Filamentation Of Femtosecond Pulses Research Articles

An Experimental Determination of Critical Power for Self-Focusing of Femtosecond Pulses in Air Using Focal-Spot Measurements

The filamentation of femtosecond pulses has attracted significant attention, owing to its unique characteristics and related applications. The self-focusing critical power of femtosecond pulses is one of the key parameters in the filamentation process and its application. However, the experimental determination of this power remains a challenging task. In this study, we propose an experimental approach to investigating the critical power for self-focusing of both femtosecond Gaussian and vortex beams with relatively low topological charges by analyzing the changes in the focal spot at different propagation distances. Our work offers a practical and convenient method for determining the self-focusing critical power of femtosecond pulses.

Multiple conical odd harmonics from filament-inscribed nanogratings

We report on the observation of conical third, fifth, seventh, and ninth harmonics that gradually emerge during the supercontinuum generation by filamentation of femtosecond midinfrared pulses in lithium strontium hexafluoroaluminate crystal. We show that the generation of conical odd harmonics is an optical signature of light-driven material reorganization in the form of volume nanogratings at the site irradiated by repetitive femtosecond filaments. The angle-resolved spectral measurements demonstrate remarkably broad spectra of individual odd harmonics, benefiting from a spectrally broadened pump pulse (supercontinuum), and reveal that filament-inscribed nanogratings represent photonic structures that are able to provide ultrabroad phase-matching bandwidths covering the wavelength range from the ultraviolet to the near infrared. We propose a scenario that interprets the generation of conical fifth, seventh, and ninth harmonics as nanograting phase-matched cascaded noncollinear four-wave mixing processes.

Manipulation of Long-Distance femtosecond laser Filamentation: From physical model to acoustic diagnosis

The promising application of femtosecond laser filamentation in atmospheric remote sensing brings imperative demand for diagnosing and controlling the spatiotemporal dynamics of long-distance filamentation. Here, the acoustic method was adopted to quantitatively diagnose the long-distance filamentation of femtosecond pulses manipulated by energy and temporal domain. The onset and length of filament can be conveniently obtained from a simple analytic formula based on reasonable approximation and further demonstrated by the experimental results. The influence of pulse energy and initial pulse chirp under three different focal lengths (∼10 m, 20 m, and 30 m) on the filamentation in the air were studied experimentally. These findings provide a guiding significance for the optimal control of the long-distance propagation of filament, thereby laying a firm foundation for femtosecond laser-based atmospheric remote sensing.

Filamentation-based, two-beam, 1-kHz, and single-shot N2 vibrational CARS thermometry in high-temperature flames

We achieve two-beam, 1-kHz, and single-shot N2 vibrational coherent anti-Stokes Raman scattering (CARS) thermometry using hybrid femtosecond/picosecond CARS in high-temperature flames. Ultrabroadband pump/Stokes pulses are generated employing the filamentation of high-power femtosecond pulses and can excite the Raman shift up to ∼3000 cm−1. This configuration can avoid the need of a long hollow-core fiber and an additional supercontinuum compressor to obtain ultrabroadband pulses. The quasi-common-path second harmonic bandwidth compression system is utilized to generate narrow-band and sideband-free picosecond pulses, which are employed as the probe pulses in the CARS process. The accuracy and the precision of the fitting results from 1000 consecutive single-shot spectra at a target temperature of 1706 K are 0.66% and 4.47%, respectively. The results demonstrate that the simplified two-beam system has the potential for dynamic and accurate temperature measurement in practical applications.

Conical third harmonic generation from volume nanogratings induced by filamentation of femtosecond pulses in transparent bulk materials

We report on observations of conical third harmonic emission that emerges during supercontinuum generation produced by self-focusing and filamentation of high (20-200 kHz) repetition rate 180 fs, 1035 nm pulses from an amplified Yb:KGW laser in various nonlinear crystals and glasses: YAG, sapphire, YLF, LiF, CaF2, MgF2, LiSAF, fused silica and BK-7 glass. We show that conical third harmonic generation is a phase-matched four-wave mixing process, where noncollinear phase matching is achieved by means of reciprocal lattice vector, inversely proportional to the period of nanograting, which is inscribed by femtosecond filament in the volume of nonlinear material. The existence of a particular period required to phase match conical third harmonic generation was indirectly verified by investigations of periodicity features of high and low spatial frequency laser-induced periodic surface structures, in which matter is reorganized in a similar fashion.

Interference effects in the formation of the light bullet spectrum under axicon focusing

Specific features of the anti-Stokes band of the light bullet supercontinuum spectrum under filamentation of 1900-nm femtosecond pulses, focused by an axicon in fused silica and calcium fluoride, have been experimentally, numerically, and analytically investigated. It is found that the formation of two or more light bullets in a filament leads to modulation of spectral components’ intensity. The oscillation period of spectral components, defined as the wavelength interval between adjacent intensity minima, decreases with decreasing their wavelength. On a logarithmic scale, the relative change in the oscillation period in the spectrum is proportional to the spectral component wavelength and is independent of the pulse energy and the position of the light bullet formation region in the filament. It is established that the modulation in the spectrum is due to the interference of the broadband supercontinuum emitted by light bullets, and its period decreases with increasing interval between them.

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.

Filamentation in Atmospheric Air with Tunable 1100\u20132400\u2009nm Near-Infrared Femtosecond Laser Source

Intense femtosecond pulse filamentation in open-air has been utilized for long distance optical communication and remote sensing, but it results in nonlinear-effect driven eye hazards which are not addressed by current eye safety standards. A systematic study of filamentation in atmospheric air was performed using a tunable 100 fs near-infrared laser (1100 nm–2400 nm). While undergoing filamentation, each source wavelength was spectrally broadened resulting in supercontinuum and third harmonic generation in the visible and near-IR spectrum. We record the spectra at the center and fringes of the supercontinuum as it is imaged onto a planar surface. In a full beam collection regime, we report the energy of the sub-1000 nm light generation for source wavelengths from 1100 nm to 1600 nm and compare the energy density to the maximum permissible exposure values under the ANSI Z136.1 laser safety standard.

Supercontinuum generation by femtosecond flat-top laser pulses in fused silica

In this work we report the numerical results on the filamentation and supercontinuum (SC) generation of femtosecond flat-top pulses with different steep edges propagating in fused silica. Results show that the filamentation process is determined mainly by the self-steepening effect and ionization, and the SC generation is influenced mainly by the self-steepening effect. The filamentation onset and termination is determined by the leading edge and the tailing edge, respectively. The steeper the leading edge is, the earlier the filamentation onset is; the steeper the tailing edge is, the earlier the filamentation termination is. The cutoff wavelength in the blue-side region of the SC has no obvious difference for the pulses with different leading and tailing edges. But the spectral intensity of the blue extension is much stronger for the pulses with the steeper leading edges. The stronger spectral intensity results from the formation of more steep descending edges during filamentation.

Origin and Dynamics of the Shortwave Wing in the Spectrum of a Supercontinuum Generated by a Light Bullet of the Mid-IR Range

The results are presented from investigating the light bullet spectrum in the filamentation of femtosecond pulses in calcium fluoride. A physical scenario is proposed for the generation of the anti-Stokes wing of the supercontinuum during the formation of the light bullet in the process of femtosecond filamentation in the mid-IR range.

Contributions of leading and tailing pulse edges to filamentation and supercontinuum generation of femtosecond pulses in air

By simulating the filamentation of femtosecond laser pulses with flat top and different steep leading and tailing edges in air, the influence of pulse edges on the filamentation dynamics and supercontinuum (SC) generation is analyzed. It is found that the filamentation length is determined by the leading edge. The different roles of the pulse edges in the SC generation are further revealed. Under a relative lower laser power, the spectral broadening of the laser pulse originates mainly from the self-phase modulation of both pulse edges induced by the Kerr effect, while under a relative higher power condition, the leading edge plays a dominant role in the blue-side extension of the SC due to electron generation.

Testing the Role of Recollision in N_{2}^{+} Air Lasing.

It has been known for many years that during filamentation of femtosecond light pulses in air, gain is observed on the B to X transition in N_{2}^{+}. While the gain mechanism remains unclear, it has been proposed that recollision, a process that is fundamental to much of strong field science, is critical for establishing gain. We probe this hypothesis by directly comparing the influence of the ellipticity of the pump light on gain in air filaments. Then, we decouple filamentation from gain by measuring the gain in a thin gas jet that we also use for high harmonic generation. The latter allows us to compare the dependence of the gain on the ellipticity of the pump with the dependence of the high harmonic signal on the ellipticity of the fundamental. We find that gain and harmonic generation have very different behavior in both filaments and in the jet. In fact, in a jet we even measure gain with circular polarization. Thus, we establish that recollision does not play a significant role in creating the inversion.

Supercontinuum spectrum upon filamentation of laser pulses under conditions of strong and weak anomalous group velocity dispersion in transparent dielectrics

Using experiment and numerical simulation, we study the effect of anomalous group velocity dispersion (AGVD) on the formation of a visible supercontinuum band and light bullets arising in the course of filamentation of mid-IR femtosecond pulses in fused silica and fluorides. It is found that the anti-Stokes shift of the visible band increases with increasing energy of the pulse, the centre wavelength of which lies in the region of weak AGVD, and does not depend on this energy in the region of strong AGVD. A criterion is introduced for assessment of the AGVD ‘strength’, at which the stable visible supercontinuum band accompanies the formation of a robust light bullet in the mid-IR filament.

Localized light structures with high intensity at multiple filamentation of femtosecond pulses of a Ti:Sapphire laser along an air path

Localized light structures with high intensity at multiple filamentation of femtosecond pulses of a Ti:Sapphire laser along an air path

Conical third-harmonic generation of optical vortex through ultrashort laser filamentation in air.

We experimentally generate third-harmonic (TH) vortex beams in air by the filamentation of femtosecond pulses produced in a lab-built Ti:sapphire chirped pulse amplifier. The generated TH beam profile is shown to evolve with increasing pump energy. At a sufficiently high pump energy, we observe a conical TH emission of the fundamental vortex and confirm that the conical radiation follows the conservation law for orbital angular momentum. We further investigate the far-field angularly resolved spectra of the TH wave to analyze the conical emission angle. We theoretically verify that the formation of the conical TH vortex results from the phase-matching between the fundamental and TH waves during the filamentation process.

Extension of filament propagation in water with Bessel-Gaussian beams

We experimentally studied intense femtosecond pulse filamentation and propagation in water for Bessel-Gaussian beams with different numbers of radial modal lobes. The transverse modes of the incident Bessel-Gaussian beam were created from a Gaussian beam of a Ti:sapphire laser system by using computer generated hologram techniques. We found that filament propagation length increased with increasing number of lobes under the conditions of the same peak intensity, pulse duration, and the size of the central peak of the incident beam, suggesting that the radial modal lobes may serve as an energy reservoir for the filaments formed by the central intensity peak.

Nonlinear space-time focusing and filamentation of annular femtosecond pulses in dielectrics

Filamentation during simultaneous space-time focusing in bulk fused silica is investigated numerically. We model the use of a pair of concentric gratings to transform a common femtosecond laser pulse with a Gaussian spatial profile into a radially chirped, annular beam shape that is focused into the bulk of silica by a lens. By varying the energy and/or time-chirp of the incident pulse, we capture the pulse dynamics and material response that yields material modification and damage in the nonlinear focus. The results show rich pulse dynamics, enhanced damage/modification site localization, and suggest novel approaches to laser machining of solids.

Quasi-steady-state air plasma channel produced by a femtosecond laser pulse sequence.

A long air plasma channel can be formed by filamentation of intense femtosecond laser pulses. However, the lifetime of the plasma channel produced by a single femtosecond laser pulse is too short (only a few nanoseconds) for many potential applications based on the conductivity of the plasma channel. Therefore, prolonging the lifetime of the plasma channel is one of the key challenges in the research of femtosecond laser filamentation. In this study, a unique femtosecond laser source was developed to produce a high-quality femtosecond laser pulse sequence with an interval of 2.9 ns and a uniformly distributed single-pulse energy. The metre scale quasi-steady-state plasma channel with a 60–80 ns lifetime was formed by such pulse sequences in air. The simulation study for filamentation of dual femtosecond pulses indicated that the plasma channel left by the previous pulse was weakly affected the filamentation of the next pulse in sequence under our experimental conditions.

Femtosecond filament array generated in air

Patterning multiple filamentation of femtosecond pulses in air is studied using a microlens array for modulation of the spatial profile and a single lens for power concentration. We generate a stable array of filaments containing a maximum of five hotspots per mm2 from a modest 68-GW input power. The evolution of the pattern along the axis of propagation as well as the means to control the inter-filament spacing is discussed. It is also shown in numerical simulations that besides the filamentation in the proximities of the focus, there is a region of early ionization around the central hotspots in the beam profile and a revival afterwards, caused by the spatial distribution of the laser energy.

Filamentation of terawatt laser pulses along hundred-meter atmospheric paths

Results of the experimental study of filamentation of terawatt femtosecond pulses of a Ti:Sapphire laser along an atmospheric path 106 m long using different spatial focusing and pulse power are presented. The control of filamentation region position and length by means of changing the initial laser beam focusing is shown to be highly effective. Dependencies are derived of filamentation region position and length on the initial degree of focusing, pulse power, and the number of filaments along the filamentation region. The obtained data on the filamentation region length and the number of filaments are compared with the results of our previous experiments and data from other authors.