Filamentation Of Femtosecond Laser Pulses Research Articles

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.

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.

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.

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.

Filamentation of Double Femtosecond Laser Pulses in Fused Silica

Filamentation of Double Femtosecond Laser Pulses in Fused Silica

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.

Diffraction-Ray Model of Single Filamentation of Femtosecond Laser Pulses for Estimating the Characteristics of the Multiple Filamentation Domain in Air

Diffraction-Ray Model of Single Filamentation of Femtosecond Laser Pulses for Estimating the Characteristics of the Multiple Filamentation Domain in Air

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.

Estimation of the Characteristics of the Domain of Multiple Filamentation of Femtosecond Laser Pulses in Air Based on the Single Filamentation Model

Characteristics of the domain of multiple filamentation in air are estimated based on the numerical solution of a nonlinear Schrodinger equation in the single filamentation statement. The method of diffraction-ray tubes was used to describe the single filamentation of laser pulses. The efficiency of this method for interpreting experimental results and predicting effects is shown, which is important when planning experiments. The characteristic size of small-scale intensity inhomogeneities in the laser centimeter-radius beam profile, which induce a multiple filamentation of femtosecond pulses, is shown to be several millimeters. An increase in the initial laser beam radius during telescoping increases the sizes of the initial small-scale intensity inhomogeneities, and reduces their power. This leads to an increase in the distance to the start of filamentation. An increase in the initial beam power promotes the elongation of the filaments and increases their number.

Observation of extremely efficient terahertz generation from mid-infrared two-color laser filaments

Extreme nonlinear interactions of THz electromagnetic fields with matter are the next frontier in nonlinear optics. However, reaching this frontier in free space is limited by the existing lack of appropriate powerful THz sources. Here, we experimentally demonstrate that two-color filamentation of femtosecond mid-infrared laser pulses at 3.9 μm allows one to generate ultrashort sub-cycle THz pulses with sub-milijoule energy and THz conversion efficiency of 2.36%, resulting in THz field amplitudes above 100 MV cm−1. Our numerical simulations predict that the observed THz yield can be significantly upscaled by further optimizing the experimental setup. Finally, in order to demonstrate the strength of our THz source, we show that the generated THz pulses are powerful enough to induce nonlinear cross-phase modulation in electro-optic crystals. Our work paves the way toward free space extreme nonlinear THz optics using affordable table-top laser systems.

Control of Multiple Filamentation of Femtosecond Laser Pulses in Air

The results are presented of experiments on controlling the characteristics of the multiple filamentation domain of femtosecond laser pulses along atmospheric paths via variations in the initial spatial focusing, beam radius, and pulse energy, as well as the optical field structure at the initial beam aperture.

Ultrafast Broadband Nonlinear Spectroscopy of a Colloidal Solution of Gold Nanoparticles

The generation of supercontinuum radiation at the filamentation of femtosecond laser pulses (800 nm) with a supercritical gigawatt peak power in pure water and colloidal solutions of plasmon gold nanoparticles (perturbative regime) has been studied experimentally. After correction to the extinction of the fluid behind a filament, the supercontinuum radiation yields from the active filamentation zone in water (broadband radiation source) and in colloidal solution (broadband radiation source modified by various in situ interactions with nanoparticles) have been compared and analyzed for various peak radiation powers. As a result, the proposed method of ultrafast broadband nonlinear spectroscopy has allowed the observation of effects of the saturated two-photon interband absorption of gold nanoparticles in the spectral range of supercontinuum generation and the enhancement of this generation in the range of plasmon resonance of nanoparticles.

The Effect of Phase Aberrations on the Position and Length of the Filamentation Domain

The results of experimental and theoretical studies of femtosecond laser pulse filamentation with use of a bimorph deformable mirror are presented. The mirror allows controlling the position of the filamentation domain throughout a model path due to phase distortions of different parts of a laser beam, determining localization of filaments and high-intensity channels in a beam cross section, and forming long (>100 m) high-intensity (1011–1012 W/cm2) weakly diverging plasma-free channels.

Control of characteristics of multiple filamentation of femtosecond laser pulses in air

Control of characteristics of multiple filamentation of femtosecond laser pulses in air

The role of heat effects in the process of formation of color centers in LiF during filamentation of femtosecond laser pulses

The possibility of the influence of thermal effects on the formation of color centers during laser femtosecond filamentation in a LiF crystal has been investigated theoretically. The conditions are formulated under which the influence of thermal effects can be noticeable.

Evaluation of the characteristics of the domain of multiple filamentation of femtosecond laser pulses in air based on the single filamentation model

Evaluation of the characteristics of the domain of multiple filamentation of femtosecond laser pulses in air based on the single filamentation model

Self-reconstruction of single-cycle light bullet in linear (free-space) propagation regime

The results of investigation of extremely compressed wave packets penetration of air gap under single femtosecond laser pulse filamentation in LiF at anomalous group velocity dispersion are presented. It is revealed by laser coloration method that a single cycle light bullet formed before the gap up to 0.5 mm was recovered after passing some distance behind the gap which increased nonlinearly with the gap length.

Controlling TW-laser pulse long-range filamentation in air by a deformable mirror.

The results of experiments and theoretical modeling of the multiple filamentation of terawatt-power femtosecond laser pulses on a 137m long air path are presented. We use a multielement optical setup consisting of a Galilean telescope and a deformable bimorph mirror, which allows construction for the desired pulse wavefront at the optical path entrance. By introducing controlled aberrations of the pulse phase profile, we demonstrate the wide-ranging manipulations on the position and spatial structure of the filamentation region. For the first time, to the best of our knowledge, the stable wide-aperture (5cm in diameter) ring-shaped spatial lattice of high-intense light channels is experimentally realized, which can persist over hundreds of meters in air.