Excitation Regime Research Articles

Electronic structure of reduced CeO2(111) surfaces interacting with hydrogen as revealed through electron energy loss spectroscopy in comparison with theoretical investigations

Based on both, ab-initio cluster calculations as well as periodic density functional based loss function calculations, we have assigned the origins of the valence electron excitation regime in electron energy loss spectra of well-ordered ceria films in (111) orientation at various states of reduction as well as after exposing the reduced films to hydrogen from the gas phase (Li et al, Angew. Chem. Int. Ed., 58 (2019) 14686–14693). The explicit calculation of intensity distributions using the dipole approximation allow us to draw conclusions about the nature of oxygen vacancies, which occur upon reduction and how those interact with hydrogen to form hydride species.The present study also reports a brief discussion of vibrational excitations of hydrogen loaded ceria and corroborates previous interpretations, based on inelastic neutron scattering.

Analysis of THz generation using the tilted-pulse-front geometry in the limit of small pulse energies and beam sizes.

Optical rectification in lithium niobate using the tilted-pulse-front geometry is one of the most commonly used techniques for efficient generation of energetic single-cycle THz pulses and the details of this generation scheme are well understood for high pulse energy driving lasers, such as mJ-class, kHz-repetition rate Ti:Sa amplifier systems. However, as modern Yb-based laser systems with ever increasing repetition rate become available, other excitation regimes become relevant. In particular, the use of more moderate pulse energies (in the few µJ to multi-10 µJ regime), available nowadays by laser systems with MHz repetition rates, have never been thoroughly explored. As increasing the repetition rate of THz sources for spectroscopy becomes more relevant in the community, we present a thorough numerical analysis of this regime using a 2+1-D numerical model. Our work allows us to confirm experimental trends observed in this unusual excitation regime and shows that the conversion efficiency is naturally limited by the small pump beam sizes as a consequence of spatial walk-off between the pump and THz beams. Based on our findings, we discuss strategies to overcome the current limitations, which will pave the way for powerful THz sources approaching the watt level with multi-MHz repetition rates.

Synthesis, Fluorescence and Nonlinear Optical Properties of Phthalocyanine Derivatives in the Continuous-Wave and Pulsed Excitation Regimes

Synthesis, Fluorescence and Nonlinear Optical Properties of Phthalocyanine Derivatives in the Continuous-Wave and Pulsed Excitation Regimes

Quasi-optical theory of relativistic Cherenkov surface-wave oscillators with oversized cylindrical waveguides

Within the quasi-optical approach, we investigate the propagation of azimuthally symmetric TM waves in periodically corrugated cylindrical waveguides and their excitation by relativistic electron beams. Presenting the field as two, forward and backward, quasi-optical wavebeams coupled at the shallow corrugation, we obtain a dispersion equation for normal waves and thus a criterion of existence of the surface wave. For a finite-length corrugation section, the spectrum of axial evanescent eigenmodes is estimated; the spatial structure and the quality factor of the fundamental mode are found at an eigenfrequency close to the Bragg frequency. A self-consistent system of equations describing the interaction of electromagnetic waves with a rectilinear electron beam injected into the system is derived. Based on this model, we recognize two oscillation regimes, namely, the π-mode excitation regime and the regime of backward surface wave oscillator. We demonstrate the viability of practical implementation of relativistic surface wave oscillators with a power level of up to 140 MW in the sub-millimeter wavelength band.

Linear and nonlinear excitation induced ultrafast absorption dynamics in laser ablated and chemically synthesized gold nanoparticle colloids

Abstract Study of excited state electron dynamics on gold nanoparticles produced by femtosecond laser ablation and chemical synthesis routes are conducted through the pump-probe technique under nonlinear (800 nm, two-photon region) and linear (350 nm, one-photon region) excitations. The temporally delayed broadband probe in the plasmonic region signifies the occurrence of dominant ground state bleach and photo-induced absorption. The excited state electrons, specifically those corresponding to below-resonant excitation, undergo relaxation as a brisk tapering curve due to electron-electron coupling followed by a gradual rise attributed to electron-phonon coupling events. The gold colloids show longer duration of electron-phonon process at 350 nm excitation than at 800 nm with an almost linear dependence of decay time with increasing excitation energy. The resonant excitation regime of chemically synthesized sample shows a visible red-shift compared to their laser ablated counterparts. This study proves efficient in better understanding energy dependent electron dynamics at different excitation wavelengths.

Long-range interaction effects on coupled excitable nodes: traveling waves and chimera state

In this paper, analytical and numerical studies of the influence of the long-range interaction parameter on the excitability threshold in a ring of FitzHugh-Nagumo (FHN) system are investigated. The long-range interaction is introduced to the network to model regulation of the Gap junctions or hemichannels activity at the connexins level, which provides links between pre-synaptic and post-synaptic neurons. Results show that the long-range coupling enhances the range of the threshold parameter. We also investigate the long-range effects on the network dynamics, which induces enlargement of the oscillatory zone before the excitable regime. When considering bidirectional coupling, the long-range interaction induces traveling patterns such as traveling waves, while when considering unidirectional coupling, the long-range interaction induces multi-chimera states. We also studied the difference between the dynamics of coupled oscillators and coupled excitable neurons. We found that, for the coupled system, the oscillation period decreases with the increasing of the coupling parameter. For the same values of the coupling parameter, the oscillation period of the Oscillatory dynamics is greater than the oscillation period of the excitable dynamics. The analytical approximation shows good agreement with the numerical results.

Higher Harmonic Generation of Coherent Sub-THz Radiation at Lifshitz Transition in Gapped Bilayer Graphene

The microscopic quantum theory of nonlinear interaction of strong coherent electromagnetic radiation with gapped bilayer graphene is used for consideration of the high harmonic generation at low-energy photon excitation Lifshitz transition. The Liouville–von Neumann equation for the density matrix is solved numerically at the nonadiabatic multiphoton excitation regime. By numerical solutions, we examine the rates of the second and third harmonics generation at the particle-hole annihilation at Lifshitz transitions in the linearly polarized coherent electromagnetic wave. The obtained results show that the gapped bilayer graphene can serve as an effective medium for the generation of even and odd high harmonics in the sub-THz domain of frequencies.

Математичне моделювання жорстких режимів збудження кавітаційних ав-токоливань у системі живлення рідинних ракетних двигунів

Hard self-oscillation excitation differs from soft excitation in that self-oscillations are set up only if the initial departure of an oscillating system from equilibrium is strong enough. Experimental studies of cavitation oscillations in hydraulic systems with cavitating pumps of liquid-propellant rocket engines ((LPREs) include works that describe hard excitation of cavitation oscillations. By mow, hard excitation regimes have not been explained theoretically, to let alone their mathematical simulation. This paper presents a mathematical model of hard excitation of cavitation oscillations in a LPRE feed system, which comprises a mathematical model of cavitation self-oscillations in a LPRE feed system that accounts for pump choking and an external disturbance model. A mechanism of hard excitation of cavitation oscillations in a LPRE feed system is proposed. It is well known that hard excitation of cavitation self-oscillations may take place in cases where the pump feed system is near the boundary of the cavitation self-oscillation region. In this case, the self-oscillation amplitudes are small, and they are limited only by one nonlinearity (cavity volume vs. pump inlet pressure and flow relationship). Under excitation of sufficient intensity, the pump inlet pressure and flow find themselves in the choking characteristic; this may be responsible for choking and developed cavitation self-oscillations, which remain of interrupted type and do not go into the initial small-amplitude oscillations even after excitation removal. A mathematical simulation of hard excitation of cavitation self-oscillations was conducted to determine the parameters of cavitation self-oscillations in a bench feed system of a test pump. The simulation results show that without an external disturbance the pump system exhibits small-amplitude self-oscillations. On an external disturbance, developed (interrupted) cavitation oscillations are set up in the system, which is in agreement with experimental data. The proposed mathematical model of hard excitation of cavitation self-oscillations in a LPRE feed system allows one to simulate a case observed in an experiment in which it was possible to eliminate cavitation self-oscillations by an external disturbance.

Optical signatures of shear collective modes in strongly interacting Fermi liquids

The concept of Fermi liquid lays a solid cornerstone to the understanding of electronic correlations in quantum matter. This ordered many-body state rigorously organizes electrons at zero temperature in progressively higher momentum states, up to the Fermi surface. As such, it displays rigidity against perturbations. Such rigidity generates Fermi-surface resonances which manifest as longitudinal and transverse collective modes. Although these Fermi-liquid collective modes have been analyzed and observed in electrically neutral liquid helium, they remain unexplored in charged solid-state systems up to date. In this paper I analyze the transverse shear response of charged three-dimensional Fermi liquids as a function of temperature, excitation frequency and momentum, for interactions expressed in terms of the first symmetric Landau parameter. I consider the effect of momentum-conserving quasiparticle collisions and momentum-relaxing scattering in relaxation-time approximation on the coupling between photons and Fermi-surface collective modes, thus deriving the Fermi-liquid optical conductivity and dielectric function. In the high-frequency, long-wavelength excitation regime the electrodynamic response entails two coherent and frequency-degenerate polaritons, and its spatial nonlocality is encoded by a frequency- and interaction-dependent generalized shear modulus; in the opposite high-momentum low-frequency regime anomalous skin effect takes place. I identify observable signatures of propagating shear collective modes in optical spectroscopy experiments, with applications to the surface impedance and the optical transmission of thin films.

The impact of memristive coupling initial states on travelling waves in an ensemble of the FitzHugh–Nagumo oscillators

A ring of the FitzHugh–Nagumo (FHN) oscillators coupled via memristive elements is studied. The impact of the memristive element initial states on the travelling wave characteristics is established for the self-oscillatory and excitable dynamics of the individual oscillators. It is shown that the initial state effect is especially evident in the excitable regime. In such a case the initial distribution of the memristive element states affects on the wave spatial profile as well as on the frequency of the pulse excitation. The study of the ensemble with ideal memristor coupling is complemented by the exploration of the model implying the interaction through the memristor characterized by ‘forgetting’ the initial states over time. It is demonstrated through this approach that the dependence of the oscillation characteristics on the memristor initial states persists in the presence of the weak memristor imperfectness.

Canard resonance: on noise-induced ordering of trajectories in heterogeneous networks of slow-fast systems

We analyse the dynamics of a network of semiconductor lasers coupled via their mean intensity through a non-linear optoelectronic feedback loop. We establish experimentally the excitable character of a single node, which stems from the slow-fast nature of the system, adequately described by a set of rate equations with three well separated time scales. Beyond the excitable regime, the system undergoes relaxation oscillations where the nodes display canard dynamics. We show numerically that, without noise, the coupled system follows an intricate canard trajectory, with the nodes switching on one by one. While incorporating noise leads to a better correspondence between numerical simulations and experimental data, it also has an unexpected ordering effect on the canard orbit, causing the nodes to switch on closer together in time. We find that the dispersion of the trajectories of the network nodes in phase space is minimized for a non-zero noise strength, and call this phenomenon canard resonance.

Tuning the nonlinear optical properties by engineering donor-acceptor configurations in nitrochalone derivatives

Development of tunable photonic devices having molecular dimensions is an active research area in nonlinear optics which continues to evolve and impact many technologies. Here, we demonstrate the tailoring of nonlinear optical properties of nitrochalcone derivatives by engineering the donor-acceptor configurations on either end of the molecules, to control the intramolecular charge transfer process. The central π-bridge in chalcones provides delocalized electrons which lead to the nonlinear polarizability observed in these molecules. Optical nonlinearity is studied by means of second harmonic generation (SHG) and open aperture Z-scan measurements. Samples show optical limiting behaviour in the nanosecond excitation regime due to excited state absorption (ESA) and two-photon absorption (2PA) phenomena. Our study highlights the importance of structure-property correlations in molecular designs.

Temperature-dependent energy diffusion in chaotic spin chains

We study the temperature dependence of energy diffusion in two chaotic gapped quantum spin chains, a tilted-field Ising model and an XZ model, using an open system approach. We introduce an energy imbalance by coupling the chain to thermal baths at its boundary and study the non-equilibrium steady states of the resulting Lindblad dynamics using a matrix product operator ansatz for the density matrix. We define an effective local temperature profile by comparing local reduced density matrices in the steady state to those of a uniform thermal state. We then measure the energy current for a variety of driving temperatures and extract the temperature dependence of the energy diffusion constant. For the Ising model, we are able to study temperatures well below the energy gap and find a regime of dilute excitations where rare three-body collisions control energy diffusion. A kinetic model correctly predicts the observed exponential increase of the energy diffusion constant at low temperatures. For the XZ model, we are only able to access intermediate to high temperatures relative to the energy gap and we show that the data is well described by an expansion around the infinite temperature limit. We also discuss the limitations of the particular driving scheme and suggest that lower temperatures can be accessed using larger baths.

Dissipative preparation of qutrit entanglement via periodically modulated Rydberg double antiblockade.

We propose a mechanism of Rydberg double antiblockade by virtue of a resonant dipole-dipole interaction between a pair of Rydberg atoms placed at short distances scaling as 1/R3. By combining this novel excitation regime with microwave-driven fields and dissipative dynamics, a stationary qutrit entangled state can be obtained with high quality, the corresponding steady-state fidelity and purity are insensitive to the variations of the dynamical parameters. Furthermore, we introduce time-dependent laser fields with periodically modulated amplitude to speed up the entanglement creation process. Numerical simulations reveal that the order of magnitude of the shortened convergence time is about 103 in units of ω0, and the acceleration effect appears valid in broad parametric space. The present results enrich the physics of the Rydberg antiblockade regimes and may receive more attention for the experimental investigations in dissipative dynamics of neutral atoms.

Coherence resonance in neuronal populations: Mean-field versus network model.

The counterintuitive phenomenon of coherence resonance describes a nonmonotonic behavior of the regularity of noise-induced oscillations in the excitable regime, leading to an optimal response in terms of regularity of the excited oscillations for an intermediate noise intensity. We study this phenomenon in populations of FitzHugh-Nagumo (FHN) neurons with different coupling architectures. For networks of FHN systems in an excitable regime, coherence resonance has been previously analyzed numerically. Here we focus on an analytical approach studying the mean-field limits of the globally and locally coupled populations. The mean-field limit refers to an averaged behavior of a complex network as the number of elements goes to infinity. We apply the mean-field approach to the globally coupled FHN network. Further, we derive a mean-field limit approximating the locally coupled FHN network with low noise intensities. We study the effects of the coupling strength and noise intensity on coherence resonance for both the network and the mean-field models. We compare the results of the mean-field and network frameworks and find good agreement in the globally coupled case, where the correspondence between the two approaches is sufficiently good to capture the emergence of coherence resonance, as well as of anticoherence resonance.

Thermal and non-thermal intensity dependent optical nonlinearities in ethanol at 800 nm, 1480 nm, and 1560 nm

Intensity dependent self-action of a continuous wave (CW) or pulsed optical beam can lead to spatial or spectral effects upon propagation through a nonlinear medium, which can be described as an intensity dependence of the refractive index, known as self-phase modulation (SPM). In this work, we revisit the nonlinear optical propagation of a CW and a CW mode-locked (CW-ML) high repetition rate [ ∼ megahertz (MHz)] laser propagating through pure ethanol in regions of very low optical absorption (800 nm) or very high absorption (1480 nm, 1560 nm). Spatial and spectral SPM and Z -scan experiments were performed to clarify the origin of the third-order nonlinear optical response in the different optical excitation regimes. From spatial SPM and Z -scan at either CW or CW-ML MHz regime, a thermal nonlinear response was determined to be the origin of the nonlinearity at 800 nm or 1480 nm, 1560 nm region, with the nonlinear refractive index response of the order of 10 − 4 − 10 − 9 c m 2 / W . From the spectral SPM, the non-thermal origin of the nonlinearity, arising from electronic or nuclear processes in the ethanol, was determined, and values of the order of 10 − 13 − 10 − 16 c m 2 / W were obtained, depending upon the spectral region. The results were supported by theoretical calculations using the nonlinear Schrödinger equation for the spectral behavior or Fresnel–Kirchhoff integral for the spatial results and clarify some of the misinterpreted results reported in the literature, besides complementing other nonlinear refraction data available at different wavelengths from 355 nm to 1560 nm.

Interaction of two-dimensional atomic lattices with a single surface plasmon polariton

We study the polaritonic bandstructure of a two-dimensional (2D) atomic lattice coupled to a surface plasmon polariton mode in the single excitation regime. We adopt a Dirichlet-to-Neumann map based computational technique which can accurately model non-Markovian dynamics as well as narrow-bandwidth features associated with any periodic atom-photon system in general. Using this technique, we design a 2D atomic lattice using only two-level atoms, which has an isolated flat polaritonic band where the magnitude of the group velocity for the modes in the band approach zero across the whole Brillouin zone. Such a system could be employed to slow, store, and manipulate single photons in a 2D geometry.

Cellular automata dynamics of nonlinear optical processes in a phase-change material

Changes in the arrangement of atoms in matter, known as structural phase transitions or phase changes, offer a remarkable range of opportunities in photonics. They are exploited in optical data storage and laser-based manufacturing, and have been explored as underpinning mechanisms for controlling laser dynamics, optical and plasmonic modulation, and low-energy switching in single nanoparticle devices and metamaterials. Comprehensive modeling of phase-change processes in photonics is, however, extremely challenging as it involves a number of entangled processes including atomic/molecular structural change, domain and crystallization dynamics, change of optical properties in inhomogeneous composite media, and the transport and dissipation of heat and light, which happen on time and length scales spanning several orders of magnitude. Here, for the first time, we show that the description of such complex nonlinear optical processes in phase-change materials can be reduced to a cellular automata model. Using the important example of a polymorphic gallium film, we show that a cellular model based on only a few independent and physically-interpretable parameters can reproduce the experimentally measured behaviors of gallium all-optical switches over a wide range of optical excitation regimes. The cellular automata methodology has considerable heuristic value for the study of complex nonlinear optical processes without the need to understand details of atomic dynamics, band structure, and energy conservation at the nanoscale.

Universal Regimes in Long-Time Asymptotic of Multilevel Quantum System Under Time-Dependent Perturbation.

In the framework of an exactly soluble model, one considers a typical problem of the interaction between radiation and matter: the dynamics of population in a multilevel quantum system subject to a time dependent perturbation. The algebraic structure of the model is taken richly enough, such that there exists a strong argument in favor of the fact that the behavior of the system in the asymptotic of long time has a universal character, which is system-independent and governed by the functional property of the time dependence exclusively. Functional properties of the excitation time dependence, resulting in the regimes of resonant excitation, random walks, and dynamic localization, are identified. Moreover, an intermediate regime between the random walks and the localization is identified for the polyharmonic excitation at frequencies given by the Liouville numbers.

Synthesis, Fluorescence and Nonlinear Optical Properties of Phthalocyanine Derivatives in the Continuous-wave and Pulsed Excitation Regimes-=SUP=-*-=/SUP=-

In this work, the photo-physical properties of seven different phthalocyanine derivatives are studied, with a focus on their transparency spectral range. Two compounds among them are recently synthesised. The investigations include synthesis, photoluminescence, absorptive and refractive nonlinear properties, and optical limiting performances. The compounds are found to be emitting interesting broad fluorescence bands in the visible range. The nonlinear optical properties are investigated with the help of Z-scan technique, with both a continuous laser at 488, 514.5 and 632.8 nm, and with a pulsed laser at 532 nm. The sign and the magnitude of the nonlinear refractive index, third-order susceptibility, and nonlinear absorption coefficient have been evaluated in the seven compounds. The optical limiting performance of each compound was also evaluated in the continuous and pulsed laser excitation regimes. Keywords: