2D Excitons Research Articles

Probing Spin Dynamics of 2D Excitons with Twisted Light

We propose a mechanism of intravalley spin-flip scattering in spin-valley coupled two dimensional systems by transferring momentum of light into exciton center of mass using optical vortex (OV) beams. By varying the dispersion of light using the topological charge of OV beam, we demonstrate a unique approach to control the intra-valley spin-flip scattering rate of excitons. From our photoluminescence measurements, we demonstrate that the intra-valley scattering rate in W-based TMDs can be tuned externally by OV beams. Variation of photoluminescence intensity with topological charges shows a crossover temperature (> 150 K), indicating competitions among time scales involving radiative recombination, spin-flip scattering, and thermal relaxations. Our proposed technique utilizing a structured light beam can open up a new approach to explore the physics of excitons in 2D systems.

Spontaneous Exciton Collapse in a Strongly Flattened Ellipsoidal InSb Quantum Dot

Electronic and excitonic states in an InSb strongly flattened ellipsoidal quantum dot (QD) with complicated dispersion law are theoretically investigated within the framework of the geometric adiabatic approximation in the strong, intermediate, and weak quantum confinement regimes. For the lower levels of the spectrum, the square root dependence of energy on QD sizes is revealed in the case of Kane’s dispersion law. The obtained results are compared to the case of a parabolic (standard) dispersion law of charge carriers. The possibility of the accidental exciton instability is revealed for the intermediate quantum confinement regime. For the weak quantum confinement regime, the motion of the exciton's center-of-gravity is quantized, which leads to the appearance of additional Coulomb-like sub-levels. It is revealed that in the case of the Kane dispersion law, the Coulomb levels shift into the depth of the forbidden band gap, moving away from the quantum confined level, whereas in the case of the parabolic dispersion law, the opposite picture is observed. The corresponding selection rules of quantum transitions for the interband absorption of light are obtained. New selection rules of quantum transitions between levels conditioned by 2D exciton center of mass vertical motion quantization in a QD are revealed. The absorption threshold behavior characteristics depending on the QDs geometrical sizes are also revealed.

Prediction of Strong Transversal s(TE) Exciton-Polaritons in C60 Thin Crystalline Films.

If an exciton and a photon can change each other’s properties, indicating that the regime of their strong bond is achieved, it usually happens in standard microcavity devices, where the large overlap between the ’confined’ cavity photons and the 2D excitons enable the hybridization and the band gap opening in the parabolic photonic branch (as clear evidence of the strong exciton–photon coupling). Here, we show that the strong light–matter coupling can occur beyond the microcavity device setup, i.e., between the ’free’ s(TE) photons and excitons. The s(TE) exciton–polariton is a polarization mode, which (contrary to the p(TM) mode) appears only as a coexistence of a photon and an exciton, i.e., it vanishes in the non-retarded limit (). We show that a thin fullerene C crystalline film (consisting of N C single layers) deposited on an AlO dielectric surface supports strong evanescent s(TE)-polarized exciton–polariton. The calculated Rabi splitting is more than meV for , with a tendency to increase with N, indicating a very strong photonic character of the exciton–polariton.

Giant excitonic upconverted emission from two-dimensional semiconductor in doubly resonant plasmonic nanocavity

Phonon-assisted upconverted emission is the heart of energy harvesting, bioimaging, optical cryptography, and optical refrigeration. It has been demonstrated that emerging two-dimensional (2D) semiconductors can provide an excellent platform for efficient phonon-assisted upconversion due to the enhanced optical transition strength and phonon-exciton interaction of 2D excitons. However, there is little research on the further enhancement of excitonic upconverted emission in 2D semiconductors. Here, we report the enhanced multiphoton upconverted emission of 2D excitons in doubly resonant plasmonic nanocavities. Owing to the enhanced light collection, enhanced excitation rate, and quantum efficiency enhancement arising from the Purcell effect, an upconverted emission amplification of >1000-fold and a decrease of 2~3 orders of magnitude in the saturated excitation power are achieved. These findings pave the way for the development of excitonic upconversion lasing, nanoscopic thermometry, and sensing, revealing the possibility of optical refrigeration in future 2D electronic or excitonic devices.

Theoretical Methods for Excitonic Physics in 2D Materials

In this Perspective article, the reader is introduced to several theoretical methods of determining the exciton wave functions and the corresponding eigenenergies. The methods covered are either analytical, semianalytical, or numeric. All the details associated with the different methods are made explicit, thus allowing newcomers to do research on their own, without experiencing a steep learning curve. The Perspective starts with a variational method and ends with a simple semianalytical approach to solve the Bethe–Salpeter equation (BSE) in gapped 2D materials. For the first methods addressed in this Perspective, the authors focus on a single layer of hexagonal boron nitride (hBN) and of transition metal dichalcogenide (TMD), as these are exemplary materials in the field of 2D excitons. For explaining the Bethe–Salpeter method, the biased bilayer graphene, which presents a tunable bandgap is chosen. The system has the right amount of complexity (without being excessive). This allows the presentation of the solution in a context that can be easily generalized to more complex systems or to apply it to simpler models.

Molding 2D Exciton Flux toward Room Temperature Excitonic Devices

AbstractDevices operating with excitons have promising prospects for overcoming the dilemma of response time and integration in current generation of electron‐ or/and photon‐based elements and devices. In combination with the advantages of emerging twistronics and valleytronics, the atomically thin transition metal dichalcogenide semiconductors open up new opportunities for pursuing practical excitonic devices, where the strong exciton binding energy enables operating exciton at room temperature. The essential and foremost step toward exciton devices is the control of spatiotemporal exciton flux, which is density‐dependent and affected by the complex many‐body interactions. It can be effectively controlled by the strain, electric field, electron‐doping, and local dielectric environment. Intriguingly, exotic phenomena such as exciton condensation, electron‐hole liquid, exciton Hall effects, and exciton halo effects can be occurred in 2D exciton system, providing new possibilities for excitonic devices. Up to now, the proof‐of‐principle of room temperature exciton devices, including excitonic switching and transistor, exciton guides, and excitonic nanolaser, have been realized. Here the authors review the recent advances in molding 2D exciton flux from basic principle, manipulation, exotic phenomena to promising applications and discuss the opportunities and challenges in pushing the frontiers of room temperature excitonic devices.

Long-range transport of 2D excitons with acoustic waves

Excitons are elementary optical excitation in semiconductors. The ability to manipulate and transport these quasiparticles would enable excitonic circuits and devices for quantum photonic technologies. Recently, interlayer excitons in 2D semiconductors have emerged as a promising candidate for engineering excitonic devices due to their long lifetime, large exciton binding energy, and gate tunability. However, the charge-neutral nature of the excitons leads to weak response to the in-plane electric field and thus inhibits transport beyond the diffusion length. Here, we demonstrate the directional transport of interlayer excitons in bilayer WSe2 driven by the propagating potential traps induced by surface acoustic waves (SAW). We show that at 100 K, the SAW-driven excitonic transport is activated above a threshold acoustic power and reaches 20 μm, a distance at least ten times longer than the diffusion length and only limited by the device size. Temperature-dependent measurement reveals the transition from the diffusion-limited regime at low temperature to the acoustic field-driven regime at elevated temperature. Our work shows that acoustic waves are an effective, contact-free means to control exciton dynamics and transport, promising for realizing 2D materials-based excitonic devices such as exciton transistors, switches, and transducers up to room temperature.

Superior optical Kerr effects induced by two-dimensional excitons

Materials with strong optical Kerr effects (OKEs) are crucial for a broad range of applications, such as all-optical data processing and quantum information. However, the underlying OKE mechanism is not clear in 2D materials. Here, we reveal key insights of the OKE associated with 2D excitons. An admirably succinct formalism is derived for predicting the spectra and the magnitude of the nonlinear refractive index ( n 2 ) of 2D materials. The predicted n 2 spectra are consistent with reported experimental data and exhibit pronounced excitonic resonances, which is distinctively different from bulk semiconductors. The n 2 value is predicted to be 3 × 10 − 10 cm 2 / W for a 2D layered perovskite at low temperature as 7 K, which is four orders of magnitude larger than those of bulk semiconductors. The superior OKE induced by 2D excitons would give rise to a narrow refractive index-near-zero region for intense laser light. Furthermore, we demonstrate that the 2D layered perovskite should exhibit the best OKE efficiency ( W FOM = 1.02 , T FOM = 0.14 ) at 1550 nm, meeting the material requirements for all-optical switching. Our findings deepen the understanding of the OKE of 2D semiconducting materials and pave the way for highly efficient all-optical excitonic devices.

Picoseconds-Limited Exciton Recombination in Metal-Organic Chalcogenides Hybrid Quantum Wells.

Metal-organic species can be designed to self-assemble in large-scale, atomically defined, supramolecular architectures. A particular example is hybrid quantum wells, where inorganic two-dimensional (2D) planes are separated by organic ligands. The ligands effectively form an intralayer confinement for charge carriers resulting in a 2D electronic structure, even in multilayered assemblies. Air-stable layered transition metal organic chalcogenides have recently been found to host tightly bound 2D excitons with strong optical anisotropy in a bulk matrix. Here, we investigate the excited carrier dynamics in the prototypical metal-organic chalcogenide [AgSePh]∞, disentangling three excitonic resonances by low temperature transient absorption spectroscopy. Our analysis suggests a complex relaxation cascade comprising ultrafast screening and renormalization, interexciton relaxation, and self-trapping of excitons within a few picoseconds (ps). The ps-decay provided by the self-trapping mechanism may be leveraged to unlock the material's potential for ultrafast optoelectronic applications.

Optical and dielectric properties of MoO3 nanosheets for van der Waals heterostructures

Two-dimensional (2D) insulators are a key element in the design and fabrication of van der Waals heterostructures. They are vital as transparent dielectric spacers whose thickness can influence the photonic, electronic, and optoelectronic properties of 2D devices. Simultaneously, they provide the protection of active layers in the heterostructure. For these critical roles, hexagonal boron nitride (hBN) is the dominant choice due to its large bandgap, atomic flatness, low defect density, and encapsulation properties. However, the broad catalogue of 2D insulators offers exciting opportunities to replace hBN in certain applications that require transparent thin layers with additional optical degrees of freedom. Here, we investigate the potential of single-crystalline molybdenum oxide (MoO3) as an alternative 2D insulator for the design of nanodevices that require precise adjustment of the light polarization at the nanometer scale. First, we measure wavelength-dependent refractive indices of MoO3 along its three main crystal axes and determine the in-plane and out-of-plane anisotropy of its optical properties. We find that the birefringence in MoO3 nanosheets compares favorably with other 2D materials that exhibit strong birefringence, such as black phosphorus, ReS2, or ReSe2, in particular in the visible spectral range, where MoO3 has the unique advantage of transparency. Finally, we demonstrate the suitability of MoO3 for dielectric encapsulation by reporting linewidth narrowing and reduced inhomogeneous broadening of 2D excitons and optically active quantum emitters, respectively, in a prototypical monolayer transition-metal dichalcogenide semiconductor. These results show the potential of MoO3 as a 2D dielectric layer for manipulation of the light polarization in vertical 2D heterostructures.

Long-living excited states of a 2D diamagnetic exciton

Hydrogenic excited states of a 2D exciton are degenerate. In the presence of a weak magnetic field, the S-states with a zero momentum of the center of mass get coupled to the P-states with finite momentum of the center of mass. This field-induced coupling leads to a strong modification of the dispersion branches of the exciton spectrum. Namely, the lower branch acquires a shape of a “mexican hat” with a minimum at a finite momentum. At certain magnetic field, exciton branches exhibit a linear crossing, similarly to the spectrum of a 2D electron in the presence of spin–orbit coupling. While spin is not involved, degenerate S and P states play the role of the spin projections. Lifting of degeneracy due to diamagnetic shifts and deviation of electron–hole attraction from purely Coulomb suppresses the linear crossing.

Exciton-Dominated Ultrafast Optical Response in Atomically Thin PtSe2.

Strongly bound excitons are a characteristic hallmark of 2D semiconductors, enabling unique light-matter interactions and novel optical applications. Platinum diselenide (PtSe2 ) is an emerging 2D material with outstanding optical and electrical properties and excellent air stability. Bulk PtSe2 is a semimetal, but its atomically thin form shows a semiconducting phase with the appearance of a band-gap, making one expect strongly bound 2D excitons. However, the excitons in PtSe2 have been barely studied, either experimentally or theoretically. Here, the authors directly observe and theoretically confirm excitons and their ultrafast dynamics in mono-, bi-, and tri-layer PtSe2 single crystals. Steady-state optical microscopy reveals exciton absorption resonances and their thickness dependence, confirmed by first-principles calculations. Ultrafast transient absorption microscopy finds that the exciton dominates the transient broadband response, resulting from strong exciton bleaching and renormalized band-gap-induced exciton shifting. The overall transient spectrum redshifts with increasing thickness as the shrinking band-gap redshifts the exciton resonance. This study provides novel insights into exciton photophysics in platinum dichalcogenides.

Cavity exciton polaritons in two-dimensional semiconductors from first principles

Two-dimensional (2D) semiconducting microcavity, where exciton-polaritons can be formed, constitues a promising setup for exploring and manipulating various regimes of light-matter interaction. Here, the coupling between 2D excitons and metallic cavity photons is studied by using first-principles propagator technique. The strength of exciton-photon coupling is characterised by its Rabi splitting to two exciton-polaritons, which can be tuned by cavity thickness. Maximum splitting of 128 meV is achieved in phosporene cavity, while remarkable value of about 440 meV is predicted in monolayer hBN device. The obtained Rabi splittings in WS$_2$ microcavity are in excellent agreement with the recent experiments. Present methodology can aid in predicting and proposing potential setups for trapping robust 2D exciton-polariton condensates.

Non-Hygroscopic, Self-Absorption Free, and Efficient 1D CsCu2I3 Perovskite Single Crystal for Radiation Detection.

A novel all-inorganic CsCu2I3 single-crystalline perovskite as a nonhygroscopic and efficient X-ray and γ-ray scintillator is described herein. It is featured by a one-dimensional (1D) perovskite structure with an orthorhombic system and a space group of Cmcm. The CsCu2I3 crystal emits yellow light peaking at 570 nm originated from strongly localized 1D exciton emission. It appears self-absorption free because of the large Stokes shift of 1.54 eV. The photophysics process of the self-trapped exciton was studied using temperature dependent photoluminescene spectra and decay kinetics measurements. The CsCu2I3 crystal exhibits an extremely low afterglow level of 0.008% at 10 ms under X-ray excitation. Under 137Cs γ-ray irradiation, its light yield is 16 000 photons/MeV with an energy resolution of 7.8% at 662 keV.

Chisosiamens A–E, five new ring-intact limonoids with isomerized furan ring from the fruit of Chisocheton siamensis

Five new ring-intact limonoids with isomerized furan ring, chisosiamens A–E (1–5), along with four known compounds (6–9) were isolated from the fruit of Chisocheton siamensis Craib. Their structures were elucidated based on 1D and 2D NMR spectroscopic data, HRESIMS, circular dichroism, and exciton chirality method. The biological activities screening showed that new limonoid 5 exhibited significant NO inhibitory activity in LPS-activated RAW 264.7 macrophages (IC50: 10.13 ± 1.40 μM) and 1, 2, 5, and 9 effectively reversed the resistance in MCF-7/DOX cells with the range IC50 values of 10.20–15.06 μM (RI: 4.05–5.98).

Optical analog of valley Hall effect of 2D excitons in hyperbolic metamaterial

The robust spin and momentum valley locking of electrons in two-dimensional semiconductors makes the valley degree of freedom of great utility for functional optoelectronic devices. Owing to the difference in optical selection rules for the different valleys, these valley electrons can be addressed optically. The electrons and excitons in these materials exhibit the valley Hall effect, where the carriers from specific valleys are directed to different directions under electrical or thermal bias. Here we report the optical analog of valley Hall effect, where the light emission from the valley-polarized excitons in a monolayer W S 2 propagates in different directions owing to the preferential coupling of excitonic emission to the high momentum states of the hyperbolic metamaterial. The experimentally observed effects are corroborated with theoretical modeling of excitonic emission in the near field of hyperbolic media. The demonstration of the optical valley Hall effect using a bulk artificial photonic media without the need for nanostructuring opens the possibility of realizing valley-based excitonic circuits operating at room temperature.

Dielectric confinement for designing compositions and optoelectronic properties of 2D layered hybrid perovskites.

Two dimensional (2D) layered hybrid lead halide perovskites are a fascinating class of semiconductors displaying a plethora of interesting optoelectronic properties with potential for application in solar cells, light emitting diodes, etc. Most of these properties can be linked to their repeating quantum well-like structures providing 2D excitons. In this perspective, we discuss how dielectric confinement of excitons originates in these layered hybrid perovskites, and then, how it can be used to tune the excitonic properties. In particular, we discuss the recent theoretical and experimental advances correlating dielectric confinement with chemical composition, excitonic binding energy, and optoelectronic property. The freedom from the restrictions of the Goldsmith tolerance factor allows the synthesis of hundreds of compositions of 2D layered hybrid perovskites by independently varying the organic and inorganic layers. We envisage that the combination of this compositional flexibility with the concepts of dielectric confinement discussed in this perspective would be a path forward for designing novel optoelectronic materials.

Realization of strong coupling between 2D excitons and cavity photons at room temperature.

Two-dimensional (2D) semiconductors of graphene, as well as transition-metal dichalcogenides, have performed strong interaction with light. Here the strong light-matter interaction between monolayer tungsten disulphide (WS2) excitons and microcavity photons at room temperature is well studied by the introduction of a gain material embedded dielectric optical microcavity structure. A Rabi splitting of about 36 meV is observed in angle-resolved reflectance spectra at room temperature, which agrees well with the theoretical results simulated by using the transfer matrix method. Since the cavity structures and 2D semiconductors can be prepared, the cavity and the gain materials, respectively, can be optimized separately in this platform. An all-dielectric Fabry-Pérot microcavity provides a simple but effective way to study the room temperature strong coupling between cavity photons and 2D excitons.

Optical dispersion of valley-hybridised coherent excitons with momentum-dependent valley polarisation in monolayer semiconductor

2D excitons in transition-metal dichalcogenides (TMDCs) offer interesting effects related to the valley pseudo-spin degree of freedom and long-range exchange interactions, as well as the coupling with light states. Several theoretical predictions have claimed that the neutral exciton of TMDCs splits into a transversal and longitudinal exciton branch, with the longitudinal one, which is the upper branch, exhibiting an extraordinary strong dispersion in the meV range within the light cone. Historically, this was linked for semiconductor quantum wells to strong far-field optical dipole coupling, or strong electronic long-range exchange interactions. Recently, experiments utilizing Fourier-space spectroscopy have shown that the exciton (exciton–polariton) dispersion can indeed be measured for high-quality hexagonal-BN-encapsulated WSe2 monolayer samples and can confirm the energy scale. Here, the exciton fine-structure’s pseudo-spin and the valley polarisation are investigated as a function of the centre-of-mass-momentum and excitation-laser detuning. For quasi-resonant excitation, a strong dispersion featuring a pronounced momentum-dependent helicity is observed. By increasing the excitation energy step-wise towards and then above the electronic band gap and the B-exciton level, the dispersion and the helicity systematically decrease due to contributions of incoherent excitons and emission from plasma. The decline of the helicity with centre-of-mass momentum can be phenomenologically modelled by the Maialle–Silva–Sham mechanism using the exciton splitting as the source of an effective magnetic field. By contributing to a better understanding of valley decoherence effects and the role of hybridised states in the optoelectronic properties, polarisation-sensitive Fourier-space investigations can support the development of future ‘optical-valleytronic’ devices.

Switchable, Tunable, and Directable Exciton Funneling in Periodically Wrinkled WS2.

The extreme elastic strain of monolayer transition metal dichalcogenides provides an ideal platform to achieve efficient exciton funneling via local strain modulation; however, studies conducted thus far have focused on the use of substrates with fixed strain profiles. We prepared 1L-WS2 on a flexible substrate such that the formation of topographic wrinkles could be switched on or off, and the depth or the direction of the wrinkle could be modified by external strain, thereby providing full control of the periodic undulation of the band gap profile of 1L-WS2 in the range 0-57 meV. Nanoscale photoluminescence (PL) imaging unambiguously evinced that the photoexcited excitons of 1L-WS2 were accumulated at the top regions of the wrinkles with less band gap than the valley region. Our results of broad tunability of the two-dimensional (2D) exciton funneling suggest a promising route to control exciton drift for enhanced optoelectronic performances and future 2D exciton devices.