arXiv subjects

Maksym V. Kovalenko

Publications and source records attributed to Maksym V. Kovalenko.

At least 37 records · Page 2Linked to original sources

arXiv2024

Effect of crystal symmetry of lead halide perovskites on the optical orientation of excitons

The great variety of lead halide perovskite semiconductors represents an outstanding platform for studying crystal symmetry effects on the spin-dependent properties. Access to them is granted through optical orientation of exciton and carrier spins by circularly polarized photons. Here, the exciton spin polarization is investigated at $1.6$\,K cryogenic temperature in four lead halide perovskite crystals with different symmetries: (almost) cubic in FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$ and FAPbBr$_3$, and orthorhombic in MAPbI$_3$ and CsPbBr$_3$. Giant optical orientation of 85\% is found for the excitons in FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$, MAPbI$_3$, and CsPbBr$_3$, while it amounts to 20\% in FAPbBr$_3$. For all studied crystals, the optical orientation is robust to detuning of the laser photon energy from the exciton resonance, remaining constant for high energy detunings up to 0.3~eV, above which it continuously decreases to zero for detunings exceeding 1~eV. No acceleration of the spin relaxation for excitons with large kinetic energy is found in the cubic and orthorhombic crystals. This evidences the absence of the Dyakonov-Perel spin relaxation mechanism, which is based on the Rashba-Dresselhaus splitting of spin states at finite $k$-vectors. This indicates that the spatial inversion symmetry is maintained in perovskite crystals, independent of the cubic or orthorhombic phase.

cond-mat.mtrl-sci↗

arXiv2024

Transition between cooperative emission regimes in giant perovskite nanocrystals

Interactions between emitters within an ensemble can give rise to cooperative processes that significantly alter the properties of the emitted light. One such process is superfluorescence (SF), where excited electric dipoles spontaneously couple coherently and effectively radiate as one macroscopic emitter. It requires low energetic disorder, high temporal coherence and oscillator strength, and sub-wavelength volumes of material can be sufficient. Conversely, amplified spontaneous emission (ASE) originates from an avalanche-like stimulated amplification of initially spontaneously emitted photons and does not necessitate temporally coherent interactions among the emitters, but rather requires spatially long enough light propagation within the material to harvest the optical gain. Cesium lead halide perovskite nanocrystals (NCs) are one of the very few materials where both ASE (in disordered films) and SF (in ordered assemblies) were observed, however leaving unclear whether and how these regimes could be connected. Here, we demonstrate that temperature and excitation density can drive the transition between both regimes in a thin film of giant CsPbBr3 perovskite NCs. At temperatures below 45 K, excitonic SF was observed, whereas above a transition range between 45 K and 100 K, ASE prevails, but requires increased optical excitation and emitter density. Our results work out the different collective effects present in lead halide perovskites, providing fundamental insights into cooperative phenomena and important guidance for the development of compact and bright (quantum) light sources.

cond-mat.mes-hall↗

arXiv2024

Size-dependent multiexciton dynamics governs scintillation from perovskite quantum dots

The recent emergence of quantum confined nanomaterials in the field of radiation detection, in particular lead halide perovskite nanocrystals, offers potentially revolutionary scalability and performance advantages over conventional materials. This development raises fundamental questions about the mechanism of scintillation itself at the nanoscale and the role of particle size, arguably the most defining parameter of quantum dots. Understanding this is crucial for the design and optimisation of future nanotechnology scintillators. In this work, we address these open questions by theoretically and experimentally studying the size-dependent scintillation of CsPbBr3 nanocrystals using a combination of Monte Carlo simulations, spectroscopic, and radiometric techniques. The results reveal and unravel a complex parametric space where the fine balance between the simultaneous effects of size-dependent energy deposition, (multi-)exciton population, and light emission under ionizing excitation, typical of confined particles, combine to maximize the scintillation efficiency and time performance of larger nanocrystals due to greater stopping power and reduced Auger decay. The remarkable agreement between theory and experiment produces a fully validated descriptive model that unprecedentedly predicts the scintillation yield and kinetics of nanocrystals without free parameters, providing the first fundamental guide for the rational design of nanoscale scintillators.

physics.app-ph↗

arXiv2024

Room-temperature cavity exciton-polariton condensation in perovskite quantum dots

The exploitation of the strong light-matter coupling regime and exciton-polariton condensates has emerged as a compelling approach to introduce strong interactions and nonlinearities into numerous photonic applications, ranging from low-threshold topological lasers to ultrafast all-optical logic devices. The use of colloidal semiconductor quantum dots with strong three-dimensional confinement as the active material in these microcavities would be highly advantageous due to their versatile structural and compositional tunability and wet-chemical processability, as well as potentially enhanced, confinement-induced polaritonic interactions. Yet, to date, cavity exciton-polariton condensation has neither been achieved with epitaxial nor with colloidal quantum dots. Here, we demonstrate room-temperature polariton condensation in a thin film of monodisperse, colloidal CsPbBr$_3$ quantum dots placed in a tunable optical resonator with a Gaussian-shaped deformation serving as wavelength-scale potential well for the polaritons. The onset of polariton condensation under pulsed optical excitation is manifested in emission by its characteristic superlinear intensity dependence, reduced linewidth, blueshift, and extended temporal coherence. Our results, based on this highly engineerable class of perovskite materials with unique optical properties, pave the way for the development of polaritonic devices for ultrabright coherent light sources and photonic information processing.

physics.optics↗

arXiv2024

On characterizing X-ray detectors for low-dose imaging

The last decade has seen a renewed exploration of semiconductor materials for X-ray detection, foremost focusing on lead-based perovskites and other metal halides as direct-conversion materials and scintillators. However, the reported performance characteristics are often incomplete or misleading in assessing the practical utility of materials. This Perspective offers guidelines for choosing, estimating and presenting the relevant figures of merit. We also provide ready-to-used tools for calculating these figures of merit: MATLAB application, Mathcad worksheet and a website. The X-ray detectors for medical imaging are at focus for their increasing societal value and since they bring about the most stringent requirements as the image shall be acquired at as low as reasonably attainable (i.e. ALARA principle) dose received by the patient.

physics.ins-det↗

arXiv2024

Perovskite nanocrystal self-assemblies in 3D hollow templates

Highly ordered nanocrystal (NC) assemblies, namely superlattices (SLs), have been investigated as novel building blocks of optical and optoelectronic devices due to their unique properties based on interactions among neighboring NCs. In particular, lead halide perovskite NC SLs have attracted significant attention, owing to their extraordinary optical characteristics of individual NCs and collective emission processes like superfluorescence (SF). So far, the primary method for preparing perovskite NC SLs has been the drying-mediated self-assembly method, in which the colloidal NCs spontaneously assemble into SLs during solvent evaporation. However, this method lacks controllability because NCs form random-sized SLs at random positions on the substrate rendering NC assemblies in conjunction with device structures such as photonic waveguides or microcavities challenging. Here, we demonstrate template-assisted self-assembly to deterministically place perovskite NC SLs and control their geometrical properties. A solution of CsPbBr3 NCs is drop-casted on a substrate with lithographically-defined hollow structures. After solvent evaporation and removal of excess NCs from the substrate surface, NCs only remain in the templates thereby defining the position and size of these NC assemblies. We performed photoluminescence (PL) measurements on these NC assemblies and observed signatures of SF, similar as in spontaneously assembled SLs. Our findings are crucial for optical devices that harness embedded perovskite NC assemblies and prepare fundamental studies on how these collective effects can be tailored through the SL geometry.

cond-mat.soft↗

arXiv2024

Phonon-driven wavefunction localization promotes room-temperature, pure single-photon emission in large organic-inorganic lead-halide quantum dots

In lead halide perovskites (APbX3), the effect of the A-site cation on optical and electronic properties has initially been thought to be marginal. Yet, evidence of beneficial effects on solar cell performance and light emission is accumulating. Here, we report that the A-cation in soft APbBr3 colloidal quantum dots (QDs) controls the phonon-induced localization of the exciton wavefunction. Insights from ab initio molecular dynamics and single-particle fluorescence spectroscopy demonstrate that anharmonic lattice vibrations and the resulting polymorphism act as an additional confinement potential. Avoiding the trade-off between single-photon purity and optical stability faced by downsizing conventional QDs into the strong confinement regime, dynamical phonon-induced confinement in large organic-inorganic perovskite QDs enables bright (10^6 photons/s), stable (> 1h), and pure (> 95%) single-photon emission in a widely tuneable spectral range (495-745 nm). Strong electron-phonon interaction in soft perovskite QDs provides an unconventional route toward the development of scalable room-temperature quantum light sources.

cond-mat.mtrl-sci↗

arXiv2024

Spin-dependent exciton-exciton interactions in a mixed lead halide perovskite crystal

We investigate the two-pulse photon echo response of excitons in the mixed lead halide perovskite crystal \sample in dependence on the excitation intensity and polarization of the incident laser pulses. Using spectrally narrow picosecond laser pulses, we address localized excitons with long coherence times $T_2 \approx 100\,$ps. This approach offers high sensitivity for the observation of excitation-induced changes in the homogeneous linewidth $Γ_2=2\hbar/T_2$ on the $μ$eV scale. Through intensity-dependent measurements, we evaluate the increase of $Γ_2$ by 10~$μ$eV at an exciton density of 10$^{17}$~cm$^{-3}$ being comparable with the intrinsic linewidth of 14$\,μ$eV. We observe that the decay of the photon echo and its power dependence are sensitive to the polarization configuration of the excitation pulses, which indicates that spin-dependent exciton-exciton interactions contribute to excitation-induced dephasing. In cross-linear polarization, the decay is faster and its dependence on exciton density is stronger as compared to the co-polarized configuration. Using a two-exciton model accounting for different spin configurations we are able to reproduce the experimental results.

cond-mat.mes-hall↗

arXiv2024

Circularly Polarized Luminescence Without External Magnetic Fields from Individual CsPbBr3 Perovskite Quantum Dots

Lead halide perovskite quantum dots (QDs), the latest generation of colloidal QD family, exhibit outstanding optical properties which are now exploited as both classical and quantum light sources. Most of their rather exceptional properties are related to the peculiar exciton fine-structure of band-edge states which can support unique bright triplet excitons. The degeneracy of the bright triplet excitons is lifted with energetic splitting in the order of millielectronvolts, which can be resolved by the photoluminescence (PL) measurements of single QDs at cryogenic temperatures. Each bright exciton fine-structure-state (FSS) exhibits a dominantly linear polarization, in line with several theoretical models based on the sole crystal field, exchange interaction and shape anisotropy. Here, we show that in addition to a high degree of linear polarization, the individual exciton FSS can exhibit a non-negligible degree of circular polarization even without external magnetic fields by investigating the four Stokes parameters of the exciton fine-structure in individual CsPbBr3 QDs through Stokes polarimetric measurements. We observe a degree of circular polarization up to ~38%, which could not be detected by using the conventional polarimetric technique. In addition, we found a consistent transition from left- to right-hand circular polarization within the fine-structure triplet manifold, which was observed in magnetic field dependent experiments. Our optical investigation provides deeper insights into the nature of the exciton fine-structures and thereby drives the yet-incomplete understanding of the unique photophysical properties of this novel class of QDs, potentially opening new scenarios in chiral quantum optics.

physics.app-ph↗

arXiv2024

Disentangling the Effects of Structure and Lone-Pair Electrons in the Lattice Dynamics of Halide Perovskites

Metal halide perovskites have shown great performance as solar energy materials, but their outstanding optoelectronic properties are paired with unusually strong anharmonic effects. It has been proposed that this intriguing combination of properties derives from the "lone pair" 6$s^2$ electron configuration of the Pb$^{2+}$ cations, and associated weak pseudo-Jahn-Teller effect, but the precise impact of this chemical feature remains unclear. Here we show that in fact an $ns^2$ electron configuration is not a prerequisite for the strong anharmonicity and low-energy lattice dynamics encountered in this class of materials. We combine X-ray diffraction, infrared and Raman spectroscopies, and first-principles molecular dynamics calculations to directly contrast the lattice dynamics of CsSrBr$_3$ with those of CsPbBr$_3$, two compounds which bear close structural similarity but with the former lacking the propensity to form lone pairs on the 5$s^0$ octahedral cation. We exploit low-frequency diffusive Raman scattering, nominally symmetry-forbidden in the cubic phase, as a fingerprint to detect anharmonicity and reveal that low-frequency tilting occurs irrespective of octahedral cation electron configuration. This work highlights the key role of structure in perovskite lattice dynamics, providing important design rules for the emerging class of soft perovskite semiconductors for optoelectronic and light-harvesting devices.

cond-mat.mtrl-sci↗

arXiv2024

Dark-Bright Exciton Splitting Dominates Low-Temperature Diffusion in Halide Perovskite Nanocrystal Assemblies

Semiconductor nanocrystals could replace conventional bulk materials completely in displays and light-emitting diodes. Exciton transport dominates over charge carrier transport for materials with high exciton binding energies and long ligands, such as halide perovskite nanocrystal films. Here, we investigate how beneficial superlattices - nearly perfect 3D nanocrystal assemblies of nanocrystals are to exciton transport. Surprisingly, the high degree of order is not as crucial as the individual nanocrystal size, which strongly influences the splitting of the excitonic manifold into bright and dark states. At very low temperatures, the energetic splitting is large for the smallest nanocrystals, and dark levels with low oscillator strength effectively trap excitons inside individual nanocrystals, suppressing diffusion. The effect is reversed at elevated temperatures, and the larger NC size becomes detrimental to exciton transport due to enhanced exciton trapping and dissociation. Our results reveal that the nanocrystal size must be strongly accounted for in design strategies of future optoelectronic applications.

cond-mat.mes-hall↗

arXiv2024

AFM-IR of EHD-Printed PbS Quantum Dots: Quantifying Ligand Exchange at the Nanoscale

Colloidal quantum dots (cQDs) recently emerged as building blocks for semiconductor materials with tuneable properties. Electro-hydrodynamic printing can be used to obtain sub-micrometre patterns of cQDs without elaborate and aggressive photolithography steps. Post-deposition ligand exchange is necessary for the introduction of new functionalities into cQD solids. However, achieving a complete bulk exchange is challenging and conventional infrared spectroscopy lacks the required spatial resolution. Infrared nanospectroscopy (AFM-IR) enables quantitative analysis of the evolution of vibrational signals and structural topography on the nano-metre scale upon ligand substitution on lead sulphide (PbS) cQDs. A solution of ethane-dithiol in acetonitrile demonstrated rapid (~60 s) and controllable exchange of approximately 90% of the ligands, encompassing structures up to ~800 nm in thickness. Prolonged exposures (>1 h) led to the degradation of the microstructures, with a systematic removal of cQDs regulated by surface-to-bulk ratios and solvent interactions. This study establishes a method for the development of devices through a combination of tuneable photoactive materials, additive manufacturing of microstructures, and their quantitative nanometre-scale analysis.

physics.app-ph↗

arXiv2023

Electric-field-resolved detection of localized surface plasmons at petahertz-scale frequencies

We present a novel electric-field-resolved approach for probing ultrafast dynamics of localized surface plasmons in metallic nanoparticles. The electric field of the broadband carrier-envelope-phase stable few-cycle light pulse employed in the experiment provides access to time-domain signatures of plasmonic dynamics that are imprinted on the pulse waveform. The simultaneous access to absolute spectral amplitudes and phases of the interacting light allows us obtaining a complex spectral response associated with localized surface plasmons. We benchmark our findings against the absorbance spectrum obtained with a spectrometer as well as the extinction cross-section modeled by a classical Mie scattering theory.

physics.optics↗

arXiv2023

Scaling of Hybrid QDs-Graphene Photodetectors to Subwavelength Dimension

Emerging colloidal quantum dot (cQD) photodetectors currently challenge established state-of-the-art infrared photodetectors in response speed, spectral tunability, simplicity of solution processable fabrication, and integration onto curved or flexible substrates. Hybrid phototransistors based on 2D materials and cQDs, in particular, are promising due to their inherent photogain enabling direct photosignal enhancement. The photogain is sensitive to both, measurement conditions and photodetector geometry. This makes the cross-comparison of devices reported in the literature rather involved. Here, the effect of device length L and width W scaling to subwavelength dimensions (sizes down to 500 nm) on the photoresponse of graphene-PbS cQD phototransistors was experimentally investigated. Photogain and responsivity were found to scale with 1/LW, whereas the photocurrent and specific detectivity were independent of geometrical parameters. The measurements were performed at scaled bias voltage conditions for comparable currents. Contact effects were found to limit the photoresponse for devices with L < 3 μm. The relation of gate voltage, bias current, light intensity, and frequency on the photoresponse was investigated in detail, and a photogating efficiency to assess the cQD-graphene interface is presented. In particular, the specific detectivity values in the range between 10^8 to 10^9 Jones (wavelength of 1550 nm, frequency 6 Hz, room temperature) were found to be limited by the charge transfer across the photoactive interface.

cond-mat.mtrl-sci↗

arXiv2023

Exciton-polaritons in CsPbBr$_3$ crystals revealed by optical reflectivity in high magnetic fields and two-photon spectroscopy

Cesium lead bromide (CsPbBr$_3$) is a representative material of the emerging class of lead halide perovskite semiconductors that possess remarkable optoelectronic properties. Its optical properties in the vicinity of the band gap energy are greatly contributed by excitons, which form exciton-polaritons due to strong light-matter interactions. We examine exciton-polaritons in solution-grown CsPbBr$_3$ crystals by means of circularly-polarized reflection spectroscopy measured in high magnetic fields up to 60 T. The excited 2P exciton state is measured by two-photon absorption. Comprehensive modeling and analysis provides detailed quantitative information about the exciton-polariton parameters: exciton binding energy of 32.5 meV, oscillator strength characterized by longitudinal-tranverse splitting of 5.3 meV, damping of 6.7 meV, reduced exciton mass of $0.18 m_0$, exciton diamagnetic shift of 1.6 $μ$eV/T$^2$, and exciton Landé factor $g_X=+2.35$. We show that the exciton states can be well described within a hydrogen-like model with an effective dielectric constant of 8.7. From the measured exciton longitudinal-transverse splitting we evaluate the Kane energy of $E_p=15$ eV, which is in reasonable agreement with values of $11.8-12.5$ eV derived from the carrier effective masses.

cond-mat.mes-hall↗

arXiv2023

Strong light-matter coupling in lead halide perovskite quantum dot solids

Strong coupling between lead halide perovskite materials and optical resonators enables both the polaritonic control of the photophysical properties of these emerging semiconductors and the observation of novel fundamental physical phenomena. However, the difficulty to achieve optical-quality perovskite quantum dot (PQD) films showing well-defined excitonic transitions has prevented the study of strong light-matter coupling in these materials, central to the field of optoelectronics. Herein we demonstrate the formation at room temperature of multiple cavity exciton-polaritons in metallic resonators embedding highly transparent Cesium Lead Bromide quantum dot (CsPbBr3-QD) solids, revealed by a significant reconfiguration of the absorption and emission properties of the system. Our results indicate that the effects of biexciton interaction or large polaron formation, frequently invoked to explain the properties of PQDs, are seemingly absent or compensated by other more conspicuous effects in the CsPbBr3-QD optical cavity. We observe that strong coupling enables a significant reduction of the photoemission linewidth, as well as the ultrafast switching of the optical absorption, controllable by means of the excitation fluence. We find that the interplay of the polariton states with the large dark state reservoir play a decisive role in determining the dynamics of the emission and transient absorption properties of the hybridized light-quantum dot solid system. Our results open the route for the investigation of PQD solids as polaritonic optoelectronic materials.

cond-mat.mtrl-sci↗

arXiv2023

Giant optical orientation of exciton spins in lead halide perovskite crystals

Optical orientation of carrier spins by circularly polarized light is the basis of spin physics in semiconductors. Here, we demonstrate strong optical orientation of 85\%, approaching the ultimate limit of unity, for excitons in FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$ lead halide perovskite bulk crystals. Time-resolved photoluminescence allows us to distinguish excitons with 60~ps lifetime from electron-hole recombination in the spin dynamics detected via coherent spin quantum beats in magnetic field. We reveal electron-hole spin correlations through linear polarization beats after circularly polarized excitation. Detuning of the excitation energy from the exciton resonance up to 0.5~eV does not reduce the optical orientation, evidencing clean chiral selection rules in agreement with atomistic calculations, and suppressed spin relaxation of electrons and holes even with large kinetic energies.

cond-mat.mes-hall↗

arXiv2023

Long-lived exciton coherence in mixed-halide perovskite crystals

Compositional engineering of the optical properties of hybrid organic-inorganic lead halide perovskites is one of the cornerstones for the realization of efficient solar cells and tailored light-emitting devices. We study the effect of compositional disorder on coherent exciton dynamics in a mixed FA$_{0.9}$Cs$_{0.1}$PbI$_{2.8}$Br$_{0.2}$ perovskite crystal using photon echo spectroscopy. We reveal that the homogeneous linewidth of excitons can be as narrow as 16$μ$eV at a temperature of 1.5K. The corresponding exciton coherence time of $T_2=83$ps is exceptionally long being attributed to the localization of excitons due to variation of composition at the scale of ten to hundreds of nanometers. From spectral and temperature dependences of the two- and three-pulse photon echo decay we conclude that for low-energy excitons, pure decoherence associated with elastic scattering on phonons is comparable with the exciton lifetime, while for excitons with higher energies, inelastic scattering to lower energy states via phonon emission dominates.

cond-mat.mes-hall↗