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The generation of high-voltage ultrafast electrical pulses has significant potential for application in ultrawideband microwave sources, terahertz technology, and inertial confinement fusion. However, there is still a lack of the generation of high-voltage ultrafast electrical pulses with greater peak amplitudes and faster pulse widths and ultrafast characteristics. In this paper, a linear low-temperature gallium arsenide photoconductive semiconductor switch (LT-GaAs PCSS) is developed using the ps-scale carrier lifetime of low-temperature gallium arsenide (LT-GaAs). The generation of ultrafast electrical pulses with a pulse width of 0.5 ns and a voltage amplitude of 1 kV is achieved by triggering a pulsed laser with a wavelength of 1064 nm, a single pulse energy of 70 μJ, and a pulse width of 30 ps. The impact of parameters, such as bias electric field, laser pulse energy, and transmission line length, on the ultrafast characteristics of the output electric pulse of the LT-GaAs PCSS is investigated through a synthesis of experimental and simulation approaches. The results show that (1) in linear conditions, the output pulse amplitude is mainly determined by the bias voltage and laser pulse energy, and the output pulse amplitude is also affected by conductor and dielectric attenuation in the transmission line; (2) the falling edge is determined by the carrier lifetime. Because of the absorption depth at 1064 nm and the carrier lifetime of semi-insulating gallium arsenide material, the output electrical pulse has a trailing edge; (3) the bias voltage and laser pulse energy can improve voltage transmission efficiency, and the saturation of voltage transmission efficiency is caused by the saturation of photon absorption in LT-GaAs material. [ABSTRACT FROM AUTHOR]

We present velocity-map imaging (VMI) of photoelectrons detached from anions using an optical parametric amplifier operating at a repetition rate as high as 100 kHz. The light source generates femtosecond (fs) laser pulses tunable from near-infrared to ultraviolet (310–2600 nm), which interact synchronously with mass-selected anion bunches. We demonstrate this technique by measuring two-dimensional projections of photoelectrons ejected from silver trimer anions, Ag3−, across a photon energy range from 2.43 to 4.00 eV (509–310 nm), with an average power of 50–300 mW. This high-repetition-rate VMI setup allows rapid data acquisition of photoelectron spectra and laboratory-frame photoelectron angular distributions of anions at various photon energies, facilitating investigation of their electronic and geometric structures. Taking advantage of the fs pulses, this approach will also enable time-resolved photoelectron imaging for tracking electronic and nuclear dynamics of anions with high efficiency. [ABSTRACT FROM AUTHOR]

Vibrational wave packets are created in the lowest triplet state 1 3 Σ u + of K2 and Rb2 residing on the surface of helium nanodroplets, through non-resonant stimulated impulsive Raman scattering induced by a moderately intense near-infrared laser pulse. A delayed, intense 50-fs laser pulse doubly ionizes the alkali dimers via multiphoton absorption and thereby causes them to Coulomb explode into a pair of alkali ions Ak+. From the kinetic energy distribution P(Ekin) of the Ak+ fragment ions, measured at a large number of delays, we determine the time-dependent internuclear distribution P(R, t), which represents the modulus square of the wave packet within the accuracy of the experiment. For both K2 and Rb2, P(R, t) exhibits a periodic oscillatory structure throughout the respective 300 and 100 ps observation times. The oscillatory structure is reflected in the time-dependent mean value of R, ⟨R⟩(t). The Fourier transformation of ⟨R⟩(t) shows that the wave packets are composed mainly of the vibrational ground state and the first excited vibrational state, in agreement with numerical simulations. In the case of K2, the oscillations are observed for 300 ps, corresponding to more than 180 vibrational periods with an amplitude that decreases gradually from 0.035 to 0.020 Å. Using time-resolved spectral analysis, we find that the decay time of the amplitude is ∼260 ps. The decrease is ascribed to the weak coupling between the vibrating dimers and the droplet. [ABSTRACT FROM AUTHOR]

Silicon surface amorphization by short pulse laser irradiation is a phenomenon of high importance for device manufacturing and surface functionalization. To provide insights into the processes responsible for laser-induced amorphization, a multiscale computational study combining atomistic molecular dynamics simulations of nonequilibrium phase transformations with continuum-level modeling of laser-induced melting and resolidification is performed. Atomistic modeling provides the temperature dependence of the melting/solidification front velocity, predicts the conditions for the transformation of the undercooled liquid to the amorphous state, and enables the parametrization of the continuum model. Continuum modeling, performed for laser pulse durations from 30 ps to 1.5 ns, beam diameters from 5 to 70 μm, and wavelengths of 532, 355, and 1064 nm, reveals the existence of two threshold fluences for the generation and disappearance of an amorphous surface region, with the kinetically stable amorphous phase generated at fluences between the lower and upper thresholds. The existence of the two threshold fluences defines the spatial distribution of the amorphous phase within the laser spot irradiated by a pulse with a Gaussian spatial profile. Depending on the irradiation conditions, the formation of a central amorphous spot, an amorphous ring pattern, and the complete recovery of the crystalline structure are predicted in the simulations. The decrease in the pulse duration or spot diameter leads to an accelerated cooling at the crystal–liquid interface and contributes to the broadening of the range of fluences that produce the amorphous region at the center of the laser spot. The dependence of the amorphization conditions on laser fluence, pulse duration, wavelength, and spot diameter, revealed in the simulations, provides guidance for the development of new applications based on controlled, spatially resolved amorphization of the silicon surface. [ABSTRACT FROM AUTHOR]

Ytterbium lasers make possible shot-to-shot data collection of two-dimensional infrared (2D IR) spectra at 100 kHz and higher repetition rates. At those rates, the power absorbed by the sample is appreciable and creates a steady state temperature rise and an accumulated thermal grating artifact in the spectra that can obscure weak or low concentration IR chromophores. We report the magnitude of the temperature rise, the pulse ordering by which it is created, and ways to mitigate it. Using a calibrant molecule, we measured a steady-state temperature up to 32.5 and 45 °C for laser light at 4 µm in H2O and 6 µm in D2O, respectively, for a typical optical density used in 2D IR experiments. The temperature reached a steady state in ∼60 s. The temperature rise scales with the integrated optical density of the sample across the laser spectrum. By cooling the sample cell, we returned the steady state temperature to room temperature within the laser focus. For samples that undergo rotation, the accumulated thermal grating artifact is removed using a perpendicular ⟨XXYY⟩ polarization because the permuted time-orderings of the thermal grating artifact has the orientational response ⟨XYXY⟩, which decays to zero during the delay between consecutive laser pulses. The procedure described in this study can be used to characterize and minimize the thermal effects in experiments where repetition rate and/or pulse energy cause an appreciable temperature rise. [ABSTRACT FROM AUTHOR]

A quantum-electrodynamics approach is presented to describe the dynamics of electrons that exchange energy with both photon and phonon baths. Our ansatz is a dissipative quantum Liouville equation, cast in the Redfield form, with two driving terms associated with radiative and vibrational relaxation mechanisms, respectively. Remarkably, within the radiative contribution, there is a term that exactly replicates the expression derived from a semiclassical treatment where the power dissipated by the electronic density is treated as the emission from a classical dipole [Bustamante et al., Phys. Rev. Lett. 126, 087401 (2021)]. Analysis of the distinct contributions to the total radiation shows that the semiclassical emission depends on the coherences, with the remainder of the quantum-electrodynamics driving term determined by the excited populations, thus accounting for the relaxation of eigenstates or incoherent mixed states. This approach is used to investigate the response of the Su–Schrieffer–Heeger model for trans-polyacetylene to both pulsed and continuous laser irradiation. Upon excitation with a short pulse and in the absence of the vibrational mechanism, the conducting band population exhibits a stepwise relaxation, characterized by cycles of exponential decay followed by a transient subradiant state. The latter arises from the collective coupling between Bloch states featuring a quasi-continuum energy spectrum in reciprocal space. The separate examination of the semiclassical dynamics reveals that it is this contribution that is responsible for the collective behavior. If vibrational dissipation is active, following the laser pulse, the excited electrons rapidly populate the minimum of the conduction band, and the emission spectrum shifts to lower frequencies with respect to absorption. Meanwhile, continuous irradiation drives the system to a stationary state with a broad emission spectrum. [ABSTRACT FROM AUTHOR]

We propose a real-time time-dependent ab initio approach within a configuration-interaction-singles ansatz to decompose the high-harmonic generation (HHG) signal of molecules in terms of individual molecular-orbital (MO) contributions. Calculations have been performed by propagating the time-dependent Schrödinger equation with complex energies, in order to account for ionization of the system, and by using tailored Gaussian basis sets for high-energy and continuum states. We have studied the strong-field electron dynamics and the HHG spectra in aligned CO2 and H2O molecules. Contribution from MOs in the strong-field dynamics depends on the interplay between the MO ionization energy and the coupling between the MO and the laser-pulse symmetries. Such contributions characterize different portions of the HHG spectrum, indicating that the orbital decomposition encodes nontrivial information on the modulation of the strong-field dynamics. Our results correctly reproduce the MO contributions to HHG for CO2 as described in the literature experimental and theoretical data and lead to an original analysis of the role of the highest occupied molecular orbitals HOMO, HOMO-1, and HOMO-2 of H2O according to the polarization direction of the laser pulse. [ABSTRACT FROM AUTHOR]

A monolayer of 4-nitrothiophenol adsorbed on an Au substrate was heated by illuminating the substrate with a 19 ps laser pulse of 532 nm wavelength. Within 91 ps, the temperature of the sample increased from room temperature by 113 K. Vibrational sum frequency spectroscopy was used to characterize the adsorption geometry of the molecules in the ordered domains in the monolayer film. Upon heating, the initially ordered monolayer largely lost its structure. While the molecules are initially tilted by about 50° with respect to the surface normal, the analysis indicates that the mean tilt angle increased to 80° with a spread for individual molecules of up to a tilt angle of 40° upon heating. The evolution of this loss of order lagged about 100 ps behind the temperature rise of the substrate. [ABSTRACT FROM AUTHOR]

Strain engineering is an important way to control the physical properties of manganite thin films. Here, the effect of strain mode on the transverse thermoelectric (TTE) effect of inclined La1xCaxMnO3 (LCMO) thin films is investigated. The compressive strain enhances the electrical conductivity of the LCMO thin film, resulting in larger laser-induced voltage and faster response speed, which is suitable for high-energy pulse laser detection. Compared to that, the LCMO thin film on the STO substrate is ideal for heat-flux detection due to higher heat-flux sensitivity originating from lower conductivity. Still, it also has to withstand the loss of response speed. Moreover, it is first observed that the TTE voltage polarity of the LCMO thin film changes under different strain states. The measured results of LCMO thin films deposited LaAlO3 (LAO) substrate buffered with the SrTiO3 (STO) thin film and STO substrate buffered with the LAO thin film strongly support that the above results are indeed caused by the lattice strain. This work may be suitable for other material systems to modulate thermoelectric transport anisotropy and the TTE effect by strain. [ABSTRACT FROM AUTHOR]

In a recent work, assuming a Beer–Lambert optical absorption and a Gaussian laser time profile, it was shown that the exact solutions for a 1D photoacoustic (PA) boundary-value-problem predict a null pressure for optically strong absorbent materials. In order to overcome this inconsistency, a heuristic correction was introduced by assuming that heat flux travels a characteristic length during the duration of the laser pulse [M. Ruiz-Veloz et al., J. Appl. Phys. 130, 025104 (2021)] τ p. In this work, we obtained exact analytical solutions in the frequency domain for a 1D boundary-value-problem for the Dual-Phase-Lag (DPL) heat equation coupled with a 1D PA-boundary-value-problem via the acoustic wave equation. Temperature and pressure solutions were studied by assuming that the sample and its surroundings have a similar characteristic thermal lag response time τ T ; therefore, the whole system is assumed to have a similar thermal relaxation. A second assumption for τ T is that it is considered as a free parameter that can be adjusted to reproduce experimental results. Solutions for temperature and pressure were obtained for a one-layer 1D system. It was found that for τ T < τ p , the DPL temperature has a similar thermal profile of the Fourier heat equation; however, when τ T ≥ τ p , this profile is very different from the Fourier case. Additionally, via a numerical Fourier transform, the wave-like behavior of DPL temperature is explored, and it was found that as τ T increases, thermal wave amplitude is increasingly attenuated. Exact solutions for pressure were compared with experimental PA signals, showing a close resemblance between both data sets, particularly in time domain, for an appropriated value of τ T ; the transference function was also calculated, which allowed us to find the maximum response in frequency for the considered experimental setup. [ABSTRACT FROM AUTHOR]

This study addresses the kinetic process of field evaporation of MgO assisted by ultrafast laser pulses combining density functional theory and molecular dynamics. A quantitative model is presented to describe the competitive evaporation of Mg and O ions under various conditions by comparing the activation barriers. The coordination number has a significant impact on the evaporation kinetics. The evaporation ratio of Mg to O rises with increasing DC field strength and laser intensity. Moreover, the energetics of evaporation is in correlation with photo-induced field ionization, revealing distinct mechanisms of evaporation for Mg and O. While Mg undergoes further ionization and field evaporation simultaneously, the evaporation of O is coupled with the relaxation of excited carriers. The final charge state of evaporated O is determined by the DC field strength rather than the laser intensity. Our findings provide insights into laser–matter interactions in ionic compounds and contribute to the development of atom probe techniques. [ABSTRACT FROM AUTHOR]

We present a model to calculate the resonant energy absorption of a laser with finite number of pulses impinging on a doped porous dielectric slab. Analytical reflection R and transmission T coefficients are first derived as a function of 0 < α ≤ 1 to account for porosity with α = 1 denotes a perfect ideal slab, which are verified using an electromagnetic solver. Based on the Drude model with resonant line due to impurities, we calculate the resonant energy absorption as a function of doping concentration, quality factor of the resonant line, porosity, length of the slab, and laser pulse length. It is important to note that simulating the combined effects of these parameters is challenging using existing models. The energy absorption efficiency is maximized for a certain degree of doping concentration at a given pulse length and also for a certain pulse length at a given doping concentration. At small doping concentration, the absorption efficiency increases with smaller α (high porosity) and the trend is reversed at larger α (low porosity). Dimensionless parameters are constructed, allowing the calculated results to be applicable over a wide range of frequencies and pulse durations. Thus, this model serves as a useful tool to characterize the amount of energy absorption due to these combined effects, which are important for many applications in plasmonics, optoelectronics, high power microwaves breakdown, and organic materials. Some possible experiments are suggested for future verification of the model. [ABSTRACT FROM AUTHOR]

Generating and manipulating magnetic skyrmions at ultrafast time scales is essential for future skyrmion-based racetrack memory and logic gate applications. Using the atomistic spin dynamics simulations, we demonstrate the nucleation of ultra-small field-free magnetic skyrmions in amorphous GdFeCo at picosecond time scales by femtosecond laser heating. The ultrafast nature of laser heating and subsequent cooling from a high-temperature state is crucial for forming magnetic skyrmion. The magnon localization and magnon coalescence are the key driving mechanisms responsible for stabilizing the magnetic skyrmions at zero-field conditions. The polarization and, hence, the topological charge can be switched by exploiting the all-optical switching observed in GdFeCo. The skyrmion sizes and numbers can be controlled by varying pulse width and fluence of incident laser pulses. Applying an external magnetic field provides an additional degree of freedom to tune the skyrmion radius during the ultrafast optical creation of magnetic skyrmions. Our results provide a detailed understanding of the ultrafast creation of magnetic skyrmions using femtosecond laser pulses, a vital step in advancing next-generation skyrmion-based memory technologies. [ABSTRACT FROM AUTHOR]

Integration of photonic crystal (PhC) configurations onto the surfaces of light-emitting diodes (LEDs) can play an important role in enhancing light extraction efficiency. While the literature is rich with various PhC fabrication approaches, there is a need for high throughput methods that are appropriate for low-cost devices. In this paper, we report the use of single pulse laser interference lithography (LIL) for the fabrication of photonic crystal structures on LEDs. The use of brief nanosecond pulse exposures offers significant benefits for high-throughput production. In our study, we have applied single pulse LIL on GaAs/AlGaAs LED structures to achieve high-quality photoresist arrays and then have used inductively coupled plasma etching to create nanoholes into the epitaxial structure. The resulting array forms an effective PhC, controlling surface transmission. Electroluminescence (EL) analyses confirm that these structures enhance the average EL intensity of the LED by up to 3.5 times at room temperature. This empirical evidence underscores the efficacy and potential of this fabrication approach in advancing the functional capabilities of semiconductor-based light-emitting devices. [ABSTRACT FROM AUTHOR]

Stimulated Raman Scattering (SRS), pumped by a broadband (i.e., compared to the bandwidth of the material excitation) chirped 50-ps pulse, with Stokes seeding by a 20-ns narrowband pulse, is experimentally and theoretically investigated. In the experiments, a femtosecond-class 0.95 μm Ti:sapphire laser system and a Q-switched 1.064 μm Nd:YAG laser were used for pumping and seeding SRS in a calcite (CaCO3) crystal. This material was chosen because its Raman resonance frequency (∼1089 cm−1) is near to the frequency difference between the pump and seed radiation. It is shown that, despite a narrowband seed, the generated Stokes pulse spectrum mimics the pump pulse spectral width. The observed SRS conversion efficiency saturates at 40%, with a weak dependence on the seed pulse energy and on the detuning of the pump-seed frequency from the Raman resonance. Theoretical modeling confirms the observed effects and permits prediction of the characteristics of the investigated system as its parameters are varied. [ABSTRACT FROM AUTHOR]

We computationally studied the effect of nuclear-quadrupole interactions on the field-free impulsive alignment of different asymmetric-top molecules. Our analysis is focused on the influence of the hyperfine- and rotational-energy-level structures. These depend on the number of nuclear spins, the rotational constants, and the symmetry of the tensors involved in the nuclear spin and external field interactions. Comparing the prototypical large-nuclear-spin molecules iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, and 2,5-diiodobenzonitrile, we demonstrate that the magnitude of the hyperfine splittings compared to the rotational-energy splittings plays a crucial role in the spin-rotational dynamics after the laser pulse. Moreover, we point out that the impact of the quadrupole coupling on the rotational dynamics decreases when highly excited rotational states dominate the dynamics. [ABSTRACT FROM AUTHOR]

Laser pulses were applied to a target mounted on a ballistic pendulum to study the momentum imparted by a laser shock impact. Photonic Doppler Velocimetry was used to assess the momentum imparted by each laser pulse. To increase the momentum produced, a layer of polymer transparent to the laser wavelength was applied to the surface of the targets to confine the plasma generated as a result of the laser–matter interaction. This yielded momentum coupling coefficients one hundred times higher than those obtained for equivalent laser parameters in the classical direct regime configuration. The study was completed by simulating the experiments with the one-dimensional Lagrangian hydrodynamics code ESTHER, which showed good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Laser annealing was employed to trigger the solid-state reaction of a thin Co film (2.5 nm) with undoped Si. A metastable disilicide layer was obtained after one laser pulse close to the melt threshold. Its diffraction pattern, relaxed lattice parameter, and residual resisitivity are consistent with the formation of the defective CsCl structure. The CoSi2 phase was found after prolonging the thermal treatment with additional pulses or rapid thermal annealing. Because CoSi is skipped in the phase sequence, CoSi2 layers are more uniform in thickness, have an increased superconductivity and a reduced formation temperature. This approach is compatible with the SALICIDE process and can be used to form smooth contacts in superconducting or regular transistors. [ABSTRACT FROM AUTHOR]

An investigation into second-harmonic generation from magnetized anharmonic carbon nanotubes (CNTs) mounted on a silica substrate is conducted using a Gaussian laser pulse with modulated amplitude. An intense amplitude-modulated laser pulse incident on the array of CNTs displaces their electrons generating a restoration force. This restoration force is the nonlinear function of displacement of electrons, which ensures the anharmonic behavior of CNTs. Using the paraxial ray approximation, the nonlinear interaction of the incident pulse with CNTs is expressed in terms of a wave equation derived using the perturbative technique. The nonlinear current at second-harmonic frequency arises due to the perturbation of the electron density of CNTs by the ponderomotive force exerted on them by an incident pulse. Numerical outcomes validate the enhanced efficiency of the generated harmonic when considering the phenomenon of self-focusing. With the substantial optical nonlinearities of an anharmonic CNT structure, the plasmon resonance is broadened and high efficiency in generating harmonics is attained. It is seen that the augmenting of the amplitude-modulated parameter of the pulse and the external magnetic field strength enhances system nonlinearity, resulting in increased amplitude of generated second harmonics. Additionally, the effect of the heating rate of CNTs on the efficiency of the generated harmonic is also discussed. [ABSTRACT FROM AUTHOR]

Electron dynamics of the Na10 chain and the Na10–N2 complex locally excited by an atomistic optical near-field are investigated using real-time time-dependent density functional theory calculations on real-space grids. Ultrafast laser pulses were used to simulate the near-field excitation under on- and off-resonance conditions. Off-resonance excitation did not lead to the propagation of the excitation through the Na10 chain. In contrast, under the resonance conditions, the excited state is delocalized over the entire Na chain. Analysis of the local dipole moment of each atom in Na10 indicates that this behavior is consistent with the transition density. Adding an N2 molecule to the opposite end of the local excitation region results in energy transfer via the Na10 chain. The energy transfer efficiency of the N2 molecule is well correlated with the absorption spectrum of Na10. The present study paves the way for realizing remote excitation and photonic devices at the atomic scale. [ABSTRACT FROM AUTHOR]

Quartz material irradiated by femtosecond laser has increasingly attracted widespread attention for the micro-fabrication of photonic devices. Mechanism exploration is beneficial for accelerating the digital progress of laser processing. However, the mechanism between femtosecond laser and quartz is complicated and needs further theoretical investigation. This paper established the theoretical model based on the ionization model with the Drude equation to study the space–time evolution of free electron density and its influence on the absorption coefficient, reflectivity, and ablation depth. In addition, we achieved a 10 × 10 micro-holes array with a pore size less than 10 μm, cone angle less than 2° in a 0.25 mm thick quartz on the condition of a laser pulse energy Ep = 3 μJ, scanning velocity v = 0.1 mm/s, and defocusing distance Δf = −0.3 mm via the bottom-up femtosecond laser processing. The work gives a new insight into further understanding the ablation mechanism of transparent materials etching by the femtosecond laser. It provides a practical technical scheme for preparing commercial quartz photonic devices. [ABSTRACT FROM AUTHOR]

Here, we investigate the hypothesis that despite the existence of at least two high-density amorphous ices, only one high-density liquid state exists in water. We prepared a very-high-density amorphous ice (VHDA) sample and rapidly increased its temperature to around 205 ± 10 K using laser-induced isochoric heating. This temperature falls within the so-called "no-man's land" well above the glass-liquid transition, wherein the IR laser pulse creates a metastable liquid state. Subsequently, this high-density liquid (HDL) state of water decompresses over time, and we examined the time-dependent structural changes using short x-ray pulses from a free electron laser. We observed a liquid–liquid transition to low-density liquid water (LDL) over time scales ranging from 20 ns to 3 μs, consistent with previous experimental results using expanded high-density amorphous ice (eHDA) as the initial state. In addition, the resulting LDL derived both from VHDA and eHDA displays similar density and degree of inhomogeneity. Our observation supports the idea that regardless of the initial annealing states of the high-density amorphous ices, the same HDL and final LDL states are reached at temperatures around 205 K. [ABSTRACT FROM AUTHOR]

Ultrafast laser technology presents the unique capacity to process glass materials with an outstanding processing quality; however, combining high quality and high throughput is still a crucial issue because glass is brittle and highly heat sensitive. One strategy to overcome this limitation is to split in space the main laser beam into multiple beams for process parallelization. In the present paper, the simultaneous interaction of several femtosecond laser beams at the surface of fused silica targets is addressed experimentally and theoretically. This work is devoted to highlight the beams cooperation for inducing stress in the material. The experiment consists in irradiating the target with multiple laser pulses with a wavelength of 1030 nm and a duration of 500 fs. The induced stress is observed through post-mortem cross-polarized microscopy. A multiscale and multiphysics model describing laser energy deposition into the material and its mechanical response is developed. The influence of various laser parameters is studied: number and position of laser beams, repetition rate, and fluence. Both experimental and modeling results, which are in a good agreement, show significant cooperative effects for stress formation with large enough laser energy deposition, possibly leading to detrimental cracks. [ABSTRACT FROM AUTHOR]

Lanthanide-doped upconversion (UC) luminescent materials display multicolor emissions, making them ideal for a variety of applications, such as multi-channel biological imaging, fluorescence encryption, anti-counterfeiting, and 3D display. Manipulating the UC emissions of the luminescent materials with a fixed composition is crucial for their applications. Herein, we propose a facile strategy to achieve pulse-width-dependent multicolor UC emissions in NaYF4:Yb/Er/Tm nanocrystals. Upon excitation with a 980 nm continuous-wave laser diode, Er3+ ions in NaYF4:20%Yb,15%Er,1%Tm nanocrystals exhibited UC emissions with a red-to-green (R/G) ratio of 11.3. Nevertheless, by employing a 980 nm pulse laser with pulse widths from 0.1 to 10 ms, the UC R/G ratio can be easily adjusted from 0.9 to 11.3, resulting in continuous and remarkable color transformation from green, yellow, orange, to red. By virtue of the dynamic luminescence color variation of these NaYF4:20%Yb,15%Er,1%Tm nanocrystals, we demonstrated their potential applications in the areas of anti-counterfeiting and information encryption. These findings provide deep insights into the excited-state dynamics and energy transfer of Er3+ in NaYF4:Yb/Er/Tm nanocrystals upon 980 nm pulse excitation, which may pave the way for designing multicolor UC materials toward versatile applications. [ABSTRACT FROM AUTHOR]

Water can be vitrified if it is cooled at high rates, which makes it possible to outrun crystallization in so-called no man's land, a range of deeply supercooled temperatures where water crystallizes rapidly. Here, we study the reverse process in pure water samples by flash melting amorphous ice with microsecond laser pulses. Time-resolved electron diffraction reveals that the sample transiently crystallizes despite a heating rate of more than 5 × 106 K/s, even though under the same conditions, vitrification can be achieved with a similar cooling rate of 107 K/s. Moreover, we observe different crystallization kinetics for amorphous solid water and hyperquenched glassy water. These experiments open up new avenues for elucidating the crystallization mechanism of water and studying its dynamics in no man's land. They also add important insights into the laser melting and revitrification processes that are integral to the emerging field of microsecond time-resolved cryo-electron microscopy. [ABSTRACT FROM AUTHOR]

Transient reflectivity measurements are used to probe the strain waves induced by ultrashort laser pulses in bulk [100] germanium. The measurement signals are compared to purely analytical model functions based on the known material parameters for germanium. The modeling includes (i) a derivation of analytical solutions to the wave equation for strain waves coupled to the diffusion equation for heat and charge carriers and (ii) an expression for the impact on reflection coefficients that are caused by perturbations to the dielectric function but extended to cover a non-isotropic, uniaxial dielectric tensorial form. The model is held up against transient reflectivity measurements with an s- and a p-polarized probe and with a probe wavelength in the range of 502–710 nm. Excellent agreement is found when comparing the oscillatory shape of the measurement signals to the models. As for the magnitude of the oscillations, the models reproduce the overall trends of the experiment when using the previously published values for the elasto-optical tensor measured under static strain. [ABSTRACT FROM AUTHOR]

Electro-optic sampling of terahertz waves by noncollinearly propagating femtosecond laser pulses in electro-optic crystals can provide high efficiency and high spectral resolution of terahertz detection with various types of crystals and laser wavelengths, unlike the conventional collinear scheme. We develop an analytical theory of noncollinear electro-optic sampling detection technique that describes the modulation of the probe laser beam polarization as a result of nonlinear interaction between the optical and terahertz fields. The theory accounts for finite widths of the terahertz and probe beams. It is found that noncollinear scheme operates as a low-pass terahertz filter with the frequency cut-off determined by the width of the probe beam and the crossing angle of the terahertz and probe beams. We apply the theory to two practical situations: sampling of terahertz waves by fiber laser pulses (1.55 μ m wavelength) in a GaAs crystal and sampling by Ti:sapphire laser pulses (800 nm wavelength) in a LiNbO 3 crystal. [ABSTRACT FROM AUTHOR]

Interaction of intense ultrashort laser pulse with gases generates a transient spatiotemporal electron density distribution via field ionization, which may lead to the spatiotemporal reshaping of the pulse, viz., its beam profile, pulse width, etc. Here, we present an experimental study on ultrashort laser pulse interaction with argon gas sheath in an ionization saturation intensity regime (∼1015–1017 W/cm2). The present investigation has been performed using a 6 mJ, 1 kHz, and 55−60 fs Ti:Sapphire laser pulse interaction with a ∼2.5 mm long argon sheath. After the laser gas interaction, the laser spatial profile exhibits a multi-ring structure around a central maximum spot. Laser gas interaction parameters, such as laser intensity, gas pressure, etc., affect the ring pattern significantly. Under optimum parameter conditions, the laser pulse has two rings in spatial profile, and the pulse width of the central spot is self-compressed to ∼35 fs. A theoretical calculation reveals that the laser beam's spatiotemporal profile evolves as it propagates inside the gas sheath. The calculation also demonstrates that the gas ionization profile plays a crucial role in the spatiotemporal reshaping and self-compression of the laser beam. The calculation also shows that the generation of concentric ring patterns in the spatial profile is mainly due to the ionization of argon atoms into Ar+, Ar2+, and Ar3+ species in the interaction region. Such self-compressed laser pulses with concentric ring beam profiles may be useful for high-harmonic generation and shorter attosecond pulse trains. [ABSTRACT FROM AUTHOR]

This work is part of a prediction challenge that invited theoretical/computational chemists to predict the photochemistry of cyclobutanone in the gas phase, excited at 200 nm by a laser pulse, and the expected signal that will be recorded during a time-resolved megaelectronvolt ultrafast electron diffraction (MeV-UED). We present here our theoretical predictions based on a combination of trajectory surface hopping with XMS-CASPT2 (for the nonadiabatic molecular dynamics) and Born–Oppenheimer molecular dynamics with MP2 (for the athermal ground-state dynamics following internal conversion), coined (NA+BO)MD. The initial conditions were sampled from Born–Oppenheimer molecular dynamics coupled to a quantum thermostat. Our simulations indicate that the main photoproducts after 2 ps of dynamics are CO + cyclopropane (50%), CO + propene (10%), and ethene and ketene (34%). The photoexcited cyclobutanone in its second excited electronic state S 2 can follow two pathways for its nonradiative decay: (i) a ring-opening in S 2 and a subsequent rapid decay to the ground electronic state, where the photoproducts are formed, or (ii) a transfer through a closed-ring conical intersection to S 1 , where cyclobutanone ring opens and then funnels to the ground state. Lifetimes for the photoproduct and electronic populations were determined. We calculated a stationary MeV-UED signal [difference pair distribution function— Δ PDF (r) ] for each (interpolated) pathway as well as a time-resolved signal [ Δ PDF (r , t) and Δ I / I (s , t) ] for the full swarm of (NA+BO)MD trajectories. Furthermore, our analysis provides time-independent basis functions that can be used to fit the time-dependent experimental UED signals [both Δ PDF (r , t) and Δ I / I (s , t) ] and potentially recover the population of photoproducts. We also offer a detailed analysis of the limitations of our model and their potential impact on the predicted experimental signals. [ABSTRACT FROM AUTHOR]

The investigation of the highest surface temperature and damage region of silicon-based photodiodes (PIN) was conducted through irradiation with millisecond (ms) pulse lasers. The convex spots on the surface of the biased photodiode were observed to be diminished by a millisecond pulse laser for the first time. The experimental results presented herein demonstrate the presence of a bump, even in cases where the maximum surface temperature of the damaged area does not exceed the melting point. The mechanism underlying this phenomenon was elucidated through the integration of simulation and experimentation in our study. The irradiation of silicon-based semiconductor detectors with lasers generates internal Joule heat, causing the temperature at the junction depth to initially reach the melting point. The expansion resulting from the Si phase transition induces outward pressure on Si3N4, leading to the eventual formation of a convex morphology. The findings of our study present a novel approach to enhance the security of photodetectors. [ABSTRACT FROM AUTHOR]

Dual-laser beams interacting with small droplets of liquid tin are currently the most efficient systems for producing the 13.5 nm EUV photon radiation source required for the next generation microchips. Usually, EUV light is produced during the second main-pulse stage, while the pre-pulse (PP) is used for target preparation, i.e., droplet preheating, vaporization, and target deformation. However, the PP laser energy can be utilized more efficiently if the EUV producing plasma is being developed during the PP stage as well. In this work, we study the ways of optimization of the PP laser temporal shape to achieve conditions for maximum EUV output during the pre-pulse. The size of the deformed droplet is kept optimized for the following main laser pulse. Our simulations showed a significant increase in the EUV output at the pre-pulse stage when a ramping profile is used for the laser temporal shape. Using the ramped square pre-pulse produces 24% gain in the EUV output in comparison with the standard Gaussian temporal profile (i.e., regular Nd:YAG shape) for the same energy of the laser pulse. [ABSTRACT FROM AUTHOR]

This study presents photothermal imaging results of subsurface material defects within fiber metal laminates utilizing the virtual wave concept. Therefore, we theoretically analyze the propagation of the virtual wave signal in a hybrid composite laminate via the method of images. For provoking local material damage, the hybrid composite sample is subjected to a defined impact loading. The results obtained from photothermal defect imaging, utilizing rectangular laser pulse excitation, are compared with results obtained from 3D x-ray computed tomography. To sum up, we demonstrate a fast, non-invasive, and easily interpretable reconstruction of defects within macroscopic hybrid composite laminates based on the virtual wave concept. [ABSTRACT FROM AUTHOR]

The N–H photodissociation dynamics of the pyridinyl radical upon continuous excitation to the optically bright, first excited ππ* electronic state by an ultra-violet (UV) laser pulse has been investigated within the mathematical framework of optimal control theory. The genetic algorithm (GA) is employed as the optimization protocol. We considered a three-state and three-mode model Hamiltonian, which includes the reaction coordinate, R (a1 symmetry); the coupling coordinates (namely, out-of-plane bending coordinate of the hydrogen atom of azine group), Θ (b1 symmetry); and the wagging mode, Q9 (a2 symmetry). The three electronic states are the ground, ππ*, and πσ* states. The πσ* state crosses both the ground state and the ππ* state, and it is a repulsive state on which N–H dissociation occurs upon photoexcitation. Different vibrational wave functions along the coupling coordinates, Θ and Q9, of the ground electronic state are used as the initial condition for solving the time-dependent Schrödinger equation. The optimal UV laser pulse is designed by applying the GA, which maximizes the dissociation yield. We obtained over 95% dissociation yield through the πσ* asymptote using the optimal pulse of a time duration of ∼30 000 a.u. (∼725.66 fs). [ABSTRACT FROM AUTHOR]

Controlling molecular reactivity by shaped laser pulses is a long-standing goal in chemistry. Here, we suggest a direct optimal control approach that combines external pulse optimization with other control parameters arising in the upcoming field of vibro-polaritonic chemistry for enhanced controllability. The direct optimal control approach is characterized by a simultaneous simulation and optimization paradigm, meaning that the equations of motion are discretized and converted into a set of holonomic constraints for a nonlinear optimization problem given by the control functional. Compared with indirect optimal control, this procedure offers great flexibility, such as final time or Hamiltonian parameter optimization. A simultaneous direct optimal control theory will be applied to a model system describing H-atom transfer in a lossy Fabry–Pérot cavity under vibrational strong coupling conditions. Specifically, optimization of the cavity coupling strength and, thus, of the control landscape will be demonstrated. [ABSTRACT FROM AUTHOR]

Two-dimensional terahertz–infrared–visible (2D TIRV) spectroscopy directly measures the coupling between quantum high-frequency vibrations and classical low-frequency modes of molecular motion. In addition to coupling strength, the signal intensity in 2D TIRV spectroscopy can also depend on the selection rules of the excited transitions. Here, we explore the selection rules in 2D TIRV spectroscopy by studying the coupling between the high-frequency CH3 stretching and low-frequency vibrations of liquid dimethyl sulfoxide (DMSO). Different excitation pathways are addressed using variations in laser pulse timing and different polarizations of exciting pulses and detected signals. The DMSO signals generated via different excitation pathways can be readily distinguished in the spectrum. The intensities of different excitation pathways vary unequally with changes in polarization. We explain how this difference stems from the intensities of polarized and depolarized Raman and hyper-Raman spectra of high-frequency modes. These results apply to various systems and will help design and interpret new 2D TIRV spectroscopy experiments. [ABSTRACT FROM AUTHOR]

Stimulated Raman Adiabatic Passage (STIRAP) and its variants, such as M-type chainwise-STIRAP, allow for efficiently transferring the populations in a multilevel system and have widely been used to prepare molecules in their rovibrational ground state. However, their transfer efficiencies are generally imperfect. The main obstacle is the presence of losses and the requirement to make the dynamics adiabatic. To this end, in the present paper, a new theoretical method is proposed for the efficient and robust creation and detection of deeply bound molecules in three-level Λ-type and five-level M-type systems via "Invariant-based shortcut-to-adiabaticity." In the regime of large detunings, we first reduce the dynamics of three- and five-level molecular systems to those of effective two- and three-level counterparts. By doing so, the major molecular losses from the excited states can be well suppressed. Consequently, the effective two-level counterpart can be directly compatible with two different "Invariant-based Inverse Engineering" protocols; the results show that both protocols give a comparable performance and have a good experimental feasibility. For the effective three-level counterpart, by considering a relation among the four incident pulses, we show that this model can be further generalized to an effective Λ-type one with the simplest resonant coupling. This generalized model permits us to borrow the "Invariant-based Inverse Engineering" protocol from a standard three-level Λ-type system to a five-level M-type system. Numerical calculations show that the weakly bound molecules can be efficiently transferred to their deeply bound states without strong laser pulses, and the stability against parameter variations is well preserved. Finally, the detection of ultracold deeply bound molecules is discussed. [ABSTRACT FROM AUTHOR]

The described method is intended for application as a diagnostic tool for a nonstationary, short-lived plasma (in particular, for the laser-produced plasma). It is based on taking into account the lifetime of a laser-produced plasma, which is so short (several nanoseconds) that it is not enough for the ionization equilibrium to be established. Among mechanisms leading to appearance of an ion with a given charge Z in the plasma, only the electron-collisional ionization is considered, because contributions of other phenomena turn out to be negligible. The method is discussed as an example of a plasma excited on the Xe gas-jet target. The necessary collisional cross sections of ions from+7Xe to+16Xe have been calculated specifically for this study using a quantum-mechanical numerical simulation, with its principles and features being also presented in the paper. To demonstrate capabilities of the method, it has been applied to one of the experimental cases when the plasma was produced by the laser beam focused on the Xe gas-jet target. The time-integrated energy of laser radiation absorbed in the plasma was measured, and the absorption coefficient, μ, was derived from it with a correction for the plasma lifetime, which was several times shorter than the laser pulse. Using the method described here, the values of ⟨ Z ⟩ and then μ were calculated as a function of temperature. The time-averaged plasma temperature, T, in the above-mentioned experiment was believed to be equal to that at which the calculated and experimentally determined values of μ coincided. The following results were obtained: T = 42 eV, ⟨ Z ⟩ = 10.2. [ABSTRACT FROM AUTHOR]

Photoelectron angular distributions (PADs) produced from the photoionization of chiral molecules using elliptically polarized light exhibit a forward/backward asymmetry with respect to the optical propagation direction. By recording these distributions using the velocity-map imaging (VMI) technique, the resulting photoelectron elliptical dichroism (PEELD) has previously been demonstrated as a promising spectroscopic tool for studying chiral molecules in the gas phase. The use of elliptically polarized laser pulses, however, produces PADs (and consequently, PEELD distributions) that do not exhibit cylindrical symmetry about the propagation axis. This leads to significant limitations and challenges when employing conventional VMI acquisition and data processing strategies. Using novel photoelectron image analysis methods based around Hankel transform reconstruction tomography and machine learning, however, we have quantified—for the first time—significant symmetry-breaking contributions to PEELD signals that are of a comparable magnitude to the symmetric terms in the multiphoton ionization of (1R,4R)-(+)- and (1S,4S)-(−)-camphor. This contradicts any assumptions that symmetry-breaking can be ignored when reconstructing VMI data. Furthermore, these same symmetry-breaking terms are expected to appear in any experiment where circular and linear laser fields are used together. This ionization scheme is particularly relevant for investigating dynamics in chiral molecules, but it is not limited to them. Developing a full understanding of these terms and the role they play in the photoionization of chiral molecules is of clear importance if the potential of PEELD and related effects for future practical applications is to be fully realized. [ABSTRACT FROM AUTHOR]

Verrucous venous malformation (VVM) is a rare vascular malformation with hyperkeratosis. Concomitant laser and surgery are first line treatments, but evidence establishing efficacy is limited. We assess the efficacy of laser alone for VVM. Retrospective analysis of patients receiving laser treatment for VVM between 1994–2023. Of 83 patients, 29 were excluded due to missing data, 7 due to concomitant surgical excision and 47 were evaluated. Pulse dye laser (PDL) and combined dual PDL-neodymium-doped yttrium aluminium garnet (PDL-Nd:YAG) lasers were used. Outcomes were difference in percentage surface area (SA) reduction and colour improvement after laser treatment. Two blinded healthcare professionals independently graded the VVMs. Tools used included a numerically graded colour chart, and a 10 × 10 surface area grid to assess pre/post photos printed to the same scale. Mean Joules delivered with PDL; 9.29 and PDL-Nd:YAG; 9.16. Spot size (mm) for PDL; 8.58 and PDL-Nd:YAG; 9.63. Mean number of treatments was 5.27 and 3.68 in red and purple lesions respectively. Mean SA reduction for red and purple lesions; PDL; 71.11% and 6.67%, and PDL-Nd:YAG; 54.30% and 32.35%. Mean colour improvement in red and purple; PDL; 53.13% and 8.59%, and PDL-Nd:YAG; 46.88% and 40.81%. Red responded better than purple (p = 0.0014 and p = 0.024), for SA and colour respectively. There was no statistical significance in better outcomes; age at first treatment or number of treatments. Laser alone is an effective non-invasive method for improvement of colour and SA. Red lesions responded better to laser. PDL-Nd:YAG laser is preferred in purple lesions. [ABSTRACT FROM AUTHOR]

Spontaneous emission has been studied by physicists for decades and continues to reveal exciting physics. We describe an experiment to study the spontaneous emission of a monoatomic vapor at room temperature as a fundamental experiment for upper-level physics undergraduates. The experiment begins by exciting a population of Rb atoms in a room temperature vapor cell with a laser pulse shorter than the average lifetime of the excited states. The resulting fluorescence signal is recorded as a function of time, and the excited state lifetime can be determined by measuring the decay rate. By analyzing the Fourier transform of the time-dependent polarized fluorescence signal, quantum interference (i.e., quantum beating) is observed among the hyperfine energy levels. This experiment can be completed by upper-level undergraduates in physics to demonstrate and connect hands-on experiments with concepts in atomic and quantum physics classes. Editor's Note: This paper presents a modern take on the quantum beat experiment. The authors use a commercially available distributed feedback laser, along with electro-optical and acousto-optical modulators, to generate short laser pulses. These laser pulses are used to excite rubidium atoms into a superposition of multiple atomic states, and the resulting polarized fluorescence decay data are collected. The necessary quantum theory for analyzing these data is presented and then used to determine the lifetime of the Rb excited state. In addition, the oscillatory behavior present in the decay data is analyzed via a Fourier transform, yielding values for the frequency differences of the excited Rb hyperfine levels. This project offers a clear demonstration of important quantum concepts and is suitable as an instructional laboratory for advanced undergraduates. [ABSTRACT FROM AUTHOR]

Tungsten and tungsten carbide were damaged in ambient air with varying incident angles (0, 30, 45, and 60 deg) for approximately 5000 shots. The goal of these experiments was to observe the macroscopic surface modification in tungsten and tungsten carbide surfaces in harsh environments. At low pulse numbers (one to eight laser pulses on the same spot), tungsten aerial surface damage was less than tungsten carbide damage; however, at very high pulse numbers (5000), the opposite was true. Surface damage was mostly in the form of craters that were near circular at low impact angles and became more elongated at higher laser pulse impact angles. On the tungsten surface, a cluster of tungsten oxide debris formed. During laser exposure, laser-induced periodic surface structures and grooves were formed, and their geometries varied with laser intensity and laser impact angle. The period of laser-induced surface changes increased as the incident angle increased for both tungsten and tungsten carbide surfaces. More mass was lost in tungsten than tungsten carbide, which agrees with the morphological responses. [ABSTRACT FROM AUTHOR]

The laser frequency spectrum, whose full width at half-maximum in the frequency domain is the laser bandwidth, plays a critical role in atomic multi-step photoionization processes and has a significant influence on isotopic selective photoionization results. In this study, two phase-independent frequency spectrum models, that is, the mode jitter model (MJ model) based on the mode jitter phenomenon and the multi-longitudinal mode model (MLM model) based on multi-longitudinal mode output, are proposed to describe the spectral characteristics in the frequency domain. In the MJ model, it is assumed that there is a time-varying longitudinal mode in each laser pulse, with different central frequencies between adjacent pulses in the pulse train; in the MLM model, it is assumed that multiple longitudinal modes are output simultaneously with fixed central frequencies. Selective photoionization properties of these two frequency spectrum models and the chaotic field model are simulated and compared with each other under the Lorentzian frequency spectrum condition. Influences of the excitation intensity, laser frequency spectral profile and cutoff frequency on selective photoionization processes are calculated and compared among the above-mentioned three frequency spectrum models. Finally, the measurement of the laser frequency spectrum and the introduction of individual longitudinal modes' bandwidths are discussed. [ABSTRACT FROM AUTHOR]

Owing to the wide tuning range, high-temperature stability, flexible design, and fast recovery dynamics, quantum dots (QDs) have been widely used in many research fields, such as lasers, photodetectors, solar cells, and displays. Among these, III–V InAs/GaAs QDs are especially suited for infrared optoelectronic applications due to their strong working stability and enhanced light output efficiency. The doping technique has been admitted as a very effective method to enhance the modal gain and temperature insensitivity for InAs/GaAs QDs-based devices. Additionally, doping could modulate critical optical responses, like light absorption and carrier recovery dynamics, making these QDs ideal for ultrafast applications. In this work, p-type (Be) doping and n-type (Si) doping were introduced into multilayer QDs to mitigate the adverse effects of grown-in interface states, which were verified by temperature-dependent photoluminescence (PL) characterization. The results show a significant increase in PL intensity for n-doped samples, which was attributed to the effective suppression of interface states. This enhancement correlates with doping levels, with PL intensity increasing from 1.71 to 2.72 times as Si concentration rises from 1 × 1018/cm3 to 3 × 1018/cm3. In contrast, p-doped samples show a slight decrease in PL intensity and a 20 nm blueshift in PL emission peak, indicating different interdiffusion behaviors between In and Ga atoms compared to that in n-doped ones. By integrating with distributed Bragg mirrors, QD semiconductor saturable absorption mirrors were developed, where n-doped QDs presented superior performance in mode-locked ultrafast lasers with the shortest pulse width and highest output power. [ABSTRACT FROM AUTHOR]

The time-resolved detection of mid-to far-infrared electric fields absorbed and emitted by molecules is among the most sensitive spectroscopic approaches and has the potential to transform sensing in fields such as security screening, quality control, and medical diagnostics. However, the sensitivity of the standard detection approach, which relies on encoding the far-infrared electric field into amplitude modulation of a visible or near-infrared probe laser pulse, is limited by the shot noise of the latter. This constraint cannot be overcome without using a quantum resource. Here, we show that this constraint can be overcome using a two-mode squeezed state. Quantum-correlated ultrashort pulses, generated by parametric down-conversion, enhance the sensitivity of far-infrared detection beyond the classical limit, achieving a twofold reduction in measured noise. This advancement paves the way for further development of ultrafast quantum metrology, moving toward quantum-enhanced time-resolved electric field spectroscopy with sensitivities beyond the standard quantum limit. [ABSTRACT FROM AUTHOR]

The propagation of an intense, femtosecond, mid-infrared laser pulse in a gaseous medium results in the efficient generation of spectrally overlapping low-order harmonics, whose optical carrier phases are linked to the carrier-envelope phase (CEP) of the mid-infrared driver pulse. Random peak-power fluctuations of the driver pulses, converted to the fluctuations of the nonlinear phases, acquired by the pulses on propagation, cause this phase correlation to smear out. We show that this seemingly irreversible loss of phase can be recovered, and that the complete information needed for the phase correction is contained in the harmonic spectra itself. The optical phases of the intense driver pulse and its harmonics, as fragile as they appear to be against even weak disturbances, evolve deterministically during highly nonlinear propagation through the extended ionization region. Shot-to-shot fluctuations of peak power of intense, mid-infrared laser pulses, propagating in a gaseous medium, result in large shifts of the optical phases of the pulses. The authors show that the phase information is, in fact, preserved and is recoverable by suitable post-processing of spectra of the low-order harmonics generated on propagation. [ABSTRACT FROM AUTHOR]

Laser-plasma acceleration of protons offers a compact, ultra-fast alternative to conventional acceleration techniques, and is being widely pursued for potential applications in medicine, industry and fundamental science. Creating a stable, collimated beam of protons at high repetition rates presents a key challenge. Here, we demonstrate the generation of multi-MeV proton beams from a fast-replenishing ambient-temperature liquid sheet. The beam has an unprecedentedly low divergence of 1° (≤20 mrad), resulting from magnetic self-guiding of the proton beam during propagation through a low density vapour. The proton beams, generated at a repetition rate of 5 Hz using only 190 mJ of laser energy, exhibit a hundred-fold increase in flux compared to beams from a solid target. Coupled with the high shot-to-shot stability of this source, this represents a crucial step towards applications. Applications of laser-plasma accelerated protons in fundamental, applied and medical sciences crucially depend on the creation of stable collimated beams with high repetition rates. Here the authors demonstrate the generation of multi-MeV protons at 5 Hz, with low (degree-level) proton beam divergence from a laser pulse focused onto a water sheet target, potentially mitigating the need for beam capturing techniques. [ABSTRACT FROM AUTHOR]

Diffraction-before-destruction imaging with ultrashort X-ray pulses can visualize non-equilibrium processes, such as chemical reactions, with sub-femtosecond precision in the native environment. Here, a nanospecimen diffracts a single X-ray flash before it disintegrates. The sample structure can be reconstructed from the coherent diffraction image (CDI). State-of-the-art X-ray snapshots lack high spatial resolution because of weak diffraction signal. Bleaching effects from photo-ionization significantly restrain image brightness scaling. We find that non-linear transient ion resonances can overcome this barrier if X-ray laser pulses are shorter than in most experiments. We compared snapshots from individual ≈ 100 nm Xe nanoparticles as a function of pulse duration and incoming X-ray fluence. Our experimental results and Monte Carlo simulations suggest that transient resonances can increase ionic scattering cross sections significantly beyond literature values. This provides a novel avenue towards substantial improvement of the spatial resolution in CDI in combination with sub-femtosecond temporal precision at the nanoscale. Diffraction-before-destruction of ultrashort X-ray pulses can visualize non-equilibrium processes at the nanoscale with sub-femtosecond precision. Here, the authors demonstrate how the brightness and the spatial resolution of such snapshots can be substantially increased despite ionization. [ABSTRACT FROM AUTHOR]

Laser-directed energy deposition (L-DED) is known for its high cooling and solidification rates in the molten pool, which is influenced by the self-quenching effect. This rapid cooling, combined with the layer-by-layer deposition approach, results in highly directional grain growth and anisotropy in the deposited components. This study examines how modulating the laser in pulse mode affects anisotropy in two deposition strategies: overlaying (thin wall deposition with single tracks) and overlapping plus overlaying (thick wall deposition with overlapping tracks and overlaying layers). By varying the duty cycle (DC) from 40 to 100% in 20% increments, while maintaining a laser frequency of 100 Hz, the impact on the molten pool's state was analyzed. An IR pyrometer, operating at 1 kHz, monitored the transition between liquid and solid states. For a 40% DC with pulse durations of 4 ms on-time and 6 ms off-time, the molten pool was found to change its state completely from liquid to solid, creating multiple solidification fronts and columnar dendrites with varied orientations while continuous wave mode exhibited directional grain growth. Modulated mode, with intermittent cooling, also led to finer microstructures and enhanced hardness. The mechanical strength and ductility improved, and anisotropy was found to reduce with deposition in laser-modulated mode. Digital image correlation (DIC) analysis of tensile tests revealed that strain localization was more uniform in modulated mode at 40% DC across all orientations, whereas continuous wave mode showed pronounced strain localization mainly at 0°, with broader zones at 45° and 90°, reflecting the impact of fusion lines on anisotropy. [ABSTRACT FROM AUTHOR]