Session M55: Exciton Physics in Transition Metal Dichalcogenides: Experimental Probes and Manipulation

Optically controllable magnetism in atomically thin semiconductors

Correlated phases including Wigner-crystals, Mott insulators, charge density waves, and superconductivity have been demonstrated in 2D semiconductors. Magnetic phases have also been theoretically predicted and experimentally demonstrated under magnetic fields. Here, we experimentally demonstrate that a mesoscopic ferromagnetic order can be generated and controlled by local optical pumping in WSe2 at zero applied magnetic field. Under a circularly-polarized, off-resonance optical pump, a mesoscopic magnetic order that extends as large as 8 µm x 5 µm, bounded by sample edges and folds, is built up and probed by circular dichroism(CD) in reflectivity with CD magnitude exceed 20% at excitonic resonances. The helicity of the optical pump determines the orientation of the magnetic order, allows optical control of the magnetic order. The research demonstrates a versatile way to manipulate long-range magnetic order in 2D semiconductors, enables sophisticated control of correlated electron phases in two-dimensional electron gases (2DEGs). The convenient manipulation of the magnetic order and the large CD amplitude also unlocks new applications of monolayer semiconductors in spintronics and nanophotonics.   

Signatures of strong exciton-surface plasmon coupling in monolayer transition metal dichalcogenide gold heterostructures

The strong light-matter interactions of excitons in the monolayer W- and Mo- transition metal dichalcogenides has led them to be among the most well-studied semiconductor excitations in recent years. Many rich phenomena has been revealed including multi-body exciton species as well as quantum confined excitons in strain potentials [1].  In addition, strong exciton-photon coupling inside optical cavities has been extensively studied, however their strong coupling with metal plasmons remains relatively unexplored. In this presentation, we will show recent work on the study of strong coupling signatures between excitons in monolayer WSe₂ and Au surface plasmon-polaritons, so-called plexciton states. Using tip-enhanced photoluminescence, we see peak splitting of strain localized excitons in WSe₂ nanobubbles coupled to gap-mode plasmons between an Au probe and substrate. Further, using WSe₂ nanobubbles as a reporter we observe signs of propagating surface plasmon-exciton in the spatial signal of the recorded photoluminescence. Our results highlight the promise of engineering plexcitons for use in the next generation of on-chip opto-electronic devices.

[1] Darlington, T.P., Carmesin, C., Florian, M. et al. Nat. Nanotechnol. 15, 854–860 (2020). https://doi.org/10.1038/s41565-020-0730-5   

Trends in wavefunction overlap, moiré-patterns and lattice reconstruction studied by interlayer exciton absorption spectroscopy

Interlayer excitons (ILXs), correlated electron-hole pairs split across two vertically stacked semiconducting monolayers, are strongly tunable through electrical field and through twist-angle [1,2,3]. While absorption measurements remain a robust exciton characterization tool, due to the weak oscillator strength of ILXs, to date most ILX characterization has focused on the photoluminescence measurements. Unfortunately, the photoluminescence data can be strongly modified by small strains and defects. Here, we present the ILX absorption spectra for the WSe₂/MoSe₂ heterostructure obtained using an electromodulation technique for samples at different twist-angles and under out-of-plane electric fields. These measurements have allowed us to extract intrinsic radiative lifetimes of ILXs in a robust manner. Looking at their electric field dependence, we obtain a signature of changing electron-hole wavefunction-overlap. In examining the oscillator strength of the ILX for different twist-angles, we find evidence both of a deep moiré potential and of reconstruction of the two materials for small twist angles.

References: [1] A. Jauregui et al., Science 366 (2019). [2] Rivera et al. Nat. Nanotech. (2018) [3] Choi et al. PRL. (2021)   

Exciton dynamics in plasmonic cavities coupled to monolayer WSe2

Tailoring light–matter interactions in atomically thin transition metal dichalcogenides (TMDs) is a subject of considerable current interest to unveil the physics of exciton dynamics in these systems. Here, we report our findings on the selective enhancement of exciton emission by coupling hBN encapsulated monolayer WSe₂ to plasmonic nanocavities. Our nanocavities, formed by a small gap between a flat gold surface and a gold nanocube, support a strong out-of-plane electric field enhancement. hBN encapsulated monolayer WSe₂ embedded in the nanocavity is shown to exhibit rich excitonic states at low temperature. Based on the plasmonic cavity geometry, we can selectively enhance exciton emission and study their unique dynamics. The possible mechanisms underlying these unusual light-matter interactions are unveiled through the assistance of tunable electric doping and magnetic field bias. Our results provide new progress on the understanding of exciton-plasmon interactions in 2D TMDs, paving the way for applications in novel optoelectronic devices.   

In-Vivo Control of Excitons in TMDs

Excitons in heterostructures of atomically thin transition metal dichalcogenides (TMDs) have recently attracted attention as an appealing platform for optoelectronic and valleytronic devices, with a high degree of in-situ tunability provided through electrostatic gating. A major challenge in achieving such applications is the realization of dynamical control schemes, in which the properties of excitons are not pre-determined at the time of excitation but can also be modulated during their lifetime. Here, we demonstrate such "in-vivo" control of excitons by combining high-frequency electrical control with long-lived interlayer excitons in MoSe2/WSe2 heterobilayers. Through temporally-resolved photoluminescence measurements, we show in-vivo control of various exciton properties, including lifetime and resonance wavelength. These capabilities open the door for novel studies of exciton dynamics and for technological applications such as on-demand photon retrieval and the manipulation of information stored in TMD-based memory devices.   

Coherence properties of interlayer exciton electroluminescence in atomically thin semiconductor heterostructures

Atomically thin semiconductor heterostructures based on transition metal dichalcogenides offer an electrically tunable platform for developing coherent on-chip optoelectronic devices. Interlayer excitons in these systems form out-of-plane dipoles and exhibit long lifetimes owing to the spatial separation of electrons and holes. The bosonic nature of excitons and strong dipolar interactions make them ideal candidates to search for Bose-Einstein condensation in 2D materials. Here, we study the optoelectronic properties of MoSe₂/hBN/WSe₂ devices and compare their emission profile to that of non-hBN spaced samples. In the former system, we observe a threshold in the electroluminescence of interlayer excitons as we increase the applied forward bias. We further characterize the nature of this transition by performing measurements of the second-order correlation function. Lastly, we discuss how first-order correlation measurements can reveal the coherence properties of the interlayer excitons.   

Tuning the exciton-phonon coupling in 2D vdW heterostructure by pressure

Van der Waals (vdW) heterostructures of 2D crystals opened a new platform to explore novel properties emerged from layer-layer interaction. For example, the interplay between electron correlations and atomic structure in Moiré superlattices can lead to exotic phenomena ranging from correlated insulators superconductor, and topological states. In addition to the electron interaction, the interlayer electron–phonon interaction is also ubiquitous in vdw heterostructure. In WSe₂-hBN heterostructures, optically silent hBN phonons could emerge in Raman spectra through resonant coupling to WSe₂ electronic transitions. Here, we demonstrate that such an interfacial exciton-phonon coupling could be tuned by pressure. In hBN sandwiched monolayer WSe₂, we observed a giant shift of the resonance of exciton-phonon complex, up to 150 meV, by applying hydrostatic pressure up to 5 GPa.     

Tunable quantum confinement of excitons using electric fields and exciton-charge interactions

Achieving fully tunable quantum confinement of optical excitations such as excitons has been a long-standing goal in optoelectronics and quantum photonics. In this talk, we will discuss our recent experimental results demonstrating, for the first time, electrically controlled 1D quantum confinement of neutral excitons in a monolayer transition metal dichalcogenide semiconductor. This confinement relies on a combination of dc Stark effect induced by inhomogeneous in-plane electric fields and a novel polaronic confinement mechanism arising from interactions between excitons and itinerant charge carriers. Quantization of excitonic motion shows up in optical spectroscopy as discrete gate voltage-dependent, energy-split lines. We will also discuss some future prospects of these electrically confined in-plane dipolar excitons. First, electrical confinement of excitons with in-plane dipole moment is expected to enhance exciton-exciton interactions while allowing for hybridization with a microcavity mode. Strong interactions in a 1D wire could enable the realization of a Tonks-Girardeau gas with photon correlations providing signatures of fermionization. Furthermore, proper design of gate electrodes may enable novel confinement geometries such as quantum dots, wires and rings. We anticipate that such electrically quantum confined excitons could become building blocks for scalable arrays of identical, independently tuned quantum emitters and have implications for ongoing efforts towards realizing strongly correlated photonic systems.   

Quantum confined excitons in crossed electric and magnetic fields

Achieving fully tunable quantum confinement of excitons has been a long-standing goal in optoelectronics and quantum photonics. We have recently demonstrated electrically controlled 1D quantum confinement of neutral excitons in a monolayer transition metal dichalcogenide semiconductor. This confinement relies on a combination of dc Stark effect induced by inhomogeneous in-plane electric fields and a novel polaronic confinement mechanism arising from interactions between excitons and itinerant charge carriers.

Here, we demonstrate how the combined effect of strong in-plane electric fields and out-of-plane magnetic fields leads to unexpected consequences for excitons confined in these unique potentials. Specifically, we find that the quantum confined excitons exhibit unusually large diamagnetic coefficients, which can be electrically tuned and reach values up to 2.5 µeV/T². These values imply an exciton size ~6 nm, which is strikingly enhanced compared to the 1s exciton Bohr radius ~1 nm. We envision that these unique properties could be harnessed to enhance exciton-exciton interactions and enable the realization of artificial gauge fields.   

Engineering of Interlayer Electronic and Mechanical Coupling in Bilayer MoS2 by Applying Gigapascal High Pressure

Interlayer electronic and mechanical coupling plays a critical role in determining the novel electronic properties of van der Waal (vdW) bilayers. In particular, the coupling strength is determined by the stacking configuration and the interlayer spacing. While the former has been well established, the influence of interlayer spacing remains largely unexplored. Here, by measuring the optical absorption and Raman mode of the bilayer MoS₂ in a high-pressure diamond anvil cell, we quantitatively determined the coupling strength of the electronic band and phonon mode. All experimental results are supported by theoretical calculations. Our work has confirmed the great potential in tailoring vdW bilayers through gigapascal high pressure.   

Diffusion of excitons immersed in the Fermi sea of free charges in monolayer semiconductors

Interactions of excitons with free charge carriers represents an intriguing area of research in semiconductor physics. In monolayer transition metal dichalcogenides tightly bound excitons are mobile at both room temperature and crypgenic temperatures and strongly interact with free electrons and holes. Here we employ transient microscopy in a hBN-encapsulated monolayer WSe₂ field-effect transistor studying diffusion of long-lived dark exciton states in the presence of free charge carriers. We find a non-monotonous behavior in the diffusivity revealing the strong impact of free electron and hole scattering in the perturbative doping regime and investigate trion and Fermi-polaron propagation at elevated charge carrier densities. Our study opens up avenues to investigate transport of Fermi-Bose quasiparticle mixtures and offers insight into the interactions of mobile excitons with free charge carriers.   

Highly-confined 2D Exciton-polaritons in Monolayer Semiconductors

We predict the existence of a new type of in-plane propagating exciton-polaritons that are supported by monolayer transition-metal-dichalcogenides (TMDs), which can carry large momentum in the visible spectrum. This 2D exciton-polariton (2DEP) is a result of the coupling between the electromagnetic light field and collective oscillations of excitons in TMDs. We experimentally show that the conditions required for the excitation of the 2DEP are attainable if the TMD is encapsulated with hexagonal-boron-nitride (hBN) and is cryogenically cooled. In addition, a comparison of the properties of the 2DEP with those of surface-plasmon-polaritons at the same spectral range reveals that the 2DEP exhibits two orders-of-magnitude larger wavelength confinement.   

Auger induced tunable trion-trion annihilation in monolayer WS2

Strong Coulomb interaction in monolayers (1L) of transition metal dichalcogenides (TMDCs) gives rise to exciton many-body effects. Exciton-exciton annihilation (EEA) is a known phenomenon [1,2] where one exciton non-radiatively recombines and provides its energy to another one, which forms free ‘hot’ electron-hole pair. In this regard, trion-trion annihilation (TTA) is nontrivial because of their like charges and has not been hitherto observed in these 1L-TMDCs.

In this work, we demonstrate TTA in a gated WS₂ sample by using a pulsed excitation at low temperature (5 K). In particular, we observe that (1) the negative trion and biexciton exhibit an ‘anti-crossing’ like behavior with increasing ; (2) the trion PL intensity reduces gradually (while the biexciton intensity increases) in the spectral overlap region between trion and biexciton; and (3) the TRPL FWHM in the same region also indicates a faster trion decay. These three experimental observations indicate towards spectrally tunable Auger interaction between a bright and dark trion. We model the phenomenon using the Auger rate equation, which suggests a strong enhancement of the Auger rate when the spectral overlap between the trion and the charged biexciton is large.

References:

1. Daniel Erkensten et al, PRB, 103(4):045426, 2021.

2. Darwin F Cordovilla Leon et al, PRB, 100(24):241401, 2019.   

Transient electron recoil effect in electrically tunable MoSe2 monolayers

Dynamics of interacting electron-hole mixtures, such as excitons, trions, or Fermi polarons are a topic of intense study in systems with strong Coulomb interactions. In our joint experimental and theoretical study we utilize the electron recoil effect, the radiative recombination under simultaneous excitation of an electron to an unbound state, to access non-equilibrated states with arbitrary center-of-mass momenta. By applying this approach to electrically-tunable MoSe₂ monolayers, we directly monitor cooling dynamics of the non-equilibrated exciton populations dressed by free charge carriers. We find strong impact of both lattice temperature and free carrier density on the relaxation timescales that range from 10’s to a few picoseconds. Theoretically, we present a description of the recoil effect in both trion and Fermi-polaron pictures, supporting the experimental analysis.   

Highly luminescent, fully polarized, narrow linewidth exciton peaks in WS2

On generation, the excitons relax to the lowest energy 1s state by scattering through various phonon modes in multiple pathways. We use a simple technique in which, by tuning the excitation laser wavelength, the excitons resonantly come down to the 1s state in a single-step manner through scattering with a specific phonon mode. This relaxation process is highly efficient due to the strong resonance of the energy separation between the initial and the final exciton state with a single phonon mode in the system.

We demonstrate that such a single-step relaxation mechanism strongly modulates the 1s exciton PL emission features in a monolayer WS₂ sample sandwiched between few-layer graphene flakes. Using this technique at 5K, we obtain exciton peaks that are: (1) highly luminescent, (2) highly linearly polarized - demonstrating near-perfect valley coherence, and (3) ultra-narrow - due to a reduction in the inhomogeneous broadening.

The lowest linewidth of the 1s exciton peak obtained using this technique is 1.5 meV, which after deconvolution with the excitation laser gives an upper bound of 0.23 meV on the homogeneous broadening of the exciton peak. We show the retention of the above features all the way from cryogenic temperature to room temperature, demonstrating the robustness of the suggested technique with temperature.