Electrical control of excitons in a gated two-dimensional semiconductor

The emergence of two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides (TMDs), and the ability to build artificial van der Waals heterostructures (vdWHs) by stacking different combinations of materials, has opened a new route for engineering quantum systems. 2D TMDs support bound electron-hole pairs, or excitons, that have particularly large binding energies, such that excitons dominate their optical properties, even at room temperature. The ability to electrically tune their properties via external electric gates is essential for various interesting opto-electronic applications. A crucial feature of semiconductor nanostructures is the quantum confined Stark effect (QCSE), the change in optical energy on applying an electric field perpendicular to the layers. Using a gated vdWH, we demonstrated that in monolayer MoS$_2$ optical absorption is strong, but the transition energy is not tunable as the neutral exciton has essentially no permanent out-of-plane electric dipole and is only slightly polarizable. The electrical control of excitons via the QCSE requires larger polarizabilities or a non-zero dipole moment as observed in heterobilayers where the bound electrons and holes reside in different layers. However, the coupling to light in these systems is considerably reduced. To combine best of both worlds, a polarizable yet strong optical dipole, we integrated homobilayer MoS$_2$ in a dual-gate device structure. In its natural bilayer form, we discovered interlayer excitons which exhibit both a high oscillator strength and highly tunable energies in an applied electric field. Owing to their very large dipole moments, we were able to bring these interlayer excitons energetically close to resonance with the excitons confined to the single layers, and study exciton-exciton interactions in these systems. Equipping MoS$_2$ with gates allows electrons to be injected, creating a 2D electron gas. By probing the electronic ground state at various electron densities, we presented experimental evidence for a spontaneous spin-polarization in monolayer MoS$_2$. Significantly, the extremely small Bohr radius of an electron in this material suggests that Coulomb effects play an important role at experimental relevant electron densities. The ability to control the properties of thin semiconductors by electrical means makes these systems a versatile platform for rich exciton physics and unique opto-electronic applications

Spin-Polarized Electrons in Monolayer MoS2_2

The optical susceptibility is a local, minimally-invasive and spin-selective probe of the ground state of a two-dimensional electron gas. We apply this probe to a gated monolayer of MoS$_2$. We demonstrate that the electrons are spin polarized. Of the four available bands, only two are occupied. These two bands have the same spin but different valley quantum numbers. We argue that strong Coulomb interactions are a key aspect of this spontaneous symmetry breaking. The Bohr radius is so small that even electrons located far apart in phase space interact, facilitating exchange couplings to align the spins

Quantum confined Stark effect in a MoS2_2 monolayer van der Waals heterostructure

The optics of dangling-bond-free van der Waals heterostructures containing transition metal dichalcogenides are dominated by excitons. A crucial property of a confined exciton is the quantum confined Stark effect (QCSE). Here, such a heterostructure is used to probe the QCSE by applying a uniform vertical electric field across a molybdenum disulfide (MoS$_2$) monolayer. The photoluminescence emission energies of the neutral and charged excitons shift quadratically with the applied electric field provided the electron density remains constant, demonstrating that the exciton can be polarized. Stark shifts corresponding to about half the homogeneous linewidth were achieved. Neutral and charged exciton polarizabilities of (7.8~\pm~1.0)\times 10^{-10}~\tr{D~m~V}^{-1} and (6.4~\pm~0.9)\times 10^{-10}~\tr{D~m~V}^{-1} at relatively low electron density (8 \times 10^{11}~\tr{cm}^{-2}) have been extracted, respectively. These values are one order of magnitude lower than the previously reported values, but in line with theoretical calculations. The methodology presented here is versatile and can be applied to other semiconducting layered materials as well

First-order magnetic phase-transition of mobile electrons in monolayer MoS2_2

Evidence is presented for a first-order magnetic phase transition in a gated two-dimensional semiconductor, monolayer-MoS$_2$. The phase boundary separates a spin-polarised (ferromagnetic) phase at low electron density and a paramagnetic phase at high electron density. Abrupt changes in the optical response signal an abrupt change in the magnetism. The magnetic order is thereby controlled via the voltage applied to the gate electrode of the device. Accompanying the change in magnetism is a large change in the electron effective mass

Optical second harmonic generation in encapsulated single-layer InSe

We report the observation of optical second harmonic generation (SHG) in single-layer indium selenide (InSe). We measure a second harmonic signal of $>10^3$ $\textrm{cts/s}$ under nonresonant excitation using a home-built confocal microscope and a standard pulsed pico-second laser. We demonstrate that polarization-resolved SHG serves as a fast, non-invasive tool to determine the crystal axes in single-layer InSe and to relate the sharp edges of the flake to the armchair and zigzag edges of the crystal structure. Our experiment determines these angles to an accuracy better than $\pm$ $0.2^{\circ}$. Treating the two-dimensional material as a nonlinear polarizable sheet, we determine a second-order nonlinear sheet polarizability $| \chi_{\textrm{sheet}}^{(2)}|=(17.9 \pm 11.0)\times 10^{-20}$ $\textrm{m}^2 \textrm{V}^{-1}$ for single-layer InSe, corresponding to an effective nonlinear susceptibility value of $| \chi_\textrm{eff}^{(2)}| \approx (223 \pm 138) \times 10^{-12}$ $\textrm{m} \textrm{V}^{-1}$ accounting for the sheet thickness ($\textrm{d} \approx 0.8$ $\textrm{nm}$). We demonstrate that the SHG technique can also be applied to encapsulated samples to probe their crystal orientations. The method is therefore suitable for creating high quality van der Waals heterostructures with control over the crystal directions

Controlling interlayer excitons in MoS2 layers grown by chemical vapor deposition

Combining MoS$_2$ monolayers to form multilayers allows to access new functionalities. In this work, we examine the correlation between the stacking order and the interlayer coupling of valence states in MoS$_2$ homobilayer samples grown by chemical vapor deposition (CVD) and artificially stacked bilayers from CVD monolayers. We show that hole delocalization over the bilayer is allowed in 2H stacking and results in strong interlayer exciton absorption and also in a larger A-B exciton separation as compared to 3R bilayers, where both holes and electrons are confined to the individual layers. Comparing 2H and 3R reflectivity spectra allows to extract an interlayer coupling energy of about $t_\perp=49$ meV. Obtaining very similar results for as-grown and artificially stacked bilayers is promising for assembling large area van der Waals structures with CVD material, using interlayer exciton absorption and A-B exciton separation as indicators for interlayer coupling. Beyond DFT calculations including excitonic effects confirm signatures of efficient interlayer coupling for 2H stacking in agreement with our experiments.Comment: 10 pages including supplement, 3 + 4 figure

Giant Stark splitting of an exciton in bilayer MoS2

Transition metal dichalcogenides (TMDs) constitute a versatile platform for atomically thin optoelectronics devices and spin-valley memory applications. In monolayers optical absorption is strong, but the transition energy is not tunable as the neutral exciton has essentially no out-of-plane electric dipole. In contrast, interlayer exciton transitions in heterobilayers are widely tunable in applied electric fields, but their coupling to light is considerably reduced. Here, we show tuning over 120 meV of interlayer excitons with high oscillator strength in bilayer MoS2. These shifts are due to the quantum confined Stark effect, here the electron is localised to one of the layers yet the hole is delocalised across the bilayer. We optically probe the interaction between intra- and interlayer excitons as they are energetically tuned into resonance. This allows studying their mixing supported by beyond standard density functional theory calculations including excitonic effects. In MoS2 trilayers our experiments uncover two types of interlayer excitons with and without in-built electric dipoles, respectively. Highly tunable excitonic transitions with large oscillator strength and in-built dipoles, that lead to considerable exciton-exciton interactions, hold great promise for non-linear optics with polaritons.Comment: final author versio

Capacitively and Inductively Coupled Excitons in Bilayer MoS 2

Main text: 6 pages, 4 figures; Supplementary Information: 17 pages, 15 figuresInternational audienceThe interaction of intralayer and interlayer excitons is studied in a two-dimensional semiconductor, homobilayer MoS$_2$. It is shown that the measured optical susceptibility reveals both the magnitude and the sign of the coupling constants. The interlayer exciton interacts capacitively with the intralayer B-exciton (positive coupling constant) consistent with hole tunnelling from one monolayer to the other. Conversely, the interlayer exciton interacts inductively with the intralayer A-exciton (negative coupling constant). First-principles many-body calculations show that this coupling arises via an intravalley exchange-interaction of A- and B-excitons