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The use of semiconductor nanocrystals in scalable quantum technologies requires characterization of the exciton coherence dynamics in an ensemble of electronically isolated crystals in which system–bath interactions are nevertheless strong. In this communication, we identify signatures of Fano-like interference between excitons and photocarriers in the coherent two-dimensional photoluminescence excitation spectral lineshapes of mixed lead-halide perovskite nanocrystals in dilute solution. Specifically, by tuning the femtosecond-pulse spectrum, we show such interference in an intermediate coupling regime, which is evident in the coherent lineshape when simultaneously exciting the exciton and the free-carrier band at higher energy. We conclude that this interference is an intrinsic effect that will be consequential in the quantum dynamics of the system and will thus dictate decoherence dynamics, with consequences in their application in quantum technologies. [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]

When used as pump pulses in transient absorption spectroscopy measurements, femtosecond laser pulses can produce oscillatory signals known as quantum beats. The quantum beats arise from coherent superpositions of the states of the sample and are best studied in the Fourier domain using Femtosecond Coherence Spectroscopy (FCS), which consists of one-dimensional amplitude and phase plots of a specified oscillation frequency as a function of the detection frequency. Prior works have shown ubiquitous amplitude nodes and π phase shifts in FCS from excited-state vibrational wavepackets in monomer samples. However, the FCS arising from vibronic-exciton states in molecular aggregates have not been studied theoretically. Here, we use a model of vibronic-exciton states in molecular dimers based on displaced harmonic oscillators to simulate FCS for dimers in two important cases. Simulations reveal distinct spectral signatures of excited-state vibronic-exciton coherences in molecular dimers that may be used to distinguish them from monomer vibrational coherences. A salient result is that, for certain relative orientations of the transition dipoles, the key resonance condition between the electronic coupling and the frequency of the vibrational mode may yield strong enhancement of the quantum-beat amplitude and, perhaps, also cause a significant decrease of the oscillation frequency to a value far lower than the vibrational frequency. Future studies using these results will lead to new insights into the excited-state coherences generated in photosynthetic pigment–protein complexes. [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]

Alkali trimers, Ak3, located on the surface of He nanodroplets are triply ionized following multiphoton absorption from an intense femtosecond laser pulse, leading to fragmentation into three correlated Ak+ ions. Combining the information from threefold covariance analysis of the emission direction of the fragment ions and their kinetic energy distributions P(Ekin), we find that Na3, K3, and Rb3 have an equilateral triangular structure, corresponding to that of the lowest lying quartet state A 2 ′ 4 , and determine the equilibrium bond distance Req(Na3) = 4.65 ± 0.15 Å, Req(K3) = 5.03 ± 0.18 Å, and Req(Rb3) = 5.45 ± 0.22 Å. For K3 and Rb3, these values agree well with existing theoretical calculations, while for Na3, the value is 0.2–0.3 Å larger than the existing theoretical results. The discrepancy is ascribed to a minor internuclear motion of Na3 during the ionization process. In addition, we determine the distribution of internuclear distances P(R) under the assumption of fixed bond angles. The results are compared to the square of the internuclear wave function |Ψ(R)|2. [ABSTRACT FROM AUTHOR]

In this study, the hydrodynamic expansion and splitting effect of ultrafast laser-induced plasma plume for multi-element alloys were studied. A fully coupled hydrodynamic model for femtosecond laser ablation of multi-element alloys was presented to study the ambipolar electric field during the plume expansion process. The model utilized a level-set equation to capture the interface between the condensed phase and the gaseous phase and accounted for the reaction between different species. A time-gate direct fluorescence measurement was conducted for the target material of brass, a Cu–Zn alloy, to validate the simulation results. The simulation results showed good agreement with the experimental results and were able to predict the plume-splitting effect. The ambipolar electric field induced by charged particle distribution difference was studied in detail. It was found that the ambipolar electric field was the dominant cause for the acceleration of ions, which yielded plume splitting during the expansion process. The fully coupled HD model was further used to explore the effect of the beam spot size, laser fluence, and the pulse width on plasma plume splitting. [ABSTRACT FROM AUTHOR]

This paper presents a self-consistent multi-dimensional mathematical model and a numerical approach for simulating the low-temperature plasma induced by the femtosecond laser filament. Addressing limitations in current models, we analyze key aspects of the laser plasma behavior, including plasma generation, detailed chemical kinetics, energy exchange channels, total energy balance, and hydrodynamics. The developed model and LOTASFOAM code are applied to study the temporal and spatial decay of the plasma produced by a femtosecond laser pulse in pure nitrogen at atmospheric pressure. The paper also includes a discussion on the spatial and temporal dynamics of electronically excited states of nitrogen in the decaying laser plasma. [ABSTRACT FROM AUTHOR]

The potential of KD 2 PO 4 (or DKDP) crystal as a terahertz generator is demonstrated, despite its extremely strong terahertz absorption. By combining the Cherenkov radiation scheme and surface-emitting geometry, femtosecond Ti:sapphire laser pulses of ∼ 200 − 300 μ J energy were converted to broadband (∼ 4 THz bandwidth) terahertz transients with the efficiency of ∼ 8 × 10 − 6 . The transients produced electro-optic signals of a ∼ 0.15 modulation depth and ∼ 10 3 dynamic range in a 1-mm thick ZnTe detector crystal. This suggests DKDP as a potential generator crystal for terahertz time-domain spectroscopy. From our measurements, we estimated the nonlinear coefficient d 36 ≈ 3 pm/V of DKDP for optical rectification, which is an order of magnitude larger than the value known from second-harmonic-generation experiments. At the same time, the obtained value of d 36 is an order of magnitude smaller than the value calculated from the clamped electro-optic coefficient r 63 for the MHz frequency range. At high pump intensities, we observed multiple filamentation of the pump beam and white light generation, spatially separated from terahertz generation. [ABSTRACT FROM AUTHOR]

Ultrafast optically excited ferromagnetic (FM)/nonmagnetic (NM) multilayer heterostructures have been demonstrated recently as efficient, high-power, and broadband sources of terahertz (THz) electromagnetic radiation. Since these spintronic THz emitters exploit the conversion from ultrafast spin to charge current, either in bulk or at the interface, the THz pulses inhere all the characteristics of the involved mechanisms and dynamics associated with spin-charge interconversion processes. Deconvolving the same requires meticulous and careful experimentation and analysis. In this article, we review the current state-of-the-art in this field and provide a perspective on the emerging phenomena, which are prospering as new research avenues and demonstrate application potential for futuristic THz technologies. In the process of developing efficient spintronic THz emitters by optimizing various conditions including those with material parameters and excitation light, it turns out that THz emission spectroscopy itself can be a unique experimental tool for probing microscopic dynamical magnetic and spintronic effects, induced by femtosecond laser pulse excitation, in a noncontact and noninvasive manner. Several breakthroughs can be listed from the literature in this regard from the last decade. Just recently, ultrafast orbitronics is another dimension that is taking shape and will impact the field immensely. A fair account to this topic is also presented in the article. [ABSTRACT FROM AUTHOR]

Ultrafast spectroscopy is a powerful technique that utilizes short pulses on the femtosecond time scale to generate and probe coherent responses in molecular systems. While the specific ultrafast methodologies vary, the most common data analysis tools rely on discrete Fourier transformation for recovering coherences that report on electronic or vibrational states and multi-exponential fitting for probing population dynamics, such as excited-state relaxation. These analysis tools are widely used due to their perceived reliability in estimating frequencies and decay rates. Here, we demonstrate that such "black box" methods for parameter estimation often lead to inaccurate results even in the absence of noise. To address this issue, we propose an alternative approach based on Bayes probability theory that simultaneously accounts for both population and coherence contributions to the signal. This Bayesian inference method offers accurate parameter estimations across a broad range of experimental conditions, including scenarios with high noise and data truncation. In contrast to traditional methods, Bayesian inference incorporates prior information about the measured signal and noise, leading to improved accuracy. Moreover, it provides estimator error bounds, enabling a systematic statistical framework for interpreting confidence in the results. By employing Bayesian inference, all parameters of a realistic model system may be accurately recovered, even in extremely challenging scenarios where Fourier and multi-exponential fitting methods fail. This approach offers a more reliable and comprehensive analysis tool for time-resolved coherent spectroscopy, enhancing our understanding of molecular systems and enabling a better interpretation of experimental data. [ABSTRACT FROM AUTHOR]

We report on the observation of a nonlinear intensity dependence of the terahertz radiation-induced ratchet effects in bilayer graphene with asymmetric dual-grating gate lateral lattices. These nonlinear ratchet currents are studied in structures of two designs with dual-grating gates fabricated on top of boron nitride encapsulated bilayer graphene and beneath it. The strength and sign of the photocurrent can be controllably varied by changing the bias voltages applied to individual dual-grating subgates and the back gate. The current consists of contributions insensitive to the radiation's polarization state, defined by the orientation of the radiation electric field vector with respect to the dual-grating gate metal stripes, and the circular ratchet sensitive to the radiation helicity. We show that intense terahertz radiation results in a nonlinear intensity dependence caused by electron gas heating. At room temperature, the ratchet current saturates at high intensities of the order of hundreds to several hundreds of kW cm − 2 . At T = 4 K, the nonlinearity manifests itself at intensities that are one or two orders of magnitude lower; moreover, the photoresponse exhibits a complex dependence on the intensity, including a saturation and even a change of sign with increasing intensity. This complexity is attributed to the interplay of the Seebeck ratchet and the dynamic carrier-density redistribution, which feature different intensity dependencies and nonlinear behavior of the sample's conductivity induced by electron gas heating. The latter is demonstrated by studying the THz photoconductivity. Our study demonstrates that graphene-based asymmetric dual-grating gate devices can be used as terahertz detectors at room temperature over a wide dynamic range, spanning many orders of magnitude of terahertz radiation power. Therefore, their integration together with current-driven read-out electronics is attractive for the operation with high-power pulsed sources. [ABSTRACT FROM AUTHOR]

Femtosecond laser excitation of crystal materials can produce coherent longitudinal acoustic phonons (CLAPs), which possess the capability to interact with various quasiparticles and influence their dynamics. The manipulation of CLAPs' behavior is thus of significant interest for potential applications, particularly in achieving ultrafast modulations of material properties. In this study, we present our findings on the propagation of laser-induced CLAPs at thin-film interfaces and heterojunctions using ultrafast optical reflectivity and ultrafast x-ray diffraction measurements. We observe that CLAPs can efficiently propagate from a LaMnO3 thin-film to its SrTi O 3 substrate due to the matching of their acoustic impedance, and the oscillation period increases from 54 to 105 GHz. In contrast, in ultrafast x-ray diffraction experiments, we discover that CLAPs are partially confined within an Au (111) thin film due to the mismatch of acoustic impedance with the substrates, leading to an oscillation period of 122 ps. However, interestingly, when examining L a 0.7 C a 0.175 S r 0.125 Mn O 3 / B a 0.5 S r 0.5 Ti O 3 bilayers, no oscillations are observed due to the favorable impedance matching between the layers. Our findings demonstrate that acoustic impedance can serve as an effective means to control coherent phonons in nanometer-thin films and may also play a crucial role in phonon engineering at interfaces or heterostructures. [ABSTRACT FROM AUTHOR]

Multiphoton ionization (MPI) of alkyl iodides (RI, R = CnH2n+1, n = 1–4) has been investigated with femtosecond laser pulses centered at 800 and 400 nm along with photoelectron imaging detection. In addition, the ultraviolet (UV)–vacuum ultraviolet (VUV) absorption spectra of gas-phase RIs have been measured in the photon energy range of 5–11 eV using the VUV Fourier transform spectrometer at the VUV DESIRS beamline of the synchrotron SOLEIL facility. The use of high-laser-field strengths in matter–radiation interaction generates highly non-linear phenomena, such as the Stark shift effect, which distorts the potential energy surfaces of molecules by varying both the energy of electronic and rovibrational states and their ionization energies. The Stark shift can then generate resonances between intermediate states and an integer number of laser photons of a given wavelength, which are commonly known as Freeman resonances. Here, we study how the molecular structure of linear and branched alkyl iodides affects the UV–VUV absorption spectrum, the MPI process, and the generation of Freeman resonances. The obtained results reveal a dominant resonance in the experiments at 800 nm, which counter-intuitively appears at the same photoelectron kinetic energy in the whole alkyl iodide series. The ionization pathways of this resonance strongly involve the 6p(2E3/2) Rydberg state with different degrees of vibrational excitation, revealing an energy compensation effect as the R-chain complexity increases. [ABSTRACT FROM AUTHOR]

We investigate the electron emission from 3D chiral silver alloy nanohelices initiated by femtosecond laser pulses with a central photon energy of hν = 1.65 eV, well below the work function of the material. We find hot but thermally distributed electron spectra and a strong anisotropy in the electron yield with left- and right-circularly polarized light excitations, which invert in sign between left- and right-handed helices. We analyze the kinetic energy distribution and discuss the role of effective temperatures. Measurements of the reflectance and simulations of the absorbance of the helices based on retarded field calculations are compared to the anisotropy in photoemission. We find a significant enhancement of the anisotropy in the electron emission in comparison to the optical absorption. Neither simple thermionic nor a multiphoton photoemission can explain the experimentally observed asymmetries. Single photon deep-UV photoemission from these helices together with a change of the work function suggests a contribution of the chirally induced spin selectivity effect to the observed asymmetries. [ABSTRACT FROM AUTHOR]

This paper is concerned with a theoretical analysis of the behavior of optically excited spin currents in bilayer and multilayer systems of ferromagnetic and normal metals. As the propagation, control, and manipulation of the spin currents created in ferromagnets by femtosecond optical pulses is of particular interest, we examine the influence of different thicknesses of the constituent layers for the case of electrons excited several electronvolts above the Fermi level. Using a Monte-Carlo simulation framework for such highly excited electrons, we first examine the spatiotemporal characteristics of the spin current density driven in a Fe layer, where the absorption profile of the light pulse plays an important role. Further, we examine how the combination of light absorption profile, spin-dependent transmission probabilities, and iron layer thickness affects spin current density in a Fe/Au bilayer system. For high-energy electrons studied here, the interface and secondary electron generation have a small influence on spin transport in the bilayer system. However, we find that spin injection from one layer to another is most effective within a certain range of iron layer thicknesses. [ABSTRACT FROM AUTHOR]

Water-assisted ultrashort laser pulse processing of semiconductor materials is a promising technique to diminish heat accumulation and improve process quality. In this study, we investigate femtosecond laser ablation of deep trenches in GaAs, an important optoelectronic material, using water and ambient air environments at different laser processing regimes. We perform a comprehensive analysis of ablated trenches, including surface morphological analysis, atomic-resolution transmission electron microscopy imaging, elemental mapping, photoluminescence, and Raman spectroscopy. The findings demonstrate that GaAs ablation efficiency is enhanced in a water environment while heat-accumulation-related damage is reduced. Raman spectroscopy reveals a decrease in the broad feature associated with amorphous GaAs surface layers during water-assisted laser processing, suggesting that a higher material quality in deep trenches can be achieved using a water environment. [ABSTRACT FROM AUTHOR]

The application of nonlinear optical effects in optoelectronic devices is still scarce because the irradiance threshold necessary to induce a specific effect is very high. In this context, knowing the frequency-resolved first order molecular hyperpolarizability (β) is essential to identifying regions where this response is intense enough to allow for applications in commercial devices. Thus, herein, we have determined the β spectral dependence of five new push–pull cinnamylidene acetophenone derivatives using femtosecond laser-induced Hyper-Rayleigh Scattering (HRS). A considerable increase in β values was observed in molecules. We found remarkable β values in regions near the two-photon resonance, which are mediated by electron withdrawing and donating groups. This effect was mapped using wavelength-tunable femtosecond Z-scan technique. Furthermore, it was modeled in light of the sum-over-states approach for the second- and third-order nonlinearities. Finally, our outcomes suggest a strategy to obtain large β values mediated by the 2PA transition. [ABSTRACT FROM AUTHOR]

The excitation of sound by a femtosecond laser pulse in a metal layer on a dielectric substrate has been studied. The modulations of the reflection coefficient of the metal, which arise due to the propagation of sound in it, are described in detail. It is shown that, in addition to oscillations corresponding to odd harmonics of sound waves, oscillations corresponding to even harmonics can be present on the profile of the Fourier image of the reflection coefficient change. The efficiency of even harmonic generation depends on the substrate material and the metal film thickness. The dependence of the reflection coefficient change on time has been studied. It has been established that if the electron heat flux reaches the metal–dielectric interface before the energy of the electrons is transferred to the lattice, then modulations are present both due to sound that occurs at the metal–vacuum interface and at the metal–dielectric interface. The wavelength of probe radiation also affects the reflection coefficient change. If real and imaginary parts of the permittivity at the wavelength of probing radiation are comparable in magnitude, then the Fourier image of the reflection coefficient change contains a smaller number of maxima, and the change in the reflection coefficient over time is accompanied by splitting of peaks. [ABSTRACT FROM AUTHOR]

We theoretically investigate the polarized terahertz (THz) generation in laser–gas interaction using circularly polarized (CP) two-color laser pulses with uncommon frequencies. We show that these laser pulses can be applied to generate polarized THz radiation, and the polarization of THz radiation, which ranges from linear to circular, can be controlled in a flexible manner by manipulating the laser parameters, such as the relative phase and phase angle. Furthermore, in contrast to traditional harmonic frequency combinations, the polarization states of the incommensurate lasers undergo complex evolution with the phase angle, resulting in a noteworthy influence on the THz yield. The obtained results could deepen our understanding of polarized THz radiation generation. [ABSTRACT FROM AUTHOR]

In semiconductor‐based microlasers, the generated excited carriers can affect both the optical absorption coefficient and refractive index, influencing the lasing behavior and device performance. In this study, the lasing behavior of individual MAPbBr3 microrods pumped by femtosecond laser pulses is investigated using an ultrafast optical Kerr gating technique. By analyzing the spectral and temporal evolution of the lasing behavior, it is found that the band‐filling (BF) effect can cause a significant change in the refractive index of the material, which results in an unfavorable red‐shift and broadening of the resonant modes, deteriorating the laser linewidth and quality factor. The hot carrier cooling process can provide a buffer for alteration of the energy level occupation state, resulting in a small transient refractive index change and slight red‐shift. These results offer insights into the lasing behavior driven by photogenerated carrier dynamics and provide an optimization strategy for semiconductor‐based microlasers. [ABSTRACT FROM AUTHOR]

The generation of topological spin textures under ultrafast laser pulse excitations and field-free manipulation of topological transitions between different spin textures has attracted enormous interest from the perspective of spintronic applications. Here, we utilize ultrafast electron microscopy to showcase the femtosecond laser pulses excitation on magnetic materials and have generated multiple topological spin textures in an amorphous ferrimagnetic TbFeCo film. Furthermore, the following field-free topological transitions between skyrmions and bubbles with diversified topology are identified via in situ heating the sample in Lorentz transmission electron microscopy. The critical role of uniaxial anisotropy variation on changing the magnetic textures during the heating process is confirmed by micromagnetic simulations. Our results provide a perspective on the generation and transformation of topological spin textures. [ABSTRACT FROM AUTHOR]

We performed a comprehensive multi-scale phonon-mediated thermal transport study of nano-porous silicon (np-Si) films with average porosities in the range of φ = 30%–70%. This depth-resolved thermal characterization involves a combination of optical methods, including femtosecond laser-based time-domain thermo-reflectance (TDTR) with MHz modulation rates, opto-thermal micro-Raman spectroscopy, and continuum laser wave-based frequency domain thermo-reflectance (FDTR) with kHz modulation rates probing depths of studied samples over 0.5–1.2, 2–3.2, and 23–34 μm, respectively. We revealed a systematic decrease in thermal conductivity (k) with the rise of φ , i.e., with the lowering of the Si crystalline phase volumetric fraction. These data were used to validate our semi-classical phonon Monte Carlo and finite element mesh simulations of heat conduction, taking into account disordered geometry configurations with various φ and pore size, as well as laser-induced temperature distributions, respectively. At high φ , the decrease in k is additionally influenced by the disordering of the crystal structure, as evidenced by the near-surface sensitive TDTR and Rutherford backscattering spectroscopy measurements. Importantly, the k values measured by FDTR over larger depths inside np-Si were found to be anisotropic and lower than those detected by the near-surface sensitive TDTR and Raman thermal probes. This finding is supported by the cross-sectional scanning electron microscopy image indicating enhanced φ distribution over these micrometer-scale probed depths. Our study opens an avenue for nano-to-micrometer scale thermal depth profiling of porous semiconducting media with inhomogeneous porosity distributions applicable for efficient thermoelectric and thermal management. [ABSTRACT FROM AUTHOR]

The availability of new-generation femtosecond lasers capable of delivering pulses with energies in the hundreds of mJ, or even in the joules range, has called for a revision of the effect of scaling spot size on the material distribution within the plasma plume. Employing a state-of-the-art Szatmári-type hybrid dye-excimer laser system emitting 248 nm pulses with a maximum energy of 20 mJ and duration of 600 fs, copper films were grown in the classical pulsed laser deposition geometry. The exceptionally clean temporal profile of the laser pulses yielded a femtosecond component of 4.18 ± 0.19 mJ, accompanied by a 0.22 ± 0.01 mJ ASE pedestal on the target surface. While varying the spot sizes, the plasma plume consistently exhibited an extremely forward-peaked distribution. Deposition rates, defined as peak thickness per number of pulses, ranged from 0.030 to 0.114 nm/pulse, with a gradual narrowing of the thickness distribution as the spot area increased from 0.085 to 1.01 mm2 while keeping the pulse energy constant. The material distribution on the silicon substrates was characterized using the f(Θ) = AcoskΘ + (1 − A)cospΘ formalism, revealing exponents characterizing the forward-peaked component of the thickness profile of the film material along the axes, ranging from k = 15 up to exceptionally high values exceeding 50, as the spot area increased. Consequently, spot size control and outstanding beam quality ensured that majority of the ablated material was confined to the central region of the plume, indicating the potential of PLD (pulsed laser deposition) for highly efficient localized deposition of exotic materials. [ABSTRACT FROM AUTHOR]

We report the optimization of the high-order harmonic generation (HHG) in the laser-induced gold plasma. It includes determination of the characteristics of heating pulses allowing formation of the gold plasma possessing suitable characteristics (plasma density, electron density, electron temperature, velocity of plasma), as well as determination of the optimal delay between heating picosecond pulses and driving femtosecond pulses. Time-resolved Au plasma spectra and images allowed determining the velocity of plasma spreading (2 × 104 m s-1), electron density (3 × 1016 cm-3), and electron temperature (0.8 eV) of gold plasma at the optimal fluence of heating 150 ps pulses allowing HHG up to the 53rd order at the conversion efficiency of 1 × 10− 5. We also report the modification of the blueshift of harmonics at different regimes of HHG and plasma formation. The harmonic splitting onto strong blue component and weak red component appeared at 3 × 1014 W cm-2 intensity of 806 nm pump. [ABSTRACT FROM AUTHOR]

The use of femtosecond laser radiation pulses is a very important tool used for metals or cell ablation, structural changes of materials optical properties and many other applications. In this light, we investigate on the generation of continuous-waves solitary structures induced by the dynamics of laser radiation propagation in transparent media with passively mode-locked saturable absober. Thus, understanding the characteristic properties of the dynamics of laser radiations turns out to be a pertinent and important step towards the optimization of their uses, as well as enhancement of tecnology. Our model is characterized by a complex Ginzburg–Landau (CGL) equation with a strong nonlinearity. In full nonlinear regime, numerical simulations of the nonlinear equation revealed multi-periodic wave structures which turn out to be multi-pulse trains. When the number of photon K increases, an increase of the Saturation Power influences considerably the pulse train. The results clearly prove the existence of multi-pulse structures and bound-state solitons with strong nonlinearity on-like cubic-quintic and saturable nonlinearities. [ABSTRACT FROM AUTHOR]

In the current study, pulsed laser ablation in a liquid process was used to create gold nanoparticles (Au NPs) in the deionized distilled water. The pulsed laser ablation in liquid (PLAL) technique was used to create the NPs in two different processes. In first method, the Au nanoparticles were prepared with different laser fluences (2.2, 4.4, 6.64, 8.85, and 11.1) J.cm−2, and the number of pulses was fixed at 300 pulses for each fluence. The second method uses different number of laser pulses (100, 200, 300, 400, and 500) while maintaining a consistent laser fluence (4.4 J.cm−2). The characterization of nanoparticles was investigated using different techniques like UV-VIS, FTIR, XRD analysis, FE-SEM, and PL spectra. The UV-visible absorbed spectrum of Au nanoparticles revealed an absorption peak at about 530 nm and another peak at about 433 nm. The FTIR measurements have successfully shown that the entire (Au NPs), which are being generated. The X-ray diffraction pattern of the nanoparticles indicated the presence of Au nanomaterials and 111, 200, and 311 planes that correspond to Au NPs. The images of FE-SEM show that the Au NPs exhibited a spherical shape framework, agglomeration, and aggregation. With increasing laser fluence, the highest level of fluorescence emission was seen at 380 and 370 nm for Au NPs. The antibacterial activities of Au NPs were investigated using well diffusion method. Two gram-positive and two gram-negative bacteria of four different species have each been exposed to antibiotic activity. Au NPs have revealed a higher activity against tested bacteria. The results revealed that the 1250 µg/mL has high activity against Pseudomonas aeruginosa and Staphylococcus aureus, while Streptococcus mutans has been affected by all concentrations of Au NPs, and Acinetobacter baumannii has been highly affected by 1000 µg/mL and 1250 µg/mL. [ABSTRACT FROM AUTHOR]

The main aim of the presented paper is to obtain the exact solutions of the generalized Sasa–Satsuma equation using two different methods, the modified auxiliary equation method and (G ′ G , 1 G) -expansion method. As a result of these methods, new soliton solutions of the given model are extracted. These solutions include rational solutions, hyperbolic solutions and periodic solutions. The necessary constraint conditions for the existence and validity of the constructed solutions are also been provided. Moreover, for appropriate parametric values, acquired findings are displayed via contour, 2D and 3D graphics that show the physical relevance and dynamical behaviors of the proposed equation. The dynamics of constructed solutions of the proposed methods help a lot in describing the propagation of the femtosecond pulses in the systems of optical fiber transmission. [ABSTRACT FROM AUTHOR]

The existence of pathogenic microorganisms is a significant contributor to high morbidity rates in both developed and developing countries. Numerous human diseases have been related to Escherichia coli and Enterococcus faecalis. Fast diagnosis of these bacteria is crucial for limiting the progress of the associated diseases. The current study investigated the potential application of femtosecond laser-induced fluorescence spectroscopy (LIF) in monitoring bacteria. The INSPIRE HF100 laser system, which was pumped by a mode-locked femtosecond Ti: sapphire MAI TAI HP laser, delivered 100 femtosecond laser pulses to the samples. E. coli and E. faecalis were exposed to femtosecond laser pulses, and a spectrometer was used to monitor the fluorescence emission spectra. The measured LIF spectra of the two bacteria showed that E. coli and E. faecalis had fluorescence peaks at 502 and 512.5 nm, respectively. When the excitation wavelength varied from 350 to 400 nm, E. coli and E. faecalis fluorescence peak intensities linearly decreased. Furthermore, the LIF peaks revealed that the full width at half maximum of E. coli is 91.9 nm and that of E. faecalis is 101.3 nm. The current study findings suggest that femtosecond laser-induced fluorescence spectroscopy is a reliable, accurate, and sensitive approach for rapidly and directly identifying pathogenic bacteria. [ABSTRACT FROM AUTHOR]

In an optical fiber, we report on our study of pulse compression using an analytical method. A significant role for the eighth order nonlinear Schrödinger (NLS) equation may be seen in the study of ultra-short pulses, in particular extremely nonlinear optical phenomena. As a result of our study, which plays a significant role in reviving potent mathematical procedures, such as the extended rational sinh–cosh method for obtaining the dark soliton solution and the cosine method for solving the NLS equation to attain M-shaped, W-shaped soliton structure, dark and bright solitonic structure. Solitons are shown to have a strictly chirp-free structure, which facilitates effective compression. According to the results, wavevector may effectively regulate the shape and propagation behavior of soliton. [ABSTRACT FROM AUTHOR]

Understanding the photodamage mechanism underlying the highly nonlinear dynamic of femtosecond laser pulses at the second transparent window of tissue is crucial for label-free microscopy. Here, we report the identification of two cavitation regimes from 1030 nm pulses when interacting with the central nervous system in zebrafish. We show that at low repetition rates, the damage is confined due to plasma-based ablation and sudden local temperature rise. At high repetition rates, the damage becomes collateral due to plasma-mediated photochemistry. Furthermore, we investigate the role of fluorescence labels with linear and nonlinear absorption pathways in optical breakdown. To verify our findings, we examined cell death and cellular responses to tissue damage, including the recruitment of fibroblasts and immune cells after irradiation. These findings contribute to advancing the emerging nonlinear optical microscopy techniques and provide a strategy for inducing precise, and localized injuries using near-infrared femtosecond laser pulses. When a pulsed laser interacts with tissue, the molecules in the sample get excited to a higher energy state and relax either nonradiatively, leading to thermal damage, or via de-excitation processes, frequently associated with photodamage. Here, the authors explore how different photodamage mechanisms unfold across a spectrum of intense near-infrared femtosecond pulses. [ABSTRACT FROM AUTHOR]

The ability to study chemical dynamics on ultrafast timescales has greatly advanced with the introduction of X-ray free electron lasers (XFELs) providing short pulses of intense X-rays tailored to probe atomic structure and electronic configuration. Fully exploiting the full potential of XFELs requires specialized experimental endstations along with the development of techniques and methods to successfully carry out experiments. The liquid jet endstation (LJE) at the Linac Coherent Light Source (LCLS) has been developed to study photochemistry and biochemistry in solution systems using a combination of X-ray solution scattering (XSS), X-ray absorption spectroscopy (XAS), and X-ray emission spectroscopy (XES). The pump–probe setup utilizes an optical laser to excite the sample, which is subsequently probed by a hard X-ray pulse to resolve structural and electronic dynamics at their intrinsic femtosecond timescales. The LJE ensures reliable sample delivery to the X-ray interaction point via various liquid jets, enabling rapid replenishment of thin samples with millimolar concentrations and low sample volumes at the 120 Hz repetition rate of the LCLS beam. This paper provides a detailed description of the LJE design and of the techniques it enables, with an emphasis on the diagnostics required for real-time monitoring of the liquid jet and on the spatiotemporal overlap methods used to optimize the signal. Additionally, various scientific examples are discussed, highlighting the versatility of the LJE. [ABSTRACT FROM AUTHOR]

Ultrafast control of both electric and spin currents triggered by femtosecond laser pulse has attracted much attention due to future applications for broadband THz emitter as well as high‐speed spintronic devices. Optically generated spin current is converted to charge current via multiple spin‐charge conversion mechanisms, generating THz wave emission in magnetic multilayers. However, to date, quantitative and comparative investigation of THz emission originating from spin‐charge conversion mechanisms has not yet been fully explored. Here, direct and straightforward nondestructive probing to measure THz emission is provided at original Co/Pt and Co/Ta interfaces embedded in Pt/Co/Ta multilayers with polarization analysis of both optical pump and THz emission. These results allow a fundamental understanding of various spin‐charge conversion phenomena, which is a key basis for future spintronic THz source development. [ABSTRACT FROM AUTHOR]

The commissioning of multi-petawatt class laser facilities around the world is gathering pace. One of the primary motivations for these investments is the acceleration of high-quality, low-emittance electron bunches. Here we explore the interaction of a high-intensity femtosecond laser pulse with a mass-limited dense target to produce MeV attosecond electron bunches in transmission and confirm with three-dimensional simulation that such bunches have low emittance and nano-Coulomb charge. We then perform a large parameter scan from non-relativistic laser intensities to the laser-QED regime and from the critical plasma density to beyond solid density to demonstrate that the electron bunch energies and the laser pulse energy absorption into the plasma can be quantitatively described via the Zero Vector Potential mechanism. These results have wide-ranging implications for future particle accelerator science and associated technologies. [ABSTRACT FROM AUTHOR]

The manifestation of giant persistent photoconductivity (GPPC) is demonstrated with a fs (femtosecond) Ti:sapphire laser pulse that has a duration of 40 fs and a central wavelength of 400 nm. The femtosecond laser pulse was irradiated on a two-terminal VO2 device fabricated on a corning glass substrate. Under the applied voltages of 9–12 V, the GPPC takes place within 8.6–15 μs after the laser irradiation. The photocurrent from the GPPC in the VO2 device remains stable with the current decreasing slope of ∼0.003%/minute. With one-dimensional thermal model, the temperature (TIR) of the irradiated area is estimated as a function of time, indicating that TIR is above the insulation-to-metal transition temperature of VO2 thin film prior to the onset of GPPC. The ultrafast onset of GPPC of VO2 device can be utilized for ultrafast optoelectronic switch and memory device. [ABSTRACT FROM AUTHOR]

Modern laser micromachining utilizes ultrashort optical fields, such as femtosecond lasers, to perform high-precision processings on solid materials, including cutting, drilling, ablation, polishing, and scripturing. Femtosecond laser-based material processings can induce a plasma of free electrons whose density depends on physical phenomena such as single-electron diffusions, multiphoton ionization, and electron–hole radiative recombinations. In this work, we examine the dynamics of femtosecond lasers in transparent materials with non-Kerr nonlinearity, taking into account the generation of an electron plasma. In these specific materials, a balance between the nonlinearity and the group-velocity dispersion of the optical medium can favor the formation of optical filaments propagating with a permanent shape by virtue of their "solitonic" features. We are interested in the effects of the competition between electron–hole radiative recombination and single-electron diffusion processes on the spatiotemporal profiles of the propagating optical field and of the plasma density. The model features a complex Ginzburg–Landau equation with an optical nonlinearity of a general saturable form and a Kth-order nonlinearity term accounting for K-photon ionization processes, coupled to a rate equation for the electron plasma density where the present terms are representing avalanche ionizations, single-electron diffusion, and electron–hole radiative recombination processes. The modulational-instability analysis suggests that the continuous-wave regime will be stabilized by strong electron–hole radiative recombination processes for a fixed value of the single-electron diffusion coefficient, a stability enhanced by an increase in K. In the nonlinear regime, numerical simulations of the model equations for different combinations of the nonlinearity-saturation exponents and different values of the photon number K unveil soliton train structures forming from the laser field propagation and the time evolution of the plasma density. These structures turn out to be either dissipative soliton trains in the absence of electron–hole radiative recombinations or soliton crystals when electron–hole radiative recombination processes are taken into consideration to balance the damping effect caused by single-electron diffusions. [ABSTRACT FROM AUTHOR]

Organic terahertz (THz) crystals consist of polar molecules with various functional substituents, resulting in numerous molecular phonon vibrations in the THz frequency range. However, the effect of incorporating substituents into organic THz crystals on the resulting THz absorption is rather an unexplored area. In this work, by choosing an appropriate model system consisting of two benchmark organic nonlinear optical crystals, exhibiting an identical type of phenolic cation–anion assembly but different substituents, their ultra‐broadband THz wave generation characteristics are demonstrated and investigated. Compared to the unsubstituted crystals, the substituted crystals possess one more methoxy substituent on highly polar cationic chromophores. Their vibrational modes and THz absorption characteristics are investigated theoretically by periodic density functional theory and experimentally by ultra‐broadband THz spectroscopy up to 12 THz. In phenolic parallel‐type cation–anion assembled organic THz crystals, the existence of a methoxy substituent differently influences the molecular vibrations in low and high THz frequency ranges. In the lower THz frequency range, the unsubstituted crystals exhibit additional strong entire molecular vibrations, the so‐called cation‐on‐cation slipping vibrations, which do not exist in the substituted crystals. In contrast, the methoxy substituent leads to additional strong intramolecular vibrations in the higher THz frequency range. [ABSTRACT FROM AUTHOR]

Chiroptical response, demonstrating chiral interaction between optical vortex and chiral structure, plays an important role in variety of fields like optics and material science. However, the flexibility and efficiency of chiral structure fabrication are limited due to mask requirement and a point‐by‐point constructing strategy. In this paper, a novel chiral lithography method is proposed that utilizes optical vortex phase beam shaping to achieve chiral structure processing with high efficiency and flexibility. By programming topological charges of the vortex phase, chiral structures with adjustable appearance, rotation, and chirality can be produced using femtosecond laser single pulse exposure. Transmittance measurements of fabricated chiral structures array confirm a 66% helical dichroism that is predicted by simulation. Moreover, with the aid of convolutional neural networks (CNN), an accuracy of 98% in Orbital Angular Momentum (OAM) recognition can be achieved. This chiral lithography method provides an alternative for chiral structure fabrication and holds promise in the development of chiral optics, optical communications, and next‐generation optical devices. [ABSTRACT FROM AUTHOR]

Femtosecond fiber lasers with a high peak power are of great interest due to their various real-world applications. This work uses a double-clad ytterbium-doped fiber (DC-YDF) as the gain medium to successfully generate mode-locked pulses with a high peak power of hundreds of kilowatts at the 1 μm wavelength. The pulse width of the mode-locked pulses was as short as 108 fs, with a fundamental frequency of 1.9 MHz. The highest average power output, energy pulse, and peak power of the mode-locked pulses were 114.5 mW, 58.4 nJ, and 540.7 kW, respectively. The mode-locked fiber laser operates at a center wavelength of 1052 nm with normal net-cavity dispersion, which shows characteristics of a dissipative soliton. The mode-locked pulses exhibit good stability, as demonstrated by the measurement of the signal-to-noise ratio (SNR) of 40 dB. It is the very first instance of high-power short pulses mode-locked in the 1 μm range being shown utilizing a DC-YDF with a two-dimensional (2D) material-based saturable absorber (SA) into the laser cavity. [ABSTRACT FROM AUTHOR]

We demonstrate a broadband second-harmonic generator for the green spectral range based on a periodical polarity GaN waveguide structure. We grew the structure with a periodicity of 4 μ m using an MOCVD (metal-organic chemical vapor deposition) reactor. We tested the structure using femtosecond laser pulse pumping and obtained the second harmonic ( λ SH =512 nm) conversion efficiency 0.79 · 10 - 7 % · W - 1 for the pump peak power of 440 kW inside the waveguide. Based on the theoretical modeling of the dispersion curves of the pump and the second harmonic, we found multiple crossings, including an extended crossing. We relate this fact to the experimentally measured broadened second harmonic spectrum. [ABSTRACT FROM AUTHOR]

Surface morphology is a key factor that determines the quality of laser-based micromachining processes. However, the governing laws of surface morphology in the laser processing process are yet to be clarified, and optimization of processing parameters has to rely on trial and error. Specifically, under multiple-pulse irradiation, it has been difficult to quantify the evolution of the surface morphology because the surface morphology changes with each pulse irradiation, and the ablation process changes accordingly. In this study, we investigated the evolution of surface morphology under femtosecond laser irradiation. Copper and silicon were used as targets, whose surface morphology changes exhibited seemingly opposite behaviors with respect to fluence. Using thousands of datasets, we obtained an evolution equation for surface morphology in terms of surface area, which acts as a good probe of the residual surface energy after ablation. Our model successfully quantifies the cumulative effect of multiple-pulse irradiation on surface morphology changes. [ABSTRACT FROM AUTHOR]

Femtosecond laser inscriptions in optical media current offer the most reliable optical technology for processing of transparent materials, among which is the laser micromachining technology. In this process, the nonlinearity of the transparent medium can be either intrinsic or induced by multiphoton ionization processes. In this work, a generic model is proposed to describe the dynamics of femtosecond laser inscription in transparent materials characterized by a saturable nonlinearity. The model takes into account multiphoton ionization processes that can induce an electron plasma of inhomogeneous density and electron diffusions. The mathematical model is represented by a one-dimensional complex Ginzburg–Landau equation with a generalized saturable nonlinearity term in addition to the residual nonlinearity related to multiphoton ionization processes, coupled to a rate equation for time evolution of the electron plasma density. Dynamical properties of the model are investigated focusing on the nonlinear regime, where the model equations are transformed into a set of coupled first-order nonlinear ordinary differential equations, which are solved numerically with the help of a sixth-order Runge–Kutta algorithm with a fixed time step. Simulations reveal that upon propagation, spatiotemporal profiles of the optical field and of the plasma density are periodic pulse trains, the repetition rates and amplitudes of which are increased with an increase of both the multiphoton ionization order and the saturable nonlinearity. When electron diffusions are taken into account, the system dynamics remains qualitatively unchanged; however, the electron plasma density gets strongly depleted, leaving almost unchanged the amplitude of pulses composing the femtosecond laser soliton crystals. [ABSTRACT FROM AUTHOR]

We experimentally studied the three-body fragmentation dynamics of a noble gas cluster (ArKr2) upon its multiple ionization by an intense femtosecond laser pulse. The three-dimensional momentum vectors of correlated fragmental ions were measured in coincidence for each fragmentation event. A novel comet-like structure was observed in the Newton diagram of the quadruple-ionization-induced breakup channel of ArKr 2 4 + → Ar+ + Kr+ + Kr2+. The concentrated head part of the structure mainly originates from the direct Coulomb explosion process, while the broader tail part of the structure stems from a three-body fragmentation process involving electron transfer between the distant Kr+ and Kr2+ ion fragments. Due to the field-driven electron transfer, the Coulomb repulsive force of the Kr2+ and Kr+ ions with respect to the Ar+ ion undergoes exchange, leading to changes in the ion emission geometry in the Newton plot. An energy sharing among the separating Kr2+ and Kr+ entities was observed. Our study indicates a promising approach for investigating the strong-field-driven intersystem electron transfer dynamics by using the Coulomb explosion imaging of an isosceles triangle van der Waals cluster system. [ABSTRACT FROM AUTHOR]

The modern means of controlled irradiation by femtosecond lasers or swift heavy ion beams can transiently produce such energy densities in samples that reach collective electronic excitation levels of the warm dense matter state, where the potential energy of interaction of the particles is comparable to their kinetic energies (temperatures of a few eV). Such massive electronic excitation severely alters the interatomic potentials, producing unusual nonequilibrium states of matter and different chemistry. We employ density functional theory and tight binding molecular dynamics formalisms to study the response of bulk water to ultrafast excitation of its electrons. After a certain threshold electronic temperature, the water becomes electronically conducting via the collapse of its bandgap. At high doses, it is accompanied by nonthermal acceleration of ions to a temperature of a few thousand Kelvins within sub-100 fs timescales. We identify the interplay of this nonthermal mechanism with the electron–ion coupling, enhancing the electron-to-ions energy transfer. Various chemically active fragments are formed from the disintegrating water molecules, depending on the deposited dose. [ABSTRACT FROM AUTHOR]

The use of metasurfaces to engineer the response of magnetic materials is of utmost importance in the field of magnon-spintronics. Here, we demonstrate a method to fabricate one- and two-dimensional magnonic crystals with arbitrary symmetry and use it to engineer the amplitude-frequency characteristic of magnetostatic surface spin waves excited in a magnetic material. The technique is based on the gentle microablation of the sample surface by focused femtosecond laser pulses. Tightly focused illumination allows using modest pulse energy while achieving micrometer precision. By raster scanning the incident laser spot on the sample surface, we control the shape and size of the building blocks constituting the unit cell of the crystal along with its symmetry and lattice parameter. Remarkable and controlled changes in the measured transmission characteristics reveal the strong and complex symmetry-dependent interaction of the spin waves with Bravais and non-Bravais lattices. The described single-step microfabrication method facilitates and speeds up the realization of integrated spintronics components and provides an efficient tool to explore complex magnetic dynamics in scattering lattices. [ABSTRACT FROM AUTHOR]

We present an experimental and theoretical energy- and angle-resolved investigation on the non-dissociative photoionization dynamics of near-resonant, one-color, two-photon, single valence ionization of neutral O2 molecules. Using 9.3 eV femtosecond pulses produced via high harmonic generation and a 3-D momentum imaging spectrometer, we detect the photoelectrons and O 2 + cations produced from one-color, two-photon ionization in coincidence. The measured and calculated photoelectron angular distributions show agreement, which indicates that a superposition of two intermediate electronic states is dominantly involved and that wavepacket motion on those near-resonantly populated intermediate states does not play a significant role in the measured two-photon ionization dynamics. Here, we find greater utility in the diabatic representation compared to the adiabatic representation, where invoking a single valence-character diabat is sufficient to describe the underlying two-photon ionization mechanism. [ABSTRACT FROM AUTHOR]

Lead‐free metal halide double perovskites have emerged as promising scintillators owing to their superior optoelectronic properties, low‐cost and solution processability. However, it is still challenging to develop high‐performance flexible X‐ray scintillators based on the B(I)‐site alkali metal modulation in lead‐free double perovskite nanocrystals (NCs). Herein, a series of novel Cs2B(I)GdCl6 (B: Li, Na, K) of double perovskite A2B(I)M(III)X6 NCs structures are successfully synthesized, the optical and scintillator properties of which are significantly sensitive to the B(I)‐site alkali metals with the doping of Sb3+. They showed efficient self‐trapped exciton (STE) emission with high photoluminescence quantum yield (PLQY) and exhibited the highest X‐ray detection limit (86 nGyair s−1) and excellent spatial resolutions (>15 lp mm−1). The femtosecond transient absorption measurement and theoretical analysis further revealed that the B(I)‐site alkali metal fundamentally balanced the exciton–phonon coupling with appropriate STEs formation energy barrier and the electron localization and thus improved the optical and scintillator properties. This B(I)‐site alkali metal engineering offers a strategy to develop bright luminescent double perovskite NCs for X‐ray detection and imaging devices. [ABSTRACT FROM AUTHOR]