Coupled forward-backward trajectory approach for non-equilibrium electron-ion dynamics

We introduce a simple ansatz for the wavefunction of a many-body system based on coupled forward and backward-propagating semiclassical trajectories. This method is primarily aimed at, but not limited to, treating nonequilibrium dynamics in electron-phonon systems. The time-evolution of the system is obtained from the Euler-Lagrange variational principle, and we show that this ansatz yields Ehrenfest mean field theory in the limit that the forward and backward trajectories are orthogonal, and in the limit that they coalesce. We investigate accuracy and performance of this method by simulating electronic relaxation in the spin-boson model and the Holstein model. Although this method involves only pairs of semiclassical trajectories, it shows a substantial improvement over mean field theory, capturing quantum coherence of nuclear dynamics as well as electron-nuclear correlations. This improvement is particularly evident in nonadiabatic systems, where the accuracy of this coupled trajectory method extends well beyond the perturbative electron-phonon coupling regime. This approach thus provides an attractive route forward to the ab-initio description of relaxation processes, such as thermalization, in condensed phase systems

Nonlinear polarization evolution using time-dependent density functional theory

We propose a theoretical and computational approach to investigate temporal behavior of a nonlinear polarization in perturbative regime induced by an intense and ultrashort pulsed electric field. First-principles time-dependent density functional theory is employed to describe the electron dynamics. Temporal evolution of third-order nonlinear polarization is extracted from a few calculations of electron dynamics induced by pulsed electric fields with the same time profile but different amplitudes. We discuss characteristic features of the nonlinear polarization evolution as well as an extraction of nonlinear susceptibilities and time delays by fitting the polarization. We also carry out a decomposition of temporal and spatial changes of the electron density in power series with respect to the field amplitude. It helps to get insight into the origin of the nonlinear polarization in atomic scale.Comment: 11 pages, 9 figure

Ab Initio Simulation of Electrical Currents Induced by Ultrafast Laser Excitation of Dielectric Materials

We theoretically investigate the generation of ultrafast currents in insulators induced by strong few-cycle laser pulses. Ab initio simulations based on time-dependent density functional theory give insight into the atomic-scale properties of the induced current signifying a femtosecond-scale insulator-metal transition. We observe the transition from nonlinear polarization currents during the laser pulse at low intensities to tunnelinglike excitation into the conduction band at higher laser intensities. At high intensities, the current persists after the conclusion of the laser pulse considered to be the precursor of the dielectric breakdown on the femtosecond scale. We show that the transferred charge sensitively depends on the orientation of the polarization axis relative to the crystal axis suggesting that the induced charge separation reflects the anisotropic electronic structure. We find good agreement with very recent experimental data on the intensity and carrierenvelope phase dependence [A. Schiffrin et al., Nature (London) 493, 70 (2013)].Comment: Accepted versio

Controlling ultrafast currents by the non-linear photogalvanic effect

We theoretically investigate the effect of broken inversion symmetry on the generation and control of ultrafast currents in a transparent dielectric (SiO2) by strong femto-second optical laser pulses. Ab-initio simulations based on time-dependent density functional theory predict ultrafast DC currents that can be viewed as a non-linear photogalvanic effect. Most surprisingly, the direction of the current undergoes a sudden reversal above a critical threshold value of laser intensity I_c ~ 3.8*10^13 W/cm2. We trace this switching to the transition from non-linear polarization currents to the tunneling excitation regime. We demonstrate control of the ultrafast currents by the time delay between two laser pulses. We find the ultrafast current control by the non-linear photogalvanic effect to be remarkably robust and insensitive to laser-pulse shape and carrier-envelope phase

Strain and process development for poly(3HB-co-3HP) fermentation by engineered Shimwellia blattae from glycerol

Poly(3-hydroxybytyrate-co-3-hydroxypropionate), poly(3HB-co-3HP), is a possible alternative to synthetic polymers such as polypropylene, polystyrene and polyethylene due to its low crystallinity and fragility. We already reported that recombinant strains of Shimwellia blattae expressing 1,3-propanediol dehydrogenase DhaT as well as aldehyde dehydrogenase AldD of Pseudomonas putida KT2442, propionate-CoA transferase Pct of Clostridium propionicum X2 and PHA synthase PhaC1 of Ralstonia eutropha H16 are able to accumulate up to 14.5% (wtPHA/wtCDW) of poly(3-hydroxypropionate), poly(3HP), homopolymer from glycerol as a sole carbon source (Appl Microbiol Biotechnol 98:7409-7422, 2014a). However, the cell density was rather low. In this study, we optimized the medium aiming at a more efficient PHA synthesis, and we engineered a S. blattae strain accumulating poly(3HB-co-3HP) with varying contents of the constituent 3-hydroxypropionate (3HP) depending on the cultivation conditions. Consequently, 7.12, 0.77 and 0.32 gPHA/L of poly(3HB-co-3HP) containing 2.1, 8.3 and 18.1 mol% 3HP under anaerobic/aerobic (the first 24 hours under anaerobic condition, thereafter, aerobic condition), low aeration/agitation (the minimum stirring rate required in medium mixing and small amount of aeration) and anaerobic conditions (the minimum stirring rate required in medium mixing without aeration), respectively, were synthesized from glycerol by the genetically modified S. blattae ATCC33430 strains in optimized culture medium

Local SiC photoluminescence evidence of non-mutualistic hot spot formation and sub-THz coherent emission from a rectangular Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta} mesa

From the photoluminescence of SiC microcrystals uniformly covering a rectangular mesa of the high transition temperature $T_c$ superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, the local surface temperature $T({\bm r})$ was directly measured during simultaneous sub-THz emission from the $N\sim10^3$ intrinsic Josephson junctions (IJJs) in the mesa. At high bias currents $I$ and low bath temperatures $T_{\rm bath}\lesssim~35$ K, the center of a large elliptical hot spot with $T({\bm r})> T_c$ jumps dramatically with little current-voltage characteristic changes. The hot spot doesn't alter the ubiquitous primary and secondary emission conditions: the ac Josephson relation and the electromagnetic cavity resonance excitation, respectively. Since the intense sub-THz emission was observed for high $T_{\rm bath}\gtrsim~50$ K in the low $I$ bias regime where hot spots are absent, hot spots can not provide the primary mechanisms for increasing the output power, the tunability, or for promoting the synchronization of the $N$ IJJs for the sub-THz emission, but can at best coexist non-mutualistically with the emission. No $T({\bm r})$ standing waves were observed

Observation of molecular dipole excitations by attosecond self-streaking

We propose a protocol to probe the ultrafast evolution and dephasing of coherent electronic excitation in molecules in the time domain by the intrinsic streaking field generated by the molecule itself. Coherent electronic motion in the endohedral fullerene \Necsixty~is initiated by a moderately intense femtosecond UV-VIS pulse leading to coherent oscillations of the molecular dipole moment that persist after the end of the laser pulse. The resulting time-dependent molecular near-field is probed through the momentum modulation of photoemission from the central neon atom by a time-delayed attosecond XUV pulse. Our ab-initio time-dependent density functional theory and classical trajectory simulations predict that this self-streaking signal accurately traces the molecular dipole oscillations in real time. We discuss the underlying processes and give an analytical model that captures the essence of our ab-initio simulations

Ab-initio multi-scale simulation of high-harmonic generation in solids

High-harmonic generation by a highly non-linear interaction of infrared laser fields with matter allows for the generation of attosecond pulses in the XUV spectral regime. This process, well established for atoms, has been recently extended to the condensed phase. Remarkably well pronounced harmonics up to order ~30 have been observed for dielectrics. We present the first ab-initio multi-scale simulation of solid-state high-harmonic generation. We find that mesoscopic effects of the extended system, in particular the realistic sampling of the entire Brillouin zone, the pulse propagation in the dense medium, and the inhomogeneous illumination of the crystal have a strong effect on the formation of clean harmonic spectra. Our results provide a novel explanation for the formation of clean harmonics and have implications for a wide range of non-linear optical processes in dense media

Frequency-resolved microscopic current density analysis of linear and nonlinear optical phenomena in solids

We perform a frequency-resolved analysis of electron dynamics in solids to obtain microscopic insight into linear and nonlinear optical phenomena. For the analysis, we first compute the electron dynamics under optical electric fields and evaluate the microscopic current density as a function of time and space. Subsequently, we perform the Fourier transformation on the microscopic current density and obtain the corresponding quantity in the frequency domain. The frequency-resolved microscopic current density provides insight into the microscopic electron dynamics in real-space at the frequency of linear and nonlinear optical responses. We apply frequency-resolved microscopic current density analysis to light-induced electron dynamics in aluminum, silicon, and diamond based on the first-principles electron dynamics simulation according to the time-dependent density functional theory. Consequently, the nature of delocalized electrons in metals and bound electrons in semiconductors and insulators is suitably captured by the developed scheme