Laser-induced Air Breakdown Research Articles

Laser energy partitioning in nanosecond pulsed laser-induced air breakdown: effect of incident laser energy.

Air breakdown is generated by a 1064nm nanosecond pulsed laser beam, and laser energy deposited in the breakdown (E d), transmitted through the plasma region (E t) and carried away by the shock wave (E s) is estimated for the incident laser energy (E i) range of 60-273mJ. The E d is approximately 85% of E i at 60mJ, rapidly increasing to 92% at 102mJ. The shock wave front velocity and radius are measured as a function of E i and propagation distance. The shock wave velocity nicely follows the v∝E i0.3 trend predicted by the laser-supported detonation wave model. The Sedov-Taylor theory is used to estimate E s, which rapidly increases with E i, but E i to E s conversion linearly decreases from 83% to 48%. At lower values of E i, most of the laser energy is carried away by the shock wave, whereas the laser energy used in plasma heating or released in the form of electromagnetic and thermal radiation becomes important at higher laser energies. This implies that laser energy partitioning is highly dependent on the value of incident laser energy. These findings provide important insights into the fundamental physics of air breakdown and will be useful in a variety of applications such as laser-induced breakdown spectroscopy, laser ignition, and laser propulsion.

Spatial, spectral, and temporal properties of laser-induced plasma in air near an aqueous solution and comparison with ambient air

Laser-induced breakdown and subsequent plasma are produced in ambient air in the proximity of an aqueous surface using a Nd:YAG Q-switched laser at 1064 nm with a pulse width of 9 ns and a delivered focused input energy of 170 mJ. The distance between the focal point of a 10 cm convex lens and the aqueous surface is 4 mm with laser propagation perpendicular to the surface. Using an intensified CCD camera attached to a 1-m spectrometer, spatial and wavelength-resolved plasma emission data are obtained for delay times after breakdown ranging from 50 ns to 10 μs with a gate window typically 5 ns. Plasma electron density is determined by applying Lorentzian fitting and FWHM extraction to three Stark-broadened spectral lines: N II 3P-3Do multiplet (593.85 nm), Hα (656.27 nm), and the Na D doublet (589.00 and 589.59 nm). One-dimensional spatially resolved measurements of the total emission intensity and electron density are obtained by binning the camera image along the laser axis in intervals of 250 μm and are reported as a function of time from 50 ns to 10 μs. Two plasmas are ignited from a single laser pulse; one from laser breakdown at the water surface and the other a few nanoseconds later from laser-induced air breakdown at the focal point of 4 mm above the water surface. Comparisons between the evolution of the air plasma near and far from the water surface are presented along with data for the water surface plasma itself.

A database of near-field head-related transfer functions based on measurements with a laser spark source

This paper presents a database of near-field head-related transfer functions (HRTFs) of an artificial head, measured at four distances (0.2, 0.3, 0.4 and 0.5 m), with 49 positions recorded at each distance, for a total of 196 measurement points. The HRTFs were recorded using an acoustic pulse created by a laser-induced breakdown of air (LIB), which realizes a close to ideal, massless, monopole sound source. While the LIB produces a high amplitude pressure pulse, the amplitude decays toward low frequencies, which introduced a low frequency limit of about 200Hz for this particular setup. Thus, a spherical head model based on the analytical expression for scattering by a rigid sphere was fitted to the measured data, and used to extend the low frequency range of the measurements. A brief evaluation of the processed dataset was undertaken, considering interaural time and level differences. The measured and processed database, as well as the low frequency extension procedure are made publicly available to support future research into nearby sound localization, and virtual/augmented reality applications.

Investigating the material properties of HfO2/SiO2-based anti-reflection coatings during 1064 nm laser-induced breakdown in air and vacuum conditions

It is important to understand the damage process of optical coatings subjected to nanosecond laser ablation. However, investigating the high power laser interaction with optical coatings in a short time and space resolution is challenging. The dynamic evolution of materials ejection and layer peeling-off processes during damage of HfO2/SiO2-based anti-reflection coatings induced by 1064 nm pulsed laser were systematically examined by employing the time-resolved shadowgraph technique. Therefore, materials response imaging was performed by shadowgraphy to better understand the interaction between coatings and laser beam in air and vacuum conditions. The coatings damage in vacuum was not influenced by the plasma plume, which showed markedly different responses to the laser-coatings interaction. By comparing the form of damage crater and layer peeling-off through SEM analysis and step profiler, it was found that more laser energy dissipated into the target with the decrease in ambient air pressure. For the reverse damage process in air and vacuum, materials ejection by the continuous impact of the laser energy was observed and the forms of materials ejection and damaged craters were the same. Investigating the materials response under different air pressure conditions provided valuable insights into the damage mechanisms during the nanosecond laser-induced damage of optical coatings.

The influence of more concentrated energy distribution formed by an aspheric lens with eliminated spherical aberration on laser-induced air breakdown

Using an aspheric lens for laser focusing solves the energy dispersion caused by spherical aberration, reducing the plasma fluctuations.

Investigating the Threshold Conditions of Air Breakdown with Mode-Locked Q-Switched Laser Pulses, and the Temporal Dynamics of Induced Plasma with Self-Scattering Phenomenon

A Q-switched Nd:YAG laser with mode-locked modulations is utilized to explore the laser-induced air breakdown. The various modulation depths of the mode-locking within the Q-switched pulse can be utilized to investigate the threshold conditions. With the GHz high-speed detectors to accurately measure the temporal pulse shape pulse by pulse, it is verified that the air breakdown threshold is crucially determined by the peak-power density instead of the energy density from the statistic results, especially for mode-locked Q-switched lasers. The stability of the system for laser-induced breakdown can be evaluated by threshold width through fitting the statistical result. Otherwise, by measuring the temporal characteristics of the excitation pulse and the induced plasma, it is further found that the plasma radiation displays a few-nanoseconds time delay to the excitation pulse and shows a decaying tail to be 10 times longer than the plasma build-up time. Moreover, the incident laser pulse is observed to be self-scattered by the air breakdown, and a rapidly modulated scattering rate is found with a slight delay time to the excitation mode-locked subpulse modulations.

On the correlation of light and sound radiation following laser-induced breakdown in air

Laser-induced breakdown in ambient air by short optical pulses is accompanied by light and sound emission from the excited volume. In this work, these secondary radiative phenomena are jointly studied and their correlation is analyzed and demonstrated both experimentally and analytically. Simultaneous systematic measurements of the light and acoustic signals produced by nanosecond laser pulses of different energies are presented at sufficiently large time windows that allow for the observation of the complete phenomena. The contribution originating from the raised temperature (thermal radiation) in the excited volume is isolated out of the full optical signal and reconstructed via signal processing techniques. Through this analysis it becomes evident that the thermal radiation and the acoustic radiation take place at the same time scales. Moreover, the thermal radiation signal is used as a source in a theoretical model to estimate the frequency spectra of the generated acoustic pulses. The model’s evaluations show very good agreement with the experimental measurements of the laser-induced breakdown acoustic spectra for different laser pulse energies. The presented method demonstrates experimentally and analytically the correlation between light and sound radiation following laser-induced breakdown in the ambient air and proves that the emitted sound pulses can be directly evaluated from the respective optical signal.

Numerical simulations of near-field head-related transfer functions: Magnitude verification and validation with laser spark sources

Despite possessing an increased perceptual significance, near-field head-related transfer functions (nf-HRTFs) are more difficult to acquire compared to far-field head-related transfer functions. If properly validated, numerical simulations could be employed to estimate nf-HRTFs: the present study aims to validate the usage of wave-based simulations in the near-field. A thorough validation study is designed where various sources of error are investigated and controlled. The present work proposes the usage of a highly-omnidirectional laser-induced breakdown (LIB) of air as an acoustic point source in nf-HRTF measurements. Despite observed departures from the linear regime of the LIB pressure pulse, the validation results show that asymptotically-estimated solutions to a lossless model (wave-equation and rigid boundaries) agree in magnitude with the LIB-measured nf-HRTF of a rigid head replica approximately within 1-2 dB up to about 17 kHz. Except a decreased reliability in notch estimation, no significant shortcoming of the continuous model is found relative to the measurements below 17 kHz. The study also shows the difficulty in obtaining accurate surface boundary impedance values for accurate validation studies.

Gas dynamics and vorticity generation in laser-induced breakdown of air.

Research has shown that the ignition characteristics of laser-induced plasmas in fuel-air mixtures are influenced by the gas dynamics effects induced during the gas breakdown stage. Here, we present the numerical modeling of the fluid mechanics induced by breakdown (plasma formation) from a nanosecond near-infrared (NIR) laser pulse in air. The simulations focus on the post-discharge kernel dynamics with the goal of developing a better understanding of how vorticity is generated during the kernel cooling phase. Initial conditions (ICs) of kernel shape, temperature, and pressure (corresponding to the end of the laser pulse) are found from experimental Rayleigh scattering data. It is shown that this method for determining ICs is preferred versus the use of the Taylor-Sedov blast wave theory as it provides a more accurate description of the starting field. Past experimental observations have revealed that the gas dynamics of nanosecond laser sparks typically lead to the formation of an asymmetric torus with a frontal lobe propagating towards the laser source. We show that the development of the asymmetric torus is governed by strong vorticity generated through baroclinic torque arising from the blast wave that forms at the kernel boundary. Initially, the blast takes the shape of the teardrop kernel but then evolves into a spherical front during the first ∼10 µs because the blast wave strength varies along its circumference. This spatial variation leads to a misalignment between the pressure and density gradients and generation of vorticity by baroclinic torque. Ultimately, the observed flow-field is dictated by how the energy was initially deposited around the beam waist during breakdown. As such, one can tailor the aerodynamics induced during the cooling and recombination phase by controlling the energy deposition profile.

Dynamics and its modulation of laser-induced plasma and shockwave in femtosecond double-pulse ablation of silicon

The dynamics of laser-induced plasma and shockwave was studied using time-resolved shadowgraphy in femtosecond double-pulse ablation of silicon. The morphology and expansion distances of laser-induced plasma and shockwave can be controlled by adjusting the pulse delay. The underlying mechanism was interpreted in terms of suppressed laser-induced air breakdown and enhanced laser energy deposition. The former is proposed to reduce the expansion distance in the longitudinal direction for small pulse delay, while the latter is the dominant mechanism for increasing the expansion distance in the longitudinal and radial directions for pulse delay exceeding a few picoseconds.

Random lasing in ZnO self-organized nanoparticles produced by laser induced breakdown

A simple laser-assisted method to transform metallic Zn to a ZnO-based random lasing medium is reported. The method uses Nd+3 laser-induced air breakdown to treat the surface of a Zn target in oxygen atmosphere, and thus produce self-organized nanoparticles (tubes, needles etc.) over nearby area. Room temperature photoluminescence spectra of the nanoparticles exhibit the emission band at 395 nm with semi-regular narrow spikes (<1 nm) with low threshold and non-linear growth with power greater 2 at photo excitation increase, which is typical for multi-mode lasing. Estimation of electron-hole pair concentration and temperature dependence of the average lasing energy prove that the lasing is originated from a scattering of free electrons on electron-hole pairs correlated. It was shown that self-organized nanoparticles exhibited better conditions for the lasing through stronger light scattering and guiding.

Plasma and flow induced by single- and dual-pulse laser-induced breakdown in stationary air

The plasma and flow induced by laser-induced breakdown (LIB) were experimentally investigated in a stationary atmospheric air using deposited energy measurement, unfiltered plasma imaging, and high-speed schlieren imaging. Single- and dual-pulse LIB were studied to explore the effects of a laser pulsing strategy on the induced plasma and flow and their potential practical applications. For dual-pulsed LIB, various time intervals (50 ns, 150 ns, 40 μs, and 100 μs) and energy combinations (10–10, 20–20, 30–30, 30–10, and 10–30 mJ) were used to change the initial conditions for the second breakdown. Laser energy deposition, plasma generation, and hot plume generation were compared between SPLIB and DPLIB. For SPLIB, the plasma and flow were self-similar while the size of the plasma and flow increased with increasing laser energy. For DPLIB, the characteristics of the plasma and the flow (hot plume) and the laser energy deposition were strongly dictated by the time interval and energy combination. For a time interval of 150 ns, strong pulse-to-pulse coupling induced a second toroidal flow structure that suppressed third lobe generation. Depending on the time interval, the interactions between the hot plumes and shockwaves generated much finer flow structures compared to SPLIB. The uneven energy combinations (30–10 and 10–30 mJ) created coarser and finer flow structures, respectively, for relatively long time intervals (40 and 100 μs) compared to the other energy combinations.

Spatiotemporal evolution of the plasma from dual-pulsed laser-induced breakdown in an atmospheric air

The spatiotemporal evolution of dual-pulsed laser-induced breakdown (DPLIB) was experimentally investigated in a quiescent atmospheric air. DPLIB with time intervals between pulses of 30–100 ns and pulse energies of 10–30 mJ were compared to a single laser-induced breakdown of the same total energy. An intensified CCD camera was used to capture direct images from the onset of the breakdown until 300 ns after the breakdown. The spatial and temporal temperature and electron number density were obtained from the N II spectrum at 500 nm. With increasing laser pulse energy, the breakdown perimeter, temperature and electron number density all increase. For DPLIB, the second breakdown occurs on the outside axial edges of the first breakdown increasing the breakdown perimeter. The uneven distribution of energy to the plasma creates an uneven temperature distribution. The electron number density is increased at the edges of the DPLIB for times less than 150 ns after the first breakdown. Depending on the electron number density of the first breakdown and the focal point location of the second breakdown, the second breakdown could occur on either the near side (closest to the focusing lens) or the far side (furthest from the focusing lens) of the focal point in the axial direction of the laser beam.

Experimental investigation of the shock loss and temporal evolution of hot plume resulting from dual-pulse laser-induced breakdown in quiescent air

To optimize a laser ignition scheme, absorption rate measurements and Schlieren visualizations are performed on dual-pulse laser-induced breakdowns (LIBs) at incident energies from 50 mJ to 200 mJ and pulse intervals that range from 20 ns to 250 μs in quiescent air at atmospheric pressure. For comparison, experiments on single-pulse LIBs are also conducted. The shock loss is determined using a semi-empirical model (Jones' model), and quantitative information on the spatial distribution of the hot plume is extracted from Schlieren images using in-house code. The results reveal that multi-location laser ignition can be achieved without reducing the energy absorption or strengthening the shock loss only when the energy of each laser pulse exceeds 200 mJ. This requirement is because the absorption rate of single-pulse LIB decreases significantly when the laser energy is lower than 200 mJ, and the shock loss of single-pulse LIB invariably accounts for approximately 80% of the absorbed laser energy at various incident energies. Compared with single-pulse LIB, dual-pulse LIB with a pulse interval of less than 200 ns is slightly inferior in terms of energy absorption and shock loss; however, the advantages of a larger initial plasma volume and lower energy dissipation can compensate for this deficiency. Therefore, dual-pulse laser ignition is a promising alternative to single-pulse laser ignition. Moreover, ignition reliability can be enhanced by initially releasing the laser pulse with higher energy when the energies of the successive pulses are not the same because of higher energy absorption and lower shock loss. In addition, the spatial distribution of the resulting hot plume is relatively centralized, which helps to reduce energy and radical dissipation. However, a pulse interval longer than 200 ns should be avoided for dual-pulse LIB because the laser energy cannot be utilized efficiently.

Enhancing the expansion of a plasma shockwave by crater-induced laser refocusing in femtosecond laser ablation of fused silica

The dynamics of plasma and shockwave expansion during two femtosecond laser pulse ablation of fused silica are studied using a time-resolved shadowgraph imaging technique. The experimental results reveal that during the second pulse irradiation on the crater induced by the first pulse, the expansion of the plasma and shockwave is enhanced in the longitudinal direction. The plasma model and Fresnel diffraction theory are combined to calculate the laser intensity distribution by considering the change in surface morphology and transient material properties. The theoretical results show that after the free electron density induced by the rising edge of the pulse reaches the critical density, the originally transparent surface is transformed into a transient high-reflectivity surface (metallic state). Thus, the crater with a concave-lens-like morphology can tremendously reflect and refocus the latter part of the laser pulse, leading to a strong laser field with an intensity even higher than the incident intensity. This strong refocused laser pulse results in a stronger laser-induced air breakdown and enhances the subsequent expansion of the plasma and shockwave. In addition, similar shadowgraphs are also recorded in the single-pulse ablation of a concave microlens, providing experimental evidence for the enhancement mechanism.

Effect of laser intensity on temporal and spectral features of laser generated acoustic shock waves: ns versus ps laser pulses.

Evolution of acoustic shock wave (ASW) properties generated during nanosecond (ns) and picosecond (ps) laser-induced breakdown (LIB) of atmospheric air at different input intensities is presented. The intensity is varied by changing the focal geometry of ns and ps pulses. The ASW pressures are observed to follow the dynamic interplay between the plasma density and recombination of plasma species. The conversion of laser energy to acoustic energy has increased from loose to tight focusing conditions. The central frequencies have moved toward the lower side with increasing laser intensities for both ns-LIB (76-48kHz) and ps-LIB (111.2-92.1kHz). The angular distribution of acoustic emissions was observed to follow the laser-induced plasma spark in both ns- and ps-LIB.

ZnO-based random lasing on nanoparticles realized by laser induced breakdown

A simple laser-assisted method to transform metallic Zn to a ZnO-based random lasing medium is reported. The method uses Nd+3 laser-induced air breakdown to treat the surface of a Zn target and thus produce ZnO nanoparticles randomly scattered over the broken area. Photoluminescence spectra of some places of the area at room temperature under resonant powerful photoexcitation exhibit the emission band at 395nm with regularly spacing narrow spikes (<1nm), which is typical of multi-mode lasing. The band is suggested to be originated from electron-hole plasma arising at high excitation in ZnO-based structures. The micro lasers revealed are possibly composed of ZnO nanoparticles scattered in chains inside self-made random resonators, with Zn flinders being mirrors.

Schlieren visualization of blast extinguishment with laser-induced breakdown

After occurrence of a very large scale earthquake, multiple simultaneous fires often break out. The author proposes to use blast extinguishment as an emergency firefighting technique and damage mitigation measure for post-disaster fires. However, the blowout mechanism of blast extinguishment has not been made clear yet. In the present study, in order to clarify the blowout mechanism of blast extinguishment, extinguishing experiments of a diffusion flame have been performed with a blast wave produced by laser-induced air breakdown, and the extinguishing processes have been visualized using the schlieren technique. From the experimental results, it is found that when the breakdown point is placed at the position where the hot gas bubble driving the blast wave interacts with the flame base, its extinguishing effectiveness becomes larger than when the blast wave only affects the flame.

Quantitative measurement of electron number in nanosecond and picosecond laser-induced air breakdown

Here we present quantitative measurements of total electron numbers in laser-induced air breakdown at pressures ranging from atmospheric to 40 barg by 10 ns and 100 ps laser pulses. A quantifiable definition for the laser-induced breakdown threshold is identified by a sharp increase in the measurable total electron numbers via dielectric-calibrated coherent microwave scattering. For the 10 ns laser pulse, the threshold of laser-induced breakdown in atmospheric air is defined as the total electron number of ∼106. This breakdown threshold decreases with an increase of pressure and laser photon energy (shorter wavelength), which is consistent with the theory of initial multiphoton ionization and subsequent avalanche processes. For the 100 ps laser pulse cases, a clear threshold is not present and only marginal pressure effects can be observed, which is due to the short pulse duration leading to stronger multiphoton ionization and minimal collisional avalanche ionization.

Effect of pulse duration on the acoustic frequency emissions during the laser-induced breakdown of atmospheric air.

Acoustic shock waves (ASWs) in the frequency range of 30-120kHz generated during laser-induced breakdown (LIB) of ambient air using 7ns and 30ps pulse durations are studied. The specific frequency range and peak amplitudes are observed to be different for nanosecond (ns) and picosecond (ps) LIB. The ASW frequencies for ps-LIB lie between 90 and 120kHz with one dominant peak, whereas for ns-LIB, two dominant peaks with frequencies in the 30-70kHz and 80-120kHz range are observed. These frequencies are observed to be laser pulse intensity dependent. With increasing energy of ns laser pulses, acoustic frequencies move toward the audible frequency range. The variation in the acoustic parameters, such as peak-to-peak pressures, signal energy, frequency and acoustic pulse widths as a function of laser energy, for two different pulse durations are presented in detail and compared. The acoustic emissions are observed to be higher for ns-LIB than ps-LIB, indicating higher conversion efficiency of optical energy into mechanical energy.