Pulse Filamentation Research Articles

Role of coherent resonant nonlinear processes in the ultrashort KrF laser pulse propagation and filamentation in air

Recent experiments on multiple filamentation of sub-picosecond terawatt-level KrF laser pulse in air and multi-photon ionization of air revealed an extremely low electron density in filaments, which is out of the conventional filamentation model considering Kerr self-focusing and plasma de-focusing. We propose here the coherent resonant scattering and ionization processes at the pulse durations significantly less than the polarization relaxation time to be possible explanation of the observed filamentation peculiarities. Namely, we argue that the plasma production results from the resonance enhanced (2+1)-photon ionization of the oxygen molecules through the two-photon excitation of the 3s metastable Rydberg state. Coherent Raman self-scattering at rotational transitions of nitrogen molecules provides self-induced focusing of the ultrashort UV laser pulse and filament formation.

Formation of luminescence centers and nonlinear optical effects in silver-containing glasses under femtosecond laser pulses

It is experimentally shown that irradiation of silver-containing silicate glass by IR femtosecond laser pulses leads to generation of free electrons in glass as a result of multiphoton ionization. Free electrons, being captured by charged molecular silver clusters, transform them into the neutral state, as a result of which the luminescence intensity along the laser-pulse track significantly increases. When a laser pulse propagates in glass, cubic nonlinearity dominates in the initial portion of the path, which leads to pulse self-focusing and filamentation. In the final portion of the path, the dominant process is self-defocusing, which is caused by the local decrease in the refractive index as a result of multiphoton generation of free electrons.

Multiple filamentation of collimated Ti:Sapphire laser beams in water

The results of experimental studies of femtosecond laser pulse filamentation in water are described. The dependencies of the number of filaments, spectrum width, distance of nonlinear focusing, and diameter of filamentation zone on the laser pulse power are measured. Existence of a segment on the scale of relative power is marked out, where the number of filaments increases dramatically.

Erratum: Compression, spectral broadening, and collimation in multiple, femtosecond pulse filamentation in atmosphere [Phys. Rev. A86, 033834 (2012)

Erratum: Compression, spectral broadening, and collimation in multiple, femtosecond pulse filamentation in atmosphere [Phys. Rev. A<b>86</b>, 033834 (2012)

Effect of nonlinearity in the pass-through optics on femtosecond laser filament in air

The influence of pass-through optics on femtosecond laser pulse filamentation in ambient air is analyzed for the first time both experimentally and numerically. Propagation of a high-power femtosecond laser pulse through solid optical elements introduces spatiotemporal phase modulation due to the Kerr effect. This modulation may have a strong effect on the pulse filamentation in air. We demonstrated that the phase modulation obtained in the thin pass-through dielectric plate reduces the distance to the filament onset and increases the plasma channel length.

Dynamics of CO(2) laser pulse filamentation in air influenced by spectrally selective molecular absorption.

The theoretical aspects of self-focusing and filamentation of high-power pulsed CO(2) laser radiation with carrier wavelength 10.6 μm in air are considered. The spectrally selective molecular absorption of realistic atmospheric air is included in the theoretical model. In the conditions of strong pulse self-phase modulation and pulse spectral broadening, the supercontinual radiation spectrum is substantially influenced by the selective atmospheric absorption that destabilizes the filamentation process and results in considerable shortening of the filamentation length.

Spectral shifts of the fundamental and third harmonic radiation in air induced by self-focused femtosecond laser pulses

The large spectral shifts of the fundamental and third harmonic waves (up to 20nm and 50nm, respectively) during femtosecond laser pulse filamentation in air have been demonstrated by using focusing lenses of long focal length. It was shown that in a wide range of laser intensities this process does not saturate and linearly depend on the pump pulse energy.

Generation of high harmonics and attosecond pulses with ultrashort laser pulse filaments and conical waves

Results illustrating the nonlinear dynamics of ultrashort laser pulse filamentation in gases are presented, with particular emphasis on the filament properties useful for developing attosecond light sources. Two aspects of ultrashort pulse filaments are specifically discussed: (i) numerical simulation results on pulse self-compression by filamentation in a gas cell filled with noble gas. Measurements of high harmonics generated by the pulse extracted from the filament allows for the detection of intensity spikes and subcycle pulses generated within the filament. (ii) Simulation results on the spontaneous formation of conical wavepackets during filamentation in gases, which in turn can be used as efficient driving pulses for the generation of high harmonics and isolated attosecond pulses.

Filament-Based Fabrication and Performance Analysis of Fiber Bragg Grating Sensors Using Ultrashort Pulse Laser

The method of ultrashort pulse filamentation induced refractive index modification is employed to inscribe fiber Bragg grating (FBG) in single-mode optical fiber (SMF). Line-by-line index inscription technique is used to write refractive index modulation in the core of SMF. The proposed pulse filamentation based index modification enables controlled and flexible writing of FBGs in optical fibers. Performance analysis of the fabricated FBG has been carried out for temperature, contact force, pressure, and axial strain sensing. The in-fiber FBG exhibits sensing performance very similar to FBGs commercially available to date. Then, the written FBG is engineered to demonstrate highly sensitive contact force sensor.

Single and multiple filamentation of multi-terawatt CO_2-laser pulses in air: numerical simulations

We numerically simulated the nonlinear propagation of high-power subpicosecond laser radiation at 10.6 μm carrier wavelength in air under conditions of pulse Kerr self-focusing and single and multiple filamentation. For the first time, to the best of our knowledge, we show that in contrast to the well-known femtosecond pulse filamentation in the near-infrared domain (800 nm), the self-action of mid-infrared radiation is characterized by the formation of millimeter-wide filaments accompanied by nearly continuous along the propagation direction and elongated plasma columns. The physical cause is the change of the air ionization regime by the CO2-laser pulse in favor of impact ionization and electron avalanche development. This alters the character of plasma influence on the laser radiation by enhancing the role of physical processes attributed to the liberated hot electrons.

Electric field measurements during filament-guided discharge

One application of ultrashort pulse filamentation is the coupling of external electric fields to filament plasmas and guiding of high-voltage discharges. However, the full physics of the guiding mechanism is still in question. Several models have been presented and explanations have been suggested to capture the full physics of the discharge event. For the first time, measurements of the electric field dynamics between two electrodes during filament-guided discharges are presented here, to the best of our knowledge. The electric field dynamics show an exponential growth region, a plateau, followed by a sharp drop off coinciding with the discharge event. We believe these results will ultimately answer the questions regarding the guiding mechanism.

Infrared pulse characterization using four-wave mixing inside a few cycle pulse filament in air

We demonstrate a four-wave mixing (FWM) technique to measure near- and mid-infrared (IR) laser pulse shapes in time domain. Few cycle 800 nm laser pulses were synchronized with the IR pulse and focused colinearly to generate a plasma filament in air. Second harmonic radiation around 400 nm was generated through FWM, with a yield proportional to the IR pulse intensity. Excellent signal to noise ratio was observed from 2.1 μm to 18 μm. With proper phase stabilization of the IR beam, this technique is a promising step toward direct electric field sensing of near-IR pulses in air.

Single-shot tomographic movies of evolving light-velocity objects

Tomography—cross-sectional imaging based on measuring radiation transmitted through an object along different directions—enables non-invasive imaging of hidden stationary objects, such as internal bodily organs, from their sequentially measured projections. Here we adapt tomographic methods to visualize—in one laser shot—the instantaneous structure and evolution of a laser-induced object propagating through a transparent Kerr medium. We reconstruct ‘movies’ of a laser pulse’s diffraction, self-focusing and filamentation from phase ‘streaks’ imprinted onto probe pulses that cross the main pulse’s path simultaneously at different angles. Multiple probes are generated and detected compactly and simply, making the system robust, easy to align and adaptable to many problems. Our technique could potentially visualize, for example, plasma wakefield accelerators, optical rogue waves or fast ignitor pulses, light-velocity objects, whose detailed space–time dynamics are known only through intensive computer simulations.

Time-Resolved Digital Holography in the Investigation of Ablation and Micro Fabrication by Femtosecond Pulses

During the last years it was shown that time-resolved digital holography (TRDH), combining the conventional offaxis digital holography and ultrashort probing laser pulses results in versatile tool suitable for ultra-fast phenomena studies. The temporal resolution of this method depends on the duration of the used ultrashort probing pulses and measurements could be performed in temporal range from ~20 fs to ~10 ns. We review features of TRDH with femtosecond pulses and its applications for the investigation of such processes occurred or used in laser micro fabrication as femtosecond pulse filamentation in transparent solids, thin film damage by femtosecond pulses and one and double pulse femtosecond laser ablation.

Formation and propagation of meter-scale laser filaments in water

We report the demonstration, characterization, and modeling of meter-scale underwater optical filaments using a nanosecond pulsed laser. We observed single filament formation for P/PCRIT = 1–5, where PCRIT ∼ 1 MW in water. We employed a variable distance water tube to characterize laser pulse evolution and filament formation. Filaments with uniform radius 50 ± 10 μm persisted for 55 cm (&amp;gt;35 Rayleigh lengths). Significant forward Stimulated Raman Scattering (up to 60%) was observed and characterized. Simulation results for propagation distances and radii were in agreement with experiment, and predict a structured plasma with peak density of 1.5 × 1018 cm−3.

Light bullets from near-IR filament in fused silica

The formation of light bullets during femtosecond laser pulse filamentation in the presence of anomalous group velocity dispersion has been recorded for the first time. The minimum experimentally detected width of the light bullet autocorrelation function is 27 fs, which corresponds to a duration of about 13.5 fs. The duration of the light bullet at a wavelength of 1800 nm is about two periods of the light field oscillation. The numerically calculated width of the autocorrelation function for such a light bullet is 23 fs, which is in good agreement with the experimental value.

Near-field modification of femtosecond laser beam to enhance single-shot pulse filamentation in glass medium

In this paper, a study of the effect of a transparent dielectric medium (soda-lime glass placed after an objective lens) on femtosecond pulse filamentation in Corning Eagle2000 glass is presented. The dielectric medium placed after the objective lens increases the effective numerical aperture of the focusing lens and tightly focuses the beam to elevate the power density in a tiny focal volume. A single-shot narrow filamentary void of diameter 3.8 μm is observed to propagate as long as 700 μm in Corning Eagle2000 glass. Elevated optical intensity confined in the small focal volume and low ionization losses per unit length in the narrow plasma column collectively enhance the filamentary propagation of the ultra-short pulse in bulk glass. The applied technique also offers an approach to control the length and width of a filamentary void structure in a transparent dielectric medium.

Spectral characterization of third-order harmonic generation assisted by a two-dimensional plasma grating in air

A dramatic spectral modulation of third-order harmonic (TH) emission generated in a near-infrared femtosecond (fs) pulse filamentation, assisted by a two-dimensional plasma grating formed by two pump femtosecond pulses, is experimentally demonstrated when their spatiotemporal overlap is achieved. It is mainly attributed to strong cross-phase modulation induced by the fundamental wave of the probe pulse and two pump ones. The delay dynamic of TH spectra indicates the influence of two retarded nonlinear responses on the TH generation. The dependencies of TH generation on the energies of probe and pump pulses, relative field polarization angle are also studied.

Femtosecond laser-induced refractive index modifications in fluoride glass

The interaction of focused 800 nm femtosecond (fs) laser pulses at low (1–45 kHz) and moderate (50–250 kHz) repetition rates with fluoride (ZBLAN-type) bulk glass is investigated in detail. It is shown that at repetition rates lower than ~50 kHz and low pulse energies, the refractive index change induced by pulse filamentation is mainly negative at the irradiated zone. At repetition rates above 50 kHz, structures are formed as a result of the fs laser pulse induced heat accumulation and subsequent melting of the glass. The refractive index profile of the structures produced in this regime is influenced by the laser writing conditions (repetition rate, pulse energy and translation speed). It is shown that waveguides with large circular cross sections and smooth positive index changes can be formed through a precise control of glass exposure. Those waveguides can exhibit low propagation losses (~0.4–2 dB/cm) and are thus good candidates for the development of mid-IR integrated photonic devices. Both absorbed energy and repetition rate thresholds for heat accumulation are determined experimentally.

The Angular Distribution of Optical Emission Spectroscopy from a Femtosecond Laser Filament in Air

Spatial characterization of a femtosecond laser pulse filament in air is experimentally investigated by detecting the optical emission spectroscopy of laser-induced plasmas. It is shown that the intensity of NI spectral lines have different angular distributions along the filament and the calculated results fit the experimental data well. The obtained degree of atomic polarization along the filament strongly depends on the electron density and the propagation characteristic of the femtosecond laser pulse.