of magneto-excitons in other emerging low-dimensionalsystems, with a spatial resolution down to 1 μm. [ABSTRACT FROM AUTHOR]