Numerical simulation study on distinguishing nonlinear propagation regimes of femtosecond pulses in fused silica.

We perform numerical simulations to investigate the nonlinear propagation dynamics of femtosecond Gaussian and vortex beams in fused silica. By analyzing the extent of spectral broadening, we are able to distinguish between the linear, self-focusing, and filamentation regimes. Additionally, the maximum intensity and fluence distribution within the cross-section of the vortex beams are analyzed for different incident laser energies. The results demonstrate a direct correlation between the spectral broadening and the peak intensity of the femtosecond laser pulse. As a result, this provides a theoretical foundation for distinguishing different propagation regimes, and determining critical powers for self-focusing and filamentation of both femtosecond Gaussian and structured beams.

Polarization of the third harmonic emission from the filamentation of femtosecond cylindrical vector beams.

We experimentally generate a third harmonic (TH) vector optical field in deep ultraviolet wavelength range using femtosecond vector laser beams. The generated TH beams are characterized by analyzing the Stokes parameters with different input laser energies. The results show that the TH predominantly preserves the vector polarization distribution of the fundamental frequency beam. Moreover, the intensity profile of the TH exhibits a multiple-ring structure. A hybrid polarization pattern is observed in the TH, where the ellipticity is influenced by the input laser energy. Our work provides an effective and straightforward method for producing TH vector optical fields, which may facilitate potential applications such as micro/nanofabrication and super-resolution microscopy.

Distinguishing the nonlinear propagation regimes of vortex femtosecond pulses in fused silica by evaluating the broadened spectrum.

The nonlinear propagation dynamics of vortex femtosecond laser pulses in optical media is a topic with significant importance in various fields, such as nonlinear optics, micromachining, light bullet generation, vortex air lasing, air waveguide and supercontinuum generation. However, how to distinguish the various regimes of nonlinear propagation of vortex femtosecond pulses remains challenging. This study presents a simple method for distinguishing the regimes of nonlinear propagation of femtosecond pulses in fused silica by evaluating the broadening of the laser spectrum as the input pulse power gradually increases. The linear, self-focusing and mature filamentation regimes for Gaussian and vortex femtosecond pulses in fused silica are distinguished. The critical powers for self-focusing and mature filamentation of both types of laser pulses are obtained. Our work provides a rapid and convenient method for distinguishing different regimes of nonlinear propagation and determining the critical powers for self-focusing and mature filamentation of Gaussian and structured laser pulses in optical media.

Experimentally determined critical power for self-focusing of femtosecond vortex beams in air by a fluorescence measurement.

The filamentation of the femtosecond vortex beam has attracted much attention because of the unique filamentation characteristics, such as annular distribution and helical propagation, and related applications. The critical power for self-focusing of the femtosecond vortex beams is a key parameter in the filamentation process and applications. But until now, there is no quantitative determination of the critical power. In this work, we experimentally determine the self-focusing critical power of femtosecond vortex beams in air by measuring fluorescence using a photomultiplier tube. The relation between the self-focusing critical power and the topological charge is further obtained. Our work provides a simple method to determine the self-focusing critical power not only for vortex beams but also for Airy, Bessel, vector, and other structured laser beams.

Influence of a pinhole diameter on the experimental determination of critical power for femtosecond filamentation in air.

Filamentation of intense femtosecond laser pulses in optical media has attracted great attention due to its various unique characteristics and potential applications. It is an important task to determine the critical power for the filamentation especially in many applications, which can be obtained by evaluating the transmitted pulse energy through a pinhole located in the filamentation region as a function of input laser energy. The pinhole diameter is very crucial to the measurement. However, there is no report on the experimental determination of critical power for filamentation in air by using the pinhole method and the influence of the pinhole diameter on the determination. In this paper, we numerically and experimentally investigate the influence of pinhole diameter on the determination of the filamentation critical power. The obtained critical power tends to a reasonable value as the decrease of the pinhole diameter, because the transmitted energy through the pinhole with a smaller diameter is more sensitive to the change of energy distribution in the beam cross section during the beginning process of filamentation. Under our experimental condition, the pinhole diameter as small as ∼50 µm is applicable to be used to determine the critical power for filamentation of femtosecond laser pulses in air.

Simultaneous spatial and temporal focusing optical vortex pulses for micromachining through optically transparent materials.

We introduce the optical vortex beam into simultaneous spatial and temporal focusing (SSTF) technique, and theoretically and experimentally demonstrate the local control of peak intensity distribution at the focus of a simultaneous spatiotemporally focused optical vortex (SSTF OV) beam. To avoid nonlinear self-focusing in the conventional focusing scheme, a spatiotemporally focused femtosecond laser vortex beam was employed to achieve doughnut-shaped ablation and high aspect ratio (∼28) microchannels on the back surface of 3 mm thick soda-lime glass and fused silica substrates.

Elongation of filamentation and enhancement of supercontinuum generation by a preformed air density hole.

The filamentation of the femtosecond laser pulse in air with a preformed density hole is studied numerically. The result shows that density-hole-induced defocusing effect can relieve the self-focusing of the pulse, and by changing the length of the density hole and relative delay time, the filamentation length, intensity, spectral energy density and broaden region can be effectively controlled. When a short density hole with millisecond delay time is introduced, a significant elongation of the filamentation and enhancement of supercontinuum intensity can be obtained. This study provides a new method to control filamentation by pulse sequence.

Intense vector supercontinuum radiation from femtosecond filamentation.

Intense vector supercontinuum (SC) radiation with spatial polarization is obtained by using 800nm femtosecond vector laser beams in the air. The SC generated by azimuthally, radially, cylindrically polarized beams, and higher-order vector beams are investigated, respectively. The results show that the SC generated by vector beams is greatly enhanced compared to that by a Gaussian beam. The energy density of SC radiation reaches the order of 1µJ/nm in a bandwidth of 258 nm from 559 nm to 817 nm and 0.1 µJ/nm from 500 nm to 559 nm. Furthermore, by checking the polarization distribution of SC in different wavelengths from visible to near-infrared bands, we find that the SC maintains nearly the same polarization distribution as pump pulses. This work provides an effective and convenient way to generate powerful SC vector beams which may facilitate potential applications including optical communication, micro/nano-fabrication, and super-resolution microscopy.

Spatial- and energy-resolved photoemission electron from plasmonic nanoparticles in multiphoton regime.

Spatial-resolved photoelectron spectra have been observed from plasmonic metallic nanostructure and flat metal surface by a combination of time-of-flight photoemission electron microscope and femtosecond laser oscillator. The photoemission's main contribution is at localized 'hot spots,' where the plasmonic effect dominates and multiphoton photoemission is confirmed as the responsible mechanism for emission in both samples. Photoelectron spectra from hot spots exponentially decay in high energy regimes, smearing out the Fermi edge in Au flat surface. This phenomenon is explained by the emergence of above threshold photoemission that is induced by plasmonic effect; other competing mechanisms are ruled out. It is the first time that we have observed the emergence of high kinetic energy photoelectron in weak field region around 'hot spot.' We attribute the emergence of high kinetic energy photoelectron to the drifting of the liberated electron from plasmonic hot spot and driven by the gradient of plasmonic field.

Ultrafast switching of photoemission electron through quantum pathways interference in metallic nanostructure.

The localized photoemission electron originating from the plasmonic "hot spots" in a metallic bowtie nanostructure can be separately switched on and off by adjusting the relative time delay between two orthogonally polarized laser pulses. The demonstrated femtosecond timing, nanometric spatial switching of multiphoton photoemission results from the interference of quantum pathways. Energy resolved measurement of the photoemission electrons further shows that the quantum pathway interference mechanism applies to control all the liberated electrons. The experimental results also show that the probability of electron emission through the quantum pathways from a plasmonic hot spot is determined by the localized emission response to the two incident laser pulses. These findings are of importance for controlling photoemission in ultrahigh spatiotemporal resolution in metallic plasmonic nanostructures.

Interference-induced filament array in fused silica.

One- and two-dimensional filament arrays are obtained in fused silica by using two and three interfered femtosecond laser beams, respectively. By modulating the number, cross angle, and azimuth of the beams, the dimension, period, orientation, and geometry of the filament-array can be controlled. The multiple beams interference method provides a convenient and effective method to generate and control the filament array in optical media with multiple degrees of freedom but without any external pulse modulation or focal element.

[Research on the dynamics of ion debris from Sn plasma by use of dual laser pulses].

Extreme ultraviolet lithography is one of the most promising technologies on the next generation of high-capacity integrated circuit manufacturing. However, techniques for ion debris mitigation have to be considered in the application of extreme ultraviolet source for lithography. In our paper the dynamics of ion debris from Sn plasma by using dual ns laser pulses were investigated. The results show that debris from plasma greatly depends on the energy of pre-pulse and the delay time between the two laser pulses. The energy of Sn ions debris was efficiently mitigated from 2. 47 to 0. 40 keV in the case of dual laser pulses, up to 6. 1 times lower than that by using single laser pulse. We also found that Sn ions debris can be mitigated at all angles by using the dual laser pulses method.

High spectral power femtosecond supercontinuum source by use of microlens array.

Generation of a high spectral power supercontinuum (SC) is reported from controlled multifilamentation of femtosecond pulses in fused silica. The use of a microlens array allows the manipulation of the filamentation pattern under very high-incident laser pulse energy without sample damage and, consequently, compared with using a single focusing lens, higher power of SC generation with a similar spectral broadening can be obtained. Moreover, the role of the interplay between diffraction pattern and proximity to the focus of the microlens array in SC generation is discussed.

Microwave guiding along double femtosecond filaments in air.

Microwave guiding along double parallel lines of femtosecond-laser-generated plasma filament has been demonstrated over a distance of about 8 cm in air, corresponding to a maximum microwave signal intensity enhancement more than sixfold the free-space propagation. It is shown that the operating frequency and the line electric width influence the propagation coefficient of microwaves propagating along this transmission line. Based on channeling microwaves along this line and by measuring and comparing the propagated microwave signals, the basic parameters of laser-generated plasma filament, namely, its electron density and conductivity, are obtained.

Laser-induced plasma cloud interaction and ice multiplication under cirrus cloud conditions.

Potential impacts of lightning-induced plasma on cloud ice formation and precipitation have been a subject of debate for decades. Here, we report on the interaction of laser-generated plasma channels with water and ice clouds observed in a large cloud simulation chamber. Under the conditions of a typical storm cloud, in which ice and supercooled water coexist, no direct influence of the plasma channels on ice formation or precipitation processes could be detected. Under conditions typical for thin cirrus ice clouds, however, the plasma channels induced a surprisingly strong effect of ice multiplication. Within a few minutes, the laser action led to a strong enhancement of the total ice particle number density in the chamber by up to a factor of 100, even though only a 10(-9) fraction of the chamber volume was exposed to the plasma channels. The newly formed ice particles quickly reduced the water vapor pressure to ice saturation, thereby increasing the cloud optical thickness by up to three orders of magnitude. A model relying on the complete vaporization of ice particles in the laser filament and the condensation of the resulting water vapor on plasma ions reproduces our experimental findings. This surprising effect might open new perspectives for remote sensing of water vapor and ice in the upper troposphere.

[The LIBS experiment condition optimization of alloy steel].

Laser induced plasma spectroscopy of alloy steel was produced by Nd : YAG pulsed laser at 1 064 nm, and the spectral signal was detected by high resolution and width controlled ICCD. Several Fe atomic spectral lines such as 404.581, 414.387, 427.176 and 438.355 nm were chosen for analysis, and the effects of different experimental parameters on LIBS spectral signal intensity were investigated. It is shown that the experimental parameters such as pulse energy, laser focus location and laser delay time have great influence on the LIBS signal. LIBS signals with high spectral intensity and signal-background ratio (SBR) as well as the optimum experiment conditions were obtained by optimizing these experiment parameters so as to make composition analysis of the alloy steel.

Control of laser filamentation in fused silica by a periodic microlens array.

Deterministic wavelength-dependent multifilamentation is controlled in fused silica by adjusting the diffraction pattern generated by a loosely focusing 2D periodic lens array. By simply translating the sample along the propagation axis the number and distribution of filaments can be controlled and are in agreement with the results of linear diffraction simulations. The loose focusing geometry allows for long filaments whose distribution is conserved along their propagation inside the sample. The effect of incident energy and polarization on filament number is also studied. Laser filamentation controlled by a microlens array could be a promising method for easy and fast 3D track writing in transparent materials.

Triggering and guiding high-voltage discharge in air by single and multiple femtosecond filaments.

The abilities to trigger and guide high-voltage discharge by using single and multiple filaments (MFs) are experimentally studied. It is shown that the discharge voltage threshold can be reduced significantly in both regimes of single and MF; however, the MF does not gain a larger reduction than a single filament. This behavior of the MF is attributed to the single discharge path rather than simultaneous multiple ones as one might expect during the discharge process.

Femtosecond laser filament-fringes in fused silica.

Linear diffraction was used to modulate intensity distribution across the femtosecond laser beam to create quasi regular arrays of filaments in fused silica. A fringe type of filament distributions (filament-fringe) were formed that could be controlled and observed over a distance of several millimeters. The difference of supercontinuum (SC) emission between individual filaments was also observed.

PCF-based cavity enhanced spectroscopic sensors for simultaneous multicomponent trace gas analysis.

A multiwavelength, multicomponent CRDS gas sensor operating on the basis of a compact photonic crystal fibre supercontinuum light source has been constructed. It features a simple design encompassing one radiation source, one cavity and one detection unit (a spectrograph with a fitted ICCD camera) that are common for all wavelengths. Multicomponent detection capability of the device is demonstrated by simultaneous measurements of the absorption spectra of molecular oxygen (spin-forbidden b-X branch) and water vapor (polyads 4v, 4v + δ) in ambient atmospheric air. Issues related to multimodal cavity excitation, as well as to obtaining the best signal-to-noise ratio are discussed together with methods for their practical resolution based on operating the cavity in a "quasi continuum" mode and setting long camera gate widths, respectively. A comprehensive review of multiwavelength CRDS techniques is also given.