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This article provides an overview of the latest results in the field of improving the properties of multiatomic inorganic oxide compounds for scintillators. A possibility to control the spatial distribution of nonequilibrium carriers in the ionization track by creating a compositional disorder in the crystalline matrix is in focus. Managing the disorder at the nanoscale level creates an opportunity for the efficient energy loss by carriers during thermalization, smaller spatial dispersion, and, consequently, more efficient binding into excitons and, further, an increase in the scintillation yield. The methods to produce multicationic crystalline scintillation materials have been discussed. The effectiveness of the approach is confirmed for both activated and self-activated scintillation materials. [ABSTRACT FROM AUTHOR]

Effectively controlling exciton–polaritons is crucial for advancing them in optical computation. In this work, we propose utilizing the valley-selective optical Stark effect (OSE) as an all-optical way to achieve the spatiotemporal control of polariton flow. We demonstrate the polarization-selective concentration of polaritons at pre-determined locations by wavefront shaping of the polaritons through an in-plane bar-code potential induced by the OSE, which helps overcome the intra-cavity disorder in potential distribution. In addition, a polariton decoder that converts binary inputs to decimal outputs is proposed by expanding the one-dimensional bar-code potential into a two-dimensional quick-response code potential offering enhanced control and encoding, whose robustness and valley selectivity are demonstrated. [ABSTRACT FROM AUTHOR]

An electric field normally increases the separation between the electron and hole in an exciton without intrinsic polarization and suppresses their Coulombic interaction, resulting in the reduction of its binding energy. Our study of one-dimensional (1D) excitons in phosphorene atomic chains, by using the exact diagonalization method, however, reveals that an electric field applied along the chain axis actually increases the exciton binding energies. Further analysis shows that the electric field tends to enhance the long-range interaction between the electron and hole while suppressing their short-range interaction by inducing an alternating charge distribution along the atomic chain. The zigzag symmetry is believed to account for this unique excitonic phenomenon in the 1D system. [ABSTRACT FROM AUTHOR]

We describe the new method that can be useful for calculation of the excitation dynamics in large molecular arrays that can be split into compartments with weak exciton coupling between them. In this method, the dynamics within each compartment is evaluated nonperturbatively using hierarchical equations of motion (HEOM), whereas transfers between the exciton states belonging to different compartments are treated by the generalized Förster (gF) theory. In a combined HEOM-gF approach, the number of equations increases linearly when adding new compartments as opposed to pure HEOM, where a depth of hierarchy exhibits strong non-linear grows when scaling the total number of molecules. Comparing the combined HEOM-gF method with an exact HEOM solution enabled us to estimate the parameters corresponding to a validity range of the proposed theory. The possibility of using the method for modeling of energy transfers in photosynthetic antenna supercomplexes is discussed. [ABSTRACT FROM AUTHOR]

Low-temperature PL analysis reveals an intriguing temperature-dependent emission pattern in WSe2: excitonic dominance above the 150 K Debye temperature, a balance between excitonic and trionic emissions at 150 K, and trionic dominance below this threshold. At lower temperatures, both excitons and trions display linearly polarized emissions, with polarization increasing from 0% at 300 K to 23% (excitons) and 7% (trions) at 150 K, and 12% for trions at 90 K. Moreover, the synthesized monolayer of WSe2 exhibits high thermal conductivity (246 W m−1 K−1 for A 1 g and 185 W m−1 K−1 for E 2 g 1 modes). This property is attributed to Se vacancies and defects at triangle edges, which redirect phonons, reducing scattering and enabling efficient heat transport along boundaries. The unveiling of these novel insights within the synthesized 2D WSe2 material holds significant promise for its potential applications in nano-optoelectronics. Its demonstrated efficiency in dissipating heat, coupled with improved thermal stability, suggests the possibility of employing it in future devices. This could facilitate compact designs and the miniaturization of advanced technological tools, showcasing the material's potential for practical implementation. [ABSTRACT FROM AUTHOR]

Broadband blue emission in zero-dimensional perovskites has received considerable attention, which is very important for the realization of stable blue-light emitters; however, the underlying formation mechanism remains unclear. Based on first-principles calculations, we have systematically studied the self-trapped excitons (STEs) behavior and luminescence properties in 0D-(DMA)4PbI6 perovskite. Our calculations show that there is a significant difference between the intrinsic STE luminescence mechanism (∼2.51 eV) and experimental observations (∼2.70 eV). In contrast, we found that the iodine vacancy (VI) is energetically accessible and exhibits a shallow charge transition level at ∼2.69 eV (0/+1) above the valence band maximum, which provides the initial local well for the STEs formation. Moreover, the low electronic dimension synergistic Jahn–Teller distortion facilitates the formation of extrinsic excitons self-trapping. Further excited state electronic structure analysis and configuration coordinate diagram calculations confirmed that the broadband blue emission in 0D-(DMA)4PbI6 is the origin of VI-induced extrinsic STEs instead of intrinsic STEs. Therefore, our simulation results rationalize the experimental phenomena and provide important insights into the formation mechanism of STEs in low-dimensional perovskite systems. [ABSTRACT FROM AUTHOR]

The competition between the radiative and nonradiative lifetimes determines the optical quantum yield and plays a crucial role in the potential optoelectronic applications of transition metal dichalcogenides (TMDCs). Here, we show that, in the presence of free carriers, an additional nonradiative decay channel opens for excitons in TMDC monolayers. Although the usual Auger decay channel is suppressed at low doping levels by the simultaneous momentum and energy conservation laws, exciton–phonon coupling relaxes this suppression. By solving a Bethe–Salpeter equation, we calculate the phonon-assisted Auger decay rates in four typical TMDCs as a function of doping, temperature, and dielectric environment. We find that even for a relatively low doping of 1012 cm−2, the nonradiative lifetime ranges from 16 to 165 ps in different TMDCs, offering competition to the radiative decay channel. [ABSTRACT FROM AUTHOR]

We recently presented a quantitative model to explain the particle-size dependence of photoluminescence (PL) quantum yields and revealed that exciton quenching is not diffusion controlled, but limited by surface reactions. However, the exciton decay kinetics has not been analyzed yet using our theoretical model. Here, we study kinetic aspects of the model and show that it should be extended to take into account subdiffusion rather than normal diffusion to maintain consistency with the observed complex decay kinetics; we also show that the PL decay kinetics is nonexponential even when the PL quenching is limited by surface reactions under subdiffusion. Our theoretical analysis of the PL quantum yield and the PL decay kinetics provides a comprehensive picture of mobile charge carriers, immobile polarons, and self-trapped excitons. [ABSTRACT FROM AUTHOR]

The layered van der Waals material CrCl3 exhibits very strongly bound ligand field excitons that control optoelectronic applications and are connected with magnetic ordering by virtue of their d-orbital origin. Time-resolved photoluminescence of these exciton populations at room temperature shows that their relaxation is dominated by exciton–exciton annihilation and that the spontaneous decay lifetime is very long. These observations allow the rough quantification of the exciton annihilation rate constant and contextualization in light of a recent theory of universal scaling behavior of the annihilation process. [ABSTRACT FROM AUTHOR]

The layered van der Waals material CrCl3 exhibits very strongly bound ligand field excitons that control optoelectronic applications and are connected with magnetic ordering by virtue of their d-orbital origin. Time-resolved photoluminescence of these exciton populations at room temperature shows that their relaxation is dominated by exciton–exciton annihilation and that the spontaneous decay lifetime is very long. These observations allow the rough quantification of the exciton annihilation rate constant and contextualization in light of a recent theory of universal scaling behavior of the annihilation process. [ABSTRACT FROM AUTHOR]

Two-dimensional electronic spectroscopy (2DES) is a powerful spectroscopic tool that allows us to study the dynamics of excited states. Exciton–exciton annihilation is at least a fifth order process, which corresponds to intrachromophoric internal conversion from the double-excited high-energy chromophoric state into the single-excited state of the same chromophore. At high excitation intensities, this effect becomes apparent in standard 2DES and can be inspected via high order n K 1 ⃗ − n K 2 ⃗ + K 3 ⃗ nonlinear processes. We calculate 2DES based on K 1 ⃗ − K 2 ⃗ + K 3 ⃗ and 2 K 1 ⃗ − 2 K 2 ⃗ + K 3 ⃗ wave mixing processes to reveal exciton–exciton annihilation (EEA) induced exciton symmetry breaking, which occurs at high excitation intensities. We present the general theory that captures all these processes for bosonic and paulionic quasiparticles in a unified way and demonstrate that the NEEs can be easily utilized for highly nonlinear two-dimensional spectra calculations by employing phase cycling for separating various phase matching conditions. The approach predicts various excitonic third- to fifth-order features; however, due to high excitation intensities, contributions of different order processes become comparable and overlap, i.e., the signals no longer can be associated with well-defined order-to-the-field contributions. In addition, EEA leads to breaking of the exciton symmetries, thus enabling population of dark excitons. Such effects are due to the local nature of the EEA process. [ABSTRACT FROM AUTHOR]

The non-radiative energy transfer process governs the transport of excitons in organic semiconductors, directly affecting the performance of organic optoelectronic devices. Successful models describe this transfer in terms of energy donor–acceptor pair sites, in contrast to experimental photophysical properties, which reflect the average behavior of the molecular ensemble. In this study, an energetic and spatial probability density function is proposed to determine the average non-radiative energy rate for homotransfer processes. This approach considers the energetic-spatial distribution typical of disordered semiconducting polymers. The average homotransfer rate is significantly dependent on the energy of the donor site, allowing the identification of the photophysical process most likely to occur. Values of the order of 1011 s−1 were predicted and are consistent with experimental results. This approach was used to evaluate how the energy transfer efficiency in heterostructures is affected by the energy and position of the energy donor site. The model presented in this study can be explored in other organic systems to investigate exciton transport mechanisms in new organic optoelectronic device architectures. [ABSTRACT FROM AUTHOR]

The generation of exciton–polaritons through strong light–matter interactions represents an emerging platform for exploring quantum phenomena. A significant challenge in colloidal nanocrystal-based polaritonic systems is the ability to operate at room temperature with high fidelity. Here, we demonstrate the generation of room-temperature exciton–polaritons through the coupling of CdSe nanoplatelets (NPLs) with a Fabry–Pérot optical cavity, leading to a Rabi splitting of 74.6 meV. Quantum–classical calculations accurately predict the complex dynamics between the many dark state excitons and the optically allowed polariton states, including the experimentally observed lower polariton photoluminescence emission, and the concentration of photoluminescence intensities at higher in-plane momenta as the cavity becomes more negatively detuned. The Rabi splitting measured at 5 K is similar to that at 300 K, validating the feasibility of the temperature-independent operation of this polaritonic system. Overall, these results show that CdSe NPLs are an excellent material to facilitate the development of room-temperature quantum technologies. [ABSTRACT FROM AUTHOR]

It has been shown theoretically that a strong quantum confinement regime in Si nanocrystals promotes highly efficient simultaneous excitation of two electron–hole pairs (biexciton) by a single photon. The rate (inverse lifetime) of biexciton generation has been calculated analytically as a function of the nanocrystal radius. The size-dependence of the rate in Si nanocrystal turns out to be sharp enough—in fact, it is inversely proportional to the sixth power of the radius. At radii values approaching a nanometer, the lifetime of biexciton generation falls into the nanosecond range. The threshold energy of this process in Si nanocrystals is exactly equal to twice the nanocrystal gap in contrast to the case of nanocrystals formed of direct-bandgap semiconductors, where the direct photon-induced creation of a biexciton with such an energy is, in fact, suppressed. [ABSTRACT FROM AUTHOR]

The use of semiconductor nanocrystals in scalable quantum technologies requires characterization of the exciton coherence dynamics in an ensemble of electronically isolated crystals in which system–bath interactions are nevertheless strong. In this communication, we identify signatures of Fano-like interference between excitons and photocarriers in the coherent two-dimensional photoluminescence excitation spectral lineshapes of mixed lead-halide perovskite nanocrystals in dilute solution. Specifically, by tuning the femtosecond-pulse spectrum, we show such interference in an intermediate coupling regime, which is evident in the coherent lineshape when simultaneously exciting the exciton and the free-carrier band at higher energy. We conclude that this interference is an intrinsic effect that will be consequential in the quantum dynamics of the system and will thus dictate decoherence dynamics, with consequences in their application in quantum technologies. [ABSTRACT FROM AUTHOR]

Van der Waals heterostructures (vdWHs) of vertically stacked two-dimensional (2D) atomic crystals have been used to elicit intriguing phenomena stemming from strong electronic correlations, magnetic textures, and interlayer excitons spawned at the heterointerface. However, vdWHs comprised of heterointerfaces between these 2D atomic crystal lattices and molecular assemblies are emerging as equally intriguing platforms supporting properties to be harnessed for photovoltaic energy conversion, photodetection, spin-selective charge injection, and quantum emission. In this perspective, we summarize recent research examining exciton dynamics in heterostructures between semiconducting 2D transition metal dichalcogenides and molecular organic semiconductors. We discuss methods for assembly of these heterostructures, the nature of interlayer or charge-transfer excitons at transition-metal dichalcogenide (TMD)-molecule interfaces, explicit exciton transfer between organics and TMDs, and other interfacial phenomena driven by the merger of these two material classes. We also suggest key new research directions extending the remit of these 2D atomic–molecular lattice heterointerfaces into the domains of condensed matter physics, quantum sensing, and energy conversion. [ABSTRACT FROM AUTHOR]

Interaction between a fixed point electric charge Q and a freely rotating point electric dipole with the magnitude P pinned near a plane interface between two dispersionless insulators with different dielectric permittivities ɛ1 and ɛ2 has been considered. It was shown that, as a result of this interaction and the interaction of the dipole with the polarization charges induced at the interface by the charge Q and the dipole itself, there arise regions where the dipole can possess either one or two equilibrium orientations. The spatial distributions of the electrostatic dipole energy Wtotal under the combined action of the charge Q and the induced interface polarization charges, as well as the equilibrium dipole orientations (orientation maps), the boundaries between the regions with different numbers of dipole orientations, and their evolution with the variation of problem parameters (the charge and dipole magnitudes, the mismatch between ɛ1 and ɛ2, and the charge–interface distance) were calculated. It was shown that there can emerge local minima of Wtotal, which may play the role of traps for dipoles (in particular, excitons in layered structures), and the corresponding requirements for the problem parameters were found. Most results were obtained in analytical form. The model can be applied to various physical systems, for instance, polar molecules, excitons, and trions in layered structures. [ABSTRACT FROM AUTHOR]

We investigate how electronic excitations and subsequent dissipative dynamics in the water soluble chlorophyll-binding protein (WSCP) are connected to features in two-dimensional (2D) electronic spectra, thereby comparing results from our theoretical approach with experimental data from the literature. Our calculations rely on third-order response functions, which we derived from a second-order cumulant expansion of the dissipative dynamics involving the partial ordering prescription, assuming a fast vibrational relaxation in the potential energy surfaces of excitons. Depending on whether the WSCP complex containing a tetrameric arrangement of pigments composed of two dimers with weak excitonic coupling between them binds the chlorophyll variant Chl a or Chl b, the resulting linear absorption and circular dichroism spectra and particularly the 2D spectra exhibit substantial differences in line shapes. These differences between Chl a WSCP and Chl b WSCP cannot be explained by the slightly modified excitonic couplings within the two variants. In the case of Chl a WSCP, the assumption of equivalent dimer subunits facilitates a reproduction of substantial features from the experiment by the calculations. In contrast, for Chl b WSCP, we have to assume that the sample, in addition to Chl b dimers, contains a small but distinct fraction of chemically modified Chl b pigments. The existence of such Chl b derivates has been proposed by Pieper et al. [J. Phys. Chem. B 115, 4042 (2011)] based on low-temperature absorption and hole-burning spectroscopy. Here, we provide independent evidence. [ABSTRACT FROM AUTHOR]

Determining the correlation between the size of a single quantum dot (QD) and its photoluminescence (PL) properties is a challenging task. In the study, we determine the size of each QD by measuring its absorption cross section, which allows for accurate investigation of size-dependent PL blinking mechanisms and volume scaling of the biexciton Auger recombination at the single-particle level. A significant correlation between the blinking mechanism and QD size is observed under low excitation conditions. When the QD size is smaller than their Bohr diameter, single CsPbI3 perovskite QDs tend to exhibit BC-blinking, whereas they tend to exhibit Auger-blinking when the QD size exceeds their Bohr diameter. In addition, by extracting bright-state photons from the PL intensity trajectories, the effects of QD charging and surface defects on the biexcitons are effectively reduced. This allows for a more accurate measurement of the volume scaling of biexciton Auger recombination in weakly confined CsPbI3 perovskite QDs at the single-dot level, revealing a superlinear volume scaling (τXX,Auger ∝ σ1.96). [ABSTRACT FROM AUTHOR]

We studied the photoluminescence decay kinetics of three nanosized anatase TiO2 photocatalysts (particle diameter: 7, 25, or 200 nm) at the pico- and nanosecond timescales for elucidating the origin of the luminescence. Luminescence spectra from these photocatalysts obtained under steady-state excitation conditions comprised green luminescence that decayed on the picosecond timescale and red luminescence that persisted at the nanosecond timescale. Among the photocatalysts with different sizes, there were marked differences in the rate of luminescence decay at the picosecond timescale (<600 ps), although the spectral shapes were comparable. The similarity in the spectral shape indicated that self-trapped excitons (STEs) directly populated in the bulk of the particle by light excitation emit the luminescence in a picosecond timescale, and the difference in the rate of luminescence decay originated from the quenching at the particle surface. Furthermore, we theoretically considered excitation light intensity dependence on the quantum yield of the luminescence and found that the quenching reaction was not limited by the diffusion of the STEs but by the reaction at the particle surface. Both the spectral shape and time-evolution of the red luminescence from the deep trapped excitons in the nanosecond timescale varied among the photocatalysts, suggesting that the trap sites in different photocatalysts have different characteristics with respect to luminescence. Therefore, the relation between trap states and photocatalytic activity will be elucidated from the red luminescence study. [ABSTRACT FROM AUTHOR]

When used as pump pulses in transient absorption spectroscopy measurements, femtosecond laser pulses can produce oscillatory signals known as quantum beats. The quantum beats arise from coherent superpositions of the states of the sample and are best studied in the Fourier domain using Femtosecond Coherence Spectroscopy (FCS), which consists of one-dimensional amplitude and phase plots of a specified oscillation frequency as a function of the detection frequency. Prior works have shown ubiquitous amplitude nodes and π phase shifts in FCS from excited-state vibrational wavepackets in monomer samples. However, the FCS arising from vibronic-exciton states in molecular aggregates have not been studied theoretically. Here, we use a model of vibronic-exciton states in molecular dimers based on displaced harmonic oscillators to simulate FCS for dimers in two important cases. Simulations reveal distinct spectral signatures of excited-state vibronic-exciton coherences in molecular dimers that may be used to distinguish them from monomer vibrational coherences. A salient result is that, for certain relative orientations of the transition dipoles, the key resonance condition between the electronic coupling and the frequency of the vibrational mode may yield strong enhancement of the quantum-beat amplitude and, perhaps, also cause a significant decrease of the oscillation frequency to a value far lower than the vibrational frequency. Future studies using these results will lead to new insights into the excited-state coherences generated in photosynthetic pigment–protein complexes. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) transition metal dichalcogenides have emerged as promising quantum functional blocks benefitting from their unique combination of spin, valley, and layer degrees of freedom, particularly for the tremendous flexibility of moiré superlattices formed by van der Waals stacking. These degrees of freedom coupled with the enhanced Coulomb interaction in 2D structures allow excitons to serve as on-chip information carriers. However, excitons are spatially circumscribed due to their low mobility and limited lifetime. One way to overcome these limitations is through the coupling of excitons with surface plasmon polaritons (SPPs), which facilitates an interaction between remote quantum states. Here, we showcase the successful coupling of SPPs with interlayer excitons in molybdenum diselenide/tungsten diselenide heterobilayers. Our results indicate that the valley polarization can be efficiently transferred to SPPs, enabling preservation of polarization information even after propagating tens of micrometers. [ABSTRACT FROM AUTHOR]

Semiconducting MXenes are an intriguing two-dimensional (2D) material class with promising electronic and optoelectronic properties. Here, we focused on recently prepared Hf-based MXenes, namely, Hf3C2O2 and Hf2CO2. Using the first-principles calculation and excited state corrections, we proved their dynamical stability, reconciled their semiconducting behavior, and obtained fundamental gaps by using the many-body GW method (indirect 1.1 and 2.2 eV; direct 1.4 and 3.5 eV). Using the Bethe–Salpeter equation, we subsequently provided optical gaps (0.9 and 2.7 eV, respectively), exciton binding energies, absorption spectra, and other properties of excitons in both Hf-based MXenes. The indirect character of both 2D materials further allowed for a significant decrease of excitation energies by considering indirect excitons with exciton momentum along the Γ-M path in the Brillouin zone. The first bright excitons are strongly delocalized in real space while contributed by only a limited number of electron–hole pairs around the M point in the k-space from the valence and conduction band. A diverse range of excitonic states in Hf3C2O2 MXene lead to a 4% and 13% absorptance for the first and second peaks in the infrared region of absorption spectra, respectively. In contrast, a prominent 28% absorptance peak in the visible region appears in Hf2CO2 MXene. Results from radiative lifetime calculations indicate the promising potential of these materials in optoelectric devices requiring sustained and efficient exciton behavior. [ABSTRACT FROM AUTHOR]

We present a detailed theoretical analysis of polarized absorption spectra and linear dichroism of cyanine dye aggregates whose unit cells contain two molecules. The studied threadlike ordered system with a molecular exciton delocalized along its axis can be treated as two chains of conventional molecular aggregates, rotated relative to each other at a certain angle around the aggregate axis. Our approach is based on the general formulas for the effective cross section of light absorption by a molecular aggregate and key points of the molecular exciton theory. We have developed a self-consistent theory for describing the orientational effects in the absorption and dichroic spectra of such supramolecular structures with nonplanar unit cell. It is shown that the spectral behavior of such systems exhibits considerable distinctions from that of conventional cyanine dye aggregates. They consist in the strong dependence of the relative intensities of the J- and H-type spectral bands of the aggregate with a nonplanar unit cell on the angles determining the mutual orientations of the transition dipole moments of constituting molecules and the aggregate axis as well as on the polarization direction of incident light. The derived formulas are reduced to the well-known analytical expressions in the particular case of aggregates with one molecule in the unit cell. The calculations performed within the framework of our excitonic theory combined with available vibronic theory allow us to quite reasonably explain the experimental data for the pseudoisocyanine bromide dye aggregate. [ABSTRACT FROM AUTHOR]

Metal-halide perovskites are a structurally, chemically, and electronically diverse class of semiconductors with applications ranging from photovoltaics to radiation detectors and sensors. Understanding neutral electron–hole excitations (excitons) is key for predicting and improving the efficiency of energy-conversion processes in these materials. First-principles calculations have played an important role in this context, allowing for a detailed insight into the formation of excitons in many different types of perovskites. Such calculations have demonstrated that excitons in some perovskites significantly deviate from canonical models due to the chemical and structural heterogeneity of these materials. In this Perspective, I provide an overview of calculations of excitons in metal-halide perovskites using Green's function-based many-body perturbation theory in the GW + Bethe–Salpeter equation approach, the prevalent method for calculating excitons in extended solids. This approach readily considers anisotropic electronic structures and dielectric screening present in many perovskites and important effects, such as spin–orbit coupling. I will show that despite this progress, the complex and diverse electronic structure of these materials and its intricate coupling to pronounced and anharmonic structural dynamics pose challenges that are currently not fully addressed within the GW + Bethe–Salpeter equation approach. I hope that this Perspective serves as an inspiration for further exploring the rich landscape of excitons in metal-halide perovskites and other complex semiconductors and for method development addressing unresolved challenges in the field. [ABSTRACT FROM AUTHOR]

Transient absorption (TA) spectroscopy of semiconductor nanocrystals (NCs) is often used for excited state population analysis, but recent results suggest that TA bleach signals associated with multiexcitons in NCs do not scale linearly with exciton multiplicity. In this manuscript, we probe the factors that determine the intensities and spectral positions of exciton and biexciton components in the TA spectra of CdSe quantum dots (QDs) of five diameters. We find that, in all cases, the peak intensity of the biexciton TA spectrum is less than 1.5 times that of the single exciton TA spectrum, in stark contrast to a commonly made assumption that this ratio is 2. The relative intensities of the biexciton and exciton TA signals at each wavelength are determined by at least two factors: the TA spectral intensity and the spectral offset between the two signals. We do not observe correlations between either of these factors and the particle diameter, but we find that both are strongly impacted by replacing the native organic surface-capping ligands with a hole-trapping ligand. These results suggest that surface trapping plays an important role in determining the absolute intensities of TA features for CdSe QDs and not just their decay kinetics. Our work highlights the role of spectral offsets and the importance of surface trapping in governing absolute TA intensities. It also conclusively demonstrates that the biexciton TA spectra of CdSe QDs at the band gap energy are less than twice as intense as those of the exciton. [ABSTRACT FROM AUTHOR]

Two-dimensional molybdenum disulfide (MoS2) has been proved to be a good candidate in photodetectors, and MoS2/graphene (MoS2/G) heterostructure has been widely used to expand the optical response wavelength of MoS2. To clarify the carrier transfer dynamics in the MoS2/G heterostructure, time-resolved transient absorption and two-color pump–probe measurements are performed. By comparing the carrier dynamics in MoS2 and MoS2/G under different pump wavelengths, we find that interfacial excitons are formed in the heterostructure, and fast hot carriers transfer (<200 fs) from graphene to MoS2 are observed. The results indicate that the formed heterostructure with graphene can not only expand the optical response wavelength of MoS2 but also improve the response time of the device in the near-infrared region. [ABSTRACT FROM AUTHOR]

A new kind of photocatalyst named xTBP-GCN, using 1, 2, 3-tribromopropane and urea as precursors, has been prepared and applied in photocatalytic hydrogen evolution. Due to extended light absorption and enhanced charge separation, the hydrogen evolution rate of 16TBP-GCN is enhanced to 1.33 mmol g−1 h−1, which is 4.3 times of pristine GCN (0.31 mmol g−1 h−1). The apparent quantum yield for the hydrogen production is determined to be 0.82% at 420 nm. Electrochemical and PL spectra results demonstrate that the introduction of 1, 2, 3-tribromopropane facilitates charge transfer process. Moreover, time-resolved PL studies illustrate the lifetime of excitons in catalyst is significantly shortened after modification. Theoretical calculations reveal that 1, 2, 3-tribromopropane regulates HOMO/LUMO distribution and thus promotes charge separation of GCN. This research serves as a reference for developing GCN-based catalysts with high photocatalytic activity for hydrogen evolution. • GCN-based photocatalyst modified by non-conjugated molecule containing halogen has been synthetized. • The absorption ability of catalyst for visible light is enhanced. • Charge separation has been improved by regulating electron structure of GCN via 1, 2, 3-tribromopropane. • Photophysical behavior of catalysts has been studied by time-resolved PL spectra. [ABSTRACT FROM AUTHOR]

The quest for platforms to generate and control exotic excitonic states has greatly benefited from the advent of transition metal dichalcogenide (TMD) monolayers and their heterostructures. Among the unconventional excitonic states, quadrupolar excitons—a superposition of two dipolar excitons with anti-aligned dipole moments—are of great interest for applications in quantum simulations and for the investigation of many-body physics. Here, we unambiguously demonstrate the emergence of quadrupolar excitons in natural MoSe2 homobilayers, whose energy shifts quadratically in electric field. In contrast to trilayer systems, MoSe2 homobilayers have many advantages, which include a larger coupling between dipolar excitons. Our experimental observations are complemented by many-particle theory calculations offering microscopic insights in the formation of quadrupolar excitons. Our results suggest TMD homobilayers as ideal platform for the engineering of excitonic states and their interaction with light and thus candidate for carrying out on-chip quantum simulations. Quadrupolar excitons — a superposition of two dipolar excitons with anti-aligned dipole moments — are of great interest for applications in quantum simulations and for the investigation of many-body physics. Here, the authors demonstrate the emergence of quadrupolar excitons in natural MoSe2 homobilayers, whose energy shifts quadratically in electric field. [ABSTRACT FROM AUTHOR]

Stretchable afterglow materials have garnered widespread attention owing to their unique combination of optical properties and mechanical flexibility. However, achieving a crystal environment to suppress the non-radiative transition of triplet excitons poses a challenge in constructing stretchable afterglow materials. Herein, we utilize an in situ supersaturated crystallization strategy to form afterglow microcrystals within a hydrogel matrix. This approach enables afterglow emission with a lifetime of 695 ms while maintaining high stretchability with tensile stress surpassing 398 kPa, extensibility over 400% and a high water content of 65.21%. Moreover, the universal supersaturated crystallization strategy allows for conferring tunable afterglow performance. Successful demonstrations in hydrogel 3D printing and anti-counterfeiting purposes showcase the potential for advanced applications of 3D printable afterglow hydrogels. This investigation provides guidelines for generally designing efficient afterglow hydrogels and addresses the inherent contradiction between flexibility and rigid in stretchable afterglow materials. [ABSTRACT FROM AUTHOR]

We report experimental evidence that MoSe2 and WS2 allow the formation of type-I and type-II interfaces, according to the thickness of the former. Heterostructure samples are obtained by stacking a monolayer WS2 flake on top of a MoSe2 flake that contains regions of thickness from one to four layers. Photoluminescence spectroscopy and transient absorption measurements reveal a type-II interface in the regions of monolayer MoSe2 in contact with monolayer WS2. In other regions of the heterostructure formed by multilayer MoSe2 and monolayer WS2, features of type-I interface are observed, including the absence of charge transfer and dominance of intralayer excitons in MoSe2. The coexistence of type-I and type-II interfaces in a single heterostructure offers opportunities to design sophisticated two-dimensional materials with finely controlled photocarrier behaviors. [ABSTRACT FROM AUTHOR]

Periodic moiré superlattice structures in transition metal dichalcogenides (TMDs) heterostructures exhibit nanoscale tunable electronic and optical properties. Much effort has been devoted to understand the twist-angle-dependent optical properties of the TMDs heterostructures. Therefore, quickly determining the stacking angle of TMDs and constructing the desired moiré superlattice structure is crucial. Here, we investigate the twist-angle-dependent optical properties of WS2/MoSe2, finding that the out-of-plane Raman mode is a reliable marker for determining the stacking angle. The interlayer exciton energy, being close to that of MoSe2 excitons, is highly sensitive to material quality and fabrication, making it unsuitable for identifying the stacking angle. In contrast, the out-of-plane Raman peaks of WS2 and MoSe2 are more sensitive to changes in the stacking angle. The out-of-plane Raman mode of WS2 is enhanced more than fivefold in near 0° or 60° WS2/MoSe2 heterostructures, while the out-of-plane mode of MoSe2 is only significantly decreased in near 0° heterostructures. Combining the intensity of out-of-plane Raman mode of WS2 and MoSe2, near 0° and 60° heterostructures can be distinguished without the need for complex optical characterizations. These typical peaks offer researchers an efficient way to construct the desired moiré superlattice structures. [ABSTRACT FROM AUTHOR]

[Display omitted] • Enhanced charge separation by phosphate modification result in H 2 O 2 oxidation by HClO but weaken excitonic effect in B001. • B010 with poor excitons generate 1O 2 via O 2 − oxidation, enhanced 1O 2 yielding in P-B010 was due to H 2 O 2 oxidation by HClO. • CBZ degradation efficiency enhanced by 1.3 and 2.2 times over B001 and B010 after phosphate modification, respectively. • The initial attack of C C bond via 1O 2 to form epoxide played important roles on CBZ degradation. 1O 2 generation over (0 0 1) or (0 1 0) facet exposed BiOCl (B001 or B010) with/without phosphate modification were studied from the aspects of excitons involved energy transfer route, the O 2 − oxidation based charge transfer route and the H 2 O 2 oxidation by HClO. Phosphate modification not only enhance charge separation thus result in H 2 O 2 oxidation by HClO to release 1O 2 but also weaken excitonic effect in the confined layer of BiOCl accordingly affect 1O 2 generation via energy transfer. Considering [0 0 1] orientation favors the formation of excitons than that of [0 1 0] direction over BiOCl, excitons loss was hardly compensated by the H 2 O 2 oxidation by HClO for 1O 2 generation over phosphate modified B001. Nevertheless, limited excitonic effect makes the O 2 − oxidation by h+ via charge transfer as dominant route for 1O 2 yielding over B010, the extra H 2 O 2 oxidation with HClO after phosphate modification significantly enhance 1O 2 generation over B010 followed with 2.2 times higher carbamazepine photodegradation activity. The initial attack of C C bond via 1O 2 to form epoxide played important roles on carbamazepine degradation. This study demonstrated that the facet-specific phosphate modification of photocatalysts can finely tune reactive 1O 2 species for superior pharmaceuticals degradations. [ABSTRACT FROM AUTHOR]

It is well-known that exciton effects are determinant to understanding the optical absorption spectrum of low-dimensional materials. However, the role of excitons in nonlinear optical responses has been much less investigated at the experimental level. Additionally, computational methods to calculate nonlinear conductivities in real materials are still not widespread, particularly taking into account excitonic interactions. We present a methodology to calculate the excitonic second-order optical responses in 2D materials relying on: (i) ab initio tight-binding Hamiltonians obtained by Wannier interpolation and (ii) solving the Bethe-Salpeter equation with effective electron-hole interactions. Here, in particular, we explore the role of excitons in the shift current of monolayer materials. Focusing on MoS2 and GeS monolayer systems, our results show that 2p-like excitons, which are dark in the linear response regime, yield a contribution to the photocurrent comparable to that of 1s-like excitons. Under radiation with intensity ~104W/cm2, the excitonic theory predicts in-gap photogalvanic currents of almost ~10 nA in sufficiently clean samples, which is typically one order of magnitude higher than the value predicted by independent-particle theory near the band edge. [ABSTRACT FROM AUTHOR]

The attainment of white emission from a light-emitting electrochemical cell (LEC) is important, since it enables illumination and facile color conversion from devices that can be cost-efficient and sustainable. However, a drawback with current white LECs is that they either employ non-sustainable metals as an emitter constituent or are intrinsically efficiency limited by that the emitter only converts singlet excitons to photons. Organic compounds that emit by thermally activated delayed fluorescence (TADF) can address these issues since they can harvest all excitons for light emission while being metal free. Here, we report on the first white LEC based on solely metal-free TADF emitters, as accomplished through careful tuning of the energy-transfer processes and the electrochemically formed doping structure in the single-layer active material. The designed TADF-LEC emits angle-invariant white light (color rendering index = 88) with an external quantum efficiency of 2.1 % at a luminance of 350 cd/m2. Current white light-emitting electrochemical cells either consist of non-sustainable metal emitters or are intrinsically efficiency limited. Here, metal-free thermally activated delayed fluorescent emitters are employed and tuned for the achievement of broadband white light emission. [ABSTRACT FROM AUTHOR]

The detailed anisotropic dispersion of the low-temperature, low-energy magnetic excitations of the candidate spin-triplet superconductor UTe2 is revealed using inelastic neutron scattering. The magnetic excitations emerge from the Brillouin zone boundary at the high symmetry Y and T points and disperse along the crystallographic b ̂ -axis. In applied magnetic fields to at least μ0H = 11 T along the c ̂ − axis , the magnetism is found to be field-independent in the (hk0) plane. The scattering intensity is consistent with that expected from U3+/U4+ f-electron spins with preferential orientation along the crystallographic a ̂ -axis, and a fluctuating magnetic moment of μeff=1.7(5) μB. We propose interband spin excitons arising from f-electron hybridization as a possible origin of the magnetic excitations in UTe2. [ABSTRACT FROM AUTHOR]

Monolayer transition metal dichalcogenides (TMDCs) have extensive applications in the field of optics and optoelectronics by virtue of their unique band structures and excitonic properties. Although possessing high absorption coefficient and emission efficiency, they suffer from low optical absorptance due to the atomic scale thickness, which limits their photoluminescence and optoelectronic performance. In spite of intense research efforts on absorption enhancement of monolayer TMDCs by optical cavities, such as plasmonic and all‐dielectric nanoresonators, there inevitably exists a competition of absorption between them because of the loss in cavities. Here, strong absorption enhancements of monolayer WS2 integrated onto a TiO2 nanohole array fabricated by colloidal lithography are reported. It achieves theoretically a tenfold and experimentally a sixfold enhancement of absorption within monolayer WS2 near the band edge when TE and TM modes of the TiO2 nanohole array spectrally overlap with the A exciton. This result can be attributed to the near perfect reflection and near field localization properties of the resonant modes in TiO2 nanohole arrays. Further, the photoluminescence of monolayer WS2 is also significantly improved owing to the increased absorption of WS2 as well as Purcell effect. Such heterostructure paves the way for designing high‐efficiency sources, photodetectors, and photovoltaic devices. [ABSTRACT FROM AUTHOR]

Dye‐sensitized down‐conversion nanoparticles (DCNPs) can significantly enhance photon absorption, bridging the gap of narrow and weak absorption cross‐section of lanthanide (Ln) ions, thus fundamentally prompting their near‐infrared (NIR) emission. However, the ideal strategy for utilization of both the singlet and triplet energy of the dyes is hindered by the nanostructures and the dependence on the heavy atom effect. Herein, thermally activated delayed fluorescence (TADF) dye is utilized with a small singlet‐triplet energy gap as the absorption antenna, achieving a 733 fold emission enhancement. This strategy facilitates efficient intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes, enabling effective energy transfer from singlet (S1) and triplet (T1) excitons to emitted energy levels of Er3+. Combining highly Erbium‐doped nanoparticles, the water‐dispersed AD‐sensitized system shows excellent hydrodynamic stability and photostability. This innovative approach marks the first report of TADF dye‐sensitized Ln nanosystems, offering a new direction for photo conversion technology. [ABSTRACT FROM AUTHOR]

Quasi‐2D perovskites are widely used in solar cells, photodetectors, and light‐emitting diodes (LEDs) due to their excellent optoelectronic properties and stability. However, much fewer all‐optical devices are realized based on quasi‐2D perovskites. Although perovskites have large nonlinear optical coefficients, the relaxation progress of excitons and carriers will slow down the response time and reduce the signal contrast of the all‐optical devices. In this paper, the exciton dynamics are investigated under off‐resonance excitation in quasi‐2D perovskite PEA2(FAPbBr3)2PbBr4. The spin‐selective optical Stark effect (OSE) is observed through circularly polarized transient absorption measurements, and a Stark shift of 3.3 meV is achieved under 1.11 eV excitation. The hot‐biexciton cooling process induced by two‐photon absorption is also observed. A spin‐based XNOR gate is demonstrated with a response time of ≈180 fs, and the logical signal contrast is enhanced through the competition of the OSE and the hot‐biexciton effect. [ABSTRACT FROM AUTHOR]

Organic‐inorganic hybrid cuprous halides (OHCHs) are intriguing candidates for the next generation of scintillators due to their environmental friendliness, simple preparation, and excellent luminescent properties. Herein, a series of efficient OHCHs based on inorganic building blocks [Cu4Br6]2− clusters, designated as (TMAA)2Cu4Br6, (EtTPPh)2Cu4Br6, and (MtTBA)2Cu4Br6 (TMAA = N,N,N‐trimethyltrimethyl‐1‐adamantylammonium; EtTPPh = ethyltriphenylphosphonium; MtTBA = methyltri‐n‐butylammonium), are synthesized via a simple solution method. The three OHCHs exhibit broadband yellow emissions. Remarkably, the 0D (TMAA)2Cu4Br6 shows a near‐unity photoluminescence quantum yield, which can be attributed to the highly localized excitons and rigid environment. Impressively, it is demonstrated to show excellent scintillation performance with a high light yield of 46700 photon MeV−1 and a low detection limit of 56.12 nGyairs−1. Photophysical studies reveal that the bright emission in (TMAA)2Cu4Br6 originates from self‐trapped excitons. This work opens an inspirational avenue in structure design for OHCHs as high‐performance X‐ray scintillators. [ABSTRACT FROM AUTHOR]

Excitons, which are Coulomb bound electron-hole pairs, are composite bosons and thus at low temperature can form a superfluid state with a single well-defined amplitude and phase. We directly image this macroscopic exciton superfluid state in an hBN-separated MoSe2-WSe2 heterostructure. At high density, we identify quasi-long-range order over the entire active area of our sample, through spatially resolved coherence measurements. By varying the exciton density and sample temperature, we map out the phase diagram of the superfluid. We observe the superfluid phase persisting to a temperature of 15 K, which is in excellent agreement with theoretical predictions. This works paves the way to realizing on chip superfluid structures capable of studying fundamental physical behaviors and quantum devices that use superfluidity. [ABSTRACT FROM AUTHOR]

Semiconductor moiré superlattices provide great platforms for exploring exotic collective excitations. Optical Stark effect, a shift of the electronic transition in the presence of a light field, provides an ultrafast and coherent method of manipulating matter states, which, however, has not been demonstrated in moiré materials. Here, we report the valley-selective optical Stark effect of moiré excitons in the WSe2/WS2 superlattice by using transient reflection spectroscopy. Prominent valley-selective energy shifts up to 7.8 meV have been observed for moiré excitons, corresponding to pseudo-magnetic fields as large as 34 T. Our results provide a route to coherently manipulate exotic states in moiré superlattices. [ABSTRACT FROM AUTHOR]

Magnetotransport of conventional semiconductor based double layer systems with barrier suppressed interlayer tunneling has been a rewarding subject due to the emergence of an interlayer coherent state that behaves as an excitonic superfluid. Large angle twisted bilayer graphene offers unprecedented strong interlayer Coulomb interaction, since both layer thickness and layer spacing are of atomic scale and a barrier is no more needed as the twist induced momentum mismatch suppresses tunneling. The extra valley degree of freedom also adds richness. Here we report the observation of fractional quantum Hall physics at 1/3 total filling for balanced layer population in this system. Monte Carlo simulations support that the ground state is also an excitonic superfluid but the excitons are composed of fractional rather than elementary charges. The observed phase transitions with an applied displacement field at this and other fractional fillings are also addressed with simulations. They reveal ground states with different topology and symmetry properties. This study explores fractional quantum Hall physics in large-angle twisted bilayer graphene, revealing a 1/3 fractional quantum Hall state driven by strong interlayer Coulomb interactions. Monte Carlo simulations confirm unique topological ground states and transitions with applied displacement fields. [ABSTRACT FROM AUTHOR]

Achieving hybrid lead halides with high stability and tunable dimensionality is essential for advancing their optoelectronic applications. While numerous studies have investigated ligand functionality to modulate lead halide dimensionality and enhance photoluminescence (PL) efficiency, most efforts remain limited to labile, ionically bound structures with stability issues. Herein, two new members of lead halide‐based metal–organic frameworks are successfully synthesized by employing dimethoxy‐functionalized benzenedicarboxylate acid (2,5‐dmbdcH2) as a bridging linker. This functionalization increases steric hindrance, effectively isolating 1D lead halide chains into 0D lead halide units. The 0D [Pb4X2]6+ (X = Cl, Br) cluster‐type units, are coordinatively linked by 2,5‐dmbdc to form two robust 3D frameworks, Pb4X2(2,5‐dmbdc)3. The structural deformation of the 0D lead halide units enhances photoluminescence quantum yields (PLQYs) of broadband emissions, increasing from 1.5–8.0% for TMOF‐5 to 33.4–34.6% for TMOF‐15, while maintaining excellent long‐term photostability and environmental durability. Mechanistic studies reveal that the strong interaction between charge carriers and phonons within the deformable [Pb4X2]6+ clusters promotes the formation of self‐trapped excitons (STEs), leading to the enhanced broadband emission observed in TMOF‐15. This work presents an effective linker‐functionalization strategy for synthesizing lead halide MOFs with both high stability and excellent PL performances. [ABSTRACT FROM AUTHOR]

Doping lanthanide ions in double perovskites (DPs) offers a promising approach to tailor optical and optoelectronic properties for versatile applications. However, achieving efficient and thermal stable DPs remains a significant challenge. In this paper, a series of Cs2KInCl6:Yb3+,Er3+ DPs with a negative thermal quenching behavior and high sensitivity are fabricated. The intrinsic luminescence properties of Cs2KInCl6 are studied with a broad intrinsic self‐trapped excitons (STEs) emission peak at 500 nm, which can be enhanced by doping Er3+ ions accompanied with greater distortion of the [InCl6]3− octahedrons as well as multiple characteristic emission modes of Er3+ under ultraviolet and near‐infrared excitation. Notably, the addition of sensitizer Yb3+ ions leads to the enhanced up‐conversion emission of Er3+ (up‐conversion quantum yield up to 1.66%), anti‐thermal quenching performance brought by the phonon‐assisted energy transfer up‐conversion process, as well as high sensitivity and reproducibility based on fluorescence intensity ratio technology. This work provides an effective strategy to enhance the STEs emission of DPs and achieve efficient optical temperature sensors through lanthanide ions doping. [ABSTRACT FROM AUTHOR]

The charge‐transfer (CT) complexes with significant electron delocalization demonstrate abundant appealing physicochemical features and unique orbital hybridization, creating great promise as artificial light‐harvesting antennas for advanced optoelectronics, such as high‐performance organic room‐temperature phosphorescence (RTP). Herein, originally, a localized CT complex is proposed in organic microcrystals for exceptional orange‐emissive RTP with photoluminescence peaks at 575 and 625 nm based on the light‐harvesting antennas of CT states in organic microcrystals with donor doping. The localized CT state antennas can capture more singlet excitons, and transition them into triplet excitons, efficiently promoting intersystem crossing (ISC). Owing to the dense packing structure and strong CT interaction, the generation and stabilization of triplet excitons under ambient conditions are facilitated, imparting effective RTP with a lasting afterglow of up to 3 s. The controlled intensity ratio of fluorescence and phosphorescence is successfully achieved via finely adjusting the donor doping ratio, presenting a tunable CIE evolution from (0.42, 0.30) to (0.25, 0.12). The potential applications of these light‐harvesting microcrystals in information encryption and flexographic printing are illustrated. [ABSTRACT FROM AUTHOR]

Establishing quantum mechanically entangled states between spatially separated 2D heterostructure offers a way to tailor novel energy transfer mechanisms at the precision of atomic level. Here, strongly coupled systems formed by monolayer WS2, spatially separated monolayer MoS2, and Ag nanoholes (Ag‐NHs) with square lattice are investigated by using an ultrafast pump‐probe approach. From transient absorption spectra of the prototypical Ag‐NHs/WS2/SiO2 (10 nm)/MoS2 heterostructures, a Rabi splitting up to 80 meV is observed, which is almost 21/2 times larger than that of each individual component. The result is as expected since Rabi splitting depends on the square root of the layer number involved, thus suggesting that the A exciton of WS2 and spatially separated B exciton of MoS2 are entangled by the Bloch‐surface plasmon polariton mode. Additionally, whether the donor or the acceptor is excited, the bleaching signals in the heterostructures all appear instantaneously and exhibit exactly the same dynamic process, further clearly highlighting the presence of quantum mechanically entangled states. From another perspective, such entangled states assist remarkably efficient energy transfer, which is also demonstrated by significantly enhanced fluorescence emission from MoS2, with an enhancement factor of 25. This research establishes the scientific foundation for developing related heterostructure optoelectronic devices. [ABSTRACT FROM AUTHOR]

Tightly bound electron-hole pairs (excitons) hosted in atomically-thin semiconductors have emerged as prospective elements in optoelectronic devices for ultrafast and secured information transfer. The controlled exciton transport in such excitonic devices requires manipulating potential energy gradient of charge-neutral excitons, while electrical gating or nanoscale straining have shown limited efficiency of exciton transport at room temperature. Here, we report strain gradient induced exciton transport in monolayer tungsten diselenide (WSe2) across microns at room temperature via steady-state pump-probe measurement. Wrinkle architecture enabled optically-resolvable local strain (2.4%) and energy gradient (49 meV/μm) to WSe2. We observed strain gradient induced flux of high-energy excitons and emission of funneled, low-energy excitons at the 2.5 μm-away pump point with nearly 45% of relative emission intensity compared to that of excited excitons. Our results strongly support the strain-driven manipulation of exciton funneling in two-dimensional semiconductors at room temperature, opening up future opportunities of 2D straintronic exciton devices. Here, the authors transfer monolayer WSe2 onto a pre-stretched substrate to achieve a sinusoidal ripple pattern that leads to alternating compressive/tensile strains. They study quantitatively how efficiently strain can induce exciton funneling and what parameters are important. [ABSTRACT FROM AUTHOR]

Fermi polarons are emerging quasiparticles when a bosonic impurity immersed in a fermionic bath. Depending on the boson-fermion interaction strength, the Fermi-polaron resonances exhibit either attractive or repulsive interactions, which impose further experimental challenges on understanding the subtle light-driven dynamics. Here, we report the light-driven dynamics of attractive and repulsive Fermi polarons in monolayer WSe2 devices. Time-resolved polaron resonances are probed using femtosecond below-gap Floquet engineering with tunable exciton-Fermi sea interactions. While conventional optical Stark shifts are observed in the weak interaction regime, the resonance shift of attractive polarons increases, but that of repulsive polarons decreases with increasing the Fermi-sea density. A model Hamiltonian using Chevy ansatz suggests the off-resonant pump excitation influences the free carriers that interact with excitons in an opposite valley, thereby reducing the binding energy of attractive polarons. Our findings may enable coherent Floquet engineering of Bose-Fermi mixtures in ultrafast time scales. The authors study the light-driven dynamics of attractive and repulsive Fermi polarons in monolayer WSe2. They show that the resonance shifts of Fermi polarons are valley-selective; the resonance shifts of attractive polarons increase with Fermi-sea density, while those of repulsive polarons decrease. [ABSTRACT FROM AUTHOR]