Moderate Excitation Research Articles

Ultrafast structural transition and electron-phonon/phonon–phonon coupling in antimony revealed by nonadiabatic molecular dynamics

Real-time time-dependent density-functional theory molecular dynamics (rt-TDDFT-MD) reveals the nonadiabatic dynamics of the ultrafast photoinduced structural transition in a typical phase-change material antimony (Sb) with Peierls distortion (PD). As the excitation intensity increases from 3.54% to 5.00%, three distinct structural transition behaviors within 1 ps are observed: no PD flipping, nonvolatile-like PD flipping, and nonstop back-and-forward PD flipping. Analyses on electron-phonon and phonon-phonon couplings indicate that the excitation-activated coherent A1gphonon mode by electron-phonon coupling drives the structural transition within several hundred femtoseconds. Then, the energy of coherent motions are transformed into that of random thermal motions via phonon-phonon coupling, which prevents the A1g-mode-like coherent structure oscillations. The electron-phonon coupling and coherent motions will be enhanced with increasing the excitation intensity. Therefore, a moderate excitation intensity that can balance the coherent and decoherent thermal movements will result in a nonvolatile-like PD flipping. These findings illustrate important roles of nonadiabatic electron-phonon/phonon-phonon couplings in the ultrafast laser-induced structural transitions in materials with PD, offering insights for manipulating their structures and properties by light.

The substituent effect on pulsed laser beam self-action manifestation in azo-azomethine PMMA composites thin films in visible and near IR ranges

Efficiency of nonlinear optical response was studied for a set of azo-azomethines dyes with substituted electron-withdrawing/donating groups, being embedded in poly(methyl methacrylate) (PMMA) matrix, via self-action effects manifestation of mode-locked Nd:YAG laser single pulses in visible at 532 nm, and in near IR at 1064 nm ranges. Photoinduced variations of refractive index and optical absorption/bleaching are crucially dependent on electron-donating/accepting substituents' properties with |Re(χ(3))| ∼ 10−7 esu and |Im(χ(3))| ∼ 10−9 esu efficiencies correspondingly at moderate laser excitation up to 10 MW/cm2. Incorporation of the azomethine group has provided tendency for fractional self-focusing effect manifestation in visible range against reference Azo dye, while at 1064 nm – self-defocusing one. For some dyes magnitudes of |Re(χ(3))| in near IR range are comparable and even higher than for the corresponding ones at 532 nm due to resonant cascaded three photon excitation into the peak vicinity of π→π∗ transitions of the dyes. It was established linear relationship between second-order hyperpolarizabilities under pulsed and CW laser excitation in visible range, being preliminary irradiated within full spectra of UV lamp for CW laser diagnostics mode. It proofs a suggestion that picosecond range single pulses transfer dyes into the same excited state as full spectrum of a mercury lamp irradiation – conventional tool for the smart molecules’ isomerization. Analysis of the experimental data has shown a high potential for the azo-azomethines dyes doped polymer composites in nonlinear optical applications.

A wavelet packet deep learning model for Energy-Based structural collapse assessment under Earthquake-Fire Scenarios: Framework and hybrid simulation

An energy-based framework is proposed for the dynamic stability assessment of structures subjected to earthquake-fire scenarios and verified through earthquake-fire hybrid simulation. In this framework, the wavelet packet Long Short-Term Memory (LSTM) model is used to separate the noise and residue caused by earthquake and fire within the structural response signals, guided by wavelet packet energy and power spectral density of signals. Moreover, a comparative analysis is performed with 4 previous signal processing models (empirical mode decomposition, variational mode decomposition, wavelet packet transform, and LSTM). Additionally, Latin hypercube sampling is employed to account for uncertainties in structural characteristics and hazards for dataset establishment and fragility analysis. The experimental findings suggest a decline in the structural dominant frequency with the emergence of high-frequency noise and residuals in structural responses due to the multi-hazard effect. The wavelet packet transform eliminates the high-frequency noise in the signal and avoids the oscillation of the LSTM prediction results. Post-earthquake fire increases the structural collapse possibility even under a moderate earthquake excitation. The proposed framework proves to be rational and has the potential to be further applied to other multi-hazard scenarios.

Dynamics of phase transformations in Si and Ge upon strong excitation with UV femtosecond laser pulses

Femtosecond laser processing of semiconductors has evolved into a mature, high-precision fabrication technique, enabling a wide range of applications. While initially most studies have employed pulses at near infrared wavelengths, the interest in using UV laser pulses is constantly increasing due to the different excitation conditions as a consequence of the much shorter optical penetration depth, leading to an improved resolution. In this context, fundamental studies on the temporal dynamics of phase transformations triggered by such pulses are necessary in order to comprehend and eventually control the complex phase transformation pathways. Here, we report a detailed time-resolved study on the phase transformation dynamics of crystalline silicon and germanium upon irradiation with single 400 nm, 100 fs laser pulses in the moderate and high excitation regime. To this end, we have employed fs-resolved optical microscopy with a probe wavelength of 800 nm to study the reflectivity evolution of the irradiated surface over a temporal window ranging from 100 fs up to 20 ns. At moderate excitation fluence, the data reveals the entire sequence of laser-induced processes, starting from the generation of a free-electron plasma, non-thermal melting, ablation onset and expansion of a semi-transparent ablation layer with sharp interfaces. At excitation with peak fluences more than 30 times the ablation threshold, an anomalous transient high-reflectivity state is observed, which might be indicative of a recoil pressure-induced liquid–liquid phase transition. Moreover, 70 nm-thick amorphous surface layers are formed in both materials after irradiation at moderate fluences. Overall, our results provide relevant information on both, transformation dynamics and final state of both materials for fs-pulse excitation in the near-UV wavelength range.

Laser-Induced Transformations in Thermally Evaporated Thin TlInSe2 Films Studied by Raman Spectroscopy

TlInSe2 films with thickness from 10 to 200 nm were thermally evaporated on silicon and silicate glass substrates. Micro-Raman spectra measured at a moderate excitation (532 nm, 4 kW/cm2) confirm the amorphous character of the films. Narrow features revealed in the spectra at an excitation power density of 40 kW/cm2 show the evidence for the formation of TlInSe2 , TlSe, and In2Se3 crystallites in the laser spot. For thin (10–30 nm) films, the rod-shaped TlInSe2 crystallites are shown to be oriented within the film plane. The crystallite formation is governed by the thermal effect of the tightly focused laser beam.

Cavitation cloud of waterjet under double excitation

This study experimentally explores the interplay of active and passive excitation on double-excited cavitating waterjet clouds. High-speed imaging and high-frequency pressure sensors are used to characterize the impact of piezoelectric transducers for active excitation and nozzle lip geometries for strong, moderate, and weak passive excitation conditions. The analysis of pressure fluctuations revealed that under active excitation, the waterjet exhibited forced oscillations characterized by an amplitude amplification exceeding that of single passive excitation by an order of magnitude. High-speed imaging, combined with proper orthogonal decomposition, allowed us to observe an expansion in the volume, size, and effective standoff distance of cavitation clouds upon introducing active excitation across all passive excitation scenarios. The synergy between strong passive excitation and harmonized frequency with active excitation resulted in the most robust cavitation cloud development, characterized by the highest intensity.

Study on electron response to near-infrared, ultraviolet and extreme ultraviolet femtosecond laser by first-principle

The phenomenon of laser-induced electronic evolution in semiconductors is an essential issue in laser fabrication, which has been extensively investigated within the near-infrared and visible spectral ranges. However, electronic evolution at short wavelength laser has neither been systematically investigated. In this paper, time-dependent density functional theory is employed to investigate the response in silicon under laser irradiation of different wavelengths. The result shows that shorter wavelength induces the excitation of electrons to higher energy levels, consequently leading to increased impact ionization rates. Such a moderate excitation of energetic electrons even at low-medium intensity is considered as a distinctive feature of extreme ultraviolet light laser, which is expected to further enhance the quality of laser processing.

Raman study of photoinduced crystallization of SnS2 in As2S3:Sn glasses

Raman spectra of synthesised As2S3:Sn samples with up to 2.7 at% Sn recorded at a moderate excitation power density (Pexc=4 kW/cm2) show their amorphous character. Raman measurements performed at a higher Pexc (40 kW/cm2) result in the local heating of the sample surface by up to 150 K, as estimated from the shift of the frequency positions of the As2S3 Raman maxima. For samples with Sn content above 2 at% a pronounced narrow peak at 311 cm–1 emerges as an evidence for SnS2 crystallites formed on the sample surface under above-bandgap illumination during the measurement. The higher-Pexc (40 kW/cm2) illumination results in a formation of a pit on the sample surface due to the material transfer and in a rapid decrease of the glass viscosity, enhancing the diffusion of atoms and facilitating their aggregation in SnS2 crystallites. This effect of mostly nonthermal nature is responsible for the SnS2-related feature observed in the Raman spectra upon illumination at Pexc= 40 kW/cm2. For As2S3 doped with 3 at% Sn the SnS2-related feature near 314 cm–1 is observed in the Raman spectra even under excitation at much lower Pexc, down to 0.4 kW/cm2. As follows from X-ray diffraction data, no crystalline SnS2 phase is present in this sample before the measurement, it appears upon illumination even at the low Pexc values.

Multi-step particle emission probabilities in superheavy nuclei at moderate excitation energies

The probabilities of xn-, pxn-, and αxn-evaporation channels in excited superheavy nuclei were evaluated using the Monte Carlo method. The calculations utilized microscopically determined nuclear level densities and were compared with results obtained from the phenomenological Jackson formula. Effective temperatures derived from the microscopic approach were incorporated into the Jackson formula for different evaporation channels at low and moderate excitation energies. Additionally, an analytical formula was introduced to estimate the average kinetic energy of emitted particles in multi-step processes.

Effect of negative stiffness nonlinearity on the vibration control effectiveness of tuned negative stiffness inerter damper

Recently, tuned negative stiffness inerter damper (TNSID) integrating the advantages of both negative stiffness and inerter elements has been proposed and verified to be effective for structural vibration control. In previous studies, the stiffness of the negative stiffness element was normally regarded as a constant for simplicity. However, it is well-known that negative stiffness is generally accompanied by nonlinearity, which may lead to undesirable nonlinear vibrations and unstable responses. Therefore, in this paper, the influence of the nonlinearity of negative stiffness, including weakening and strengthening nonlinear negative stiffness, on the performances of two representative TNSIDs with different configurations is investigated. The working mechanisms and optimal parameters of the TNSIDs with simplified linear negative stiffness are introduced firstly. Following that, based on the harmonic balance method, the dynamic responses of the two TNSIDs with different nonlinear negative stiffness are attained. The unstable problems and performance degradation caused by the nonlinearity of negative stiffness are analysed. Parametric studies are further carried out to evaluate the effects of the nonlinear coefficient, excitation amplitude, internal frequency ratio, and damping ratio on the performances of the two nonlinear TNSIDs. Moreover, to eliminate the impact of the negative stiffness nonlinearity, a new structural parameter optimization is conducted by minimizing the maximum displacement amplitude-frequency response of the TNSIDs. The results indicate that the nonlinearity of negative stiffness could lead to the bend and jump of the displacement amplitude-frequency response curves of TNSIDs, as well as superharmonic resonances, especially when the performance of TNSIDs is sensitive to the variation of negative stiffness. Besides, the unstable areas and the maximum displacement amplitude-frequency responses of TNSIDs are dependent on the excitation amplitude and nonlinear coefficient. To avoid the unstable vibration phenomenon and maintain effective vibration control performance, TNSID-P2 is recommended to be equipped with a weakening negative stiffness under both moderate and strong excitations, while TNSID-P1 should be designed with a weakening negative stiffness and work under a moderate excitation due to its performance being highly sensitive to the variation of negative stiffness.

An asymptotic approach for large amplitude motions of generic nonlinear systems

Starting from the observation that classical asymptotic methods fail to correctly describe the resonance peak of the frequency response curve of a nonlinear oscillator under moderate and large excitation amplitudes, an alternative approach is proposed to overcome this problem. The differences between the multiple time scale method (one of the most performant classical methods) and numerical simulations are initially shown with reference to on the paradigmatic Duffing equation. They are also shown some characteristics of the near peak behavior. Then, the proposed asymptotic approach is illustrated. The basic idea is that of having the zero-order problem nonlinear, while in classical methods it is linear. Thanks to the energy conservation, the zero-order problem is solved exactly. Also, the exact solution of the higher-order problems is obtained in closed-form, thus providing a fully analytical approach. Although the proposed method is valid for any kind of motion, special attention is dedicated to periodic nonlinear oscillations, because of their interest in practical applications. A simple formula for determining the exact intersection of the frequency response and backbone curves is obtained, and it is shown that it can be computed without the need of solving explicitly not even the zero-order problem. Some illustrative examples are finally reported.

Biexciton-like Auger Blinking in Strongly Confined CsPbBr3 Perovskite Quantum Dots.

Perovskite quantum dots (QDs) with high room-temperature luminescence efficiency have been applied in single-photon sources. While the optical properties of large, weakly confined perovskite nanocrystals have been extensively explored at the single-particle level, few studies have focused on single-perovskite QDs with strong quantum confinement. This is mainly due to their poor surface chemical stability. Here we demonstrate that strongly confined CsPbBr3 perovskite QDs (SCPQDs) embedded in a phenethylammonium bromide matrix exhibit a well-passivated surface and improved photostability under intense photoexcitation. We find that in our SCPQDs, photoluminescence blinking is suppressed at moderate excitation intensities, and increasing the excitation rates leads to weak photoluminescence intensity fluctuations accompanied by an unusual spectral blue-shift. We attribute this to a biexciton-like Auger interaction between excitons and trapped excitons formed by surface lattice elastic distortions. This hypothesis is corroborated by the unique repulsive biexciton interaction observed in the SCPQDs.

Selective coupling of coherent optical phonons in YBa2Cu3O7−δ with electronic transitions

We investigate coherent lattice dynamics in optimally doped ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ driven by ultrashort ($\ensuremath{\sim}12$ fs) near-infrared and near-ultraviolet pulses. Transient reflectivity experiments, performed at room temperature and under moderate ($<0.1$ mJ/${\mathrm{cm}}^{2}$) excitation fluence, reveal ${A}_{g}$-symmetry phonon modes related to the O(2,3) bending in the ${\mathrm{CuO}}_{2}$ planes and to the apical O(4) stretching at frequencies between 10 and 15 THz, in addition to the previously reported Ba and Cu(2) vibrations at 3.5 and 4.5 THz. The relative coherent phonon amplitudes are in stark contrast to the relative phonon intensities in the spontaneous Raman scattering spectrum excited at the same wavelength. This contrast indicates mode-dependent contributions of the Raman and non-Raman mechanisms to the coherent phonon generation. We show that the particularly intense coherent Cu(2) phonon, together with its initial phase, supports its generation predominately via a displacive mechanism, possibly involving the charge transfer within the ${\mathrm{CuO}}_{2}$ planes. The small amplitude of the coherent out-of-phase O(2,3) bending mode at 10 THz also suggests the involvement of non-Raman generation mechanism. The generation of the other coherent phonons can in principle be explained within the framework of Raman mechanism. When the pump light has the polarization component perpendicular to the ${\mathrm{CuO}}_{2}$ plane, the coherent O(4) mode at 15 THz is strongly enhanced compared to the in-plane excitation, corresponding to the large polarizability component associated with the hopping between the apical and the chain oxygens, O(4) and O(1).

Using Density Functional Theory for Testing the Robustness of Mobile-Proton Molecular Dynamics Simulations on Electrosprayed Ions: Structural Implications for Gaseous Proteins.

Current experiments only provide low-resolution information on gaseous protein ions generated by electrospray ionization (ESI). Molecular dynamics (MD) simulations can yield complementary insights. Unfortunately, conventional MD does not capture the mobile nature of protons in gaseous proteins. Mobile-proton MD (MPMD) overcomes this limitation. Earlier MPMD data at 300 K indicated that protein ions generated by "native" ESI retain solution-like structures with a hydrophobic core and zwitterionic exterior [Bakhtiari, M.; Konermann, L. J. Phys. Chem. B 2019, 123, 1784-1796]. MPMD redistributes protons using electrostatic and proton affinity calculations. The robustness of this approach has never been scrutinized. Here, we close this gap by benchmarking MPMD against density functional theory (DFT) at the B3LYP/6-31G* level, which is well suited for predicting proton affinities. The computational cost of DFT necessitated the use of small peptides. The MPMD energetic ranking of proton configurations was found to be consistent with DFT single-point energies, implying that MPMD can reliably identify favorable protonation sites. Peptide MPMD runs converged to DFT-optimized structures only when applying 300-500 K temperature cycling, which was necessary to prevent trapping in local minima. Temperature cycling MPMD was then applied to gaseous protein ions. Native ubiquitin converted to slightly expanded structures with a zwitterionic core and a nonpolar exterior. Our data suggest that such inside-out protein structures are intrinsically preferred in the gas phase, and that they form in ESI experiments after moderate collisional excitation. This is in contrast to native ESI (with minimal collisional excitation, simulated by MPMD at 300 K), where kinetic trapping promotes the survival of solution-like structures. In summary, this work validates the MPMD approach for simulations on gaseous peptides and proteins.

Emission Line Profile Variations in the Early Coronal Phase of the Nova V2944 Ophiuchi

V2944 Ophiuchi was a Fe ii type novae that reached moderate excitation and displayed large variations among the profiles of its emission lines. The profiles become increasingly double-peaked as the ionization level of the parent ion increases. At the extremes, H i and O i have flat-topped profiles while the coronal features [Si vii] and [Ca viii] peak in the line wings and have centers that fall to near the continuum level. He ii, N v, and [Si vi] display intermediate behavior but in the same sense that the line center depth is smallest for He ii and largest for [Si vi. A possible explanation for these profile variations is that the emission line region is ring-like in structure and becomes increasingly flattened as line excitation increases.

Photoisomerization of Heptamethine Cyanine Dyes Results in Red-Emissive Species: Implications for Near-IR, Single-Molecule, and Super-Resolution Fluorescence Spectroscopy and Imaging.

Photoisomerization kinetics of the near-infrared (NIR) fluorophore Sulfo-Cyanine7 (SCy7) was studied by a combination of fluorescence correlation spectroscopy (FCS) and transient state (TRAST) excitation modulation spectroscopy. A photoisomerized state with redshifted emission was identified, with kinetics consistent with a three-state photoisomerization model. Combining TRAST excitation modulation with spectrofluorimetry (spectral-TRAST) further confirmed an excitation-induced redshift in the emission spectrum of SCy7. We show how this red-emissive photoisomerized state contributes to the blinking kinetics in different emission bands of NIR cyanine dyes, and how it can influence single-molecule, super-resolution, as well as Förster resonance energy transfer (FRET) and multicolor readouts. Since this state can also be populated at moderate excitation intensities, it can also more broadly influence fluorescence readouts, also readouts not relying on high excitation conditions. However, this additional red-emissive state and its photodynamics, as identified and characterized in this work, can also be used as a strategy to push the emission of NIR cyanine dyes further into the NIR and to enhance photosensitization of nanoparticles with absorption spectra further into the NIR. Finally, we show that the photoisomerization kinetics of SCy7 and the formation of its redshifted photoisomer depend strongly on local environmental conditions, such as viscosity, polarity, and steric constraints, which suggests the use of SCy7 and other NIR cyanine dyes as environmental sensors. Such environmental information can be monitored by TRAST, in the NIR, with low autofluorescence and scattering conditions and on a broad range of samples and experimental conditions.

Investigation of Energy Harvesting Capabilities of Metglas 2605SA1

In the framework of electromechanical energy conversion devices for vibrational energy harvesting, magnetostrictive materials are an attractive alternative solution to the brittleness of piezoelectric materials. Electromagnetic systems have low voltage output at a low frequency while magnetostrictive materials are suitable for a larger frequency bandwidth. In this work, a special experimental emphasis is placed on Fe80Si9B11 (also known as Metglas 2605SA1) alloy. The ultimate energy conversion abilities are investigated by performing experimental Ericsson cycles as well as through theoretical predictions using a dedicated model for the magnetic curves at the material scale. Typical output magnetic energy densities ranged between 0.1 and 1 mJ/cm3/cycle under moderate stress (<100 MPa) and magnetic excitation (up to 4 kA/m). Apart from its energy conversion abilities, Metglas 2605SA1 also features attractive characteristics for realistic applications in microgenerators, such as a low price, which is an important advantage for the mass production and cost-effectiveness of the harvester. Furthermore, its soft magnetic property reduces the need for high magnetic fields and yields a well-adapted solution from a system point of view. It is therefore shown that this material is a suitable conversion material according to the available stress and magnetic excitation magnitudes, in addition to economic considerations.

Combined Fluorescence Fluctuation and Spectrofluorometric Measurements Reveal a Red-Shifted, Near-IR Emissive Photo-Isomerized Form of Cyanine 5.

Cyanine fluorophores are extensively used in fluorescence spectroscopy and imaging. Upon continuous excitation, especially at excitation conditions used in single-molecule and super-resolution experiments, photo-isomerized states of cyanines easily reach population probabilities of around 50%. Still, effects of photo-isomerization are largely ignored in such experiments. Here, we studied the photo-isomerization of the pentamethine cyanine 5 (Cy5) by two similar, yet complementary means to follow fluorophore blinking dynamics: fluorescence correlation spectroscopy (FCS) and transient-state (TRAST) excitation-modulation spectroscopy. Additionally, we combined TRAST and spectrofluorimetry (spectral-TRAST), whereby the emission spectra of Cy5 were recorded upon different rectangular pulse-train excitations. We also developed a framework for analyzing transitions between multiple emissive states in FCS and TRAST experiments, how the brightness of the different states is weighted, and what initial conditions that apply. Our FCS, TRAST, and spectral-TRAST experiments showed significant differences in dark-state relaxation amplitudes for different spectral detection ranges, which we attribute to an additional red-shifted, emissive photo-isomerized state of Cy5, not previously considered in FCS and single-molecule experiments. The photo-isomerization kinetics of this state indicate that it is formed under moderate excitation conditions, and its population and emission may thus deserve also more general consideration in fluorescence imaging and spectroscopy experiments.

Диполярные биэкситоны в латеральных ловушках в гетероструктурах Si/SiGe/Si

Si/SiGe/Si heterostructures with long-range lateral potential fluctuations that appear in the capping Si layer near the SiGe/Si heterointerface due to the presence of relaxed areas in the SiGe layer have been studied. Analysis of the low-temperature photoluminescence spectra indicates that, upon the photoexcitation of the structure, the accumulation of nonequilibrium charge carriers, formation of dipolar excitons, and their recombination take place in large-scale lateral traps formed by these fluctuations. It is found that, at temperatures T < 10 K, a new narrow line appears with an increase in the excitation level at the blue side of the broad photoluminescence band of dipolar excitons localized by short-range potential fluctuations. At temperatures T ~ 2 K, this line is dominant in the spectra even at the lowest excitation levels. It is shown that, at moderate excitation levels, this line is caused by the recombination of free dipolar biexitons in large-scale traps. At high excitation levels, the width of the new line increases by more than a factor of 2 compared to that at lower excitation levels, and under these conditions this line is associated with the recombination of dipolar electron–hole plasma in large-scale traps.

Gold-nanoparticles induced transition in YVO4 crystal: From saturable to reverse saturable absorption

The embedded gold (Au) nanoparticles (NPs) are synthesized inside the yttrium vanadate (YVO4) crystal by direct Au ion implantation. The formed Au nanospheres are with average diameter of ∼5 nm, generating a clear optical absorption band centered at 595 nm due to the localized surface plasmon resonance (LSPR). With the doping of Au NPs, the nonlinear optical properties of YVO4 crystal are also modified, endowing the substrate with new features. Using femtosecond open-aperture Z-scan investigation under variable laser intensities, the Au NPs-modified YVO4 exhibits saturable absorption (SA) property at moderate excitation irradiances due to the plasmonic effect of embedded Au NPs, whilst transits to reverse saturable absorption (RSA) under higher pump irradiances. As an example for practical applications, we demonstrate a 1-μm pulse laser generation utilizing Au NPs-embedded YVO4 crystal as the saturable absorber. This work suggests that ion beam synthesis of metallic NPs may be a suitable way to tailor properties of functional crystals to achieve diverse applications in optics.