Pulses In Air Research Articles

Diffraction-Beam Optics of Filamentation: II–Diffraction-Beam Pattern of Laser Pulse Filamentation

Based on the diffraction-ray representation of nonlinear propagation of laser pulses, some examples of visualization of the filamentation of high-power femtosecond laser pulses in air are considered in the form of evolutionary paths of diffraction beams and light tubes. It is ascertained that a filament is supported by the light energy of the part of the beam which is inside a beam tube with the initial peak power higher than the critical power of self-focusing. This light tube is an energy reservoir for the filament. The light beam periphery holds the diffraction-beam tube which includes the filament by means of formation of a “diffractive” waveguide.

Gas pressure dependence of microwave pulses generated by laser-produced filament plasmas.

The plasma arising due to the propagation of a filamenting ultrafast laser pulse in air contains currents driven by the pulse that generate radiated electromagnetic fields. We report absolutely calibrated measurements of the frequency spectrum of microwaves radiated by the filament plasma from 2-40GHz. The emission pattern of the electric field spectrum is mapped as a function of air pressure from atmosphere to 0.5Torr. For fixed laser pulse energy, duration, and focal geometry, we observe that decreasing the air pressure by a factor of approximately 103 increases the amplitude of the electric field waveform by a factor of about 40. As the air pressure decreases, the lower frequency components (<10 GHz) increase in amplitude faster than those at higher frequencies (>20 GHz). To the best of our knowledge, this behavior has not been observed before, is not predicted by existing theory, and implies the existence of a radiation mechanism in the plasma distinct from that which emits at terahertz frequencies.

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.

Experimental study on uniformity of dielectric barrier discharge generated by nanosecond pulse in atmospheric air

Dielectric barrier discharge (DBD) with better uniformity, excited by nanosecond pulses, is achieved within 8 mm air gap by using a self-designed DBD structure in airflow. The self-designed DBD structure is characterized by adding a metal rod (MR) in discharge gap, which will produce a local strong electric field and induce surface and volume discharge. It is observed from sequential single periodic discharge images that the uniformity of discharge starts from the electrode edge at the airflow inlet and moves gradually toward downstream under the action of airflow. When the uniformity of discharge is transmitted to a nearby of MR, a stable uniform and symmetrical annular discharge configuration is formed. Besides, that uniform area can be expanded by adding many MRs or MR array in the discharge gap. There is an optimum distance between every two MRs to produce a continuous and maximum discharge area with better uniformity. The above phenomena are mainly ascribed to the change of electric field distribution caused by MR and the charged particles redistribution induced by airflow.

Remote access to electrical conductivity by charge attachment from an ambient pressure plasma

A technique for investigating the electrical conductivity of condensed matter based on softly attaching charge carriers from a plasma to the front side of a sample is presented. The attachment of the charge carriers induces an electric surface potential and consequently charge migration inside the sample which is in contact with a single metal electrode at the backside. The measurement of the current allows determining the electrical conductivity of the sample. The plasma employed is generated by focusing femtosecond laser pulses in air at ambient pressure. As a proof-of-principle, we demonstrate the measurement of the ionic conductivity of a D263T glass and its activation energy. The analysis reveals very good agreement with independent previous measurements. The approach presented is operative in positive and negative polarities. It is in principle extendable to a wide range of plasma media and pressures and can thus be expected to be applicable for remote material testing.

Dual THz Wave and X-ray Generation from a Water Film under Femtosecond Laser Excitation.

Simultaneous emission of the THz wave and hard X-ray from thin water free-flow was induced by the irradiation of tightly-focused femtosecond laser pulses (35 fs, 800 nm, 500 Hz) in air. Intensity measurements of the THz wave and X-ray were carried out at the same time with time-domain spectroscopy (TDS) based on electro-optic sampling with a ZnTe(110) crystal and a Geiger counter, respectively. Intensity profiles of the THz wave and X-ray emission as a function of the solution flow position along the incident laser axis at the laser focus show that the profile width of the THz wave is broader than that of the X-ray. Furthermore, the profiles of the THz wave measured in reflection and transmission directions show different features and indicate that THz wave emission is, under single-pulse excitation, induced mainly in laser-induced plasma on the water flow surface. Under double-pulse excitation with a time separation of 4.6 ns, 5–10 times enhancements of THz wave emission were observed. Such dual light sources can be used to characterise materials, as well as to reveal the sequence of material modifications under intense laser pulses.

Circular ripple formation on the silicon wafer surface after interaction with linearly polarized femtosecond laser pulses in air and water environments

Ultrashort laser pulse interaction with the surface of silicon wafer in air and water environments is investigated. Ti:sapphire laser with 40 femtosecond laser pulses at 790 nm and 10 Hz repetition rate was used. The ablation threshold of the silicon surface in the air was determined to be about 0.28 J cm−2. The surface morphology was studied by using scanning electron microscope images. The size of the regular ripples formed in the air environment is a little smaller than the laser wavelength. Due to the nonlinear interaction and self-focusing before the target, the ripples size reduced to nearly a half of the laser wavelength in the water. Moreover, the spikes’ structure formation and their diameter in air and water were studied. Two regimes for spike formation in water are proposed that can explain the anomalous decrease of the spikes’ diameter in higher fluence. During the interaction of single linearly polarized femtosecond laser pulse with the surface, an irregular ripple formation that called circular ripple is observed. This structure which is a result of radiation pressure implies to the surface by the end of the pulse. A new physical model for interpretation of the circular ripples formation based on the ponderomotive force of an ultrashort pulse laser is proposed which can predict the size of the circular ripples. The calculated results are in accordance with our experimental findings.

Ab initio calculations of the linear and nonlinear susceptibilities of N2 , O2 , and air in midinfrared laser pulses

We present first-principles calculations of the linear and nonlinear susceptibilities of N$_2$, O$_2$, and air in the mid-infrared wavelength regime, from $1-4$ $\mu$m. We extract the frequency-dependent susceptibilities from the full time-dependent dipole moment that is calculated using time-dependent density functional theory. We find good agreement with curves derived from experimental results for the linear susceptibility, and with measurements for the nonlinear susceptibility up to 2.4 $\mu$m. We also find that the susceptibilities are insensitive to the laser intensity even in the strong field regime up to $5\times 10^{13}$ W/cm$^2$. Our results will allow accurate calculations of the long-distance propagation of intense MIR laser pulses in air.

Electric-field induced second-harmonic generation of femtosecond pulses in atmospheric air

Electric-field induced second harmonic generation with a femtosecond Ti:sapphire pump laser in atmospheric air is investigated. Phase matching between fundamental and second harmonic waves is achieved via quasi-phase matching, employing periodic electrode arrangements on printed circuit boards with a total length of about 0.5 m. The weak chromatic dispersion of the atmospheric air enables broadband phase matching conditions and makes this method tailorable for frequency doubling broadband femtosecond laser pulses. A maximum energy of 0.13 nJ is observed in a 20 nm bandwidth around 395 nm.

Visualization of differences in nanosecond laser ablation behavior for geological standard samples with different matrices

A combination of time-resolved shadowgraphy and laser-induced breakdown spectra (LIBS) was performed to investigate the ablation behavior of United States Geological Survey (USGS) and National Institute of Standards and Technology (NIST) geological standard samples when ablated by focused nanosecond laser pulses in air. The shock wave (SW) radius achieved 1535 μm at 743 ns for the BCR-2G, and 1513 μm at 759 ns for the NIST SRM 612 standard samples when a laser pulse energy of 17 mJ was used. The maximum speed, pressure, temperature, and density of the shock wave were 5435 m/s, more than 600 MPa, 3 × 105 K, and 7.76 kg/m3, respectively, for NIST SRM 612. Three SWs were generated as these standard samples were ablated. Laser-induced SWs are expected to contribute to attain an approximate matrix-matched reference material, which would render direct solid concentration analysis possible.

Filamentation of mid-IR pulses in ambient air in the vicinity of molecular resonances

Properties of filaments ignited by multi-millijoule, 90fs mid-infrared pulses centered at 3.9μm are examined experimentally by monitoring plasma density, losses, spectral dynamics and beam profile evolution at different focusing strengths. By changing from strong (f=0.25 m) to loose (f=7 m) focusing, we observe a shift from plasma-assisted filamentation to filaments with low plasma density. In the latter case, filamentation manifests itself by beam self-symmetrization and spatial self-channeling. Spectral dynamics in the case of loose focusing is dominated by the nonlinear Raman frequency downshift, which leads to the overlap with the CO2 resonance in the vicinity of 4.2μm. The dynamic CO2 absorption in the case of 3.9μm filaments with their low plasma content is the main mechanism of energy losses and, either alone or together with other nonlinear processes, contributes to the arrest of intensity.

Manipulation by multiple filamentation of subpicosecond TW KrF laser beam

A self-focusing of TW-level subpicosecond UV KrF laser pulses in ambient air produces a few 100 randomly distributed filaments over 100-m propagation distance. A control of multiple filamentation process by a number of methods was demonstrated in the present work envisaging applications for a HV discharge guiding, remote excitation of an atmospheric air laser, MW radiation transfer by virtual plasma waveguide, as well as filamentation suppression to improve short pulse parameters in direct amplification scheme. Under the laser beam focusing, a multitude of filaments coalesced into a superfilament with highly increased intensity and plasma conductivity. A superradiant forward lasing was obtained in the superfilament around 1.07-µm wavelength of atmospheric nitrogen. A regular 2D array of a 100 superfilaments was configured over 20-m distance by Fresnel diffraction on periodic amplitude masks. Effective Kerr defocusing and a subsequent filaments suppression over 50-m distance was demonstrated in Xe due to 2-photon resonance of laser radiation with 6p state being accompanied by a narrow-angle coherent conical emission at 828-nm wavelength.

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.

Controlled generation of a single Trichel pulse and a series of single Trichel pulses in air

In this paper, a simple method for the controlled generation of a single Trichel pulse or a series of single Trichel pulses of a regulated repetition frequency in air is proposed. The concept of triggering a single Trichel pulse or a series of such pulses is based on the precise controlling the voltage inception of the negative corona, which can be accomplished through the use of a ramp voltage pulse or a series of such pulses with properly chosen ramp voltage pulse parameters (rise and fall times, and ramp voltage pulse repetition frequency). The proposal has been tested in experiments using a needle-to-plate electrode arrangement in air, and reproducible Trichel pulses (single or in a series) were obtained by triggering them with an appropriately designed voltage waveform. The proposed method and results obtained have been qualitatively analysed. The analysis provides guidance for designing the voltage ramp pulse in respect of the generation of a single Trichel pulse or a series of single Trichel pulses. The controlled generation of a single Trichel pulse or a series of such pulses would be a helpful research tool for the refined studies of the fundamental processes in a negative corona discharge in a single- (air is an example) and multi-phase gaseous fluids. The controlled generation of a single Trichel pulse or a series of Trichel pulses can also be attractive for those corona treatments which need manipulation of the electric charge and heat portions delivered by the Trichel pulses to the object.

The dependence of external focusing geometries and polarization in generation of supercontinuum by femtosecond laser pulse in air

Supercontinuum (SC) generation in ambient air using different focusing geometries and laser energy under linear and circular polarization was investigated experimentally. Under the same input laser energy, the SC spectra in the visible range became wider when smaller f-number convex lens are used and the linearly polarized laser is used as the pump source, the main nonlinear mechanism for asymmetric spectral broadening of femtosecond laser pulses towards the higher frequencies in isotropic media is that the cascade generation with GHz spectral shift for gases and the four-photon parametric wave mixing processes. Besides, the temporal differential of electron density Ne(t) play a more important role in extending the SC spectra to the blue side than the interaction length l. Under the same magnitude of the optical field and the critical power for self-focusing of linearly and circularly polarized beams, the SC spectra range is wider when circularly polarized beams as the pump source.

Formation of nanoparticles from thin silver films irradiated by laser pulses in air

Some specific features of the transport of silver nanoparticles onto a SiO2 substrate under focused nanosecond IR laser pulses is experimentally investigated. A possibility of obtaining silver coatings is demonstrated. The formation of silver nanostructures as a result of pulsed laser ablation in air is studied. Nanoparticles are formed by exposing a silver film to radiation of an HTF MARK (Bulat) laser marker (λ = 1064 nm). The thus prepared nanoparticles are analysed using scanning electron microscopy and optical spectroscopy.

Broadening and shift of emission lines in a plasma of filaments generated by a tightly focused femtosecond laser pulse in air

We study the broadening and shift of N I (λ = 746.8 nm) and O I ( λ = 777.4 nm) lines in a plasma of filaments generated by a tightly focused femtosecond laser pulse (0.9 mJ, 48 fs). It is shown that the dynamic Stark effect makes a major contribution to the broadening and shift of the N I line (λ = 746.8 nm). It is found that amplified spontaneous emission of a regenerative amplifier and post-pulses break the LS coupling for the O I 3p 5P energy level, lead to the generation of side bands related to the Rabi splitting of levels and result in pulsed emission of the first positive system of N2.

Influence of ambient pressure on surface structures generated by ultrashort laser pulse irradiation

We report an experimental investigation on the surface structures induced by linearly polarized ≈ 900 fs laser pulses, at λ = 1055 nm, on silicon at different values of the ambient pressure, from ~ 10−4 mbar to one atmosphere. Our experimental findings address interesting influences of the surrounding pressure on: (1) the spatial period of ripples; (2) the formation of micro-grooves; (3) the shape of the structured area. Moreover, the effects of various states of polarization in vacuum as well as of circularly polarized pulses in air vs vacuum are also addressed. We identify as one possible key element of such experimental observations: the fact that as the pressure raises the ablated nanoparticles produced during the femtosecond ablation process of the target get deposited more and more on the sample surface covering the irradiated spot area and influencing the structuring process.

Spectral broadening and conical emission of near-infrared femtosecond laser pulses in air

Spectral and spatial properties of infrared femtosecond laser pulses with a wavelength between 1.2 and 1.6 μm propagating in air are investigated both, experimentally and theoretically. It is shown that under tight focusing conditions laser pulses experience asymmetric spectral broadening (SB) and a strong blue-shift with respect to the incident laser wavelength. The observed blue-shifted radiation is emitted conically, in contrast to the weaker red-shifted on-axis emission. The SB and frequency shifts were interpreted in terms of ionization-induced effects.

Free-beam soliton self-compression in air

We identify a physical scenario whereby soliton transients generated in freely propagating laser beams within the regions of anomalous dispersion in air can be compressed as a part of their free-beam spatiotemporal evolution to yield few-cycle mid- and long-wavelength-infrared field waveforms, whose peak power is substantially higher than the peak power of the input pulses. We show that this free-beam soliton self-compression scenario does not require ionization or laser-induced filamentation, enabling high-throughput self-compression of mid- and long-wavelength-infrared laser pulses within a broad range of peak powers from tens of gigawatts up to the terawatt level. We also demonstrate that this method of pulse compression can be extended to long-range propagation, providing self-compression of high-peak-power laser pulses in atmospheric air within propagation ranges as long as hundreds of meters, suggesting new ways towards longer-range standoff detection and remote sensing.