Biexciton Binding Energy Research Articles

(Invited, Digital Presentation) Ultrafast Spectroscopy of Inorganic Perovskite Nanocrystals and Their Assemblies: Uncovering the Multiple Exciton Generation Rate and Band-Formation

Inorganic perovskite nanocrystals (NCs) display attractive structural and optical properties, and have perspective in a wide range of optoelectronic applications. One interesting feature is their relatively slow hot carrier cooling rate, which allows to follow carrier dynamics in great detail with ultrafast spectroscopic techniques.In this talk, we present studies of the ultrafast carrier dynamics of perovskite NCs and assembled structures. Typically, when the excitation photon energy is increased, and thus the excess energy of the created carriers, the cooling time of the carriers towards the bandedge increases along. However, for CsPbI3 NCs, when the photon energy exceeds a certain threshold energy the cooling time starts to decrease, indicating that a competing relaxation pathway opens up which funnels carriers towards the bandedge. This threshold energy indicates the onset of a carrier multiplication process, in which absorption of a single photon leads to the creation of multiple electron-hole pairs. This process can also be inferred by following the carrier concentration, which shows a delayed build-up following the excitation pulse. The carrier dynamics in this system can be modeled within a framework of two competing cooling mechanisms. With the experimentally determined carrier cooling rate and threshold energy it is possible to extract the time-constant associated with the carrier multiplication process.Experiments on highly ordered CsPbBr3 NCs, so-called supercrystals, revealed how the photophysics are altered due to the inter-NC coupling as compared to the individual NCs. Next to a red-shift of the absorption and photoluminescence peak, the supercrystals show signatures of band-like states. The observation of a reduced Stokes-shift, decreased biexciton binding energy and increased carrier cooling rates, support the formation of delocalized states resulting from the coupling between individual NCs. These results open perspectives for assembled NCs for application in optoelectronic devices, with design opportunities exceeding the level of nanocrystal and bulk material.

Photon‐dressed biexciton‐mediated anomalous optical Stark effect in CsPbBr3 nanocrystals

Intense optical pumping below bandgap creates photon‐dressed states, which interact with and result in the shift of electronic levels, known as optical Stark effect (OSE). The OSE persists only for pump pulse duration implying the coherent response. Previous researches in semiconductors have shown that coherent OSE is evident only at cryogenic temperatures. Only recently lead halide perovskites (LHPs) were found to possess room temperature OSE. However, despite strong exciton–exciton interactions known in LHPs, OSE is described in non‐interacting framework that treats an exciton like isolated two‐level systems. The present work demonstrates anomalous OSE in interacting regime at room temperature in CsPbBr3 nanocrystals having exciton resonance (E0) at 2.40 eV using circularly polarized transient absorption spectroscopy. The results show that photon dressing for same pump–probe polarization (σ+σ+) lead to observation of blueshift of exciton resonance across all pump detuning energies (Ω = E0 − ħω; ħω is pump photon energy). On the contrary, under opposite pump–probe polarization (σ+σ−), the strong exciton−biexciton interaction leads to a coherent red shift and splitting of the exciton resonance as a function of the drive photon frequency. The hallmark observations under σ+σ− result from large biexciton binding energy of ~71 meV and exciton−biexciton transition dipole moment of ~25 Debye. Further, we demonstrate unusual crossover from linear to nonlinear dependence of the OSE as a function of the drive photon frequency. Our systematic study reveals crucial information on unexplored many body interacting regimes of coherent manipulation in LHPs, important for exotic quantum phase applications like quantum information processing and ultrafast switching.

Symmetry-dependent exciton-exciton interaction and intervalley biexciton in monolayer transition metal dichalcogenides

The multivalley band structure of monolayer transition metal dichalcogenides (TMDs) gives rise to intravalley and intervalley excitons. Much knowledge of these excitons has been gained, but fundamental questions remain, such as how to describe them all in a unified picture with their correlations, how are those from different valleys coupled to form the intervalley biexciton? To address the issues, we derive an exciton Hamiltonian from interpair correlations between the constituent carriers-fermions of two excitons. Identifying excitons by irreducible representations of their point symmetry group, we find their pairwise interaction depending on interacting excitons’ symmetry. It is generally repulsive, except for the case excitons from different valleys, which attract each other to form the intervalley biexciton. We establish a semianalytical relationship between the biexciton binding energy with exciton mass and dielectric characteristics of the material and surroundings. Overall, by providing insight into the nature of diverse excitons and their correlations, our theoretical model captures the exciton interaction properties permitting an inclusive description of the structure and energy features of the intervalley biexciton in monolayer TMDs.

Correlated spectral fluctuations quantified by line shape analysis of fifth-order two-dimensional electronic spectra.

Correlated spectral fluctuations were suggested to coordinate excitation transport inside natural light harvesting complexes. We demonstrate the capacities of 2D line shapes from fifth-order coherent electronic signals (R5-2D) to report on such fluctuations in molecular aggregates and present a stochastic approach to fluctuations in correlated site and bi-exciton binding energies in the optical dynamics of Frenkel excitons. The model is applied to R5-2D line shapes of a homodimer, and we show that the peak tilt dynamics are a measure for site energy disorder, inter-site correlation, and the strength of bi-exciton binding energy fluctuations.

Electronic Coupling of Highly Ordered Perovskite Nanocrystals in Supercrystals.

Assembled perovskite nanocrystals (NCs), known as supercrystals (SCs), can have many exotic optical and electronic properties different from the individual NCs due to energy transfer and electronic coupling in the dense superstructures. We investigate the optical properties and ultrafast carrier dynamics of highly ordered SCs and the dispersed NCs by absorption, photoluminescence (PL), and femtosecond transient absorption (TA) spectroscopy to determine the influence of the assembly on the excitonic properties. Next to a red shift of absorption and PL peak with respect to the individual NCs, we identify signatures of the collective band-like states in the SCs. A smaller Stokes shift, decreased biexciton binding energy, and increased carrier cooling rates support the formation of delocalized states as a result of the coupling between the individual NC states. These results open perspectives for assembled perovskite NCs for application in optoelectronic devices, with design opportunities exceeding the level of NCs and bulk materials.

Photon Echo Polarimetry of Excitons and Biexcitons in a CH3NH3PbI3 Perovskite Single Crystal

Lead halide perovskites show remarkable performance when used in photovoltaic and optoelectronic devices. However, the peculiarities of light-matter interactions in these materials in general are far from being fully explored experimentally and theoretically. Here we specifically address the energy level order of optical transitions and demonstrate photon echos in a methylammonium lead triiodide single crystal, thereby determining the optical coherence times $T_2$ for excitons and biexcitons at cryogenic temperature to be 0.79 ps and 0.67 ps, respectively. Most importantly, we have developed an experimental photon-echo polarimetry method that not only identifies the contributions from exciton and biexciton complexes, but also allows accurate determination of the biexciton binding energy of 2.4 meV, even though the period of quantum beats between excitons and biexcitons is much longer than the coherence times of the resonances. Our experimental and theoretical analysis methods contribute to the understanding of the complex mechanism of quasiparticle interactions at moderate pump density and show that even in high-quality perovskite crystals and at very low temperatures, inhomogeneous broadening of excitonic transitions due to local crystal potential fluctuations is a source of optical dephasing.

Direct measurement of biexcitons in monolayer WS2

The optical properties of atomically thin transition metal dichalcogenides are dominated by Coulomb bound quasi-particles, such as excitons, trions, and biexcitons. Due to the number and density of possible states, attributing different spectral peaks to the specific origin can be difficult. In particular, there has been much conjecture around the presence, binding energy and/or nature of biexcitons in these materials. In this work, we remove any ambiguity in identifying and separating the optically excited biexciton in monolayer WS2 using two-quantum multidimensional coherent spectroscopy (2Q-MDCS), a technique that directly and selectively probes doubly-excited states, such as biexcitons. The energy difference between the unbound two-exciton state and the biexciton is the fundamental definition of biexciton binding energy and is measured to be 26 ± 2 meV. Furthermore, resolving the biexciton peaks in 2Q-MDCS allows us to identify that the biexciton observed here is composed of two bright excitons in opposite valleys.

Room-Temperature Anomalous Coherent Excitonic Optical Stark Effect in Metal Halide Perovskite Quantum Dots.

Nonresonant optical driving of confined semiconductors can open up exciting opportunities for experimentally realizing strongly interacting photon-dressed (Floquet) states through the optical Stark effect (OSE) for coherent modulation of the exciton state. Here we report the first room-temperature observation of the Floquet biexciton-mediated anomalous coherent excitonic OSE in CsPbBr3 quantum dots (QDs). Remarkably, the strong exciton-biexciton interaction leads to a coherent red shift and splitting of the exciton resonance as a function of the drive photon frequency, similar to Autler-Townes splitting in atomic and molecular systems. The large biexciton binding energy of ∼71 meV and exciton-biexciton transition dipole moment of ∼25 D facilitate the hallmark observations, even at large detuning energies of >300 meV. This is accompanied by an unusual crossover from linear to nonlinear fluence dependence of the OSE as a function of the drive photon frequency. Our findings reveal crucial information on the unexplored many-body coherent interacting regime, making perovskite QDs suitable for room temperature quantum devices.

Observation of high-density multi-excitons in medium-size CdSe/CdZnS/ZnS colloidal quantum dots through transient spectroscopy and their optical gain properties.

Semiconductor quantum dots have extremely significant advantages in terms of optoelectronic devices. However, it is unfeasible to avoid the generation of charged exciton states during operation. Such states can change the radiation recombination rate and bring additional non-radiative Auger recombination channels. Herein, we synthesize high photoluminescence quantum yield medium-size CdSe/CdZnS/ZnS core/alloy shell/shell QDs. Their multiexciton spectra and dynamics were systematically studied by pump-power-dependent fluorescence blinking and time-correlated spectroscopy. The lifetimes of positively/negatively charged trions and biexcitons are estimated to be 0.74/6.1 and 0.16 ns, respectively. It demonstrated that the band-edge biexciton is influenced by the Coulomb interaction and Stark effect. The amplified spontaneous emission threshold is only 81 μJ cm-2 and can retain a long operation lifetime under continuous pumping. A vertical microcavity surface-emitting laser device is fabricated using these QDs. The coupling factor between the spontaneous emission and cavity mode is 0.81, which benefits the low stimulated emission threshold. This work provides a new perspective of the charged states in the multiexciton AR process in the QDs, implying a promising application prospect of such QDs as optical gain materials in "zero-threshold" laser fabrication.

Effect of intravalley and intervalley electron-hole exchange on the nonlinear optical response of monolayer MoSe2

The coherent third-order nonlinear response of monolayer transition-metal dichalcogenide semiconductors, such as ${\mathrm{MoSe}}_{2}$, is dominated by the nonlinear exciton response, as well as biexciton and trion resonances. The fact that these resonances may be spectrally close together makes identification of the signatures, for example in differential transmission (DT), challenging. Instead of focusing on explaining a given set of experimental data, a systematic study aimed at elucidating the roles of intravalley and intervalley long-range electron-hole $(e\ensuremath{-}\mathrm{h})$ exchange on the DT spectra is presented. Previous works have shown that the $e\ensuremath{-}\mathrm{h}$ long-range exchange introduces a linear leading-order term in the exciton dispersion. Based on a generalized Lippmann-Schwinger equation, we show that the presence of this linear dispersion term can reduce the biexciton binding energy to zero, contrary to the conventional situation of quadratic dispersion where an arbitrarily weak (well-behaved) attractive interaction always supports bound state(s). The effects of spin scattering and the spin-orbit interaction caused by $e\ensuremath{-}\mathrm{h}$ exchange are also clarified, and the DT line shape at the exciton and trion resonance is studied as a function of $e\ensuremath{-}\mathrm{h}$ exchange strength. In particular, as the exciton line shape is determined by the interplay of linear exciton susceptibility and the bound-state two-exciton resonance in the $T$ matrix, the line shape at the trion is similarly determined by the interplay of the linear trion susceptibility and the bound-state exciton-trion resonance in the $T$ matrix.

Probing biexciton in monolayer WS2 through controlled many-body interaction

The monolayers of semiconducting transition metal dichalcogenides host strongly bound excitonic complexes and are an excellent platform for exploring many-body physics. Here we demonstrate a controlled kinetic manipulation of the five-particle excitonic complex, the charged biexciton, through a systematic dependence of the biexciton peak on excitation power, gate voltage, and temperature using steady-state and time-resolved photoluminescence. With the help of a combination of the experimental data and a rate equation model, we argue that the binding energy of the charged biexciton is less than the spectral separation of its peak from the neutral exciton. We also note that while the momentum-direct radiative recombination of the neutral exciton is restricted within the light cone, such restriction is relaxed for a charged biexciton recombination due to the presence of near-parallel excited and final states in the momentum space.

Resolving the Controversy in Biexciton Binding Energy of Cesium Lead Halide Perovskite Nanocrystals through Heralded Single-Particle Spectroscopy.

Understanding exciton−exciton interaction in multiply excited nanocrystals is crucial to their utilization as functional materials. Yet, for lead halide perovskite nanocrystals, which are promising candidates for nanocrystal-based technologies, numerous contradicting values have been reported for the strength and sign of their exciton−exciton interaction. In this work, we unambiguously determine the biexciton binding energy in single cesium lead halide perovskite nanocrystals at room temperature. This is enabled by the recently introduced single-photon avalanche diode array spectrometer, capable of temporally isolating biexciton−exciton emission cascades while retaining spectral resolution. We demonstrate that CsPbBr3 nanocrystals feature an attractive exciton−exciton interaction, with a mean biexciton binding energy of 10 meV. For CsPbI3 nanocrystals, we observe a mean biexciton binding energy that is close to zero, and individual nanocrystals show either weakly attractive or weakly repulsive exciton−exciton interaction. We further show that, within ensembles of both materials, single-nanocrystal biexciton binding energies are correlated with the degree of charge-carrier confinement.

Impact of Dark Excitons on the Population and Relaxation Kinetics of Two-Dimensional Biexcitons in [CH3(CH2)3NH3]2Pb1-xMnxBr4 (x = 0-0.09).

Two-dimensional (2D) semiconductors have emerged as an excellent platform for studying various excitonic matter under strong quantum and dielectric confinements. However, such effects can be seriously overestimated for Coulomb binding of two excitons to form a biexciton by a naive interpretation of the corresponding photoluminescence (PL) spectrum. By using 2D halide perovskite single crystals of [CH3(CH2)3NH3]2Pb1-xMnxBr4 (x = 0-0.09) as a model system, we investigated both population and relaxation kinetics of biexcitons as a function of excitation density, temperature, polarization, and Mn doping. We show that the biexciton is formed by binding of two dark excitons, which are partially bright, but they radiatively recombine to yield a bright exciton in the final state. This renders the spectral distance between the exciton peak and the biexciton peak as very different from the actual biexciton binding energy (ϕ) because of large bright-dark splitting. We show that Mn doping introduces paramagnetism to our 2D system and improves the biexciton stability as evidenced by increase in ϕ from 18.8 ± 0.7 to 20.0 ± 0.7 meV and the increase of the exciton-exciton capture coefficient C from 2.4 × 10-11 to 4.3 × 10-11cm2/ns within our doping range. The precisely determined ϕ values are significantly smaller than the previously reported ones, but they are consistent with the instability of the biexciton against thermal dissociation at room temperature. Our results demonstrate that electron-hole exchange interaction must be considered for precisely locating the biexciton level; therefore, the ϕ values should be reassessed for other 2D halide perovskites that even do not exhibit any dark exciton PL.

Efficient Biexciton State Preparation in a Semiconductor Quantum Dot‐Metallic Nanoparticle Hybrid Structure Using Transitionless Quantum Driving

AbstractHow to efficiently prepare the biexciton state in a semiconductor quantum dot placed in the vicinity of a metal nanoparticle is investigated, with control pulses calculated using shortcuts to adiabaticity and specificaly transitionless quantum driving. The quantum dot is considered to be initialized in its ground state. It is numerically demonstrated that the proposed scheme can generate biexciton population values close to unity, for a broad span of interparticle distances and small to moderate values of the biexciton binding energy, when using short laser pulses. It is also shown that these results are robust for small errors in the nanoparticle placement. When the interparticle distance is decreased or the duration of the applied pulses is increased, the population transfer to the biexciton state is reduced. The reason is that for shorter distances the nonlinear terms in the density matrix equation, arising due to exciton‐plasmon interaction, become stronger, while for longer pulses the influence of these unwanted terms is prolonged. Similarly, the fidelity is also reduced for larger values of the biexciton binding energy and longer pulses. This work may be exploited in research fields like quantum information processing and high speed nanoswitches, where a key procedure is the efficient biexciton state preparation in quantum dots.

Multiple excitons dynamics of lead halide perovskite

Abstract The lead halide perovskite material shows its unique photoelectric properties, the resulting conversion efficiency of perovskite solar cells. However, the efficiency comes to a bottleneck owing to that mechanism research fall behind the device research. Multiple excitons dynamics play an important role, especially in the lifetime and binding energy of multiple excitons. A long multiexciton lifetime is beneficial to the application for light-emitting devices and photovoltaic devices. Large multiexciton binding energy means a large Stokes shift in exciton absorption, thus avoiding the loss of linear absorption. To conclude, discussions are presented regarding views of current multiple excitons research in terms of the biexciton lifetime and biexciton binding energy that should be considered for further advances in materials and devices.

Heralded Spectroscopy Reveals Exciton-Exciton Correlations in Single Colloidal Quantum Dots.

Multiply excited states in semiconductor quantum dots feature intriguing physics and play a crucial role in nanocrystal-based technologies. While photoluminescence provides a natural probe to investigate these states, room-temperature single-particle spectroscopy of their emission has proved elusive due to the temporal and spectral overlap with emission from the singly excited and charged states. Here, we introduce biexciton heralded spectroscopy enabled by a single-photon avalanche diode array based spectrometer. This allows us to directly observe biexciton–exciton emission cascades and measure the biexciton binding energy of single quantum dots at room temperature, even though it is well below the scale of thermal broadening and spectral diffusion. Furthermore, we uncover correlations hitherto masked in ensembles of the biexciton binding energy with both charge-carrier confinement and fluctuations of the local electrostatic potential. Heralded spectroscopy has the potential of greatly extending our understanding of charge-carrier dynamics in multielectron systems and of parallelization of quantum optics protocols.

Interplay between polarization and quantum correlations of confined polaritons

We investigate polariton quantum correlations in a coherently driven box cavity in the low driving regime, with a particular focus on accounting for the polarization degree of freedom. The possibility of having different interaction strengths between co- and cross-circularly polarized polaritons as well as a realistic linear-polarization splitting allows one to model the system as two coupled nonlinear resonators with both self- and cross-Kerr-like nonlinearities, thus making our results potentially relevant for other experimental platforms. Within an effective wave-function approach, we obtain analytical expressions for the steady-state polarization-resolved polariton populations and second-order correlation functions, which agree very well with our numerical results obtained from a Lindblad master equation. Notably, we highlight that depending on the excitation polarization (circular or linear), both the unconventional (interference-mediated) and conventional (mediated by nonlinearities) antibunchings can be investigated in a single cavity. Moreover, using our results, we argue that recent experiments on confined fiber-cavity polaritons are likely to have probed a regime where the dominant interaction is between cross-polarized polaritons, which is characteristic of the polariton Feshbach resonance. We furthermore investigate the regime close to resonance using a two-channel model, and we show that systems with large biexciton binding energies, such as atomically thin semiconductors, are promising platforms for realizing strong polariton antibunching.

Biexciton Binding Energy and Line width of Single Quantum Dots at Room Temperature.

Broadening of multiexciton emission from colloidal quantum dots (QDs) at room temperature is important for their use in high-power applications, but an in-depth characterization has not been possible until now. We present and apply a novel spectroscopic method to quantify the biexciton line width and biexciton binding energy of single CdSe/CdS/ZnS colloidal QDs at room temperature. In our method, which we term “cascade spectroscopy”, we select emission events from the biexciton cascade and reconstruct their spectrum. The biexciton has an average emission line width of 86 meV on the single-QD scale, similar to that of the exciton. Variations in the biexciton repulsion (Eb = 4.0 ± 3.1 meV; mean ± standard deviation of 15 QDs) are correlated with but are more narrowly distributed than variations in the exciton energy (10.0 meV standard deviation). Using a simple quantum-mechanical model, we conclude that inhomogeneous broadening in our sample is primarily due to variations in the CdS shell thickness.

Systematic study of the emission spectra of nanowire quantum dots

A systematic study of the emission spectra of single InAsP nanowire quantum dots in position-controlled InP photonic nanowire waveguides is presented. Using excitation power-dependent photoluminescence and correlation measurements, we distinguish between the different excitonic complexes responsible for s-shell emission. From measurements of over 40 nominally identical devices, we obtain a standard deviation of ∼5 meV in the emission energy of excitons, biexcitons, and charged exciton photons. The mean biexciton binding energy was 1.9 meV with a standard deviation of ∼0.8 meV. The experimental spectra are understood using atomistic multi-million atom theory of neutral and charged multi-exciton complexes implemented in the design tool QNANO.

Optical properties and symmetry optimization of spectrally (excitonically) uniform site-controlled GaAs pyramidal quantum dots

GaAs quantum dots (QDs) have recently emerged as state-of-the-art semiconductor sources of polarization-entangled photon pairs, however, without site-control capability. In this work, we present a systematic study of epitaxially grown GaAs/AlxGa1-xAs site-controlled pyramidal QDs possessing unrivaled excitonic uniformity in comparison to their InGaAs counterparts or GaAs QDs fabricated by other techniques. We have experimentally and systematically investigated the binding energy of biexcitons, highlighting the importance of the uniformity of all excitonic lines, rather than concentrating solely on the uniformity of the neutral exciton as a typical figure of merit, as it is normally done in the literature. We present optical signatures of GaAs QDs within a range of ∼250 meV with a remarkable uniformity within each individual sample, the ability to excite the biexciton state resonantly, and a systematic study of the fine-structure splitting (FSS) values—features important for polarization entangled photon emission. While, in general, we observe relatively large FSS distribution and associated non-uniformities, we discuss several strategies to suppress the average FSS values to <15 μeV.