After introducing nonlinear optics and a few nonlinear-optics-based spectroscopic techniques, I will argue why it is important to have theory and simulations to interpret, support and guide these experiments. I will then discuss how to predict nonlinear optical response of materials from simulations and conclude with the challenges posed by these simulations.
In recent years, machine learning (ML) approaches to circumvent quantum-mechanical calculations have become increasingly popular, allowing atomistic simulations with the accuracy of electronic structure theory to be run in a fraction of the time needed for ab inition molecular dynamics (AIMD).
I will describe the background and basics of these approaches, explaining the theory behind ML...
I discuss the evolution of computer facilities, the present status of available HPC machines and how scientific codes needs to be adapted to run on these machines. I'll show the example of the Yambo code, and show how the code evolved in recent years to be able to exploit MPI, OpenMP and more recently GPUs.
I will give an overview of the possible applications of point defects in quantum technologies, showing what are the quantity of interest that can be calculated and predicted, and I will discuss the main approaches that are currently exploited to achieve them.
In this talk I present an ab inito approach to calculate spin waves (magnons) based in the Bethe-Salpeter Equation.
The irradiation of a finite electronic system by an ultrafast XUV pulse can lead to the ultrafast creation of core-holes with strong subsequent electronic rearrangements. A standard theoretical tool to investigate such excitations that avoids the cost of the simulation of the XUV irradiation is to start the dynamics with an instantaneous core-hole excitation. In a recent series of papers...
We developed an algorithm to study ultrafast electronic relaxation and decoherence process in solid-state systems using real-time,real-space time-dependent density functional theory (RT-TDDFT) that is currently being implemented in the Octopus software package. The methodology will employ a natural orbital representation to monitor the time evolution of the electronic density matrix, enabling...
This project explores the process of light-induced structural symmetry breaking in polar semiconductors and insulators. When photoinduced electrons and holes interact with the lattice through electron-phonon interaction (EPI), they can lead to the formation of symmetry breaking structural distortions. The goal of this project is to investigate the ultrafast dynamics of electrons and phonons in...
1st TIMES Workshop joint with ETSF Real-Time collaboration-team meeting
Torsten Geirsson
Supervisor: Alejandro Molina-Sánchez
The CrSBr van der Waals layered material is a magnetic semiconductor which has gained attention due to its unique magnetic and optical properties. It is air-stable and allows for manipulation of its magnetic order with a small external field, positioning it as a...
The interaction of a finite electronic system, such as a molecule or cluster, with an intense ultrafast laser pulse induces complex electronic and ionic dynamics. A key theoretical approach to studying these dynamics is to initiate the system with an instantaneous core-hole excitation, bypassing the need for direct simulation of the laser interaction. Recently, an unexpected phenomenon has...
The present work focuses on coupling semi-classical Boltzmann dynamics with the non-equilibrium Bethe-Salpeter equation in order to describe bound state absorption/emission and intraband relaxation processes occurring in a WS2 monolayer.
The appeal stems from the direct access to the electronic/hole populations and the real-time evolution of the non-equilibrium thermal density matrix...
The prediction of polarization and optical responses are crucial for anticipating charge transfer properties and identifying functional materials involved in defect engineering and single-photon emitters.
To do so, Nickel Iodide magnetic cell is simplified to reproduce the antiferromagnetic behavior raised from the helicoidal incommensurate magnetic periodicity within the layers that...
Density Functional Theory (DFT) is a widely-used computational approach for investigating electronic properties of materials, relying heavily on the choice of appropriate exchange-correlation functionals. Among these functionals, the generalized gradient approximation (GGA) is widely used due to its effectiveness in capturing electron exchange-correlation effects beyond local approximations....
We investigate the electronic and nuclear dynamics of molecules upon photoexcitation in real time. To that end, single- and multi-trajectory Ehrenfest dynamics is employed. Different sampling techniques for the initial nuclear conditions, such as harmonic sampling or sampling with a quantum thermostat, are compared.
In recent years, two-dimensional (2D) materials have gained significant attention as promising platforms for next-generation quantum technologies[1] offering a wide range of electronic properties, optical and mechanical properties. Specifically, engineering 2D materials provides insights into the mechanisms underlying properties such as magnetism, which is of great interest for the development...
Monolayers of Transition Metal Dichalcogenides (TMDs) have become a central focus in condensed matter physics research due to their exceptional optoelectronic properties, which arise from strong light-matter interactions. Quantum confinement and reduced Coulomb screening make TMDs an ideal system for studying excitons (i.e. ground state and excited state excitons) and their many-body complexes...
In this lecture I will present a general introduction to the exciton-phonon coupling problem with some application to phonon-assisted absorption/emission.
Exciton dynamics, encompassing ultrafast photogeneration, diffusion, and thermalization, plays a crucial role in optoelectronic, photovoltaic, and photocatalytic processes.
In this talk we discuss a novel many-body approach to describe exciton dynamics from first-principles. We show that the introduction of an auxiliary exciton species, termed “irreducible exciton”, is crucial to formulate a...
Two-dimensional (2D) materials have the potential to revolutionize the quantum technology industry by enabling the development of quantum information devices [1]. Defect centers in wide-bandgap semiconductors, such as transition metal dichalcogenides (TMDs), have been proposed as quantum bits (qubits) [2] and have been shown to behave as single photon emitters (SPE) [3]. Advances in this field...
Time-resolved angle-resolved photoemission spectroscopy (tr-ARPES) provides direct access to the non-equilibrium electronic structure of solids, combining sub-femtosecond temporal resolution with momentum-resolved spectral information. On the theoretical side, time-dependent density functional theory (TDDFT) has become a key ab-initio approach for modeling ultrafast processes in real...
Using an ultrashort pulsed extreme ultraviolet laser as the photon source, angle-resolved photoemission spectroscopy (ARPES) has expanded the study of electronic structure beyond the 3D momentum space and into the temporal domain with femtosecond resolution. This advancement in studying electron dynamics greatly enhances the characterization and identification of exotic phase transitions in...
Magnetic skyrmions are vortex-like spin textures of potential interest for racetrack memories, nanoengines, but also quantum and neuromorphic computing. We consider magnetic skyrmions from localised-spin textures coupled to itinerant electrons. The electron dynamics is described either via Green's functions (NEGF-GKBA) or exact diagonalization, and the localised spins classically or quantum...
The Auger process is well known as a spectroscopic fingerprint for elemental identification, but it also provides a unique platform for studying electron entanglement. To gain theoretical insight into this latter aspect, we address entanglement formation in a model molecule following electronic transitions induced by a light-pulse, and its subsequent disappearance due to decoherence effects...
Optical femtosecond transient absorption and reflectivity spectroscopies are the most common experimental techniques used to study the ultrafast dynamics of photoexcited carriers and excitons in materials. However, unfortunately, the interpretation of the data is far from straightforward and often requires support from theoretical studies to be able give a reliable interpretation of the raw...
In this talk, I will present a theoretical study on the Ehrenfest time-dependent Hartree-Screened Exchange (TD-HSEX) equations of motion, focusing on the role of core electrons and electron-phonon (e-ph) interactions in near-equilibrium regimes. TD-HSEX, derived from non-equilibrium Green’s functions (NEGF) theory[1], accurately captures coherent excitonic dynamics beyond the limitations of...
Coherent phonons offer a powerful means to manipulate structural and electronic properties of materials on ultrafast timescales, enabling control of light-induced phase transitions and non-equilibrium dynamical phenomena.
We develop an $\textit{ab initio}$ framework to describe the excitation of coherent phonons in semimetals by combining electron-phonon coupling calculations and the...
Approaching the attosecond time scale is crucial for advancing petahertz opto-electronics, that is the capability to coherently control and manipulate the optical properties of solid-state materials when excited by PHz fields, offering processing speed much beyond current limits.
With this respect, Attosecond Transient Reflection Spectroscopy (ATRS) in solids has emerged as a reliable tool...
Sum frequency generation (SFG) and difference frequency generation (DFG) are second order nonlinear processes where two lasers with frequencies ω_1 and ω_2 combine to produce a response at frequency ω = ω_1 ± ω_2 . Compared with other nonlinear responses, such as second harmonic generation, SFG and DFG allow for tunability over a larger range and by selecting the two laser frequencies, one can...
Two-dimensional (2D) materials have the potential to revolutionize the quantum technology industry by enabling the development of quantum information devices [1]. Defect centers in wide-bandgap semiconductors, such as transition metal dichalcogenides (TMDs), have been proposed as quantum bits (qubits) [2] and have been shown to behave as single photon emitters (SPE) [3]. Advances in this field...
