arXiv subjects

Tönu Pullerits

Publications and source records attributed to Tönu Pullerits.

13 recordsLinked to original sources

arXiv2026

Phonon-Mediated Chirality Transfer from Organic Cation to Inorganic lattice in Hybrid Perovskites

Two-dimensional hybrid metal halide perovskites combine spin-orbit coupling, soft lattice dynamics, and molecular tunability, making them promising platforms for chiral optoelectronics. While chiral organic spacers are known to induce optical activity in the inorganic framework, the microscopic mechanism by which molecular chirality couples to the inorganic lattice is actively studied. Here, we investigate coherent vibrational dynamics in chiral ($R$-MBA)$_2$PbI$_4$ and its racemic analogue using circular-polarized transient absorption spectroscopy. A vibrational mode at ~5.7 meV is present in the chiral material and not observed in the racemic counterpart, confirmed as a lattice phonon by agreement between time-domain and steady-state measurements. Ab-initio calculations reveal correlated libration of the organic spacer and bending displacement of the Pb-I framework analogous to a classical Wilberforce pendulum whose sense is locked to the structural handedness of the lattice. Polarization-resolved measurements reveal a transient circular dichroism whose exact sign reversal between ($R$-MBA)$_2$PbI$_4$ and ($S$-MBA)$_2$PbI$_4$ is consistent with a periodic modulation of the excitonic rotational strength driven by the coupled lattice displacement. These results establish dynamic chirality transfer through the coupled librational-bending mechanism, with direct implications for phonon-driven chiral optical responses and spin-selective transport.

cond-mat.mtrl-sci↗

arXiv2026

Operando multidimensional spectroscopy reveals A-site-dependent carrier cooling in perovskite solar cells

Understanding how photogenerated carriers dissipate excess energy in operating perovskite solar cells is essential for connecting ultrafast photophysics with photovoltaic function and provides design principles for engineering next-generation cell architectures. Here we use photocurrent-detected two-dimensional electronic spectroscopy (PC-2DES) to resolve energy-dependent carrier relaxation in fully encapsulated, functioning metal halide perovskite solar cells. By comparing devices with identical architecture but different absorber compositions - MAPbI3, mixed FAMA, and FAPbI3 - we directly follow the redistribution of photoexcited carriers from initially populated high-energy states toward lower-energy band-edge states. The multidimensional photocurrent response reveals a cascade-like intraband cooling process whose rate depends strongly on absorber composition, with the slowest relaxation in MA-based devices, intermediate behaviour in mixed-cation devices, and fastest relaxation in FA-based devices. A reduced kinetic model incorporating phonon-mediated intraband scattering, supplemented by a phenomenological many-body contribution, captures the main energy-dependent trends. These results establish action-detected multidimensional spectroscopy as a device-level probe of ultrafast energy dissipation and show that subtle changes in perovskite composition can substantially reshape the carrier relaxation pathways that precede charge extraction.

physics.chem-ph↗

arXiv2026

Disorder-Induced Localization of Molecular Polaritons Despite Spectroscopic Strong Coupling

Molecular polaritons are hybrid light--matter quasiparticles whose collective character is often associated with molecular excitations extending over many emitters. However, molecular ensembles are intrinsically disordered and dissipative, and spectrally visible polariton peaks do not necessarily imply delocalized molecular character. Here, we theoretically examine how static energetic disorder and finite cavity and molecular linewidths affect the delocalization of electronic polaritons in cavity--coupled molecular ensembles. Using a disordered Tavis--Cummings model, we show that energetic disorder mixes polariton states with the dark-state manifold, causing a rapid loss of collective molecular character even when polaritonic spectral features remain visible. We quantify this crossover using the molecular participation ratio, a density--matrix--based coherence measure, and an energy--resolved autocorrelation function. In the lossless electronic model, preserving an extended polaritonic molecular component requires the collective Rabi splitting to exceed the disorder width by more than a factor of five, providing a stricter condition than conventional spectroscopic strong coupling. Extending the analysis to a non--Hermitian Hamiltonian shows that cavity--molecule linewidth imbalance further reduces disorder tolerance. The resulting delocalization boundary indicates that preserving an extended molecular polariton component requires a collective Rabi splitting larger than roughly eight times the disorder width plus approximately twice the cavity--molecule linewidth mismatch. These results provide a quantitative criterion for polariton delocalization under disorder and loss and show that disorder, dissipation, and collective coupling must be considered together when assessing whether molecular polaritons remain collectively extended in realistic optical cavities.

physics.chem-ph↗

arXiv2026

Helicity-Resolved Spatiotemporal Mapping of Chiral Plexcitons in Helicoids

Plasmon-exciton hybrids, or plexcitons, offer deeply subwavelength light-matter interactions with versatile pathways for energy redistribution. Incorporating chirality into such systems is particularly compelling, enabling spin-sensitive optical functionality that can operate on ultrafast timescales and within ultracompact volumes. Despite recent progress in chiral plexcitonic systems, how structural chirality and plasmon-exciton coupling determine chiroptical spectra and ultrafast energy flow remains elusive. Here we realize chiral plexcitons by functionalizing intrinsically chiral gold helicoid nanoparticles with molecular J-aggregates. Within a non-Hermitian framework, we trace the microscopic origin of the helicoid chiroptical response and its coupling to the excitonic transition, revealing how the helicity of light selectively addresses distinct hybrid responses. At the spatiotemporal extreme, we find that the gap-localized response not only enhances polarization-sensitive contrast but also strengthens the local hybrid interaction, leading to accelerated ultrafast relaxation. Together, these space-, time-, and polarization-resolved measurements provide a physically grounded and experimentally benchmarked picture of chiral plexcitonic coupling, identifying chirality as a practical control parameter for selectively steering nanoscale energy pathways and dynamics.

physics.optics↗

arXiv2025

AI-enhanced High Resolution Functional Imaging Reveals Trap States and Charge Carrier Recombination Pathways in Perovskite

Understanding and controlling charge carrier recombination dynamics is essential for enhancing the performance of metal halide perovskite optoelectronic devices. In this study, we present a machine learning-assisted intensity-modulated two-photon photoluminescence microscopy (ML-IM2PM) method to quantitatively map recombination processes in MAPbBr3 perovskite microcrystalline films at micrometer-scale resolution. To improve model accuracy, we implemented a balanced classification sampling strategy during the machine learning optimization phase. The resulting regression chain model effectively predicts key physical parameters across a 576-pixel spatial map, including exciton generation rate (G), initial trap concentration (N_TR), and trap energy barrier (E_a). These extracted parameters were subsequently used to solve a system of coupled ordinary differential equations, enabling spatially resolved simulations of carrier populations and recombination dynamics under steady-state photoexcitation. The simulations reveal significant spatial heterogeneity in exciton, electron, hole, and trap populations, along with photoluminescence and nonradiative losses. Correlation analysis delineates three distinct recombination regimes: (i) a trap-filling regime dominated by nonradiative recombination, (ii) a transitional crossover regime, and (iii) a band-filling regime characterized by markedly enhanced radiative efficiency. A critical trap density threshold of approximately 10^17 cm^-3 marks the transition between these regimes. Overall, this work establishes ML-IM2PM as a robust framework for probing carrier dynamics and informing defect passivation strategies in perovskite materials.

physics.chem-ph↗

arXiv2024

Coherent Phonons, Localization and Slow Polaron Formation in Lead-free Gold Perovskite

Lead-free metal halide perovskites are emerging as less-toxic alternatives to their lead-based counterparts. However, their applicability in optoelectronic devices is limited, and the charge transport dynamics remain poorly understood. Understanding photo-induced charge and structural dynamics is critical for unlocking the potential of these novel systems. In this work, we employ ultrafast optical and Raman spectroscopy combined with band structure calculations to investigate the coupled electronic and vibrational dynamics in Caesium gold bromide, a promising lead-free perovskite. We find that the band-edge charge transfer states are strongly coupled to Au-Br stretching phonon modes, leading to frequency modulation of absorption by impulsively excited coherent phonons. Early-stage relaxation is characterized by dynamics of delocalized charge transfer excitation and slowly decaying coherent phonons. The electronic and vibrational relaxation reveals a slow formation of a localized polaronic state in the 10-20 ps timescale. Using a displaced harmonic oscillator model, the polaronic binding energy is estimated to be ~80 meV following lattice relaxation along the phonon modes. Strong exciton-phonon coupling and slow polaron formation via coupling to lattice modes make this material a promising testbed for the control of coherent phonons and localized polaronic states using light.

cond-mat.mtrl-sci↗

arXiv2023

Machine learning regression analyses of intensity modulation two-photon spectroscopy (Mlim) in perovskite microcrystals

Perovskite thin films hold great promise for optoelectronic applications, such as solar cells and light emitting diodes. A challenge is that defects are unavoidably formed in the material. Thorough understanding of the defect formation and their dynamics has proven challenging based on traditional spectroscopy. Here we integrated the functional intensity modulation two-photon spectroscopy with artificial intelligence - enhance data analyses to obtain a deep understanding of defect-related trap states within perovskite microcrystals. We introduce a novel charge carrier recombination dynamics model that comprehensively includes exciton and electron-hole pair photoluminescence (PL) emissions, as well as the trapping and detrapping equilibrium dynamics. By varying parameters in the dynamic model, a large pool of the temperature dependent intensity modulation PL spectra can be simulated by solving the ordinary differential equations in the charge carrier dynamics model. Then, tree-based supervised machine learning methods and ensemble technique -- regression chain have been used to optimize the Machine learning intensity modulation spectroscopy (Mlim), which helps to determine the parameters of the charge carrier dynamics model based on the temperature dependent intensity modulated PL spectra in perovskite. And the reliability of the Mlim predicted trap property parameters is confirmed by directly comparing the Mlim-retrieved intensity modulation spectra with experimental data. Besides, our approach unravels valuable insights into PL emissions, including those from excitons and free electron-hole pairs, but also provides details of trapping, detrapping, and nonradiative depopulation processes, offering a comprehensive understanding of perovskite material photophysics. This study suggests that Mlim applications hold promise for studying various photoactive devices.

physics.chem-ph↗

arXiv2022

Continuous variable quantum state tomography of photoelectrons

We propose a continuous variable quantum state tomography protocol of electrons which result from the ionization of atoms or molecules by the absorption of extreme ultraviolet light pulses. Our protocol is benchmarked against a direct calculation of the quantum state of photoelectrons ejected from helium and argon in the vicinity of a Fano resonance. In the latter case, we furthermore distill ion-photoelectron entanglement due to spin-orbit splitting. This opens new routes towards the investigation of quantum coherence and entanglement properties on the ultrafast timescale.

physics.atom-ph↗

arXiv2021

Probing electronic decoherence with high-resolution attosecond photoelectron interferometry

Quantum coherence plays a fundamental role in the study and control of ultrafast dynamics in matter. In the case of photoionization, entanglement of the photoelectron with the ion is a well known source of decoherence when only one of the particles is measured. Here we investigate decoherence due to entanglement of the radial and angular degrees of freedom of the photoelectron. We study two-photon ionization via the 2s2p autoionizing state in He using high spectral resolution photoelectron interferometry. Combining experiment and theory, we show that the strong dipole coupling of the 2s2p and 2p$^2$ states results in the entanglement of the angular and radial degrees of freedom. This translates, in angle integrated measurements, into a dynamic loss of coherence during autoionization.

physics.atom-ph↗

arXiv2020

Photo-Excitation Dynamics in Electrochemically Charged CdSe Quantum Dots: from Hot Carrier Cooling to Auger Recombination of Negative Trions

Fulfilling the potential of the colloidal semiconductor quantum dots (QDs) in electrically driven applications remains a challenge largely since operation of such devices involves charged QDs with drastically different photo-physical properties compared to their well-studied neutral counterparts. In this work, the full picture of excited state dynamics in charged CdSe QDs at various time-scales has been revealed via transient absorption spectroscopy combined with electrochemistry as direct manipulation tool to control the negative charging of CdSe QDs. In trions, excited states of single charged QDs, the additional electron in the conduction band speeds up the hot electron cooling by enhanced electron-electron scattering followed by charge redistribution and polaron formation in picoseconds timescale. The trions are finally decayed by Auger process in 500 ps timescale. Double charging in QDs, on the other hand, decelerates the polaron formation process while accelerates the following Auger decay. Our work demonstrates the potential of photo-electrochemistry as a platform for ultrafast spectroscopy of charged species and paves a way for further studies to develop comprehensive knowledge of the photophysical processes in charged QDs more than the well-known Auger decay preparing their use in future optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2019

Compressed Sensing for Reconstructing Coherent Multidimensional Spectra

We apply two sparse reconstruction techniques, the least absolute shrinkage and selection operator (LASSO) and the sparse exponential mode analysis (SEMA), to two-dimensional (2D) spectroscopy. The algorithms are first tested on model data, showing that both are able to reconstruct the spectra using only a fraction of the data required by the traditional Fourier-based estimator. Through the analysis of a sparsely sampled experimental fluorescence detected 2D spectra of LH2 complexes, we conclude that both SEMA and LASSO can be used to significantly reduce the required data, still allowing to reconstruct the multidimensional spectra. Of the two techniques, it is shown that SEMA offers preferable performance, providing more accurate estimation of the spectral line widths and their positions. Furthermore, SEMA allows for off-grid components, enabling the use of a much smaller dictionary than the LASSO, thereby improving both the performance and lowering the computational complexity for reconstructing coherent multidimensional spectra.

eess.SP↗

arXiv2018

Nanophotonic enhanced two-photon excited photoluminescence of perovskite quantum dots

All-inorganic CsPbBr3 perovskite colloidal quantum dots have recently emerged as promising material for a variety of optoelectronic applications, among others for multi-photon-pumped lasing. Nevertheless, high irradiance levels are generally required for such multi-photon processes. One strategy to enhance the multi-photon absorption is taking advantage of high local light intensities using photonic nanostructures. Here, we investigate two-photon-excited photoluminescence of CsPbBr3 perovskite quantum dots on a silicon photonic crystal slab. By systematic excitation of optical resonances using a pulsed near-infrared laser beam, we observe an enhancement of two-photon-pumped photoluminescence by more than one order of magnitude when comparing to using a bulk silicon film. Experimental and numerical analyses allow relating these findings to near-field enhancement effects on the nanostructured silicon surface. The results reveal a promising approach for significant decreasing the required irradiance levels for multi-photon processes being of advantage in applications like low-threshold lasing, biomedical imaging, lighting and solar energy.

physics.optics↗

arXiv2018

Unexpectedly large delocalization of the initial excitation in photosynthetic light harvesting

Electronic 2D spectroscopy allows nontrivial quantum effects in chemistry and biology to be explored in unprecedented detail. Here, we apply recently developed fluorescence detected coherent 2D spectroscopy to study the light harvesting antenna 2 (LH2) of photosynthetic purple bacteria. The method utilizes the destructive interference between two signal components thereby uncovering cross peaks which are not visible in conventional photon-echo based 2D and transient absorption measurements. Analyses of signal generating quantum pathways leads to the conclusion that, contrary to the currently prevailing physical picture, the two weakly-coupled pigment rings of LH2 share the initial electronic excitation leading to quantum mechanical correlation between the two clearly separate bands. These results are general and have consequences for the interpretation of excited states not only in photosynthesis but in all light absorbing systems. The initial delocalization could be the key for enhancing the light harvesting efficiency via biased motion towards the energy funnel.

physics.chem-ph↗