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Light-induced deprotonation of molecules is an important process in photochemical reactions. Here, we theoretically investigate the tunneling deprotonation of H 2 + and its asymmetric isotopologues driven by circularly polarized THz laser pulses. The quasi-static picture shows that the field-dressed potential barrier is significantly lowered for the deprotonation channel when the mass asymmetry of the diatomic molecule increases. Our numerical simulations demonstrate that when the mass symmetry breaks, the tunneling deprotonation is significantly enhanced and the proton tunneling becomes the dominant dissociation channel in the THz driving fields. In addition, the simulated nuclear momentum distributions show that the emission of the proton is directed by the effective vector potential for the deprotonation channel and, meanwhile, the angular distribution of the emitting proton is affected by the alignment and rotation of the molecule induced by the rotating field. [ABSTRACT FROM AUTHOR]

We experimentally study two-body Coulomb explosions of CO2, O2, and CH3Cl molecules in intense femtosecond laser pulses. We observe an obvious variation in the ionic angular distribution of the fragments with respect to the kinetic energy releases (KERs). Using a classical model based on ab initio potential energy curves, we find that the dependence of the ionic angular distribution on the KER is relevant to the fact that the accurate potential energy deviates significantly from the value determined by applying the Coulomb interaction approximation at a relatively small internuclear distance of the molecule. We show that the KER-dependent ionic angular distribution provides an effective way to determine the critical internuclear distance at which the Coulomb interaction approximation holds or breaks down without relying on the knowledge of the accurate potential energy curves. [ABSTRACT FROM AUTHOR]

Abstract: Trefoil and quatrefoil channels between air holes of two microstructure fibers can strongly confine light field and act as highly nonlinear waveguides. Polarization-dependent blue-shifted emissions of femtosecond Ti:sapphire laser pulses were carried out in these channels. The spectra bandwidth of blue-shifted emissions in a trefoil channel is narrower than 10nm and can be tuned slightly by altering the polarization state of pump light. Polarization-dependent blue-shifted emissions from a quatrefoil channel lie in a hollow beam having potential use in optical tweezers. [Copyright &y& Elsevier]

Abstract: Single-phase β-FeSi2 thin films have been grown on quartz substrates using femtosecond laser deposition (800nm, 50fs, 1KHz) under gas pressure of 3.0×10−4 Pa. X-ray diffraction (XRD) and field-emission scanning electron microscopy (SEM) were used to determine the structural properties and surface images of the films. Typical XRD patterns of the film showed that no other diffraction peak except β-FeSi2 was found. The SEM results indicated that the films were composed of well-distributed grains, in the range 50–150nm in diameter. In addition, normal incidence spectral transmittance and reflectance data suggested that the β-FeSi2 film has a direct energy gap of about 0.85eV. The thickness of the layer and the refractive index of the film were determined by performed calculation in the wavelength range 1.9–2.7μm. Furthermore, the Raman spectra of the films were also discussed. [Copyright &y& Elsevier]

Abstract: Femtosecond laser micromachining of a Zr-based amorphous alloy in air, including measuring the ablation threshold, micro-drilling and trenching, was investigated. The threshold of ablating this amorphous alloy was determined by experiment. Laser-induced ablation and associated damage were examined by means of optical microscope, scanning electron microscopy, transmission electron microscopy and electron diffraction diagram. The results show that conventional processing method induced defects in the vicinity of machined area, such as crystallization, molten trace and spatter, were absent in femtosecond laser ablation area with selected parameters. This indicates that femtosecond laser ablation is a promising method for micromachining amorphous alloys without crystallization. [Copyright &y& Elsevier]

Abstract: The morphology of structural changes in KTP crystal induced by single femtosecond laser pulse has been investigated by means of CCD camera, scanning electron microscopy (SEM) and atom force microscopy (AFM). The structurally changed region is depressed at energies close to the threshold for producing a structural change and melting ablation morphologies are observed as pulse energy is increased. Furthermore, periodic nanostructures are formed around the edge of the laser-induced spots. [Copyright &y& Elsevier]

Abstract: Single-phase β-FeSi2 thin films were prepared on Si(100) and Si(111) wafers by using femtosecond laser deposition with a FeSi2 alloy target for the first time. X-ray diffraction (XRD), field scanning electron microscopy (FSEM), scanning probe microscopy (SPM), electron backscattered diffraction pattern (EBSD), and Fourier-transform Raman infrared spectroscopy (FTRIS) were used to characterize the structure, composition, and properties of the β-FeSi2/Si films. The orientation of β-FeSi2 grains was found to depend on the orientation of the Si substrates, and photoluminescence at wavelength of 1.53 μm was observed from the single-phase β-FeSi2/Si thin film at room temperature (20 °C). [Copyright &y& Elsevier]

Abstract: SrAl4O7 was synthesized by Pechini method using citric acid as a chelating agent and ethylene glycol as an esterification agent. The effects of the pH of the starting solutions and the molar ratio of citric acid to total metal cations concentration on the formation of SrAl4O7 were investigated. Increasing the pH of the starting solutions and the molar ratio of citric acid to total metal cations concentration promoted the formation of SrAl4O7. When the molar ratio of citric acid to total metal cations concentration was 2, well crystallized, single-phase SrAl4O7 was synthesized at 900°C for 2h for the precursor with pH 7, but it could not be obtained for the precursor with pH 1. Increasing the molar ratio of citric acid to total metal cations concentration up to 4, single-phase SrAl4O7 was also synthesized at 900°C for 2h for the precursors with pH 7, but it was obtained at 1100°C for 2h for the precursors with pH 1. The stability of SrAl4O7 as a function of time and temperature was determined. We found that SrAl4O7 can be stable up to 1400°C for 2h and it shows decomposition at 1500°C for 2h. Heating single-phase SrAl4O7 powder at 1600°C for 2h, most of SrAl4O7 decomposes to SrAl12O19 as the major phase and an unidentified phase (main reflection at d =3.0149). DSC, TG, FT-IR and XRD were used to characterize the precursors and the derived oxide powders. [Copyright &y& Elsevier]

Abstract: La(Mg1/2Ti1/2)O3 was synthesized via citric acid precursor. The effect of the pH of the starting solution and the molar ratio of citric acid to total metal cations concentration CA/M on the formation of La(Mg1/2Ti1/2)O3 was studied. It is found that increasing the pH of the starting solutions and the molar ratio of citric acid to total metal cations concentration enhances the formation of La(Mg1/2Ti1/2)O3. Single-phase La(Mg1/2Ti1/2)O3 was obtained from the precursor with CA/M=1 and pH=7 at 800°C for 2h, without formation of any intermediate phase. The precursors and derived powders were characterized by DTA, TG, X-ray diffraction and Fourier transform infrared spectroscopy. [Copyright &y& Elsevier]

Parity‐time (PT) and anti‐parity‐time (anti‐PT) symmetries have provided important guiding principles in the research of non‐Hermitian physics. However, realizations of anti‐PT symmetry in photonic systems usually rely on optical nonlinearities and indirect‐coupling approaches. Here, they apply the channel interference principle mediated by synthetic gauge‐flux biasing in open‐cavity systems to construct anti‐PT symmetries. It is shown that a specific π‐flux biasing into a looped‐resonator array can force a frequency degeneracy between pairwise Bloch modes therein. By further coupling the array into two external waveguides with tailored positions of ports, the system near the degeneracy point can be described by an anti‐PT‐symmetric Hamiltonian. When a real gauge‐flux detuning is introduced, the system undergoes a spontaneous transition between anti‐PT and anti‐PT‐broken phases, through which the two extreme cases of complete channel‐drop tunneling and complete tunneling suppression can be switched. Finally, by superimposing a PT‐symmetric term onto the anti‐PT‐symmetric Hamiltonian via applying an imaginary gauge‐flux biasing, extreme channel‐drop amplifying effects can be further realized by exciting the "lasing" mode under the critical‐coupling condition. The work bridges the physical connection between synthetic gauge field and anti‐PT symmetry. This paradigm may also find many applications from optical routing, and switching to buffering and amplifying on a chip–scale platform. [ABSTRACT FROM AUTHOR]

Mode selecting plays a vital role in the field of optoelectronics, such as optical communication, signal processing, on-chip light manipulation, mode conversion, and frequency synthesis. In this work, flexible selection and enhancement of the frequency modes in an unidirectional coupled Su–Schrieffer–Heeger (SSH) frequency lattice are obtained with Floquet exceptional points (EPs) and chiral Zener tunneling (ZT). The unidirectional coupled non-Hermitian SSH frequency lattices are synthesized by a double-ring system with complex dynamical modulations. Under an effective direct current (dc) force induced by the phase-mismatching of the modulations, the two Floquet bands of the non-Hermitian frequency lattices are degenerated and the Floquet EPs arise. Therefore, the unidirectional and irreversible frequency mode conversion takes place, which is the chiral ZT. Moreover, through perturbation analysis and numerical simulations, we prove that the frequency modes of the two-band system can be selected and enhanced by a multi-photon resonance dc force. [ABSTRACT FROM AUTHOR]

A novel method is proposed to directly characterize the exciton‐plasmon coupling in a photosensitive nanoparticle‐on‐mirror (NPoM) nanocavity. Under the illumination of UV light, the localized surface plasmon resonance (LSPR) peaks of the nanocavity can be continuously tuned over a wide range (≈65 nm), enabling precise determination of the energy dispersion relationship of plexcitons in the nanocavity. The results indicate that the energy of Rabi splitting can be actively tuned from 65 meV (weak coupling) to 170 meV (strong coupling) by adjusting the LSPR peak of a specific nanocavity (particle size L≈55‐92 nm) to the exciton wavelength (the zero detuning). In addition, the criterion of strong coupling is optimized by introducing single‐particle absorption spectroscopy. The work offers an intuitive method for the strong coupling characterization in nanocavity, paving the way for the real‐time tuning of coupling strength over a wide range (100 meV) from weak to strong coupling regimes. [ABSTRACT FROM AUTHOR]

Dynamic localization (DL) refers to a wavefunction confinement effect of electrons in periodic lattices under an ac electric field driving. Recent studies have transplanted DLs from electronic to photonic systems using periodically curved waveguide lattices, where the ac electric field is mimicked by the waveguide's bending curvature. However, due to the severe limitations of bending losses and poor tunability for highly curved, arbitrarily shaped waveguides, present studies mainly focus on DL under the simplest sinusoidal‐wave driving; its extension to an arbitrary‐wave driving remains challenging. Here, by constructing a synthetic temporal lattice with a coupled fiber‐loop circuit, the limitations of spatially curved waveguides are circumvented and generalized DLs are experimentally demonstrated under arbitrary‐wave driving. First, by applying band‐collapse criteria, the general conditions for the driving field's symmetry to achieve DLs are revealed. Then, two representative even‐function driving fields of square‐ and triangle‐waveforms are chosen, and wave‐packet revivals are observed as signatures of DLs. As a counter‐example, the breakdown of DL is also verified using odd‐function driving field of sawtooth‐waveform. The work reveals the conditions for realizing generalized DLs and experimentally verifies them using synthetic lattice schemes. This paradigm may find potential applications in versatile temporal‐waveform reshaping, pulse manipulations for optical communications, and signal processing. [ABSTRACT FROM AUTHOR]

We theoretically investigate nonadiabatic effects in tunneling ionization of a hydrogen atom induced by a few-cycle elliptically polarized laser pulse. We find a deflection angle between the maximum of the photoelectron momentum distribution and the direction of the negative vector potential corresponding to the electric-field peak. This deflection angle mainly originates from the fact that the instant of the maximal ionization rate does not exactly correspond to the instant of the electric-field peak in the few-cycle elliptically polarized laser pulse, revealing an ionization time shift between those two instants. The ionization time shift is a result of the nonadiabatic effect induced by the envelope of the few-cycle elliptically polarized laser pulse, which disappears in a long-pulsed laser. Our study is useful for understanding attoclock experiments using few-cycle elliptically polarized laser pulses. [ABSTRACT FROM AUTHOR]

The elastic properties and thermal expansion behavior of lead-free halide double perovskite, Cs 2 AgBiBr 6 , were studied first with the aid of first principles calculations, which were followed by experimental characterization. The calculated full elastic constants enable the complete mapping of Young’s and shear moduli, and Poisson’s ratios of Cs 2 AgBiBr 6 along all crystallographic orientations. Results show that significant anisotropy in them despite the fact that Cs 2 AgBiBr 6 possesses cubic symmetry. Detailed structural analysis and a spring model were utilized to rationalize the observed anisotropy. Further, the theoretical results were validated by recourse to nanoindentation and high-pressure synchrotron powder X-ray diffraction experiments. The superior mechanical behavior of Cs 2 AgBiBr 6 over the hybrid bromide analogues, MAPbBr 3 (MA = CH 3 NH 3 + ), were attributed to the higher framework stiffness of the former, which is a result of the presence of relatively stronger Ag-Br and Bi-Br bonds in it. Variable temperature single crystal X-ray diffraction reveal linear and volumetric thermal expansion coefficients to be about 20% lower than those of MAPbBr 3 , again suggesting its higher framework rigidity. The superior stiffness, thermal expansion behavior, moisture stability, and nontoxicity of Cs 2 AgBiBr 6 make it a strong candidate for potential applications in optoelectronics and photovoltaics. [ABSTRACT FROM AUTHOR]

We report an unexplored observation of multi-orbital contribution in the below- and near-threshold harmonic generation of aligned molecules. A typical pump–probe configuration is used in our experiments. By scanning the time delay and crossing angle of polarization directions between the pump and probe pulses, we find that the harmonic yield in this region exhibits abnormal intensity-dependent modulation patterns. Further comparison and analysis show that this observation can be interpreted as due to the contribution of deeper-lying molecular orbitals, which are much more prominent than in the above-threshold region and can overall surpass the contribution of the highest occupied molecular orbital, leading to reversed modulation patterns. The particular importance of the deeper-lying orbitals in this regime is closely related to the multi-photon excitation pathways involved in the generation process. Our work will advance further investigations on the mechanism of below- and near-threshold harmonic generation. [ABSTRACT FROM AUTHOR]

The electron response of nanoplasmon and nanoplasma in the laser field is greatly important for improving the functionality and efficiency of many potential applications. However, how and under what conditions the nanoplasmon‐nanoplasma transition works remains poorly understood due to radiation damage and charge buildup. Utilizing the combined aerodynamic lens and velocity map imaging spectrometer, this transition mediated by different mechanisms are demonstrated, as verified by the distinct photoelectron momentum distributions from Cu nanoparticles. Initially the polarization‐dependent distributions emphasize the domination of surface emission driven by the plasmonic field. Subsequently the transition starts and the competition of volume‐multiphoton and thermionic emission plays the intermediate state dominating the transition process. Finally, a complete plasma is generated with the leading role of thermionic emission, which is confirmed by first observing the correlated electronic decay in metal nanoparticles. These findings bridge the gap between electron emission in nanoplasmonic and nanoplasma states and identify the mechanism‐specific contributions, which will offer general principles for designing nanostructure and laser fields to control electron excitation and emission in practice. [ABSTRACT FROM AUTHOR]

The laser‐irradiated aerosolized nanostructures with complex geometries offer unique opportunities to steer orientation‐dependent photoion emission for multiple applications of directional ion sources and photocatalysis. The orientation‐dependent photoion emission is characterized by measuring the distribution and probability on differently oriented nanostructures. This unique capability is achieved by introducing momentum‐to‐space orientation discrimination in the single‐shot velocity map imaging technique with the aerodynamic lens. The effect of geometry, polarization, and concentration to control the photoion emission from differently oriented nanostructures is introduced for optimizing the potential applications of photoion emission including directional tunable nanoanode and optical sensors. The mechanism behind controlling the photoion emission is visualized by the variations in the regions of high laser intensity inside the nanostructures responding to the orientation, geometry, and polarization. This work represents an advance in characterizing the photoion emission from the randomly oriented aerosolized nanostructures and facilitates the potential for combining with the pump–probe methods to provide an intuitive understanding of their real‐time ionization dynamics. [ABSTRACT FROM AUTHOR]

Dynamically encircling the exceptional point (EP) of an anti‐parity‐time (anti‐PT) symmetric system can achieve chiral mode switching between symmetry‐broken eigenmodes, which is especially important for developing nanophotonic devices. However, the realization of anti‐PT‐symmetric systems on a chip still remains significantly challenging as precisely controlling the parameters of nanophotonic structures is difficult. Herein, a novel anti‐PT‐symmetric system is designed and constructed on a silicon‐on‐insulator chip by taking advantage of subwavelength gratings to help engineer nanophotonic structures. Consequently, on‐chip chiral switching between symmetry‐broken eigenmodes is experimentally realized by dynamically encircling the EP in parametric space of the proposed anti‐PT‐symmetric system. Moreover, the achieved chiral mode switching exhibits high output power ratios (≥ 6.4 dB) at 1550 nm, wide‐band response that covers C‐band completely, and great thermal stability up to 90 °C, which indicate desirable performance features for practical applications in optical communication. This study paves an avenue for realizing on‐chip chiral switching between symmetry‐broken modes, which shows great promise for developing nanophotonic devices, such as optical switching, logic gates, and isolators, in optical telecommunication bands. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMD) have attracted considerable attention in the field of photonic integrated circuits due to their giant optical anisotropy. However, on account of their inherent loss in the visible region and the difficulty of measuring high refractive index materials, near-field characterizations of the optical anisotropy of TMD in the visible region have inherent experimental difficulties. In this work, we present a systematical characterization of the optical anisotropy in tungsten disulfide (WS2) flakes by using scattering-type scanning near-field optical microscopy (s -SNOM) excited at 671 nm. Transverse-electric and transverse-magnetic (TM) waveguide modes can be excited in WS2 flakes with suitable thickness, respectively. With the assistance of the Au substrate, the contrast of the near-field fringes is enhanced in comparison with the SiO2 substrate. By combining waveguide mode near-field imaging and theoretical calculations, the in-plane and out-of-plane refractive indexes of WS2 are determined to be 4.96 and 3.01, respectively, indicating a high birefringence value up to 1.95. This work offers experimental evidence for the potential application of WS2 in optoelectronic integrated circuits in the visible region. [ABSTRACT FROM AUTHOR]

The sensitivity of strong-field ionization to atomic orbital helicity has attracted much attention from physicists, due to its potential application in attosecond spectroscopy and spintronics. In order to intuitively observe the physical mechanisms of helicity-dependent ionization rates during photoionization, the concept of the Wigner phase can be used to characterize the different interactions between the rotating electrons and the Coulomb potential. Here, we find that in both one- and three-photon ionization schemes, the electrons liberated more easily by the circularly polarized laser field suffer less influence of the Coulomb potential during the propagation and then accumulate less Winger phase. This result indicates that the strength of the interaction between the rotating electrons and the Coulomb potential can explain the helicity-dependent ionization for different ionization mechanisms universally, which is also supported by our classical ensemble analysis. Our work provides an intuitive perspective towards the physics picture of ionization propensity rules. [ABSTRACT FROM AUTHOR]

We propose a quantitative polarization-resolved quantum trajectory approach to analyse the solid high-order harmonic generation (HHG). By using this approach, we can distinguish the ionization channels of different polarization components in HHG. We take the HHG in hBN under two counter-rotating circularly polarized fields as an example. We find that the right and left circular polarized high harmonics are contributed by the electron trajectories with the ionization channel around the high symmetry points K and K ′ respectively. Furthermore, the intensity of the right and left circular polarization harmonics is determined by the interference between these electron trajectories. Our work offers a quantitatively polarization-resolved analysis to understand the underlying mechanism in HHG in the view of electron reciprocal space trajectory. [ABSTRACT FROM AUTHOR]

• We firstly propose a cascaded Kerr beam self-cleaning scheme that combines multiple MMFs to further improve multimode beam quality. • Radial offset between fiber 1 and fiber 2 of the cascaded multimode fiber system should be as small as possible for cascaded beam self-cleaning. • Our results also suggest that an increased number of HOMs with negligible energy along with a higher number of cascaded fibers contribute to superior multimode beam quality. Spatial beam self-cleaning has shown a nonlinear improvement in beam quality in multimode fibers (MMFs), proving its value in high-power fiber lasers and nonlinear biomedical imaging. In this Letter, we propose a novel cascaded Kerr beam self-cleaning scheme that combines multiple MMFs to further improve beam quality. Through this scheme, the initially speckled beam gradually evolves into a well-defined bell-shaped, primarily dominated by the fundamental mode, after undergoing two or three cascaded self-cleaning processes. Simulation results also suggest that an increased number of high-order modes (HOMs) with negligible energy along with a higher number of cascaded fibers contribute to superior beam quality. This phenomenon is attributed to the new excited HOMs acting as attractor and facilitating an energy cascade towards the fundamental mode. [ABSTRACT FROM AUTHOR]

Ultrafast electron pulses generated using femtosecond lasers offer a direct means for investigating ultrafast processes, but the yield of electron emission remains a significant practical limitation. This study demonstrates a substantial enhancement in yield of electron emission from carbon nanotubes induced by two-color (ω + 2 ω) pulses with an 80 MHz repetition rate and this enhancement is non-delay-dependent. Compared to the sum of photocurrents induced by each color pulse individually, the combined two-color pulses result in a remarkable enhancement in yield of electron emission by more than 25 times. We attribute this enhanced yield to carrier-assisted optical field emission: carbon nanotubes sustain high carrier concentration state under laser radiation thereby enhancing yield of electron emission. We propose the carrier migration emission model to provide a semi-quantitative description for the carrier-assisted optical field emission. This phenomenon introduces a new method to increase the yield of ultrafast electron pulses from CNTs and paves the way for next-generation ultrafast electron emitters and on chip ultrafast optoelectronic device. [Display omitted] [ABSTRACT FROM AUTHOR]

Coherent superposition of electronic states induces attosecond electron motion in molecules. We theoretically investigate the strong-field ionization of this superposition state by numerically solving the time-dependent Schrödinger equation. In the obtained photoelectron momentum distribution, an intriguing bifurcation structure appears in the strong-field holographic interference pattern. We demonstrate that this bifurcation structure directly provides complete information about the status of the transient wave function of the superposition state: the horizontal location of the bifurcation in the momentum distribution reveals the relative phase of the involved components of the superposition state and the vertical position indicates the relative coefficient. Thus, this bifurcation structure takes a snapshot of the transient electron wave packet of the superposition state and provides an intuitive way to monitor electron motion in molecules. [ABSTRACT FROM AUTHOR]

Abstract: Linear Thomson scattering by a relativistic electron of a short pulse laser has been investigated by computer simulation. Under a laser field with a pulse of 33.3-fs full-width at half-maximum, and the initial energy of an electron of , the motion of the electron is relativistic and generates an ultrashort radiation of 76-as with a photon wave length of 2.5-nm in the backward scattering. The radiation under a high relativistic energy electron has better characteristic than under a low relativistic energy electron in terms of the pulse width and the angular distribution. [Copyright &y& Elsevier]

Abstract: The characteristics of harmonic radiation due to electron oscillation driven by an intense femtosecond laser pulse are analyzed considering a single electron model. An interesting modulated structure of the spectrum is observed and analyzed for different polarization. Higher order harmonic radiations are possible for a sufficiently intense driving laser pulse. We have shown that for a realistic pulsed photon beam, the spectrum of the radiation is red shifted as well as broadened because of changes in the longitudinal velocity of the electrons during the laser pulse. These effects are more pronounced at higher laser intensities giving rise to higher order harmonics that eventually leads to a continuous spectrum. Numerical simulations have further shown that by increasing the laser pulse width broadening of the high harmonic radiations can be limited. [Copyright &y& Elsevier]

Abstract: BaTi4O9 was synthesized via polymeric precursor made of citric acid and ethylene glycol. The effects of the pH of the starting solutions and the molar ratio of citric acid to total metal cations concentration on the formation of BaTi4O9 were investigated. Increasing the pH of the starting solutions retarded the formation of BaTi4O9, but the increase of the molar ratio of citric acid to total metal cations concentration enhanced the formation of BaTi4O9. Single-phase BaTi4O9 was obtained at 1100°C for 2h when the molar ratio of citric acid to total metal cations concentration was unity, independent of the pH of the starting solutions. Increasing the molar ratio of citric acid to total metal cations concentration up to 2, single-phase BaTi4O9 was synthesized at 1000°C for 2h for the pH unadjusted precursors, but for the precursors with pH 6.0, the formation temperature of single-phase BaTi4O9 was increased to 1100°C. When the molar ratio of citric acid to total metal cations concentration was increased further up to 4, single-phase BaTi4O9 formed at 1000°C for 2h for the precursors with or without pH adjustment. DSC, TG, XRD and FESEM were used to characterize the precursors and the derived oxide powders. Details of the synthesis and characterizations of the resultant products were given. [Copyright &y& Elsevier]

Abstract: Yttrium aluminum garnet was synthesized via ethylenediaminetetraacetic acid (EDTA) precursor route. The precursors and the derived oxide powders were characterized by differential scanning calorimetry, thermogravimetric analysis, X-ray diffraction and scanning electron microscopy. The effect of the molar ratio of EDTA to total metal cation concentration (EDTA/M) on the crystallization behavior of YAG was investigated. For the EDTA/M=1 precursors, YAG was found to crystallize from an amorphous precursor beginning at temperatures of 800°C within 2h in air and completely transformed into YAG at 950°C; a certain amount of hexagonal-YAlO3 co-existed with the YAG phase between 800 and 900°C. For the EDTA/M=0.5 precursors, the phase-pure YAG was obtained at 1000°C for 2h. [Copyright &y& Elsevier]

Abstract: Single-phase La(Mg1/2Ti1/2)O3 has been prepared by ethylenediaminetetraacetic acid (EDTA) gel combustion synthesis process. The process starts with a pH adjusted aqueous solutions of La(NO3)3, Mg(NO3)2·6H2O, Ti(OC4H9)4, and EDTA. Combustion reactions occur when heating the gel to 250°C, which convert them to La(Mg1/2Ti1/2)O3 with traces of carbonaceous material directly. The derived powders were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, SEM and laser particle size distribution analysis. [Copyright &y& Elsevier]

Abstract: The spectral properties in different concentration of Yb ions (0.5–5mol%)-doped silica glasses are explored in this paper. The glasses are prepared by traditional melting method. The absorption spectra and the fluorescent lifetime (τ f) are measured at room temperature and low temperature (18K). The stimulated cross-section (σ emi) and potential laser properties (β min, I sat, I min) are calculated based on the absorption spectra. The absorption cross-section (σ abs) are in the range 1.08×10−20– 1.18×10−20 cm2 in different glasses, the fluorescence lifetime (τ f) change from 1.9 to 1.2ms with the increase of Yb3+ concentration. The potential laser properties indicate that lead silica glass is a good host for highly Yb ion doping glass. [Copyright &y& Elsevier]

Interferometry, a key technique in modern precision measurements, has been used for length measurement in engineering metrology and astronomy. An analogous time-domain interferometric technique would represent a significant complement to spatial domain applications and require the manipulation of interference on extreme time and energy scales. Here, we report an all-optical interferometer using laser-driven high order harmonics as attosecond temporal slits. By controlling the phase of the temporal slits with an external field, a time domain interferometer that preserves both attosecond temporal resolution and hundreds of meV energy resolution is implemented. We apply this exceptional temporal resolution to reconstruct the waveform of an arbitrarily polarized optical pulse, and utilize the provided energy resolution to interrogate the abnormal character of the transition dipole near the Cooper minimum in argon. This novel attosecond interferometry paves the way for high precision measurements in the time-energy domain using all-optical approaches. [ABSTRACT FROM AUTHOR]

Charge migration during light–matter interaction is one of the most fundamental processes which plays a key role in chemical and biological processes of molecule. The measurement of this process requires the extreme temporal and subatomic spatial resolutions. Here, we show that a scheme based on infrared pump and short extreme ultraviolet probe technique enables us to trace the laser-controlled electron motion with high spatiotemporal resolution. By numerically solving the time-dependent Schrödinger equation for combined with a simple double-slit model, we demonstrate that the laser-controlled charge migration can be directly reconstructed from the interference patterns of the photoelectron spectrum. [ABSTRACT FROM AUTHOR]

Nonlinear metasurfaces provide a promising platform for integrated nonlinear photonic devices owing to their unprecedented capability of nonlinear wavefront manipulation. However, the previously reported second‐harmonic (SH) metasurfaces are mainly based on the nonlinear Pancharatnam–Berry phase modulation. While this method is quite convenient and robust, it shows limitations of modulation efficiency and the difficulty in extending to multidimensional combined modulations. Here, the resonant nonlinear synthetic metasurface is proposed and experimentally demonstrated for independent phase and amplitude modulations of the SH beam. A high SH modulation efficiency of 75% (with a theoretical limit of 90%) is achieved in the polarization‐dependent SH metalens. Moreover, SH holographic imaging with phase and amplitude (at 2 and 4 levels) combined modulation is realized experimentally. Compared with the pure‐phase modulation, the signal‐to‐noise ratios are increased by 2 and 3 times when 2‐ and 4‐level amplitude controls are introduced. This is not only an important step in the improvement and innovation of holographic display technology, but also paves a distinct avenue toward multifunctional, higher efficiency, and ultracompact nonlinear optical devices. [ABSTRACT FROM AUTHOR]

We theoretically investigate the high-order harmonic generation (HHG) driven by dual-pulse lasers. The first pulse of 800 nm can create plasma in the high-density gas target. Then, the second pulse of 1800 nm is spatiotemporally reshaped by the plasma and generates soft x-ray high-order harmonics in the overdriven regime. The simulation results show that adding the first pulse can promote the phase matching of HHG in the cutoff region. As a result, a cutoff-extended HHG is achieved, which is one order of magnitude more intense than that in one pulse scheme reported previously. In dual-pulse driving lasers, the harmonic yield and the cutoff photon energy can be optimized effectively by increasing the gas pressure inside the water window region. [ABSTRACT FROM AUTHOR]

[Display omitted] A harmonic vortex beam is a typical vector beam with a helical wavefront at harmonic frequencies (e.g., second and third harmonics). It provides an additional degree of freedom beyond spin- and orbital-angular momentum, which may greatly increase the capacity for communicating and encoding information. However, conventional harmonic vortex beam generators suffer from complex designs and a low nonlinear conversion efficiency. Here, we propose and experimentally demonstrate the generation of a large second-harmonic (SH) vortex beam with quasi-nonlinear spin–orbit interaction (SOI). High-quality SH vortex beams with large topological charges up to 28 are realized experimentally. This indicated that the quasi-angular-momentum of a plasmonic spiral phase plate at the excitation wavelength (topological charge, q) could be imprinted on the harmonic signals from the attached WS 2 monolayer. The generated harmonic vortex beam has a topological charge of l n = 2 n q (n is the harmonic order). The results may open new avenues for generating harmonic optical vortices for optical communications and enables novel multi-functional hybrid metasurface devices to manipulate harmonic beams. [ABSTRACT FROM AUTHOR]

We report an experimental measurement of nonlinear absorption by using the near-infrared femtosecond laser and a crystalline MgO sample. We show that the nonlinear absorption inside MgO presents a four-fold modulation as a function of the crystal alignment to the laser polarization. In different intensities and wavelengths, we observe that the modulation amplitude is changed. We introduce a model based on the calculation of the carrier density to explain the phenomenon. It shows that the varied modulation is due to the variation of effective electron mass, which is related to the crystal structure. Our work makes a step forward in the application of nonlinear absorption in exploring solid structure. [ABSTRACT FROM AUTHOR]

The high-resolution three-dimensional photoelectron momentum distributions via above-threshold ionization (ATI) of Xe atoms are measured in an intense near circularly polarized laser field using velocity map imaging and tomography reconstruction. Compared to the linearly polarized laser field, the employed near circularly polarized laser field imposes a more strict selection rule for the transition via resonant excitation, and therefore we can selectively enhance the resonant ATI through certain atomic Rydberg states. Our results show the self-reference ionization delay, which is determined from the difference between the measured streaking angles for nonadiabatic ATI via the 4f and 5f Rydberg states, is 45.6 as. Our method provides an accessible route to highlight the role of resonant transition between selected states, which will pave the way for fully understanding the ionization dynamics toward manipulating electron motion as well as reaction in an ultrafast time scale. [ABSTRACT FROM AUTHOR]

We discuss the non-dipole effect in high-order harmonic generation from atoms driving by a linearly polarized vortex beam. Different from the selection rule under the dipole approximation, where only odd high harmonics are allowed, both even and odd harmonics are generated in the vortex beam. Moreover, the polarization states of the harmonics are modulated. Elliptically polarized high harmonics can be directly generated with a linearly polarized vortex beam. We show that these non-dipole effects arise from the orbital angular momentum and spin angular momentum of the vortex beam. [ABSTRACT FROM AUTHOR]

2D organic-inorganic lead iodide perovskites hold great promise for functional optoelectronic devices. However, their performances have been seriously limited by poor long-term stability in ambient environment. Here, we perform a systematic study for the stability improvement of a typical 2D organic-inorganic lead iodide perovskite (PEA)2PbI4. The degradation of the (PEA)2PbI4 films can be attributed to the interaction with the humidity in environment, which leads to decomposition of the perovskite components. Then, we demonstrate that polymer passivation provides an effective approach for improving the crystal quality and stability of the (PEA)2PbI4 films. Correspondingly, the photoemission of the polymer-passivated (PEA)2PbI4 films has been enhanced due to the decreased trap states. More importantly, a hydrophobic polymer (Poly(4-Vinylpyridine), PVP) will protect the (PEA)2PbI4 films from humidity in ambient environment, which can greatly improve the physical and chemical stability of the 2D perovskite films. As a result, the PVP-passivated (PEA)2PbI4 films can produce a bright emission even after long-term (>15 d) exposure to ambient environment (25 °C, 80% RH) and continuous UV illumination. This work provides a convenient and effective approach for improving the long-term stability of 2D organic-inorganic lead iodide perovskites, which shows great promise for fabricating large-area and versatile optoelectronic devices. [ABSTRACT FROM AUTHOR]

We investigate the excitation of surface plasmon polaritons (SPPs) using a metallic nanoaperture array illuminated by circularly polarized Laguerre-Gaussian (LG) vortex beams. The direction of SPP excitation is tunable by changing the circular polarization and topological charge of LG beams. The left- or right-handed circular polarization determines SPP propagation on either side of the nanoaperture array. Furthermore, varying the topological charge of LG beam will result in beam splitting of SPPs. We also utilize a composite nanoaperture array with different periods to achieve unidirectional excitation of SPPs. The study provides an interesting approach to control the excitation direction of SPPs and may find great applications in SPP generators and optical switches. [ABSTRACT FROM AUTHOR]

By numerically solving the three-dimensional time-dependent Schrödinger equation, we theoretically investigate the dynamic interference of the hydrogen atom in intense arbitrarily polarized high-frequency XUV pulses with and without the non-dipole correction included. Our results show a clear shift of the dynamic-interference peaks in the photoelectron spectra from the linearly and circularly polarized XUV pulses, which can be attributed to the different AC Stark shifts of the ground state. The non-dipole correction for the electron–light interaction produces a similar peak shift along the propagation direction in the photoelectron momentum spectra for the linear and circular pulses. But no obvious non-dipole effect for the AC Stark shift of the ground state is identified for the light pulse with intensity around 1019 W cm−2. [ABSTRACT FROM AUTHOR]

A method to produce circularly polarized attosecond pulses with tunable helicity from CO molecule by using an unidirectionally rotating laser field is proposed. It is found that broadband harmonic supercontinuum with circular polarization can be generated from the oriented CO molecule. This enables the production sub‐100 attosecond isolated pulse with the ellipticity as high as 0.9 at the macroscopic level. Moreover, the helicity of the generated high‐order harmonics and the attosecond pulse can be tuned by adjusting the orientation of the CO molecule. This method will be beneficial for the studies on chiral‐specific dynamics and magnetic circular dichroism on an attosecond time scale. [ABSTRACT FROM AUTHOR]

2D organic–inorganic hybrid perovskites are promising materials for next‐generation optoelectronic devices. Nevertheless, 2D perovskites still suffer from performance‐limiting trap states and possess fundamental photophysical effects that remain poorly understood. Herein, a photoinduced trap passivation for enhanced photoluminescence (PL) in 2D hybrid perovskites is reported. Under two‐photon excitation, the PL emission of the 2D hybrid perovskites exhibits a gradual increase in intensity and lifetime, due to the photoinduced trap passivation by interacting with oxygen in ambient environment. Interestingly, the PL increase shows distinct features at the surface and in the bulk of the 2D perovskites, which can be well revealed by a photoinduced oxygen‐diffused trap‐passivation model. Moreover, the PL increase and relaxation can recycle for several times, which suggests an excellent repeatability of the photoinduced trap passivation. The photoinduced trap passivation is critical for fundamental study of light–matter interaction in 2D perovskites, and shows great promise for improving the optoelectronic responses for functional devices. [ABSTRACT FROM AUTHOR]

We extend the functionality of nonlinear photonic crystals by fabricating a structure combining a few individual optical transformations in second harmonic generation. In particular, we employed all-optical spontaneous polarization reversal in ferroelectrics to create nonlinear holograms with curved fork-like spatial modulation of nonlinearity for the generation of second harmonic optical vortices. The curved fork-like structure represents a combination of a standard vortex harmonic generator (fork structure) and an axicon, thereby allowing us to realize the second harmonic "perfect vortices" whose diameter remains constant independently of their topological charge. [ABSTRACT FROM AUTHOR]

Experiments involving laser produced tin plasma have been carried out using a CO2 laser with an energy of 800 mJ/pulse and a full width at half maximum (FWHM) of 80 ns in vacuum. Time-integrated extreme ultraviolet spectral measurement showed that the peak of the extreme ultraviolet lithography spectrum was located at 13.5 nm and the spectrum profile's FWHM of the unresolved transition arrays was 1.1 nm. Plasma parameters of the electron temperature and density measurements in both axial and radial directions at later times had been obtained from a two-dimensional time and space resolved image spectra analysis. The axial spatial distribution of the electron density showed a 1/d2.6 decrease profile, and the radial spatial distribution of the electron density showed a 1/r1.1 profile, in which d is the axial distance from the target surface and r is the radial distance. The electron density was found to maintain symmetry across the radial distance at all delay times. Near the plasma plume center, the electron temperature Te varied slightly with increasing axial or radial distance, which was related to collisional decoupling and reheating of the ionized species in the plasma at distances longer than 3 to 4 mm. The space averaged electron temperature was measured in the range of 3.4-1.0 eV, and the space averaged electron density was measured in the range of 2.0 × 1017 to 2.2 × 1016 cm-3, as the time delay varied from 1.6 μs to 3.6 μs with respect to the pulse discharge. Time evolutions of the plasma temperature and density were found to have an apparent rise at a delay time of 2.4 μs in the corresponding time of the laser pulse tail peak. This suggests that plasma parameters and extreme ultraviolet emission intensity can be controlled by a double pulse combined laser. [ABSTRACT FROM AUTHOR]

Highlights • Our fiber curvature sensor has merits of inline compact, low cost and sensitive. • Highly sensitivity −15.33 dB/m−1 in the range 3.63–4.69 m−1 is experimentally demonstrated. • Low cross sensitivity 0.018 m−1/°C implies it is insensitive to environmental temperature change. Abstract An inline fiber structure formed by a silica tube spliced between single-mode fibers has been developed and tested for curvature measurement based on anti-resonant reflecting waveguide. A Fabry-Perot resonator with leaky modes of guided light in the silica tube is formed based on anti-resonant effect in the resonant wavelengths, which results in lossy dips in the transmission spectrum. Since the transmission power of the resonant dips is sensitive to structure bending, an inline curvature sensor can be realized. Experimental results reveal a high sensitivity up to −15.33 dB/m−1 in the wide curvature range of 3.63–4.69 m−1. The proposed sensor also demonstrates a low temperature sensitivity 0.27 dB/°C implying a small temperature cross sensitivity 0.018 m−1/°C. Our sensor has its merits of compact, low cost and high sensitivity. [ABSTRACT FROM AUTHOR]

We theoretically investigate the correlated electron dynamics in strong-field nonsequential double ionization (NSDI) of Ar by the parallel two-color pulses consisting of 800- and 400-nm laser fields with the semiclassical ensemble model. Our calculations show that the momentum distributions of Ar 2+ in the direction parallel to the polarization of laser field exhibit the single- or double-hump structure, depending on the relative phase of the two-color fields. Back analysis of the NSDI trajectories reveals that recollision of returning electron is well confined in a time window of several attoseconds, and the position of which can be controlled with attosecond precision by changing the relative phase of the two-color fields. Thus, the recollision energy is accurately controlled with the two-color fields. As a consequence, the time delay between the double ionization and recollision changes with the relative phase, resulting in the relative-phase-dependent ion momentum distributions. More interestingly, the different peaks of ion momentum spectra result from the NSDI events with different time delays between double ionization and recollision. Thus, the corresponding correlated electron dynamics for different peaks in the ion momentum distributions can be directly determined in the two-color fields. [ABSTRACT FROM AUTHOR]