arXiv subjects

Michele Guerrini

Publications and source records attributed to Michele Guerrini.

11 recordsLinked to original sources

arXiv2026

Real-time probing of quadrupolar contributions to the absorption cross section of non-periodic systems

The non-perturbative evaluation of multipolar cross sections is essential for probing atomic and molecular responses to spatially inhomogeneous electric fields characteristic of nanoscale environments where the conventional dipole approximation breaks down. Taking the hydrogen atom as an analytical and numerical benchmark, we map in real time quadrupole interactions driven by instantaneous electric field gradients. Our study explores two distinct interaction regimes. Under a uniform field, quadrupolar responses are activated by multi-step dipole transitions, responsible for sub-1~eV excited-state absorption. Conversely, under pure spatial gradients, the weak-field response is dominated by the single-photon $1s \to 3d$ quadrupole resonance at 12.1~eV, while strong gradients induce low-energy stimulated emission via transiently driven coherent populations. This dynamical analysis is complemented by a rigorous evaluation of the symmetry aspects of the quadrupole response and the applied electric field. This methodology establishes the theoretical and computational foundation for future non-perturbative multipole simulations of molecules and nanostructures in the near-field regime.

cond-mat.quant-gas↗

arXiv2025

Excited State Absorption Drives Low-Energy Optical Limiting in Oligothiophenes

Optical limiting (OL), a crucial mechanism for protecting human eyes and sensitive sensors from intense radiation, relies on understanding the optical nonlinearities acting on the systems. Assessing and disentangling the effects at play is crucial to predict and control the nonlinear optical response in real materials. In this ab initio study based on real-time time-dependent density-functional theory, we investigate non-perturbatively the absorption spectra of a set of thiophene oligomers, the building blocks of technologically relevant organic semiconductors, excited by broadband radiation of increasing intensity. Under strong electric fields, the absorption cross section grows significantly below the onset of linear excitations, exhibiting saturation typical of OL. By exciting the oligothiophenes with a train of pulses targeting the first and second excited states of each moiety and analyzing the resulting population dynamics, we reveal excited-state absorption (ESA) in the near-infrared to visible region. Our results indicate ESA as the driving mechanism for OL in oligothiophene molecules, thereby providing important insight to design novel compounds with optimized nonlinear optical characteristics.

physics.optics↗

arXiv2025

Ultrafast Photoexcitation of Semiconducting Photocathode Materials

Cs-based semiconductors like $\mathrm{Cs_3Sb}$ and $\mathrm{Cs_2Te}$ are currently used as photocathodes in particle accelerators. Their performance as electron sources critically depends on their interaction with intense laser sources. In this work, we investigate from first principles the time-dependent response of $\mathrm{Cs_3Sb}$ and $\mathrm{Cs_2Te}$ to ultrafast pulses of varying intensities, ranging from $1~\mathrm{GW/cm^2}$ to $1~\mathrm{PW/cm^2}$. Nonlinear effects, including high harmonic generation, emerge starting from $100~\mathrm{GW/cm^2}$ in $\mathrm{Cs_3Sb}$ and $200~\mathrm{GW/cm^2}$ in $\mathrm{Cs_2Te}$. Above these intensities, the numbers of absorbed photons and excited electrons saturate due to the depletion of one-photon absorption channels, with renewed increases beyond $1~\mathrm{TW/cm^2}$ for $\mathrm{Cs_3Sb}$ and $5~\mathrm{TW/cm^2}$ for $\mathrm{Cs_2Te}$ where multi-photon absorption appears. Finally, the analysis of the occupation density in $\mathrm{Cs_3Sb}$ reveals the onset of tunnel ionization at intensities above $10~\mathrm{TW/cm^2}$. Our findings provide new insights into the nonlinear optical properties of $\mathrm{Cs_3Sb}$ and $\mathrm{Cs_2Te}$, contributing to the optimization of these materials for the development of next-generation photoinjectors.

cond-mat.mtrl-sci↗

arXiv2023

Two-dimensional electronic spectroscopy from first principles

The recent development of multidimensional ultrafast spectroscopy techniques calls for the introduction of computational schemes that allow for the simulation of such experiments and the interpretation of the corresponding results from a microscopic point of view. In this work, we present a general and efficient first-principles scheme to compute two-dimensional electronic spectroscopy maps based on real-time time-dependent density-functional theory. The interface of this approach with the Ehrenfest scheme for molecular dynamics enables the inclusion of vibronic effects in the calculations based on a classical treatment of the nuclei. The computational complexity of the simulations is reduced by the application of numerical advances such as branching techniques, undersampling, and a novel reduced phase cycling scheme, applicable for systems with inversion symmetry. We demonstrate the effectiveness of this method by applying it to prototypical molecules such as benzene, pyridine, and pyrene. We discuss the role of the approximations that inevitably enter the adopted theoretical framework and set the stage for further extensions of the proposed method to more realistic systems.

cond-mat.mes-hall↗

arXiv2022

Modeling the electronic structure of organic materials: A solid-state physicist's perspective

Modeling the electronic and optical properties of organic semiconductors remains a challenge for theory, despite the remarkable progress achieved in the last three decades. The complexity of these systems, including structural (dis)order and the still debated doping mechanisms, has been engaging theorists with different backgrounds. Regardless of the common interest across the various communities active in this field, these efforts have not led so far to a truly interdisciplinary research area. In the attempt to move further in this direction, we present our perspective as solid-state theorists for the study of molecular materials in different states of matter. Considering exemplary systems belonging to well-known families of oligo-acenes and -thiophenes, we provide a quantitative description of electronic properties and optical excitations obtained with state-of-the-art first-principles methods such as density-functional theory and many-body perturbation theory. Simulating the systems as gas-phase molecules, clusters, and periodic lattices, we are able to identify short- and long-range effects in their electronic structure. While the latter are usually dominant in organic crystals, the former play an important role, too, especially in the case of donor/acceptor complexes. Furthermore, we demonstrate the viability of implicit schemes to evaluate band gaps of molecules embedded in isotropic and even anisotropic environments, in quantitative agreement with experiments. In the context of doped organic semiconductors, we show how the crystalline packing enhances the favorable characteristics of these systems for opto-electronic applications. The counter-intuitive behavior predicted for their electronic and optical properties is deciphered with the aid of a tight-binding model, which represents a connection to the most common approaches to evaluate transport properties in these materials.

cond-mat.mtrl-sci↗

arXiv2021

Polarization Resolved Optical Excitation of Charge-Transfer Excitons in PEN:PFP Co-Crystalline Films: Limits of Non-Periodic Modelling

Charge-transfer excitons (CTX) at organic donor/acceptor interfaces are considered important intermediates for charge separation in photovoltaic devices. Crystalline model systems provide microscopic insights into the nature of such states as they enable microscopic structure-property investigations. Here, we use angular-resolved UV/Vis absorption spectroscopy to characterize the CTX of crystalline pentacene:perfluoro-pentacene (PEN:PFP) films allowing to determine the polarization of this state. This analysis is complemented by first-principles many-body calculations, performed on the three-dimensional PEN:PFP co-crystal, which confirm that the lowest-energy excitation is a CTX. Analogous simulations performed on bimolecular clusters are unable to reproduce this state. We ascribe this failure to the lack of long-range interactions and wave-function periodicity in these cluster calculations, which appear to remain a valid tool for modelling properties of organic materials ruled by local intermolecular couplings.

cond-mat.mtrl-sci↗

arXiv2021

Long-Range Order Promotes Charge-Transfer Excitations in Donor/Acceptor Co-Crystals

Electronic and optical properties of doped organic semiconductors are dominated by local interactions between donor and acceptor molecules. However, when such systems are in crystalline form, long-range order competes against short-range couplings. In a first-principles study on three experimentally resolved bulk structures of quaterthiophene doped by (fluorinated) tetracyanoquinodimethane, we demonstrate the crucial role of long-range interactions in donor/acceptor co-crystals. The band structures of the investigated materials exhibit direct band-gaps decreasing in size with increasing amount of F atoms in the acceptors. The valence-band maximum and conduction-band minimum are found at the Brillouin zone boundary and the corresponding wave-functions are segregated on donor and acceptor molecules, respectively. With the aid of a tight-binding model, we rationalize that the mechanisms responsible for these behaviors, which are ubiquitous in donor/acceptor co-crystals, are driven by long-range interactions. The optical response of the analyzed co-crystals is highly anisotropic. The absorption onset is dominated by an intense resonance corresponding to a charge-transfer excitation. Long-range interactions are again responsible for this behavior, which enhances the efficiency of the co-crystals for photo-induced charge separation and transport. In addition to these results, our study clarifies that cluster models, accounting only for local interactions, cannot capture the relevant impact of long-range order in donor/acceptor co-crystals.

cond-mat.mtrl-sci↗

arXiv2020

Electronic and Optical Properties of Protonated Triazine Derivatives

The peculiar electronic and optical properties of covalent organic frameworks (COFs) are largely determined by protonation, a ubiquitous phenomenon in the solution environment in which they are synthesized. The resulting effects are non-trivial and appear to be crucial for the intriguing functionalities of these materials. In the quantum-mechanical framework of time-dependent density-functional theory, we investigate from first principles the impact of protonation of triazine and amino groups in molecular building blocks of COFs in water solution. In all considered cases, we find that proton uptake leads to a gap reduction and to a reorganization of the electronic structure, driven by the presence of the proton and by the electrostatic attraction between the positively charged protonated species and the negative counterion in its vicinity. Structural distortions induced by protonation are found to play only a minor role. The interplay between band-gap renormalization and exciton binding strength determines the energy of the absorption onsets: when the former prevails on the latter, a red-shift is observed. Furthermore, the spatial and energetic rearrangement of the molecular orbitals upon protonation induces a splitting of the lowest-energy peaks and a decrease of their oscillator strength in comparison with the pristine counterparts. Our results offer quantitative and microscopic insight into the role of protonation on the electronic and optical properties of triazine derivatives as building blocks of COFs. As such, they contribute to rationalize the relationships between structure, property, and functionality of these materials.

cond-mat.mtrl-sci↗

arXiv2019

Quantifying the Plasmonic Character of Optical Excitations in a Molecular J-Aggregate

The definition of plasmon at the microscopic scale is far from being understood. Yet, it is very important to recognize plasmonic features in optical excitations, as they can inspire new applications and trigger new discoveries by analogy with the rich phenomenology of metal nanoparticle plasmons. Recently, the concepts of plasmonicity index and the generalized plasmonicity index (GPI) have been devised as computational tools to quantify the plasmonic nature of optical excitations. The question may arise whether any strong absorption band, possibly with some sort of collective character in its microscopic origin, shares the status of plasmon. Here we demonstrate that this is not always the case, by considering a well-known class of systems represented by J-aggregates molecular crystals, characterized by the intense J band of absorption. By means of first-principles simulations, based on a many-body perturbation theory formalism, we investigate the optical properties of a J-aggregate made of push-pull organic dyes. We show that the effect of aggregation is to lower the GPI associated with the J-band with respect to the isolated dye one, which corresponds to a nonplasmonic character of the electronic excitations. In order to rationalize our finding, we then propose a simplified one-dimensional theoretical model of the J-aggregate. A useful microscopic picture of what discriminates a collective molecular crystal excitation from a plasmon is eventually obtained.

cond-mat.mtrl-sci↗

arXiv2019

Interplay between Intra- and Intermolecular Charge Transfer in the Optical Excitations of J-Aggregates

In a first-principles study based on density functional theory and many-body perturbation theory, we address the interplay between intra- and intermolecular interactions in a J-aggregate formed by push-pull organic dyes by investigating its electronic and optical properties. We find that the most intense excitation dominating the spectral onset of the aggregate, i.e., the J-band, exhibits a combination of intramolecular charge transfer, coming from the push-pull character of the constituting dyes, and intermolecular charge transfer, due to the dense molecular packing. We also show the presence of a pure intermolecular charge-transfer excitation within the J-band, which is expected to play a relevant role in the emission properties of the J-aggregate. Our results shed light on the microscopic character of optical excitations of J-aggregates and offer new perspectives to further understand the nature of collective excitations in organic semiconductors.

cond-mat.mtrl-sci↗

arXiv2018

Solid-State Effects on the Optical Excitation of Push-Pull Molecular J-Aggregates by First-Principles Simulations

J-aggregates are a class of low-dimensional molecular crystals which display enhanced interaction with light. These systems show interesting optical properties as an intense and narrow red-shifted absorption peak (J-band) with respect to the spectrum of the corresponding monomer. The need to theoretically investigate optical excitations in J-aggregates is twofold: a thorough first-principles description is still missing and a renewed interest is rising recently in understanding the nature of the J-band, in particular regarding the collective mechanisms involved in its formation. In this work, we investigate the electronic and optical properties of a J-aggregate molecular crystal made of ordered arrangements of organic push-pull chromophores. By using a time dependent density functional theory approach, we assess the role of the molecular packing in the enhancement and red shift of the J-band along with the effects of confinement in the optical absorption, when moving from bulk to low-dimensional crystal structures. We simulate the optical absorption of different configurations (i.e., monomer, dimers, a polymer chain, and a monolayer sheet) extracted from the bulk crystal. By analyzing the induced charge density associated with the J-band, we conclude that it is a longitudinal excitation, delocalized along parallel linear chains and that its overall red shift results from competing coupling mechanisms, some giving red shift and others giving blue shift, which derive from both coupling between transition densities and renormalization of the single-particle energy levels.

cond-mat.mtrl-sci↗