Filamentation Of Laser Pulses Research Articles

Nonlinear to geometric focusing transition: beam self-cleaning and self-focusing critical power

We propose an easy but effective approach to find a transition numerical aperture between the regimes of laser pulse filamentation with nonlinear focusing and with geometric focusing predominance. The suggested method based on the beam profile measurements allows correction of the data provided by spectra measurements. Using a simple semi-analytical model, we study the dependence of the transition numerical aperture on the pulse power and medium nonlinearity. The analysis shows that in condensed media the transition from the nonlinear to geometric focusing regime occurs at much tighter focusing than in air. Moreover, if the medium nonlinear refractive index is high enough, only the nonlinear focusing regime is observed even at numerical apertures close to one, which allows symmetric plasma channel formation.

Turbulence-enhanced THz generation by multiple chaotically-distributed femtosecond filaments in air

Remote generation of electromagnetic waves in the terahertz (THz) by the volume of air medium gains increasing scientific interest due to the demand for this spectral range in solving various practical problems including those belonging to the nonlinear atmospheric optics. During self-focusing and filamentation of high-power femtosecond laser pulses in air, the source of THz radiation is the plasma of laser filaments. One of the main challenges of THz generation by a laser filament is increasing the value of optical energy conversion to the long-wavelength band. In this paper, we present the results of our studies on DC-biased THz generation during single-color filamentation in air of focused femtosecond Ti:Sapphire-laser pulses (800 nm, pulse energy up to 40 mJ). The distinctive feature of our study is that a femtosecond optical pulse propagates through a spatially-localized heated air layer containing randomly inhomogeneous refractive index perturbations, which mimics strong air turbulence. For the first time to our knowledge, we show, that the turbulent air layer formed at the beginning of the optical path allows increasing the yield of THz radiation up to 1.5 times due to the formation of multiple chaotically-arranged filaments resulting from random perturbations of the optical beam energy profile that reduces the subsequent THz (re)absorption in the filament plasma.

Impact of gravitational force on high repetition rate filamentation of femtosecond laser pulses in the atmosphere

We study the influence of the gravitational force on the generation of low-density channels of air left in the path of femtosecond laser filaments at high repetition rate. We observe a more important density variation along the filament longitudinal axis in the case of a vertically created filament as compared to a horizontal one. This leads to a more important reduction of the electrical breakdown field using vertical filament. This geometry induced difference is only observed at high repetition rate because it is directly related to the cumulative effect appearing above 100 Hz.

Formation of an O-Shaped Structure in the Red Wing of the Frequency–Angular Spectrum During Filamentation on a Long Atmospheric Path

An O-shaped structure at wavelengths of 930–960 nm in the frequency–angular spectrum of the supercontinuum generated during the filamentation of a femtosecond laser pulse with a central wavelength of 740 nm on a 75-m path in air has been observed experimentally. This feature of the frequency–angular spectrum is due to the presence of the absorption band of water vapor in the range of 930–960 nm and the anomalous dispersion region associated with this absorption. This result opens prospects for the remote single-pulse detection of impurities in air.

Conical Emission Induced by the Filamentation of Femtosecond Vortex Beams in Water

Conical emission is a typical nonlinear phenomenon that occurs during the filamentation of femtosecond laser pulses in transparent media. In this work, the conical emission induced by two kinds of typical vortex beams (i.e., Laguerre–Gaussian (LG) and Bessel–Gaussian (BG) beams) in water is experimentally studied. By recording the light spots of different spectra components from the supercontinuum induced by the vortex beams, the characteristics of the conical emission induced by femtosecond vortex beams are studied. It is found that the spots of the supercontinuum induced by the two kinds of vortex beams differ greatly from each other. The spots of the supercontinuum induced by the BG beams are a set of concentric rings like a rainbow with a white center, while the white light spots in the case of the LG beams are circular white disks, which are different from the commonly observed white light spots. By measuring the maximum divergence angle, it is observed that the divergence angle increases with a decrease in the wavelength, while it is merely affected by the topological charge, which is explained by the formation mechanism of conical emission in terms of self-phase modulation. Based on the observed results, we discuss the transfer of optical angular momentum during the supercontinuum induced by the filamentation of femtosecond vortex beams. This work may help to better understand the transfer of optical angular momentum in non-optical parametric processes as well as the interaction of high-intensity pulses with matter.

Directivity Control of Terahertz Radiation from Single-Color Filament Plasma with Polypropylene Pipe

Two-dimensional angular distribution of terahertz emission from single-color filamentation of laser pulses is experimentally studied. The use of a polypropylene pipe makes it possible to transform the angular pattern of terahertz radiation from the original hollow-cone one to a more preferable unimodal structure within the terahertz frequency range. It has been shown that propagation in a dielectric pipe can significantly (up to several orders of magnitude) increase the terahertz radiation amplitude at a required distance. We have also experimentally demonstrated that bending the pipe allows us to control the direction of the terahertz radiation propagation.

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.

Control of Femtosecond Filamentation by Means of the Alignment of Gas Molecules by Short-Wavelength Infrared Laser Pulses

It has been demonstrated experimentally that both the single and multiple filamentation of a femtosecond laser pulse in gaseous nitrogen can be controlled by means of the nonadiabatic alignment of molecules by 1400-nm pulses. The spectral shifts and change in the duration of a pulse caused by changes in the refractive index in the revival regions of a rotational wave packet have been detected. The stable and reproducible localization of radiation into separate filaments with the subdiffraction divergence and broadening of the spectrum by more than an octave has been observed in the multiple filamentation regime upon the alignment of molecules in the direction perpendicular to the pulse polarization.

Self-channeling of spatially modulated femtosecond laser beams in the post-filamentation region

The propagation of high-intensity femtosecond laser pulses in air under conditions of superposed spatial phase modulation is considered theoretically. The numerical simulations are carried out on the basis of the reduced form of a nonlinear Schrödinger equation for a time-averaged electric field envelope. Initial spatial modulations are applied to pulse wavefront profiling by a staggered ( T E M 22 ) phase plate, which is simulated numerically. The dynamics of laser pulse self-focusing, filamentation, and post-filamentation self-channeling after the phase plates with variable phase jumps is studied. We show that, with specific phase modulations, the pulse filamentation region in air can be markedly shifted further and elongated compared with a nonmodulated pulse. Moreover, during the post-filamentation propagation of spatially structured radiation, the highly localized light channels are formed, possessing enhanced intensity and reduced angular divergence, which enables post-filamentation pulse self-channeling on the distance multiple exceeding the Rayleigh range.

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.

Air-Lasing: High-Resolution Spectroscopy for Atmospheric Remote Sensing

Air-lasing is a cavityless coherent radiation generated in free space from air constituents as the gain medium, featuring significant advantages such as high collimation, high coherence and high intensity. Benefited from the long-range filamentation of high-power ultrashort laser pulses propagating in air, the air-laser can be induced remotely which provides an ideal light source for atmospheric remote sensing and chemical specie-resolved detection. Thanks to the coherent atomic/molecular excitation process accompanying the generation of air laser, remote sensing based on air-laser has high spectral resolution and high detection sensitivity, which is recently proved to be a powerful tool for important applications such as trace molecule detection, greenhouse gas monitoring and industrial pollutant detection. In this short review, the physical mechanism of air laser is briefly introduced, and various applications of air laser remote sensing are reviewed emphatically, and the future research is prospected.

Air-lasing high-resolution spectroscopy for atmospheric remote sensing

Air-lasing is a cavityless coherent radiation generated in free space from air constituents as gain medium, featuring high collimation, high coherence, and high intensity. Benefited from the long-range filamentation of high-power ultrashort laser pulses propagating in air, the air-lasing can be induced remotely, providing an ideal light source for atmospheric remote sensing and chemical species-resolved detection. Owing to the coherent atomic/molecular excitation process accompanied with the generation of air laser, remote sensing based on air-lasing has high spectral resolution and high detection sensitivity, which recently proved to be a powerful tool for important applications such as in trace molecule detection, greenhouse gas monitoring and industrial pollutant detection. In this short review, the physical mechanism of air lasing is briefly introduced, and various applications of air laser remote sensing are reviewed emphatically, and the future research is prospected.

Filamentation of Double Femtosecond Laser Pulses in Fused Silica

Filamentation of Double Femtosecond Laser Pulses in Fused Silica

Polarization-dependent filamentation of femtosecond laser pulses in synthetic diamond

The filamentation process inside of the bulk of type IIa synthetic diamond with known crystallographic orientation has been studied as a function of the polarization state of the ultrashort laser pulses with a duration of 300 fs and a wavelength of 515 nm. The transmittance of the sample was measured using a photodiode, while the micro-image of the filament was recorded on the CMOS camera perpendicular to the propagation axis of the exciting laser radiation. The dependences of the transmittance and length of the filament on the polarization azimuth show a distinct modulation over the entire range of its change. Keywords: ultrashort laser pulses, synthetic diamond, photoluminescence, laser polarization, nonlinear absorption, wide-bandgap dielectrics.

Cumulative air density depletion during high repetition rate filamentation of femtosecond laser pulses: Application to electric discharge triggering

We study the influence of the laser repetition rate on the generation of low-density channels of air left in the path of femtosecond laser filament. At high repetition rates, we observe the formation of a permanent millimeter-wide low-density channel that exceeds the depth and width of the transient depletion due to a single filament. We also show that this permanent cumulative effect decreases the breakdown voltage between two electrodes and can alter the path of the discharge. By comparing this effect in air and in pure nitrogen, we show that an accumulation of O2− ions contributes to the reduction in the breakdown voltage.

Millisecond-long suppression of spectroscopic optical signals using laser filamentation.

Ultrashort laser pulse filamentation in air can extend the delivery of focused laser energy to distances greatly exceeding the Rayleigh length. In this way, remote measurements can be conducted using many standard methods of analytical spectroscopy. The performance of spectroscopic techniques can be enhanced by temporal gating, which rejects the unwanted noise and background. In the present work, we investigate the thermal relaxation of air in the wake of single-filament plasmas using shadowgraphy. We demonstrate that the transient change in refractive index associated with relaxation of the gas can be used to reject both continuous and time-varying spectroscopic signals, including emission from laser-produced plasmas. This method can augment temporal gating of simple optical detectors.

Origin of the redshifted hump in the spectrum of an ultrashort laser pulse under its filamentation in atomic and molecular gases

We experimentally compare spectrum conversion of an ultrashort laser pulse under its filamentation in various gases. The laser pulse propagation through both atomic and molecular gases is observed to result in a separated local maximum formation in the long-wavelength spectral wing. This redshifted hump is more noticeable and does have a higher amplitude under the laser pulse filamentation just in molecular gases as compared to atomic ones. Therefore, we claim that the laser pulse self-phase modulation plays a crucial role in the formation of such a hump, while the Raman scattering in molecular gases enhances this process efficiency.

Pulsewidth and ambient medium effects during ultrashort-pulse laser ablation of silicon in air and water

Comparative single-shot pulse study of ablation topographies and depths on Si surfaces excited by 1030-nm sub- (0.3 ps) and pico-picosecond (10 ps) laser pulses at different laser fluences in air and water environments was performed. Fast energy transport via 3D-diffusion of dense electron-hole plasma was revealed at picosecond timescale to extend transversely the ablation craters over the focal spot in air for laser pulse widths shorter, than electron–phonon thermalization time about 3 ps. In the given fluence range fluence-independent shallow ablation was observed in air for 10-ps pulses. Then, rather shallow laser ablation occurred in water at supercritical peak pulse powers, apparently, due to laser beam deterioration by self-focusing and filamentation. Finally, very intense, fluence-dependent material removal took place during wet ablation by 10-ps laser pulses at the sub-critical peak pulse powers. Electron-hole plasma and related energy transport on Si surface, 1030-nm laser pulse filamentation in water, pulsewidth and ambient medium effects were revealed during the surface ablation studies.

Дифракционно-лучевая модель одиночной филаментации фемтосекундных лазерных импульсов для оценок характеристик области множественной филаментации в воздухе

The characteristics of the domain of multiple filamentation of femtosecond laser pulses in air were estimated based on the single filamentation model. As the single filamentation model, the diffraction-ray model is considered. It is based on the representation of a laser beam as a set of diffraction-ray tubes nested in each other that do not intersect in space and do not exchange energy with each other. In this situation changes in tubes shape and cross section during propagation demonstrate the effect of physical processes that occur with radiation in the medium. It is shown that the use of this model for interpreting experimental results and predicting effects is effective. In particular, it was demonstrated that the radius of small-scale intensity inhomogeneities in the profile of a centimeter laser beam, forming the domain of multiple filamentation of subterawatt femtosecond laser pulses, is several millimeters. The power in these inhomogeneities varies from several units to several tens of gigawatts. Telescoping the initial laser beam, leading to an increase in its radius, also expands the sizes of the initial small-scale intensity inhomogeneities and reduces the power contained in them. As a result of this, the coordinate of the filamentation beginning shifts along the path from the source of laser pulses. As the peak power in the beam increases, the length of the filaments and their number increase.

Diffraction-Ray Optics of Femtosecond Laser Pulses Propagating in Air Under the Influence of the Normal Group Velocity Dispersion

Results of theoretical study of the femtosecond Ti:Sa laser pulse propagation in air under the influence of the normal group velocity dispersion are presented. The use of the diffraction-ray tube method for an analysis of numerical solutions of the nonlinear Schrodinger equation in chromatic dispersion medium with the Kerr and plasma nonlinearity makes it possible to determine the main regularities of the femtosecond laser pulse selffocusing and filamentation in air for various pulse durations, initial beam radii, and peak powers. It is shown that under the influence of the group velocity dispersion, the filamentation terminates with an increase in the initial radius of the laser beam even at high values of supercritical powers. With an increase in the dispersion distortions of the pulse, the radius of the energetically replenishing diffraction-ray tube, the angular divergence of the post-filamentation light channel, and the nonlinear focus coordinate normalized to the Rayleigh length increase in the central time slices of the laser pulse and its integral pattern.