Light Filaments Research Articles

Light filaments with higher-order Kerr effect

The influence mechanism of higher-order Kerr effect on the propagation of laser beam is investigated by a modified model, which indicates that a collapsing wave will transform into a universe blowup profile. The analysis of higher-order terms of the nonlinear refractive index shows that the filamentation process can be induced by Kerr self-focusing without the occurrence of the ionization effect. The determining role of the combination of self-focusing and spontaneous defocusing and the energy reservoir in formation of lengthy filament is confirmed visually.

Self-focusing of a focused femtosecond laser pulse in air

The regularities of self-focusing and filamentation of spatially focused ultra-short laser radiation in air are theoretically considered. The relationship between beam focusing sharpness and effective characteristics of the filamentation zone is investigated. The formation of ‘narrow’ and ‘wide’ light filaments compared to that of collimated radiation filamentation, upon high-power focused laser pulse propagation, is established.

Controlled interactions of femtosecond light filaments in air

We experimentally demonstrate coherent control of two light filaments in air generated by ultrafast laser pulses, as proposed by Xi et al. [Phys. Rev. Lett. 96, 025003 (2006)]. We show that depending on the relative phase and the incidence angle, the filaments can experience fusion, repulsion, energy redistribution, and spiral motion for propagation in air. By translating the initial beams on subwavelength scales, we achieve $~1$-mm transverse deflection of approximately $0.5$-mJ energy for parallel propagation and $7$${}^{\ifmmode^\circ\else\textdegree\fi{}}$ angular deflection for spiral motion in $3$-m propagation. Our approach using the relative beam phases to reliably control propagation of femtosecond light filaments is potentially applicable to remote sensing and lightning guiding.

Filamentation of high-power laser radiation in air and water: Comparative analysis

The propagation of high-power femtosecond laser pulses is considered in the self-focusing and filamentation formation regime. Similarities and differences in the character of the laser beam filamentation are analysed qualitatively and quantitatively in the atmospheric air and water. Based on the numerical results, the most important parameters (radius, intensity, free electron density) in light and plasma filaments produced by ultrashort laser pulses are shown to depend strongly on the optical parameters of the propagation medium, and, first of all, on the Kerr nonlinearity coefficients and the photoionisation rate of the medium molecules.

Interaction of light filaments in air

This paper analytically investigates the interaction of light filaments generated by a femtosecond laser beam in air. It obtains the Hamiltonian of a total laser field and interaction force between two filaments with different phase shifts and crossing angles. The property of the interaction force, which leads the attraction or repulsion of filaments, is basically dependent on the phase shift between filaments. The crossing angle between two filaments can only determine the magnitude of the interaction force, but does not change the property of the force.

Tilted-pulse time-resolved off-axis digital holography

In this Letter we present an improvement of time-resolved off-axis digital holography by the use of tilted femtosecond laser pulses. The pulse front tilting of the reference beam with respect to the phase front allows larger crossing angles to be used for recording of digital holograms without significant reduction of pulse interference area (typically limited by low temporal coherence of ultrashort pulses). Such approach increases the area of interference fringes, thus enabling the higher resolution of the reconstructed image as well as better separation of dc term. Temporal resolution is not deteriorated by this method, as only the reference pulse is tilted. The proposed technique was applied for direct intensity clamping observations of light filaments in water using the laser pulses of 30 fs duration.

Self-pinching of pulsed laser beams during filamentary propagation

Competing nonlinear optical effects that act on femtosecond laser pulses propagating in a self-generated light filament may give rise to a pronounced radial beam deformation, similar to the z-pinch contraction of pulsed high-current discharges. This self-generated spatial beam contraction is accompanied by a pulse break-up that can be beneficially exploited for on-axis temporal compression of the pulse. The pinching mechanism therefore explains the recently observed self-compression and the complicated spatio-temporal shapes typical for filament propagation experiments.

Long distance filamentation of 400 nm femtosecond laser pulses in air

The characteristics of the light filaments formed by λ=400 nm blue femtosecond laser pulses are experimentally investigated. Both pre-focused and free propagation conditions are studied. The diameter, length and electron density were measured by using fluorescence signal imaging and longitudinal diffraction method. About 2×1017 cm−3 electron density in filaments was achieved with beam pre-focusing. In the free propagation case, the multi-filamentation can be observed over 70 m propagation distance, and the energy in the filament core is only about 30 μJ. The effects of energy reservoir on the filament core have been investigated by inserting a diaphragm in a selected single filament path to block the energy of the reservoir. The evolution of filamentation has been studied experimentally as a function of diaphragm size. The energy reservoir can be extended to about 10 mm from the filament core. The results indicate that the up conversion of laser is a possible way to enhance the utilization of laser energy for the generation of long filaments.

Self-focusing and multiple filamentation of laser light in disperse media

We present the results of a computer simulation of the initiation and formation of filaments under conditions of high-power laser pulse scattering on atmospheric aerosols. We demonstrate that the femtosecond filament is robust to the perturbations induced by a disperse medium and can transport a high-fluence energy for long distances. At sufficient pulse power, the intensity perturbations, caused by the interference of light scattered on particles, initiate the generation of filaments and result in multiple filamentation. The results were obtained using the stratified-medium approach of coherent light scattering on an ensemble of aerosol particles.

Tungsten oxide fiber dissolution and persistence in artificial human lung fluids

Tungsten is a dense metal that is used in a range of industrial applications, including non-sag wire for light bulb filaments. During the conversion of tungsten oxide powder into tungsten metal powder for use as filaments, aerosols may be generated which contain tungsten sub-oxide particles having fiber morphology. To evaluate whether these fibers pose a yet unrecognized inhalation hazard due in part to their biodurability, we characterized the physicochemical properties and measured relative dissolution of fiber-containing (WO2.81, WO2.66, WO2.51) and isometric-shaped (WO3.00, WO2.98) powders in artificial lung fluids. Raman spectroscopy results present a shift in the main frequencies for tungsten oxide samples that were sonicated in surfactant, confirming a decrease in the size of the crystalline domains by de-agglomeration. Geometric mean fiber aspect ratios were 8.3 (WO2.81), 7.9 (WO2.66), and 6.9 (WO2.51). In artificial extracellular lung fluid, alkylbenzyldimethylammonium chloride (ABDC), added to prevent mold growth during experiments, inhibited (p < 0.05) dissolution of WO2.98, WO2.81, and WO2.66. Less (p < 0.05) of the fibrous WO2.66 and WO2.51 dissolved relative to W metal; however, biodurability was only modestly greater than W metal. These data are useful for understanding the inhalation dosimetry of fibrous and non-fibrous forms of tungsten oxide materials.

Optically adjustable light filaments generated by a compact laser convertor

In the paper, the geometrical parameters and energetics of the extremely narrow pseudo-nondiffracting beams with the spot size of several micrometers are examined. The main attention is focused on design, realization and testing of the set-up enabling conversion of the laser diode beam or the fiber mode to the narrow Bessel-Gauss beam whose spot can be continuously relocated across the plane perpendicular to the beam propagation direction. Application of the laser convertor to the optical manipulation is demonstrated on experiments enabling transport of microparticles along a desired trajectory.

Experimental study and numerical simulation of the propulsion of microbeads by femtosecond laser filament

The light filament formed by intense femtosecond laser pulses in air can be used to generate the effective impulse to propel a micro glass bead. In this report, through both experimental studies and the corresponding numerical simulations that involve the dynamics of the nonlinear propagation of light and the laser ablation mechanism, we confirm that this propulsion scheme is based on the laser ablation of the target material. The fundamental characteristics of laser propulsion using a single ultrafast laser filament is also revealed.

Spatial Solitons in 2D Lattices

These authors describe their observation of two-color optical solitons in a 2D parametric photonic lattice with hexagonal symmetry in lithium niobate. Their results have enabled wavelength-controlled angular steering and transverse displacement of self-trapped filaments of light.

Remote sensing of explosives using infrared and ultraviolet filaments

Light filaments in air provide a unique opportunity to perform remote spectroscopic measurements. A study has been made into the detection of dinitrotoluene (DNT) and ammonium perchlorate using IR and UV filaments. UV filaments appear to have marked advantages in being able to capture clear spectroscopic signatures of these materials, in addition to propagating over longer distances.

Accessibility and occlusion of biopolymers, ray tracing of radiating tubes, and the temperature of a tangle

We introduce a measure of complexity, an energy, for any conformation of filaments. It is the occlusion, the portion hidden when viewed from an arbitrary exterior point. By inverting we get the exposure, a first approximation of the accessibility of the filaments. Assuming the filament is a source, we get the self-irradiation, which leads to both an interpretation as the temperature and a visualization technique: ray tracing as a virtual laboratory. There is a wide variety of applications, from enzyme action on and radiation damage of biopolymers, to the geometry of light bulb filaments. Energy minimization provides automatic detangling, resulting in symmetric and pleasing conformations.

Potential fields of merging and splitting filaments in air

Two interacting light filaments with different initial phases propagating in air are investigated numerically by using a ray tracing method. The evolution of the rays of a filament is governed by a potential field. During propagation, the two potential wells of the two filaments can merge into one or repel each other, depending on the initial phase difference between the two filaments. The study provides a simple description of the interacting filaments.

MODEL FOR MOLECULAR DIRECTOR CONFIGURATION IN A LIQUID CRYSTAL CELL WITH MULTIPLE INTERFACES

The study of optical solitons and light filaments steering in liquid crystal requires the utilization of particular cells designed for top view investigation and realized with an input interface which enables the control of the molecular director configuration, preventing light scattering. Actually, the director orientation imposed by this additional interface is estimated only by experimental observations. We report a simple model describing the distribution of the director orientation inside a liquid crystal sample under the anchoring action of multiple interfaces. The model is based on the elastic continuum theory, and strong anchoring is taken into account for boundary conditions. Results are in good agreement with experimental observations.

Long range trace detection in aqueous aerosol using remote filament-induced breakdown spectroscopy

Remote filament induced breakdown spectroscopy (R-FIBS) is used for probing a cloud of microdroplets where table salt has been dissolved. These microdroplets are a good simulant for aerosols. We demonstrated experimentally that R-FIBS can efficiently be used as a ppm-level sensing technique to remotely retrieve the composition of microdroplets in clouds located at a distance. The technique has been successfully tested up to 70 m and, as revealed by extrapolation, showed great potential for kilometer-range application. The proposed technique is sensitive to the solvent as well. For the first time, to the best of our knowledge, four hydrogen bands from the Balmer series were observed in an aqueous microdroplet cloud after H2O molecules were broken by the light filaments.

Spontaneous Pattern Formation Due to Modulation Instability of Incoherent White Light in a Photopolymerizable Medium

Spontaneous pattern formation due to modulation instability was observed in a broad uniform beam of incoherent white light propagating in an optically isotropic, photopolymerizable organosiloxane. Pattern formation originates from intensity-dependent refractive index changes due to polymerization, which cause competition between the natural diffraction (broadening) and self-induced refraction of the beam. Under these nonlinear conditions, weak intensity modulations in the beam, noise, that would be negligible under linear conditions are amplified. The amplified patterns become unstable over time and spontaneously divide into individual self-trapped filaments of white light of essentially identical diameter (76 +/- 3 microm), which propagate through the medium without diffracting. In the case of noise with a weak 1-D periodic modulation, for example, the uniform beam transformed into a 1-D periodic array of self-trapped lamellae, which in turn formed a 2-D array of self-trapped cylindrical filaments. Although the rate of pattern formation varied inversely with optical power (measured from 8.4 to 59.8 mW), the uniform beam always split into discrete filaments, demonstrating that they are the most stable form of light propagation under the nonlinear conditions created by polymerization. Each filament of light retained the spectral composition and incoherence of white light, which showed that the entire polychromatic, incoherent and unpolarized wavepacket collectively participated in pattern formation. These findings are consistent with recent theoretical models of nonlinear white light propagation and with experimental observations of pattern formation in coherent and partially coherent light. Because refractive index changes due to polymerization are permanent, pattern formation imparts microstructure to the organosiloxane. Optical micrographs revealed that, after pattern formation, the initially homogeneous medium consisted entirely of a closely packed array of narrow channel waveguides induced by self-trapped filaments.

Structural modifications in fused silica induced by ultraviolet fs laser filaments

It is shown that the tight focusing of short ultraviolet laser pulses (248 nm, 450 fs) in the bulk of high bandgap transparent solids (fused silica) can result in structural modifications in the material. These can vary from small changes of the refractive index to birefringence, cracks and voids. This restructuring of the medium is due to the high laser intensities attained, and the plasma that is generated through multi-photon processes. The restructuring comes in the form of a string, which is the result of the nonlinear propagation of the laser beam in the medium as a self-trapped filament. We resume the conditions for the generation of the different types of modifications and comment on the temporal evolution and the role of the plasma strings at the trail of the light filaments.