Session G24: Focus Session: Computational Studies of Interactions between Electromagnetic Fields and Materials II

Plasmon enhanced light harvesting

Plasmon energies can be tuned across the spectrum by simply changing the geometrical shape of a nanostructure. Plasmons can efficiently capture incident light and focus it to nanometer sized hotspots which can enhance electronic and vibrational excitations in nearby structures. Another important but still relatively unexplored property of plasmons, is that they can be efficient sources of hot energetic electrons which can transfer into nearby structures and induce a variety of processes. This process is a quantum mechanical effect: the decay of plasmon quanta into electron-hole pairs. I will discuss how plasmon induced hot electrons can be used in various applications: such as to induce chemical reactions in molecules physisorbed on a nanoparticle surface; to inject electrons directly into the conduction band of a nearby substrate; to dramatically enhance the light harvesting efficiency of a photovoltaic device; and to induce local doping of a nearby graphene sheet.   

Ab Initio Simulation of Fragmentation in Polyatomic Molecules by Short Intense Laser Pulses

We study ionization and fragmentation of polyatomic molecules induced by short intense laser pulses by performing ab initio simulations within the formalism of Time Dependent Density Functional Theory. Within this formalism we investigate intra-molecular electron dynamics during a fragmentation reaction on a pre-chemistry time-scale. The time-scale of the dynamics bridges the time-domain of sub-femtoseconds, on which the electrons move, and that of the much slower motion of the heavier ions (e.g. carbon ions), which proceeds on a time-scale of tens to hundreds of femtoseconds. The kinetic energy spectrum of the fragments and the charge state of the molecule prior to fragmentation are calculated and compared to experiment.   

Laser-Induced High Harmonic Generations in Nano-Graphene Molecules

Nano graphene molecules is a promising material for the non-linear optical devices. We performed a first-principles calculation on Graphene Molecules. Two distinct signals are noticed: the integer higher-order harmonic generation (HHG) and the intrinsic emissions. Due to the small gap between HOMO and LUMO of graphene molecule, the HHG can be generated for the infrared laser pulse with the photon energy ranging from 20 meV to 1 eV. The intrinsic emission corresponds to the electron excitation between eigen states. They can be generated using a relatively low intensity laser pulse (0.05 eV/\AA) through the multiphoton process. Moreover, these signals are very sensitive to the molecule size and the hydrogen passivation. They can be the fingerprints for detecting the product in fabrication.   

Surface Integral Formulations for the Design of Plasmonic Nanostructures

Numerical formulations based on surface integral equations (SIEs) provide an accurate and efficient framework for the solution of the electromagnetic scattering problem by three-dimensional plasmonic nanostructures in the frequency domain. In this work, we present a unified description of SIE formulations with both singular and nonsingular kernel and we study their accuracy in solving the scattering problem by metallic nanoparticles with spherical and nonspherical shape. In fact, the accuracy of the numerical solution, especially in the near zone, is of great importance in the analysis and design of plasmonic nanostructures, whose operation critically depends on the manipulation of electromagnetic hot spots. Four formulation types are considered: the N-combined region integral equations, the T-combined region integral equations, the combined field integral equations and the null field integral equations. A detailed comparison between their numerical solutions obtained for several nanoparticle shapes is performed by examining convergence rate and accuracy in both the far and near zone of the scatterer as a function of the number of degrees of freedom. A rigorous analysis of SIE formulations can have a high impact on the engineering of numerous nano-scale optical devices.   

Dynamics of irradiation: from molecules to nano-objects and from material science to biology

We discuss microscopic mechanisms of irradiation in clusters and molecules considering the case of isolated molecules/clusters [1] and/or in an environment [2]. We use Time Dependent Density Functional Theory (for electrons) coupled to Molecular Dynamics (for ions) and follow explicitly in time irradiation and response of the system. Examples are taken from free metal clusters, fullerenes, molecules of biological interest and clusters deposited on a surface or embedded in a matrix [3,4]. We analyse in particular properties of emitted electrons (photo electron spectra, angular distributions...) which provide a key tool of analysis of properties of irradiated clusters and molecules [5]. We also discuss pump and probe scenarios (opening the road to manipulation at the molecular scale) with help of dedicated laser pulses, exploring in particular very short times scales down towards the attosecond domain. \\[4pt] [1] F. Calvayrac et al, Phys. Reports 337(2000)493\\[0pt] [2] P. M. Dinh et al, Phys. Reports 485 (2009) 43\\[0pt] [3] Z.P. Wang et al, Int. J. Mass Spect. 285 (2009) 1430\\[0pt] [4] U. F. NdongmuoTaffoti et al, Eur. Phys. J. D 58 (2010) 131\\[0pt] [5] Th. Fennel et al, Rev. Mod. Phys. 82 (2010) 1   

Time-dependent transition density matrix for visualizing charge-transfer excitations in photoexcited organic donor-acceptor systems

The time-dependent transition density matrix (TDM) is a useful tool to visualize and interpret the induced charges and electron-hole coherences of excitonic processes in large molecules. Combined with time-dependent density functional theory on a real-space grid (as implemented in the octopus code), the TDM is a computationally viable visualization tool for optical excitation processes in molecules. It provides real-time maps of particles and holes which gives information on excitations, in particular those that have charge-transfer character, that cannot be obtained from the density alone. Some illustration of the TDM and comparison with standard density difference plots will be shown for photoexcited organic donor-acceptor molecules.   

Density functional studies of plasmons, hybridizations and electron diffractions in carbon fullerene nanomaterials

Quantized plasma waves in carbon valence electron clouds driven by photon or charged particle fields create plasmon resonances in the ionization of fullerene nanomaterials [1]. If the materials have composite structures, like nested fullerenes (buckyonions) or fullerenes endohedrally doped by an atom (endofullerenes), then plasmonic motions dynamically hybridize, leading to spectacular effects in the emission spectra [2,3]. Further, for fast ejected electrons, diffraction type modulations in the momentum space of emission intensities enrich the ionization process which offer an unusual spectroscopic route to image the charge cloud geometry [4,5]. Using a time-dependent local density functional methodology, but smearing the ionic core into a jellium, we recently completed some studies of such processes for fullerene nanomaterials. Results have shown good agreements with measurements. [1] Madjet et al., J. Phys. B 41, 105101 (2008); [2] McCune et al., J. Phys. B Fast Track Comm. 44, 241002 (2011); [3] Madjet et al., Phys. Rev. Lett. 99, 243003 (2007); [4] Patel et al., J. Phys. B Fast Track Comm. 44, 191001 (2011); [5] Ruedel et al., Phys. Rev. Lett. 89, 125503 (2002).   

Non-Linear Optical Response Simulations for Strongly Corellated Hybrid Carbon Nanotube Systems

Hybrid carbon nanotube systems, nanotubes containing extrinsic atomic type species (dopants) such as semiconductor quantum dots, extrinsic atoms, or ions, are promising candidates for the development of the new generation of tunable nanooptoelectronic devices -- both application oriented, e.g., photovoltaic devices of improved light-harvesting efficiency, and devices for use in fundamental research. Here, we simulate non-linear optical response signals for a pair of spatially separated two-level dipole emitters (to model the dopants above) in the regime where they are coupled strongly to a low-energy surface plasmon resonance of a metallic carbon nanotube. Such a coupling makes them entangled [1], and we show that the cross-peaks in 2D photon-echo spectra are indicative of the bipartite entanglement being present in the system [2]. We simulate various experimental conditions and formulate practical recommendations for the reliable experimental observation of this unique quantum phenomenon of relevance to the solid-state quantum information science.\\[4pt] [1] I.V. Bondarev, J. Comp. Theor. Nanosci. 7, 1673 (2010).\\[0pt] [2] M.F. Gelin, I.V. Bondarev, A.V. Meliksetyan, Chem. Phys., at print.   

Dynamical density matrix renormalization group study of non-linear optical response of one-dimensional strongly correlated electron system

We studied the third-order non-linear optical response of one-dimensional Mott insulators by using the dynamical density matrix renormalization group method. We employed an one-dimensional extended Hubbard model which corresponds to the one-dimensional Mott insulators. Also, we introduced a Holstein-type electron-phonon interaction which is important for understanding the optical response in the one-dimensional Mott insulators. We calculated the non-linear optical response using the parameters corresponding to Sr$_2$CuO$_3$ which is known as a kind of the one-dimensional Mott insulators. Our calculated results show a relatively large effect of the electron-phonon interaction on the calculated third-order non-linear optical response.   

Real-time TDDFT calculations of core-hole spectral functions

Core-hole response is important in a variety of x-ray spectra, including x-ray absorption, resonant and non-resonant inelastic x-ray scattering, and x-ray photo-electron spectroscopy, but has usually been treated within the adiabatic approximation. Here we explore the dynamic response of valence electrons to the sudden appearance of a deep core-hole using real time time dependent density functional theory (RT-TDDFT). The core-hole is treated as a transient time dependent potential which excites the valence electrons, as in the edge-singularity theory of Nozieres and De Dominicis. RT-TDDFT provides an efficient approach for treating response to time-dependent external fields including interactions among the valence electrons, which has recently been applied to calculations of optical and x-ray spectra.\footnote{A. J. Lee, F. D. Vila, and J. J. Rehr, PRB {\bf 86} 115107} Here we generalize this approach to explore the role of the strength and localization of the core-hole potential and its effects on the spectral function and various x-ray spectra, together with comparisons to the adiabatic approximation.   

Quantal Density Functional Theory (QDFT) in the Presence of an Electromagnetic Field

We derive the QDFT equations of electrons in an external time-dependent field $\cal{E} ({\bf{r}} t) = -$ {\boldmath $\nabla$} $ v ({\bf{r}} t)$ and in the presence of an electromagnetic field characterized by the magnetic induction ${\bf{B}} ({\bf{r}} t) =$ {\boldmath $\nabla$} $\times {\bf{A}} ({\bf{r}} t)$ and electric field ${\bf{E}} ({\bf{r}}t) = -$ {\boldmath $\nabla$} $\Phi ({\bf{r}}t) - (1/c) \partial {\bf{A}} ({\bf{r}}t)/\partial t$. The QDFT is comprised of the mapping from this system to one of noninteracting fermions with the same density $\rho ({\bf{r}}t)$ and physical current density ${\bf{j}} ({\bf{r}}t)$. The mapping is in terms of `classical' fields representative of the different electron correlations that must be accounted for. On deriving the `quantal Newtonian' second law for the interacting and model systems, we obtain the local electron-interaction potential $v_{ee} ({\bf{r}} t)$ of the latter to be the work done in a conservative effective field $\cal{F}^{\mathrm{eff}} ({\bf{r}} t)$. The components of $\cal{F}^{\mathrm{eff}} ({\bf{r}} t)$ are representative of correlations due to the Pauli exclusion principle and Coulomb repulsion and the Correlation-(Kinetic, Current Density, Electric, and Magnetic) effects.   

Basic Variables in the Presence of a Magnetostatic Field

We present our recent understanding of the issue of what properties constitute the basic variables in quantum mechanics for electrons in the presence of external electrostatic ${\cal{E}} ({\bf{r}}) = -$ {\boldmath $\nabla$} $v ({\bf{r}})$ and magnetostatic ${\bf{B}} ({\bf{r}}) =$ {\boldmath $\nabla$} $\times {\bf{A}} ({\bf{r}})$ fields. In this case, the relationship between the potentials $\{v, {\bf{A}} \}$ and the ground state wave function $\Psi$ can be many-to-one. We discuss our prior work\footnote{Pan and Sahni, Int. J. Quantum Chem. 110, 2833 (2010); J. Phys. Chem. Solids. 73, 630 (2012).} in which we claimed that the basic variables are the ground state density $\rho ({\bf{r}})$ and physical current density ${\bf{j}} ({\bf{r}})$. We prove here more fully this to be the case for the nondegenerate ground state for which $\Psi$ is real. The proof explicitly accounts for the many-to-one relationship between $\{v, {\bf{A}} \}$ and $\Psi$. We also draw parallels between our work on the density and physical current density functional theory and those of the Hohenberg-Kohn and Percus-Levy-Lieb definitions of density functional theory.   

The magnetization of periodic solids from time-dependent current-density-functional theory

The evaluation of the macroscopic magnetization of solids is problematic when periodic boundary conditions are used because surface effects are artificially removed. This poses a problem unless surface effects can be reformulated in terms of bulk quantities. For example, in case of the macroscopic polarization one can express the contribution of the charge density accumulated at the surface in terms of the bulk current density through the continuity equation. Therefore one can work in the framework of time-dependent current-density functional theory to efficiently calculate the macroscopic polarization [1,2]. In this presentation we will show how also the magnetization can be described within this framework. \\[4pt] [1] F. Kootstra, P. L. de Boeij, and J. G. Snijders, J. Chem. Phys. 112, 6517 (2000).\\[0pt] [2] J. A. Berger, P. Romaniello, R. van Leeuwen, and P. L. de Boeij , Phys. Rev. B 74, 245117 (2006)