Filamentation In Air Research Articles

Analysis of the extension of optical filament in air based on phase-nested laser beam

Extending the length of femtosecond laser filament has always been desired for related atmospheric applications. Here we demonstrated that significant extension of filament can be achieved in air by using phase-nested beam which integrates the arise of filamentation and longitudinally extended background reservior in one optical beam. The influences of beam parameters on the length and uniformity of filament and corresponding physical mechanism are revealed. The energy extraction process from the background reservoir to the filament is investigated based on the evolution of pulse profile during the regeneration of filament. A fiber-like model is proposed to explain the capture effect of filament to the background reservior. Our theoretical results will be helpful to understand the mechanism of background reservior in the extending of filament, and may further guide the experimental design of phase-nested beam to achieve long and homogeneous filament in air.

Molecular emission dynamics from a femtosecond filament induced plasma plume

In this study, we investigated the filament-induced plasma properties and the associated molecular emission features from three different non-metallic samples. Graphite, polymethyl methacrylate, and Teflon samples have been ablated using a tightly focused filament, and their emission spectra were analyzed using a time-integrated optical emission spectroscopy technique. The temporal responses and evolution dynamics of molecular species such as CN and C2 from these samples in ambient conditions are compared. The tightly focused filament was generated by focusing the Ti:Sapphire femtosecond pulses having a pulse duration of 29 fs using a short focal length external focusing system. The time-integrated intensified charge-coupled device images of air filament show that the filament survives up to a few nanosecond time duration after the onset of air plasma. The influence of physical and chemical properties of the samples during filament ablation has also been studied by characterizing the optical emission spectra. We find that the molecular signal intensity strongly depends on the sample properties and the position of the sample in the filament. The increase in molecular emission intensity from a graphite sample as a function of incident laser intensity suggests that the tightly focused filament surpasses the intensity clamping value.

Local OAM manipulation of a terahertz wave from the air filament by chirping the few-cycle vortex pump laser

We propose a scheme to manipulate the local orbital angular momentum (OAM) of the ultra-broadband (0.1-30 THz) terahertz (THz) waves from the laser-induced short air filament via chirping the few-cycle vortex laser pump. The simulation results show that either the THz vortex pulses with linear azimuth-dependent phases or the THz angular accelerating vortex beams (AAVBs) with nonlinear azimuth-dependent phases can be produced by tuning the chirp parameter of the pump. Thus, the dominant physical mechanism for THz generation can be determined. The THz temporal and transverse spatial distributions can be also controlled by the chirp parameter. Furthermore, their local OAM density distributions present very complex structures because most of the modulated azimuthal intensity and the corresponding local angular helicity distributions are not able to cancel out completely. Via analyzing the simulated THz results at the different pump intensities, we classify the initial pump intensity into three cases. For the low intensity case, the Kerr effect comes into prominence, so the generated THz radiation shall be vortex pulses. While for the high intensity case, the leading plasma effect dominates. In contrast, when the pump intensity is at the medium level, the Kerr nonlinearity and the plasma effect may be comparable and competitive. Basically, THz AAVBs are generated for both high and medium intensity cases. Our study will provide the possibility for studying the optically induced rotation technology more intuitively from the perspective of angular momentum transfer.

Enhancement of terahertz radiation from a filament by using circularly polarized two-color laser fields

We investigate the terahertz radiation from a filament in two-color laser fields with various polarizations. It has been experimentally demonstrated that the efficiency of terahertz emission in co-rotating two-color circularly polarized laser fields is eight times as strong as that in linearly polarized two-color laser fields with parallel polarization. This enhancement is explained based on the polarization-dependent clamping intensity inside a filament and the dynamics of electrons ionizing from a single atom or molecule. Our finding provides a simple method to improve the THz intensity from an air filament in two-color fields.

High-Temperature Oxidation and Destruction of Metal Filaments in Air

The high-temperature oxidation and the thermal destruction of electrically heated tungsten and molybdenum filaments in air arestudied experimentally. The temperature histories of filaments are obtained, and the successive stages of oxidation are defined. It isshown that the phase transition points of oxides determine the limits of these stages. The filament destruction conditions are established: the temperature in and the diameter of the filament central zone, the electric current corresponding to the filament failure, as well as the burning time. The behavior of oxide particles in an electric field is studied.

Anti-Correlated Plasma and THz Pulse Generation during Two-Color Laser Filamentation in Air

The THz generation efficiency and the plasma density generated by a filament in air have been found anti-correlated when pumped by 800 nm + 1600 nm two-color laser field. The plasma density near zero delay of two laser pulses has a minimum value, which is opposite to the trend of THz generation efficiency and contradicts common sense. The lower plasma density cannot be explained by the static tunneling model according to the conventional photocurrent model, but it might be attributed to the electron trapping by the excited states of nitrogen molecule. The present work also clarifies the dominant role of the drifting velocity accelerated by the two-color laser field during the THz pulse generation process. The results promote our understanding on the optimization of the THz generation efficiency by the two-color laser filamentation.

Bessel Terahertz Pulses from Superluminal Laser Plasma Filaments

Terahertz radiation with a Bessel beam profile is demonstrated experimentally from a two-color laser filament in air, which is induced by tailored femtosecond laser pulses with an axicon. The temporal and spatial distributions of Bessel rings of the terahertz radiation are retrieved after being collected in the far field. A theoretical model is proposed, which suggests that such Bessel terahertz pulses are produced due to the combined effects of the inhomogeneous superluminal filament structure and the phase change of the two-color laser components inside the plasma channel. These two effects lead to wavefront crossover and constructive/destructive interference of terahertz radiation from different plasma sources along the laser filament, respectively. Compared with other methods, our technique can support the generation of Bessel pulses with broad spectral bandwidth. Such Bessel pulses can propagate to the far field without significant spatial spreading, which shall provide new opportunities for terahertz applications.

Time-resolved measurements of electron density and plasma diameter of 1 kHz femtosecond laser filament in air

Time-resolved measurements of electron density and plasma diameter of 1 kHz femtosecond laser filament in air

Regularities of femtosecond laser radiation filamentation in air under aberration focusing

Regularities of femtosecond laser radiation filamentation in air under aberration focusing

Balance of emission from THz sources in DC-biased and unbiased filaments in air

At the frequencies from 0.1 to 1 THz, we measured the angular distributions of terahertz (THz) emission from DC-biased femtosecond filament. The external electric field (DC bias) was increased from 0 to 3.3 kV/cm and provided continuous transition from forward conical emission, corresponding to the unbiased single-color filament, to on-axis emission, corresponding to the DC-biased one. We decomposed the measured far-field THz distributions into the quadrupole and dipole contributions, the latter being increased with increasing biasing field. The superposition of quadrupole and dipole local sources was integrated numerically over the plasma channel length and fit to the experimentally obtained angular distributions. The transition from the conical to the on-axis emission occured at the external field of (3.2 ± 0.8) kV/cm in the range of frequencies studied.

Bubble production by air filament and cavity breakup in plunging breaking wave crests

Air filaments and cavities in plunging breaking waves, generically cylinders, produce bubbles through an interface instability. The effects of gravity, surface tension and surface curvature on cylinder breakup are explored. A generalized dispersion relation is obtained that spans the Rayleigh–Taylor and Plateau–Rayleigh instabilities as cylinder radius varies. The analysis provides insight into the role of surface tension in the formation of bubbles from filaments and cavities. Small filaments break up into bubbles through a Plateau–Rayleigh instability driven through the action of surface tension. Large air cavities produce bubbles through a Rayleigh–Taylor instability driven by gravity and moderated by surface tension, which has a stabilizing effect. Surface tension, interface curvature and gravity are all important for cases between these two extremes. Predicted unstable mode wavenumber and bubble size show good agreement with direct numerical simulations of plunging breaking waves and air cylinders.

Wake dynamics of air filaments generated by high-energy picosecond laser pulses at 1 kHz repetition rate.

We investigated the filamentation in air of 7 ps laser pulses of up to 200 mJ energy from a 1.03 μm-wavelength Yb:YAG laser at repetition rates up to f=1kHz. Interferograms of the wake generated show that while pulses in a train of repetition rate f=0.1kHz encounter a nearly unperturbed environment, at f=1kHz, a channel with an axial air density hole of ∼20% is generated and maintained at all times by the cumulative effect of preceding laser pulses. Measurements at f=1kHz show that the energy deposited decreases proportional to the air channel density depletion, becoming more pronounced as the repetition rate and pulse energy increase. Numerical simulations indicate that contrary to filaments generated by shorter duration pulses, the electron avalanche is the dominant energy loss mechanism during filamentation with 7 ps pulses. The results are of interest for the atmospheric propagation of joule-level picosecond pulses from Yb:YAG lasers, of which average powers now surpass 1 kW, and for channeling other directed energy beams.

Dynamics of femtosecond synthesized coronary profile laser beam filamentation in air

Multiple filamentation in air of high-power ultrashort laser radiation with a transverse intensity profile resembling a corona composed of several annularly distributed independent top-hat sub-beams is theoretically studied. Through the numerical solution of the time-averaged nonlinear Schrödinger equation, we study the spatio-angular dynamics of a near-infrared annular combined beam (CB) along the optical path by varying the number and power of the beamlets (corona-spikes). The evident advances in the use of CB in the multiple-filamentation region manipulation and for achieving long-range filamentation are theoretically revealed. In particular, by adjusting the number and aperture of the constituting sub-beams, it makes it possible to significantly delay the CB filamentation onset distance and increase the filamentation length in air. In addition, at the post-filamentation stage of femtosecond pulse propagation under certain conditions, the CBs exhibit significantly lower angular divergence of their most intense part (post-filamentation light channel) compared to the beams with regular unimodal intensity profiles (Gaussian or plateau-like) that provide enhancement of laser power delivered to the receiver over the atmospheric links.

The propagation of femtosecond laser filaments in air with continuously varying pressures

The characteristics of the filaments generated by propagating a femtosecond Gaussian beam in a 2 m long gas cell with continuously varying pressures at different focal distances are numerically investigated. The simulated results indicate that the onset distance and the length of the filaments depend sensitively on the variation of pressure. These effects are enhanced with the increase of the focal length for the initial power Pin=5Pcr. The main reason is that some parameters (the group velocity dispersion, the nonlinear refractive index, multiphoton ionization rate and the neutral atom density) are variational with the gaseous pressure. And the stability of the filament is determined by the combined effect of the pressure and the focal distance. In addition, we discuss the differences of the filament characteristics between the fixed pressure and continuously varying pressures. Through the analyses of the temporal dynamics and supercontinuum spectra, we find that maintaining a large pressure (1 atm) and changing to a large pressure (such as 0.3∼1 atm) are beneficial to the propagation of the filament and the broadening of the spectra. It is interesting that, for fixing the initial energy Ein=1 mJ, although the critical power of laser pulse is higher at the low pressure, the filament length at the continuously varying pressure p=0.3∼1 atm is still longer than that of p=1∼0.3 atm as the focal distance increases. This research is of great significance to remotely detect atmospheric pollutant components.

Millisecond-long suppression of spectroscopic optical signals using laser filamentation.

Ultrashort laser pulse filamentation in air can extend the delivery of focused laser energy to distances greatly exceeding the Rayleigh length. In this way, remote measurements can be conducted using many standard methods of analytical spectroscopy. The performance of spectroscopic techniques can be enhanced by temporal gating, which rejects the unwanted noise and background. In the present work, we investigate the thermal relaxation of air in the wake of single-filament plasmas using shadowgraphy. We demonstrate that the transient change in refractive index associated with relaxation of the gas can be used to reject both continuous and time-varying spectroscopic signals, including emission from laser-produced plasmas. This method can augment temporal gating of simple optical detectors.

Remote triggering of air-gap discharge by a femtosecond laser filament and postfilament at distances up to 80 m

We experimentally observed laser-induced remote high-voltage discharge triggering between two needle electrodes with half-a-cm spacing. The discharge was initiated by a 744-nm, 90-fs, 6-mJ laser pulse undergoing filamentation in air. For the direct voltage below the self-breakdown threshold, triggering of air-gap discharge was synchronized with a 10-Hz laser repetition rate and occurred between 40 and 80 m of the propagation path. No discharge guiding was observed. The experimentally registered and simulated remote triggering probability was above 80% in the range of 45–60 m from laser output and about 50% in the range of 60–80 m. The probability decreases as the postfilament hot spot diverges with a simultaneous increase in stochastic laser beam wandering.

Raising the saturation point of fluorescence emitted by air optical filament via π phase plate

Femtosecond laser filament induced breakdown spectroscopy is a promising technique that can be used in the remote sensing of air pollutants. Raising the laser energy saturation point below which the laser filament fluorescence increases exponentially with the incident laser energy will significantly improve the sensing sensitivity and detection distance. In this work, a saturation point raising scheme is proposed in which a quarter-circle π phase plate is used to form four optical filaments in air. The extrapolation based on the experimental results infers that the employment of the quarter-circle π phase plate and its induced multiple filaments can lead to the increase of the laser energy saturation point by four times, which is proportional to the number of sub-regions in the phase plate.

Spatial Structure of Femtosecond Laser Radiation during Filamentation in Air

Spatial Structure of Femtosecond Laser Radiation during Filamentation in Air

Air filament contraction

We investigate numerically the contraction dynamics of a long air filament surrounded by liquid for a range of Ohnesorge numbers Oh. The contraction velocity rises to a maximum value Umax and then decreases due to the hydrodynamic drag force from the liquid medium. Umax follows a capillary-inertial scaling for low Oh while it shifts to a capillary-viscous scaling with increasing Oh. Our simulations reveal that contracting air filaments always first rupture via end-pinching mechanism before the Rayleigh–Plateau instability can develop.

Effect of focusing element-induced aberrations on filamentation and supercontinuum emission in ambient air.

Femtosecond laser pulse induced filamentation in atmosphere is susceptible to a number of input laser, focusing optics and medium characteristics. Filamentation of fs pulses in atmosphere is an intense propagation regime where the focusing geometry used to focus the fs laser pulses play an important role influencing the filament intensity and the associated supercontinuum. We identified different optical elements used for focusing the fs laser pulses leading to filamentation in air and classified them according to the induced aberrations. To clearly identify the role of aberrations, all the optical elements were taken to have same focal length. The subsequent filament structure and emissions from the filament were correlated with the aberrations induced by optical element revealed stark differences. The onset of the filamentation, its longitudinal intensity and the associated supercontinuum emission (SCE) have varied drastically with the aberrations induced by the focusing optics. A systematic study directed to choose and identify suitable optical elements according to the usage of the fs pulses for a specific filamentation regime is presented.