9 research outputs found
Saturable Absorption of Free-Electron Laser Radiation by Graphite near the Carbon K-Edge
The interaction of intense light with matter gives rise to competing nonlinear responses that can dynamically change material properties. Prominent examples are saturable absorption (SA) and two-photon absorption (TPA), which dynamically increase and decrease the transmission of a sample depending on pulse intensity, respectively. The availability of intense soft X-ray pulses from free-electron lasers (FELs) has led to observations of SA and TPA in separate experiments, leaving open questions about the possible interplay between and relative strength of the two phenomena. Here, we systematically study both phenomena in one experiment by exposing graphite films to soft X-ray FEL pulses of varying intensity. By applying real-time electronic structure calculations, we find that for lower intensities the nonlinear contribution to the absorption is dominated by SA attributed to ground-state depletion; our model suggests that TPA becomes more dominant for larger intensities (\u3e1014 W/cm2). Our results demonstrate an approach of general utility for interpreting FEL spectroscopies
Real-time exciton dynamics with time-dependent density-functional theory
Linear-response time-dependent density-functional theory (TDDFT) can describe
excitonic features in the optical spectra of insulators and semiconductors,
using exchange-correlation (xc) kernels behaving as to leading
order. We show how excitons can be modeled in real-time TDDFT, using an xc
vector potential constructed from approximate, long-range corrected xc kernels.
We demonstrate for various materials that this real-time approach is consistent
with frequency-dependent linear response, gives access to femtosecond exciton
dynamics following short-pulse excitations, and can be extended with some
caution into the nonlinear regime.Comment: 7 pages, 4 figure
Theory of Moment Propagation for Quantum Dynamics in Single-Particle Description
We present a novel theoretical formulation for performing quantum dynamics in
terms of moments within the single-particle description. By expressing the
quantum dynamics in terms of increasing orders of moments, instead of
single-particle wave functions as generally done in time-dependent density
functional theory, we describe an approach for reducing the high computational
cost of simulating the quantum dynamics. The equation of motion is given for
the moments by deriving analytical expressions for the first-order and
second-order time derivatives of the moments, and a numerical scheme is
developed for performing quantum dynamics by expanding the moments in the
Taylor series as done in classical molecular dynamics simulation. We propose a
few numerical approaches using this theoretical formalism on a simple
one-dimensional model system, for which an analytically exact solution can be
derived. Application of the approaches to an anharmonic system is also
discussed to illustrate their generality. We also discuss the use of an
artificial neural network model to circumvent the numerical evaluation of the
second-order time derivatives of the moments, as analogously done in the
context of classical molecular dynamics simulations
Velocity-gauge real-time TDDFT within a numerical atomic orbital basis set
The interaction of laser fields with solid-state systems can be modeled efficiently within the velocity-gauge formalism of real-time time dependent density functional theory (RT-TDDFT). In this article, we discuss the implementation of the velocity-gauge RT-TDDFT equations for electron dynamics within a linear combination of atomic orbitals (LCAO) basis set framework. Numerical results obtained from our LCAO implementation, for the electronic response of periodic systems to both weak and intense laser fields, are compared to those obtained from established real-space grid and Full-Potential Linearized Augmented Planewave approaches. Potential applications of the LCAO based scheme in the context of extreme ultra-violet and soft X-ray spectroscopies involving core-electronic excitations are discussed