arXiv subjects

R. E. F. Silva

Publications and source records attributed to R. E. F. Silva.

12 recordsLinked to original sources

arXiv2025

Giant enhancement of attosecond tunnel ionization competes with disorder-driven decoherence in silicon

High-harmonic generation (HHG) is a strong-field phenomenon that is sensitive to the attosecond dynamics of tunnel ionization and coherent transport of electron-hole pairs in solids. While the foundations of solid HHG have been established, a deep understanding into the nature of decoherence on sub-cycle timescales remains elusive. Furthermore, there is a growing need for tools to control ionization at the nanoscale. Here, we study HHG in silicon along a crystalline-to-amorphous (c-Si to a-Si) structural phase transition and observe a dramatic reshaping of the spectrum, with enhanced lower-order harmonic yield accompanied by quenching of the higher-order harmonics. Modelling the real-space quantum dynamics links our observations to a giant enhancement (>250 times) of tunnel ionization yield in the amorphous phase and a disorder-induced decoherence that damps the electron-hole polarization over approximately six lattice sites. HHG spectroscopy also reveals remnant order that was not apparent with conventional probes. Finally, we observe a rapid and targeted non-resonant laser annealing of amorphous silicon islands. Our results offer a unique insight into attosecond decoherence in strong-field phenomena, establish HHG spectroscopy as a probe of structural disorder, and pave the way for new opportunities in lightwave nanoelectronics.

cond-mat.mtrl-sci↗

arXiv2023

Orbital perspective on high-harmonic generation from solids

High-harmonic generation in solids allows probing and controlling electron dynamics in crystals on few femtosecond timescales, paving the way to lightwave electronics. In the spatial domain, recent advances in the real-space interpretation of high-harmonic emission in solids allows imaging the field-free, static, potential of the valence electrons with picometer resolution. The combination of such extreme spatial and temporal resolutions to measure and control strong-field dynamics in solids at the atomic scale is poised to unlock a new frontier of lightwave electronics. Here, we report a strong intensity-dependent anisotropy in the high-harmonic generation from ReS$_2$ that we attribute to angle-dependent interference of currents from the different atoms in the unit cell. Furthermore, we demonstrate how the laser parameters control the relative contribution of these atoms to the high-harmonic emission. Our findings provide an unprecedented atomic perspective on strong-field dynamics in crystals and suggest that crystals with a large number of atoms in the unit cell are not necessarily more efficient harmonic emitters than those with fewer atoms.

physics.optics↗

arXiv2023

Reconstruction of attosecond beating by interference of two-photon interband transitions in solids

The reconstruction of attosecond beating by interference of two-photon transitions (RABBIT) is one of the most widely used techniques for obtaining both the relative phases of harmonics forming an attosecond pulse train and the phase of atomic radiative transitions. If the latter is computed by theory, it allows to reconstruct the attosecond pulse train; if the former is known experimentally, it allows reconstruction of the electronic dynamics of photoionization in atomic and molecular systems with attosecond temporal resolution. As it relies on the interference of photo-electrons in vacuum, similar interference has never been contemplated inside crystals. Here we explore the applicability of this scheme to solid-state systems using a one-dimensional model and a DFT-calculated structure of 2D hexagonal boron nitride. We discuss the possibility of: (i) reconstructing the relative phases between harmonics with trivial influence of the 'atomic phase', (ii) retrieving the relative phases of two-photon transitions through different bands, which are generally challenging to obtain both experimentally and numerically. These phases are recorded in the beating of the population signal arising from interfering two-photon pathways, and can be read-out with angle-resolved photo-emission spectroscopy. Furthermore, the amplitude of the population beating decays as the pump and probe pulses are separated in time due to electron-hole decoherence, providing a simple interferometric method to extract dephasing times.

physics.optics↗

arXiv2020

Plasmonic Purcell Effect in Organic Molecules

By means of quantum tensor network calculations, we investigate the large Purcell effect experienced by an organic molecule placed in the vicinity of a plasmonic nanostructure. In particular, we consider a donor-π bridge-acceptor dye at the gap of two Ag nanospheres. Our theoretical approach allows for a realistic description of the continua of both molecular vibrations and optical nanocavity modes. We analyze both the exciton dynamics and the corresponding emission spectrum, showing that these magnitudes are not accurately represented by the simplified models used up to date. By disentangling the molecule coupling to radiative and non-radiative plasmonic modes, we also shed light into the quenching phenomenology taking place in the system.

cond-mat.mes-hall↗

arXiv2020

Lightwave topology for strong-field valleytronics

Modern light generation technology offers extraordinary capabilities for sculpting light pulses, with full control over individual electric field oscillations within each laser cycle. These capabilities are at the core of lightwave electronics - the dream of ultrafast lightwave control over electron dynamics in solids, on a few-cycle to sub-cycle timescale, aiming at information processing at tera-Hertz to peta-Hertz rates. Here we show a robust and general approach to valley-selective electron excitations in two-dimensional solids, by controlling the sub-cycle structure of non-resonant driving fields at a few-femtosecond timescale. Bringing the frequency-domain concept of topological Floquet systems to the few-fsec time domain, we develop a transparent control mechanism in real space and an all-optical, non-element-specific method to coherently write, manipulate and read selective valley excitations using fields carried in a wide range of frequencies, on timescales that can be much shorter than the valley lifetime, crucial for implementation of valleytronic devices.

physics.optics↗

arXiv2020

Cumulant expansion for the treatment of light-matter interactions in arbitrary material structures

Strong coupling of quantum emitters with confined electromagnetic modes of nanophotonic structures may be used to change optical, chemical and transport properties of materials, with significant theoretical effort invested towards a better understanding of this phenomenon. However, a full theoretical description of both matter and light is an extremely challenging task. Typical theoretical approaches simplify the description of the photonic environment by describing it as a single or few modes. While this approximation is accurate in some cases, it breaks down strongly in complex environments, such as within plasmonic nanocavities, and the electromagnetic environment must be fully taken into account. This requires the quantum description of a continuum of bosonic modes, a problem that is computationally hard. We here investigate a compromise where the quantum character of light is taken into account at modest computational cost. To do so, we focus on a quantum emitter that interacts with an arbitrary photonic spectral density and employ the cumulant or cluster expansion method to the Heisenberg equations of motion up to first, second and third order. We benchmark the method by comparing with exact solutions for specific situations and show that it can accurately represent dynamics for many parameter ranges.

quant-ph↗

arXiv2020

Polaritonic Molecular Clock: All-Optical Ultrafast Imaging of Wavepacket Dynamics without Probe Pulses

Conventional approaches to probing ultrafast molecular dynamics rely on the use of synchronized laser pulses with a well-defined time delay. Typically, a pump pulse excites a wavepacket in the molecule. A subsequent probe pulse can then dissociates or ionizes the molecule, and measurement of the molecular fragments provides information about where the wavepacket was for each time delay. In this work, we propose to exploit the ultrafast nuclear-position-dependent emission obtained due to large light-matter coupling in plasmonic nanocavities to image wavepacket dynamics using only a single pump pulse. We show that the time-resolved emission from the cavity provides information about when the wavepacket passes a given region in nuclear configuration space. This approach can image both cavity-modified dynamics on polaritonic (hybrid light-matter) potentials in the strong light-matter coupling regime as well as bare-molecule dynamics in the intermediate coupling regime of large Purcell enhancements, and provides a new route towards ultrafast molecular spectroscopy with plasmonic nanocavities.

cond-mat.mes-hall↗

arXiv2019

High harmonic generation in crystals using Maximally Localized Wannier functions

In this work, the nonlinear optical response, and in particular, the high harmonic generation of semiconductors is addressed by using the Wannier gauge. One of the main problems in the time evolution of the Semiconductor Bloch equations resides in the fact that the dipole couplings between different bands can diverge and have a random phase along the reciprocal space and this leads to numerical instability. To address this problem, we propose the use of the Maximally Localized Wannier functions that provide a framework to map ab-initio calculations to an effective tight-binding Hamiltonian with great accuracy. We show that working in the Wannier gauge, the basis set in which the Bloch functions are constructed directly from the Wannier functions, the dipole couplings become smooth along the reciprocal space thus avoiding the problem of random phases. High harmonic generation spectrum is computed for a 2D monolayer of hBN as a numerical demonstration.

cond-mat.mes-hall↗

arXiv2018

Topological strong field physics on sub-laser cycle time scale

Sub-laser cycle time scale of electronic response to strong laser fields enables attosecond dynamical imaging in atoms, molecules and solids. Optical tunneling and high harmonic generation are the hallmarks of attosecond imaging in optical domain, including imaging of phase transitions in solids. Topological phase transition yields a state of matter intimately linked with electron dynamics, as manifested via the chiral edge currents in topological insulators. Does topological state of matter leave its mark on optical tunneling and sub-cycle electronic response? We identify distinct topological effects on the directionality and the attosecond timing of currents arising during electron injection into conduction bands. We show that electrons tunnel across the band gap differently in trivial and topological phases, for the same band structure, and identify the key role of the Berry curvature in this process. These effects map onto topologically-dependent attosecond delays in high harmonic emission and the helicities of the emitted harmonics, which can record the phase diagram of the system and its topological invariants. Thus, the topological state of the system controls its attosecond, highly non-equilibrium electronic response to strong low-frequency laser fields, in bulk. Our findings create new roadmaps in studies of topological systems, building on ubiquitous properties of sub-laser cycle strong field response - a unique mark of attosecond science.

physics.optics↗

arXiv2017

Study of diatomic molecules under short intense laser pulses

In this work we report ab initio calculations for the $H_2^+$ molecule interacting with ultrashort intense laser pulses. We analyse several observables that can, in principle, be available experimentally, in order to get a deeper understanding of the strong field molecular dynamics. In particular, we will focus our attention to the interplay between electronic and nuclear dynamics and how the two motions are correlated. We have extended the Resolvent Technique to molecules and we have extracted the correlated energy spectra in different regimes of ionization: from the perturbative regime to the tunneling ionization regime. We have applied this new method, called resolvent operator method (ROM), to a 1+1D model of the $H_2^+$ molecule. We have applied this method to several photoionization regimes and we have compared with previous results in the literature. We show that the correlated spectra can provide us much more information than the integrated spectra on the electron or nuclear energy. We report the correlated energy spectrum from the multiphoton ionization to the tunneling ionization regime and how the sharing of the excess energy is different for the two regimes. We have also applied the ROM to a 3D model of the $H_2^+$ molecule in which angular distributions can be obtained. With these angular correlated spectra we show that the contributions of electrons coming from absorption of different number of photons can be disentangled. We have calculated the high harmonic spectra for a 3D model of the $H_2^+$ molecule and its isotopes. In these calculations we have observed that for sufficiently long pulses and for light molecules we observe even harmonic generation, a phenomenon that is not expected in a centrosymmetric molecule.

physics.atom-ph↗

arXiv2017

High harmonic imaging of ultrafast many-body dynamics in strongly correlated systems

This Letter brings together two topics that, until now, have been the focus of intense but non-overlapping research efforts. The first concerns high harmonic generation in solids, which occurs when intense light field excites highly non-equilibrium electronic response in a semiconductor or a dielectric. The second concerns many-body dynamics in strongly correlated systems such as the Mott insulator. Here we show that high harmonic generation can be used to time-resolve ultrafast many-body dynamics associated with optically driven phase transition, with accuracy far exceeding one cycle of the driving light field. Our work paves the way for time-resolving highly non-equilibrium many body dynamics in strongly correlated systems, with few femtosecond accuracy.

cond-mat.str-el↗

arXiv2016

Even harmonic generation in isotropic media of dissociating homonuclear molecules

Isotropic gases irradiated by long pulses of intense IR light can generate very high harmonics of the incident field. It is generally accepted that, due to the symmetry of the generating medium, be it an atomic or an isotropic molecular gas, only odd harmonics of the driving field can be produced. Here we show how the interplay of electronic and nuclear dynamics can lead to a marked breakdown of this standard picture: a substantial part of the harmonic spectrum can consist of even rather than odd harmonics. We demonstrate the effect using ab-initio solutions of the time-dependent Schrödinger equation for $H$$_2$$^+$ and its isotopes in full dimensionality. By means of a simple analytical model, we identify its physical origin, which is the appearance of a permanent dipole moment in dissociating homonuclear molecules, caused by light-induced localization of the electric charge during dissociation. The effect arises for sufficiently long laser pulses and the region of the spectrum where even harmonics are produced is controlled by pulse duration. Our results (i) show how the interplay of femtosecond nuclear and attosecond electronic dynamics, which affects the charge flow inside the dissociating molecule, is reflected in the nonlinear response, and (ii) force one to augment standard selection rules found in nonlinear optics textbooks by considering light-induced modifications of the medium during the generation process.

physics.atom-ph↗