Exciton States Research Articles (Page 1)

The Zhang-Rice (ZR) singlet is an intriguing quantum state offering potential to realize a spin-orbit-entangled bosonic quasiparticle, which gives rise to the Zhang-Rice exciton. Its formation is attributed to the correlation between a localized d-orbital of a transition metal and the p-orbitals of the neighbouring ligands. The layered two-dimensional (2D) antiferromagnetic Ni2P2S6 system provide an excellent platform to probe the ZR exciton dynamics along with the role of exciton-phonon coupling. Here, we present a comprehensive study of ZR exciton and coupling with the phonons in bulk and few-layered single crystals of Ni₂P₂S₆ using temperature, polarization and power-dependent photoluminescence (PL) spectroscopy. At cryogenic temperatures, the PL spectra reveal distinct phonon sidebands spaced by an energy difference of ~ 117 cm⁻¹, indicative of exciton-phonon hybridization. Polarization-resolved measurements demonstrate a strong optical anisotropy, with a linear polarization degree of ~ 40% at 4 K. Excitation power variation highlights linear scaling of PL intensity in the low-power regime, followed by spectral deformation at higher powers attributed to the phonon-assisted recombination and exciton saturation effects. ZR exciton and phonon side bands survival temperature decreases with decreasing flake thickness suggesting their tunability. The emergence and suppression of phonon sidebands with temperature and flake thickness emphasize dimensional sensitivity and coherence limits of excitonic states. Our findings position Ni₂P₂S₆ as a promising candidate for tunable and anisotropic optoelectronic applications, while offering insight into quasiparticle interactions in 2D magnetic systems.

Monolayer Janus transition metal dichalcogenides (TMDs) are intrinsically polarized two-dimensional (2D) semiconductors with broken mirror symmetry, offering additional degrees of freedom for exciton and spin-orbit engineering. However, controlled access to tunable excitonic ground states has remained largely inaccessible. Here, we report a composition-dependent transition from bright to dark excitonic behavior in alloyed Janus TMDs (SeMoxW1-xS), synthesized via a plasma-assisted epitaxial replacement process. This method enables the reliable transformation of MoxW1-xSe2 into structurally ordered Janus alloys across a wide compositional range. Atomic-resolution imaging and optical spectroscopy reveal that the excitonic character switches abruptly from dark to bright exciton complexes at a critical Mo concentration (∼25%), confirmed by first-principles calculations. This crossover arises from the interplay between spin-orbit coupling and band-edge alignment in the alloyed Janus lattice. Our findings demonstrate a route for engineering dark and bright excitonic ground states in Janus 2D materials and establish a broadly tunable platform for investigating spin-valley physics in 2D Janus TMDs.

Two-dimensional organic-inorganic hybrid perovskite (2D-OIHP) quantum wells exhibit a triplet of bright exciton fine structure states near the band edge, enabling the generation of transient macroscopic spin alignments with circularly polarized light. Here, we investigate the microscopic origin of photoinduced spin relaxation in 2D-OIHPs using multidimensional coherent spectroscopy together with a theoretical framework that combines time-dependent perturbation theory with the Fokker-Planck equation. Analysis of the spectral line shapes reveals highly correlated exciton fluctuations within the fine structure manifolds of a pair of 2D-OIHPs featuring different organic layer thicknesses and polaron binding energies. In particular, the Gaussian correlation coefficients determined for the two lead-iodide-based systems range from 0.67 to 0.80, while their polaron binding energies span 11.8-18.9meV. Incorporating time-coincident solvation dynamics into a stochastic model shows that these energy level correlations reduce the exciton-bath couplings and extend dephasing times for spin-flip transitions, even in spectral broadening regimes governed by Marcus-like kinetics (which are typically considered incompatible with motional narrowing). Since photoexcitation occurs on the seam of intersection between the excited-state free energy surfaces, spin relaxation can proceed without an activation barrier, provided it outpaces energy dissipation into the environment. Overall, these results demonstrate that correlated exciton fluctuations play a central role in accelerating spin depolarization in 2D-OIHPs through motional narrowing of coherences between exciton states.

Photosynthesis relies on efficient energy relaxation within the excited-state manifold of pigment-protein complexes. Since the protein scaffold is rather flexible, the resulting energetic and structural disorder gives rise to a complex excited-state energy level structure that fluctuates on all time scales. Although the impact of such fluctuations on relaxation processes is known, the precise exciton states involved in relaxation as well as the nature of the vibrational modes driving relaxation are under debate. Here, single pigment-protein complexes from a photosynthetic purple bacterium are excited with two identical ultrashort phase-locked pulses, producing two exciton wave packets that can interfere. This leads to a modulation of the emission intensity as a function of the delay time between the pulses that fades out within about ≈100 fs due to fluctuating environments on those time scales. For several single complexes, we find variations in the interference patterns on a time scale of several tens of seconds that reveal fluctuations in the energy relaxation pathways toward the lowest-energy exciton states. This relaxation is driven by temporal variations in the coupling between electronic excitations and low-frequency vibrational modes.

Abstract State transfer between different quantum systems is key for successful quantum technologies. Over long distances, photons are irreplaceable, but on short ranges in miniaturized complex devices or hybrid systems, coupling via orders of magnitude shorter-wavelength acoustic waves has great potential. With interfaces to light, acoustic waves, and more, optically active quantum dots (QDs) are essential for multi-component systems. Here, we propose a hybrid acousto-optical method for non-resonant QD charge state control, extending the recent all-optical swing-up state preparation. We show that exciton and biexciton states, or other superpositions of charge states, can be prepared. Each field can act as a trigger, allowing for the implementation of either an optically gated acoustic control or the opposite scheme, where an optical pulse controls the transition during acoustic modulation. Thus, we introduce acoustic state control into a system that lacks direct acoustic coupling between the states. The method does not rely on pulse shaping and is expected to work with arbitrary pulse shapes as long as the optical dressing is performed quasi-adiabatically. Evaluating the phonon impact, we find an almost decoherence-free exciton preparation even at elevated temperatures with current QD and acoustic technology. This approach may also pave the way for optically controlled entanglement between emitters and acoustic modes, and further on-chip state transfer via quantum acoustic buses.

Magnetic proximity interactions in van der Waals heterostructures offer a distinct means for controlling over spin and valley properties of nonmagnetic monolayers, such as transition metal dichalcogenides (TMDs). An essential aspect is to manipulate the valley-polarized excitons in TMDs through spin-polarized electron hopping at the heterointerface. While the notion of electron hopping channels well illustrates the circularly polarized photon emission, it obscures the fact that spin coupling between the magnetic layers and various excitonic states (such as neutral excitons and trions) in the TMDs could be substantially different. Individual addressing and manipulation of different excitonic valley polarizations remain challenging so far. Herein, we show that the valley polarization of excitons and trions can actually be asymmetric in the MoSe2/CrI3 heterostructure, which is triggered by the disequilibrium of intervalley excitonic conversion. The model is experimentally verified by temperature-dependent and magnetic-field-dependent polarization degrees, while the valley polarization dynamics is quantitatively fitted by the differential equation of excitonic population with intervalley conversion and scattering processes included. Finally, we demonstrate all-optical manipulation of valley polarization. Our studies provide routes to achieve dual-valley pseudospin processing channels, opening up opportunities for excitonic spin information processing in magneto-optoelectronic devices.

The high-temperature phase of Ta_textrm{2}NiSe_textrm{5}, a near-zero-gap semiconductor (E_G = 0), is a promising candidate for an excitonic insulator. Given the dome-like evolution expected for an excitonic insulator around E_G, we investigated Ta_textrm{2}NiSe_textrm{5}, the more semi-metallic Ta_textrm{2}(Ni,Co)Se_textrm{5}, and semiconducting Ta_textrm{2}NiS_textrm{5} using high-resolution single-crystal x-ray diffraction and near-edge x-ray absorption fine structure (NEXAFS). Our findings reveal a second-order structural phase transition from orthorhombic (space group: Cmcm) to monoclinic (space group: C2/c) in Ta_textrm{2}NiSe_textrm{5} and Ta_textrm{2}(Ni,Co)Se_textrm{5}, but no transition in Ta_textrm{2}NiS_textrm{5} down to 2 K. This transition breaks two mirror symmetries, enabling and enhancing the hybridization of Ta, Ni, and Se atoms, shortening bond lengths, and strengthening orbital interactions. NEXAFS data confirm stronger hybridization, significant changes in excitonic binding energies, and a key alteration in orbital character, suggesting an excitonic insulating state in Ta_textrm{2}NiSe_textrm{5} and emphasizing the crucial electronic role of orbitals in the formation of the excitonic insulator state.

Twisted bilayer transition metal dichalcogenides (TMDs) have generated diverse unusual electrical and optical phenomena and can provide a powerful platform for designing nanodevices with tunable interlayer interaction. Striving to explore novel excitons with spin response in these semiconductor systems is highly desirable, as they highlight the possibility to access complex electronic band structure and magneto-exciton effect, thereby facilitating efficient spin-based information storage via exciton degrees of freedom. Here, fabrication of bilayer WSe2/Fe5GeTe2 (FGT) heterostructures with different stacking phases is reported, and a new hybridized excitonic state T* is defined in both 3R and 2H bilayer WSe2, which exhibits strong correlations dependent on the FGT spin order. This spin-dependent hybridized exciton is demonstrated to originate from the coupling between injected spin-polarized electrons and neutral excitons, because of the spin-cross-polarized band that obstructs the normal electron-hole annihilation process. Besides, the difference in the coupling strength of the T* exciton attributed to the distinct stacking symmetries in twisted bilayer WSe2 is further unveiled. These findings open an accessible avenue for designing tailored excitonic states in twisted bilayers, thus offering prospects for the future applications of stacking-engineered opto-spintronics at the integration level.

(CdSe)13 magic‐sized clusters (MSCs) that consist of only 26 atoms represent an exciting class of materials at the boundary between molecules and quantum dots. At low temperatures, the characteristic photoluminescence (PL) shows signatures of the exciton bandgap emission at 3.65 eV, accompanied by a broad and redshifted emission at around 3.0 eV. Upon heating, the clusters reveal unique, energetically sharp PL features near the excitonic emission. For shedding light on the origin of these specific spectral fingerprints, density functional theory (DFT), time‐dependent DFT, and screened configuration interaction singles (SCIS) calculations are performed and compared with experimental data. The SCIS calculations identify excitonic fine structure states with strongly varying oscillator strengths, in excellent agreement with emissive states observed in time‐resolved PL experiments. Introducing undercoordinated Se defects to the structures used in the theoretical calculations reveals new optical states below the bandgap, which nicely fit to the temperature‐dependent, energetically sharp PL features. The broadband ≈3.0 eV emission is found to be a consequence of a polaron formation of optically generated electron–hole pairs in MSCs containing undercoordinated Se defects, leading to a rearrangement of the crystal lattice and thus to a significant lowering in PL energy with respect to the excitonic emission.

The interplay between coherence and system-environment interactions is at the basis of a wide range of phenomena, from quantum information processing to charge and energy transfer in molecular systems, biomolecules, and photochemical materials. In this work, we use a Frenkel exciton model with long-range interacting qubits coupled to a damped collective bosonic mode to investigate vibrationally assisted transfer processes in donor-acceptor systems featuring internal substructures analogous to light-harvesting complexes. We find that certain delocalized excitonic states maximize the transfer rate and that the entanglement is preserved during the dissipative transfer over a wide range of parameters. We investigate the reduction in transfer caused by static disorder, white noise, and finite temperature and study how transfer efficiency scales as a function of the number of dimerized monomers and the component number of each monomer, finding which excitonic states lead to optimal transfer. Finally, we provide a realistic experimental setting to realize this model in analog trapped-ion quantum simulators. Analog quantum simulation of systems comprising many and increasingly complex monomers could offer valuable insights into the design of light-harvesting materials, particularly in the nonperturbative intermediate parameter regime examined in this study, where classical simulation methods are resource intensive.

Regulation on the excited state of anthracene-bridged fluorophores for highly efficient blue non-doped OLEDs with ultra-low efficiency roll-off.

Abstract We investigate bilayer Janus transition metal dichalcogenides using ab initio many-body screened configuration interaction calculations and find that MoSSe–WSSe and WSSe–WSSe, with Se–S interfaces, exhibit spin-allowed interlayer exciton and trion ground states, in contrast to the spin-forbidden ground states of conventional transition metal dichalcogenides bilayers. This behavior arises from intrinsic structural asymmetry and interface-induced polarization, which rearrange the band structure. We further show that, besides the intrinsic asymmetry, also the applied external strain enables tuning of the ground state exciton brightness, allowing for detailed control of their optical properties.

Abstract Moiré superlattices in van-der-Waals heterostructures offer a versatile platform for exploring emergent quantum phenomena. In type-I MoSe2-WS2 moiré superlattices, the large lattice mismatch ensures robustness of the moiré period against twist-angle disorder. The excitonic ground state is formed by moiré-trapped MoSe2 intralayer excitons. However, a key challenge is the controlled transfer of excitonic energy across moiré sites. This work investigates gate-controlled phonon-assisted resonant energy transfer (RET) as a means to transfer excitonic energy between moiré cells. By harnessing the interplay between resonantly excited moiré excitonic complexes and single or few phonons, energy transfer pathways can be modulated via the charging state of moiré cells. We discuss two potential RET mechanisms: phonon-assisted resonant tunneling and Förster-like dipole–dipole transfer. Our findings highlight the potential of this approach for excitonic circuits and nanoscale energy transport, paving the way for future applications in quantum technologies.

In monolayer transition metal dichalcogenides bubbles-nanoscale deformations typically exhibiting a dome-like shape-Excitons are confined by the strain effect, which exhibits extraordinary emission properties, such as single photon generation, enhanced light emission, and spectrally tunable excitonic states. While the strain profiles of these bubbles are extensively studied, this work provides an approach 1) to directly visualize the associated exciton properties in bubbles formed in WSe2 monolayer, revealing an intrinsic emission wavelength shift of ≈40 nm, and 2) actively modify local strain, enabling further exciton emission tuning over a range of 50 nm. These are achieved by emission mapping and nanoindentation using a dielectric near-field probe, which enables the detection of local emission spectra and emission lifetimes within individual bubbles. Statistical analysis of 67 bubbles uncovers an emission wavelength distribution centered around 780 nm. Furthermore, saturation behavior in the power-dependent studies and the associated lifetime change reveal the localized nature of the strain-induced states. These findings provide direct insights into the strain-localized emission dynamics in bubbles and establish a robust framework for non-destructive, reversible, and predictable nanoscale emission control, presenting a potential avenue for developing next-generation tunable quantum optical sources.

Abstract Efficient photoluminescence (PL) of layered semiconductors is crucial for advancing next‐generation photonic devices. However, thermal effect‐induced destruction typically hinders the practical applications, such as biosensing and imaging. Here, the upconversion PL of multilayer GaSe is reported, which circumvents thermal damage. A high‐order multiphoton (up to 8‐photon) PL is first reported in multilayer GaSe. Both experimental and theoretical results reveal a power‐dependent redshift of the PL peak (≈40 meV, equivalent to 2% of the bandgap) and PL spectral broadening (full width at half maximum increased by ≈2 times), attributed to the hot electron–hole plasma. Time‐resolved PL resolves the multistage of carrier relaxation, revealing an ultrafast transition (≈58 ps) from electron–hole plasma to excitonic states, which establishes hot electron–hole plasma engineering as a critical mechanism for manipulating PL processes in Group‐III–VI chalcogenides. Furthermore, wavelength‐dependent two‐ and three‐photon PL spectra are explored. These results establish a microscopic framework connecting hot electron–hole plasma dynamics with macroscopic optoelectronic phenomena, providing critical insights for designing ultrafast photonic modulators and nonlinear optical devices based on 2D layered semiconductors.

Building on advances in topological photonics and computationaloptimization, we inverse-design a periodic dielectric structure surroundinga chain of interacting qubits, emulating an extended, dimerized Su–Schrieffer–Heegerexcitonic model. Our approach enables precise control over photon-mediatedinteractions, allowing us to explore the emergence of topologicaledge states in the qubit chain. By systematically tuning structuralparameters to address both coherent evolution and dissipative effects,we demonstrate that edge states remain robust and isolated from thebulk, even in the presence of long-range coupling and disorder, andpreserve key topological properties despite deviations from completechiral symmetry preservation. This work highlights the potential ofinverse design in stabilizing topological excitonic states, openingnew possibilities for advanced quantum technologies.

The manipulation of valley degrees of freedom in transition metal dichalcogenides (TMDCs) via the magnetic proximity effect (MPE) is key to advancing next-generation quantum and optoelectronic technologies. However, a comprehensive understanding of how MPE depends on layer configuration and excitonic charge state remains elusive. Here, a systematic study is reported on MPE-induced magneto-optical responses in monolayer and bilayer MoS2 interfaced with few and bulk CrBr3. Photoluminescence spectroscopy reveals a charge state dependent MPE, consistent with a spin-aligned interfacial charge transfer mechanism. Notably, CrBr3 introduces significant Zeeman splitting in 1L-MoS2, with a large splitting of 1.49 meV at zero field, consistent with the theoretical calculation of 1.8 meV. Under high magnetic fields, the g-factors of neutral excitons and trions are enhanced by factors of ≈2.3 and ≈1.8, respectively. In contrast, bilayer MoS2 exhibits a weaker response, highlighting the diminished interfacial exchange coupling in multilayer systems. The findings demonstrate a robust MPE in CrBr3-based heterostructures and provide critical insights into layer and charge state-dependent proximity effects, paving the way for tailored valleytronic functionalities in 2D materials.

The use of spectroscopic techniques to resolve the energy level structure of excitonic excited states and to elucidate the interaction mechanisms between excitons in quantum dots is of vital importance for the development and application of such materials. However, various static and dynamic one-dimensional spectroscopic techniques are limited by inhomogeneous broadening effects, making it challenging to directly observe the fine-level energy structure of quantum dots. We developed a 25kHz shot-to-shot phase-cycling two-dimensional (2D) electronic spectroscopy technique to investigate the excitonic energy structure and exciton-exciton interactions in a quantum dot aggregate. Using CdTe/CdSe/ZnS core-shell-shell quantum dots as a model system, we implemented a 36-step phase-cycling scheme to acquire rephasing 2D spectra at zero waiting time under varying excitation powers. In these spectra, both diagonal and off-diagonal features reveal spectral components that are otherwise obscured in one-dimensional spectroscopy due to inhomogeneous broadening. The presence of off-diagonal peaks in the 2D spectra indicates non-negligible interactions between excitons with different transition energies. Power-dependent 2D spectroscopy reveals that, with increasing excitation power, high-energy states exhibit greater resistance to Auger recombination. The experimental method developed in this work may contribute to advancing the theory of excited-state structures and dynamics in quantum dots and other low-dimensional materials.

Photon upconversion through high harmonic generation, multiphoton absorption, Auger recombination and phonon scattering performs a vital role in energy conversion and renormalization. Considering the reduced dielectric screening and enhanced Coulomb interactions, semiconductor monolayers provide a promising platform to explore photon upconversion at room temperature. Additionally, two-photon upconversion was recently demonstrated as an emerging technique to probe the excitonic dark states due to the extraordinary selection rule compared with conventional excitation. However, highly efficient two-photon upconversion still remains challenging due to the limited multiphoton absorption efficiency and long radiative lifetimes. Here, a 2440-fold enhancement of two-photon luminescence (TPL) is achieved in doubly resonant plasmonic nanocavities due to the amplified light collection, enhanced excitation rate, and increased quantum efficiency. To gain more insight into the attractive doubly resonant enhancement in such a plasmon−exciton coupling system, the intriguing thermally tuned excitonic upconversion and optimized amplification factor >3000 are realized at 350 K. Meanwhile, the single resonance enhanced photoluminescence (PL) (~890-fold) and second-harmonic generation (SHG) (~134-fold) are elaborately demonstrated. These results establish a foundation for developing cost-effective, high-performance nonlinear photonic devices and probing fine excitonic states via configuring plasmonic nanocavities.

Tuning the exciton fine structure of lead halide perovskites to brighten the dark excitonic ground state is crucial for enhancing their optoelectronic performance. While Rashba splitting is linked to dark-to-light exciton flipping, the specific nature of this phenomenon remains unclear. Here, we systematically studied 18 Cs2PbBr4 structures, representing 2D systems of CsPbBr3 with varying degrees of distortion, using density functional theory (DFT) and the Model-Bethe-Salpeter Equation (m-BSE). We demonstrate that spin-orbit coupling (SOC) combined with inversion symmetry breaking induces spin splitting in both the valence band (VB) and conduction band (CB), typically leading to a dark ground exciton due to band misalignment, even in the presence of Rashba splitting. However, controlled inversion symmetry breaking in states with weaker SOC─such as the VB states in perovskites─enables tunable Rashba splitting of the VBM. It was found that structures with VBM exhibiting linear Rashba and linear Dresselhaus splitting, where the Rashba coefficient exceeds the Dresselhaus coefficient, create an elliptical spin texture that aligns the VB maximum (VBM) and CB minimum (CBM), potentially brightening the excitonic ground state. This behavior is driven by significant bond angle distortions from the ideal cubic perovskite geometry that enhances Rashba splitting of the VBM through orbital noncentrosymmetry and facilitates a tetragonal-to-orthorhombic phase transition that further splits the excitonic states and flattens the VBM. These findings establish a structure-property relationship linking structural distortions and Rashba splitting that elucidates their role in brightening ground excitons and their implications for bright ground states in perovskites used in advanced optoelectronic applications.