Laser-induced Air Plasma Research Articles

Ultra-broadband detection of coherent infrared pulses by sum-frequency generation spectroscopy in reflection geometry.

We have newly developed, to the best of our knowledge, a detection method for broadband infrared pulses based on sum-frequency generation spectroscopy in reflection geometry, which can avoid a restriction of the detection bandwidth originating from the phase mismatch that is inevitable for the upconversion in transmission geometry. Using a GaAs crystal, we successfully demonstrated the ultra-broadband detection of the infrared pulses generated from a two-color laser-induced air plasma filament in a region from 300 to 3300 cm-1. With the advantage of ultra-short infrared pulses, the present detection method holds promise for application to time-resolved, ultra-broadband vibrational spectroscopy.

Two-dimensional temperature measurements in laser-induced air plasma

Two-dimensional temperature measurements in laser-induced air plasma

Optical vortex coronagraphy for high-contrast imaging of femtosecond laser filaments

A high-contrast imaging technique for phase objects based on the optical vortex coronagraph (OVC) is proposed. This method offers the unique advantage of background-free imaging due to the introduction of azimuthal phase in the Fourier plane. We employed the OVC method to detect femtosecond laser-induced air plasma and compared it with the classic diffractometry and fluorescent imaging methods. We achieved a phase sensitivity of ∼0.035 waves that surpassed the capabilities of the other two methods. The combination of this highly sensitive imaging technique with the pump–probe method holds promise for applications in ultrafast imaging of laser–material interactions.

Terahertz unipolar polarimetry by second-harmonic generation in air

Femtosecond laser-based terahertz (THz) sources have gained attention for their potential in various applications. As for any electromagnetic radiation, the wave polarization is a critical parameter, which needs to be under control in a wide class of those applications. However, characterizing the polarization of broadband THz pulses remains challenging due to their limited efficient optics. THz air-biased coherent detection has emerged as a promising candidate. The technique employed is heterodyne detection, utilizing second-harmonic generation induced by THz radiation in laser-induced air plasmas. This approach provides exceptional spectral bandwidth and an unbounded power detection limit, rendering it highly suitable for ultra-broadband and high-power THz sources. It enables a very efficient polarization measurement too. However, recent findings have revealed that the laser-induced air plasma generated in this technique can exhibit birefringence, which in turn adds systematic errors to the polarization-state determination. In this Letter, we propose a simplified approach that utilizes a weak probe beam and avoids high-voltage DC bias-fields. Unlike the terahertz (THz) air-biased coherent detection scheme, our approach yields a unipolar, intensity-proportional signal for second-harmonic generation. The experimental results reported in this Letter show the absence of induced birefringence in air and, hence, demonstrate accurate measurements of the polarization state of ultra-broadband THz pulses. Therefore, our technique may provide valuable results in applications where the polarization state, and not the full electric field waveform, is required for analysis or characterization. Finally, we discuss a possible application of our method to the emergent field of THz singular optics.

Enhanced second harmonic generation in laser-induced air plasma.

We report a systematic investigation into the processes behind a near hundred-fold enhanced second harmonic wave generated from a laser-induced air plasma, by examining the temporal dynamics of the frequency conversion processes, and the polarization of the emitted second harmonic beam. Contrary to typical nonlinear optical processes, the enhanced second harmonic generation efficiency is only observed within a sub-picosecond time window and found to be nearly constant across fundamental pulse durations spanning from 0.1 ps to over 2 ps. We further demonstrate that with the adopted orthogonal pump-probe configuration, the polarization of the second harmonic field exhibits a complex dependence on the polarization of both input fundamental beams, contrasting with most of the previous experiments with a single-beam geometry.

Coherent generation and control of tunable narrowband THz radiation from a laser-induced air-plasma filament.

We report on the proof-of-principle experiment of generating carrier-envelope phase (CEP)-controllable and frequency-tunable narrowband terahertz (THz) radiation from an air-plasma filament prescribed by the beat of a temporally stretched two-color laser pulse sequence. The pulse sequence was prepared by propagating the fundamental ultrafast laser pulse through a grating stretcher and Michelson interferometer with variable inter-arm delay. By partially frequency-doubling and focusing the pulse sequence, an air-plasma filament riding a beat note was created to radiate a THz wave with primary pulse characteristics (center frequency and CEP) under coherent control. To reproduce experimental results and elucidate complex nonlinear light-matter interaction, numerical simulation has been performed. This work demonstrates the feasibility of generating coherently controlled narrowband THz wave with high tunability in laser-induced air plasma.

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.

Single-Shot Multi-Frame Imaging of Femtosecond Laser-Induced Plasma Propagation.

Single-shot ultrafast multi-frame imaging technology plays a crucial role in the observation of laser-induced plasma. However, there are many challenges in the application of laser processing, such as technology fusion and imaging stability. To provide a stable and reliable observation method, we propose an ultrafast single-shot multi-frame imaging technology based on wavelength polarization multiplexing. Through the frequency doubling and birefringence effects of the BBO and the quartz crystal, the 800 nm femtosecond laser pulse was frequency doubled to 400 nm, and a sequence of probe sub-pulses with dual-wavelength and different polarization was generated. The coaxial propagation and framing imaging of multi-frequency pulses provided stable imaging quality and clarity, as well as high temporal/spatial resolution (200 fs and 228 lp/mm). In the experiments involving femtosecond laser-induced plasma propagation, the probe sub-pulses measured their time intervals by capturing the same results. Specifically, the measured time intervals were 200 fs between the same color pulses and 1 ps between the adjacent different. Finally, based on the obtained system time resolution, we observed and revealed the evolution mechanism of femtosecond laser-induced air plasma filaments, the multifilament propagation of femtosecond laser in fused silica, and the influence mechanism of air ionization on laser-induced shock waves.

Transmission effects of high energy nanosecond lasers in laser-induced air plasma under different pressures

When a high energy nanosecond (ns) laser induces breakdown in the air, the plasma density generated in the rarefied atmosphere is much smaller than that at normal pressure. It is associated with a relatively lower absorption coefficient and reduces energy loss of the laser beam at low pressure. In this paper, the general transmission characterizations of a Joule level 10 ns 1064 nm focused laser beam are investigated both theoretically and experimentally under different pressures. The evolution of the electron density (n e), the changes in electron temperature (T e) and the variation of laser intensity (I) are employed for numerical analyses in the simulation model. For experiments, four optical image transfer systems with focal length (f) of 200 mm are placed in a chamber and employed to focus the laser beam and produce plasmas at the focus. The results suggest that the transmittance increases obviously with the decreasing pressure and the plasma channels on the transmission path can be observed by the self-illumination. The simulation results agree well with the experimental data. The numerical model presents that the maximum n e at the focus can reach 1019 cm−3, which is far below the critical density (n c). As a result, the laser beam is not completely shielded by the plasmas.

Analysis of the characteristics of microwave-enhanced laser-induced atmospheric air plasma and ablation plasma for Al target

The investigation and comparison of air plasma generated directly in the absence as well as in the presence of solid target along with microwave enhancement were reported. The plasma characteristics including emissions, size, duration, and temperature were measured using high-speed camera, optical emission spectroscopy, and temperature approximation through spectral fitting. The laser energy required for ablation is much lower than the air breakdown energy but, the minimum energy density for air and alumina breakdown is inversely proportional to the focal lengths. The size and volume of the air plasma generated by the microwave and ablation plasma interactions were less than those of the microwave-enhanced laser-induced air plasma but was similar with standard laser-induced air plasma. When comparing the plasma lifetimes, both air plasmas (with and without a solid target) exceeded the duration of the microwave pulse input (approximately 1 ms). As the plasma lifetime is a function of electron density, shorter lifespan with the target ∼1500 ± 150 µs compared with the lifetime of the plasma without the target at ∼1800 ± 150 µs were observed. Similar results were observed for the time-series measurements of the OH and N2 second positive (N2PS) molecular emissions. OH and N2PS were not observed within the first 300 µs in the presence of the solid target; these air plasma components were only generated as a “by-product” of the microwave and ablation plasma. The rotational and vibrational temperatures were approximated by spectral fitting of simulated OH and N2PS spectra and the experimentally acquired spectra. The microwave increased the vibrational temperature in a constant rotational temperature during the expansion of the air plasma. The same trend is observed with the presence of the alumina target.

Simple system for realizing single-shot ultrafast sequential imaging based on spatial multiplexing in-line holography.

We present a simple system for realizing single-shot ultrafast sequential imaging based on spatial multiplexing in-line holography. In this system, we propose to combine a specially designed mini-reflector delay-line array with digital in-line holography. The former including a group of adjustable mini-reflectors can easily generate an array of probe sub-pulses that can be controlled independently in the propagation direction and time delays. The object beams formed by the different sub-pulses will propagate and fall on different recording regions of the image sensor to generate a single-shot spatial-multiplexing in-line hologram. The geometry of the digital in-line holography can simplify the complexity of the system and enable complex amplitude imaging. In addition, the time resolution of this system is limited only by the pulse duration, which allows this system to study the dynamic processes with the femtosecond order. In an experiment about the laser-induced air plasma, our proposed system achieves nine frames sequential holographic images with the frame rate of 7.5 trillion frames per second (Tfps).

Influence of plasma density on the cross sections of radiative recombination to configuration-averaged excited nitrogen and oxygen atoms and ions

Radiative recombination cross sections of all the charge states of nitrogen and oxygen ions are calculated in the central field and Hartree–Fock–Slater approximations. The recombining ions are considered on their ground states, to form recombined ions on different configuration-averaged excited states. The ion potential energies are calculated assuming electro-neutrality in a Wigner–Seitz cell containing bound and free electrons, and the effect of plasma density on the cross sections is investigated by varying the cell radius. When the plasma density increases up to 1020 cm−3, the bound and free wave functions are distorted which significantly impact the cross sections. These deviations from the free atom case are all the more significant as the ion charge state of the recombining ion is low and as the excitation energy of the recombined ion is high. Also, calculations of the radiative recombination rates allow for quantifying the impact of plasma density at different temperatures. It is shown, for temperatures greater than 1 (Ry), that the rates at low and high plasma densities are closed. Nevertheless, for temperatures lighter than 1 (Ry) the influence of plasma density on the rates is significant. In addition, transition probabilities between the bound levels of all the charge states of N and O are calculated, and the influence of plasma density on these probabilities is analyzed. These cross sections and rates may be used as entrance parameters in collisional-radiative models for fully ionized plasma simulations in the framework of studies concerning for example, switches in Marx generator and laser-induced plasmas in air.

Microwave-enhanced laser-induced air plasma at atmospheric pressure.

This paper investigated how microwaves affect the temperature of laser-generated air plasma. The air breakdown threshold was experimentally characterized by focusing the 1064 nm YAG laser on varied condensing lens focal lengths. Increase in focal lengths increases the focused spot diameter of the laser and decreases the laser fluence. Large spot diameter required large amount of laser fluence for breakdown. However, the plasma generated with small spot sizes found to absorb higher laser energy in compared to the plasma generated with large spot size condition. In terms of energy density, the experimental threshold breakdown was generated between 2.6∼4.9 × 1011 W/cm2. The plasma formation was then observed under a high-speed camera. The area of intensity distribution increased with the input of microwaves owing to re-excitation and microwave absorption. This led to emission intensity measurements of the elusive stable electronically excited molecular nitrogen (N2 2nd positive system) and hydroxyl radical (OH). Without the input of microwave, these molecular and radical emissions were not observed. The OH and N2 2nd positive system emission intensities were then used to measure the rovibrational temperature using the synthetic spectrum method by SPECAIR. The rotational and vibrational temperatures were not found to be equal indicating non-equilibrium plasma. The nonequilibrium and nonthermal plasma was observed from after the initial laser air breakdown using the 2.6 × 1011 W/cm2, 1.0 kW microwave power, and 1.0 ms microwave pulse width. The microwaves were not found to affect the temporal changes in the rotational temperatures, demonstrating that the intensity enhancements and plasma sustainment were caused by re-excitation and not by microwave absorption.

Enhancing Gas Solubility in Water via Femtosecond Laser-Induced Plasma.

The generation of laser-induced plasma at the gas–liquid interface provides many fundamental and interesting scientific phenomena such as ionization, sharp explosion, shock wave radiation, bubble creation, and water splitting. However, despite the extensive research in this area, there is no reference on the effect of the surrounding environment on the chemical processes that occur during the laser-induced plasma–water interaction. In this work, we investigate the effect of the surrounding gas environment on femtosecond laser-induced plasma when generated at the pure water–gas interface. Ultrashort laser pulses were applied to water in the presence of air and N2 and Ar gas environments. Formation of a significant number of nitrate-based species in water was observed after exposure to femtosecond laser-induced plasma in air and N2 environments. The detected NO3 ions formed in the laser-treated water led to the appearance of an absorption peak in the UV range, a significant decrease in the water pH value, and a significant increase in water’s electrical conductivity. All induced properties of water were stable for 3 months of monitoring after laser treatment. Our work shows that the generation of laser-induced plasma in water propagating into a gaseous medium facilitates the interaction between the two media, as a result of which the compositions of substances present in the gaseous medium can be dissolved in water without increasing the gas pressure. The presented approach may find applications in areas such as water purification, material synthesis, and environmental stewardship.

Pressure Effects on Simultaneous Optical and Acoustics Data from Laser-Induced Plasmas in Air: Implications to the Differentiation of Geological Materials.

The shockwave generated alongside the plasma is an intimately linked, yet often neglected additional input for the characterization of solid samples by laser-induced breakdown spectroscopy (LIBS). The present work introduces a dual LIBS-acoustics sensor that takes advantage of the analysis of the acoustic spectrum yielded by shockwaves produced on different geological samples to enhance the discrimination power of LIBS in materials featuring similar optical emission spectra. Six iron-based minerals were tested at a distance of 2 m using 1064 nm laser light and under pressure values ranging from 7 to 1015 mbar. These experimental parameters were selected to assess the effects of pressure, one of the main factors conditioning the propagation of sound as well as a commonly investigated influence in LIBS experiments. Moreover, precise values for carrying out the analyses were set based on one of the most exciting scenarios in which LIBS data may be enhanced by laser-induced acoustics: space exploration. This is exemplified by the tasks performed by the Mars 2020 SuperCam instrument located onboard the Perseverance rover. Authors evaluated the use of acoustic signals both in the time-domain and frequency-domain in sensitive cases for the distinguishing of minerals exhibiting LIBS spectra featuring almost the same emission lines using PCA schemes for each pressure setting. Thus, we report herein the impact of the surrounding pressure level upon this diagnostic tool. Overall, this paper seeks to show how the analytical potential of simultaneous phenomena taking place during a laser-produced plasma event is subjected to the defined operational conditions.

A spectral comparison of lightning discharge plasma and laser-induced air plasma

The high time-resolved spectrum of natural lightning is obtained by a slitless spectrograph. The spectrum of natural lightning is compared to the spectrum of laser-induced air plasma. The results show that the spectra of lightning and the spectra of laser-induced air plasma are similar, but they also have some differences. In the superimposed spectrum of lightning, the intensities of atomic lines are far stronger than those of ionic lines, and the continuous spectrum in the whole wavelength region is very strong. However, in the spectrum of laser-induced air plasma, the intensities of atomic lines are similar to those of ionic lines, and there is almost no continuous spectrum in the near-infrared region. In addition, the Hα line is strong in the lightning spectrum but very weak in the spectrum of laser-induced air plasma. The temperature, electron density, and conductivity are calculated, and their evolution with time is also given. The differences in the three physical parameters between lightning and laser-induced air plasma are analyzed. In addition, the reasons of spectral differences have been explained. These results will provide an important reference for the simulation of the lightning spectrum.

Frequency tuning for broadband terahertz emission from two-color laser-induced air plasma

Effective manipulation of broadband terahertz emission, especially on spectrum tuning, is of great importance for many applications. We demonstrate a method to realize frequency tuning of terahertz emissions from two-color laser-induced air plasmas. The terahertz central frequency is switched from 0.56 to 0.82 THz by changing the polarization state of the fundamental wave with a quarter-wave plate. Based on numerical simulation, it is found that this frequency tuning is due to the birefringence effect induced by the fundamental wave on the second harmonic inside the filament, which leads to a discrepancy on the polarization chirality of the two-color laser components. Two-color lasers with opposite chirality will emit terahertz radiation with higher central frequency compared to two-color lasers with the same chirality at moderate laser intensity.

Influence of plasma density on radiative recombination cross sections of oxygen ions

Radiative recombination cross sections of oxygen ions from O+ to O6+ are calculated by an atomic model assuming central field and Hartree–Fock–Slater approximation for exchange potential. The recombining ions are considered on their ground states, to form recombined ions on their ground states as well as on several subshells up to 4d. The effect of plasma density on the cross sections is taken into account by varying the Wigner–Seitz cell representing the atomic volume containing bound and free electrons. It is shown that the increase of plasma density significantly enhances the cross sections involving excited states of the recombined ions at atmospheric pressures and beyond. These cross sections may be used in collisional-radiative models for fully ionized plasma simulations in the frame work of, for example, switches in Marx generator and laser-induced plasmas in air.

Enhancing terahertz radiation from femtosecond laser filaments using local gas density modulation

We present a method to enhance the terahertz (THz) wave radiation from a femtosecond laser-induced plasma filament by controlling the local gas density within the filament. We develop a theoretical model for THz generation from a laser-induced air plasma filament and the subsequent propagation process, to account for a varying local gas density. By adjusting the local gas density along the filament, the transient current distribution along the filament and the resulting coherent superposition of terahertz waves can be controlled. The location of the gas jet nozzle and the relative phase between multicolor light fields both affect the transient current distribution and thus the strength of the generated THz field. Compared with the conventional terahertz generation by a two-color filament in a homogeneous gas, a three-color filament can realize an increase by 6.12 times in the generated THz pulse energy, with optimized local gas density modulation. Our results suggest that the THz amplification via local gas density modulation can be further improved with well-designed multicolor pulses.

Investigation of stagnation layer dynamics of counterpropagating laser induced air plasmas: Numerical simulations vis-à-vis experimental observations

The interaction and evolution dynamics of two counterpropagating shockwaves generated by two collinear laser induced air plasmas of equal and unequal energies are presented. The formation of a stagnation layer during the interaction of the shockwaves from the two plasma sources, namely, S1 and S2 separated by a distance of 4 mm, were studied over a duration of 30 μs, using two-dimensional FLASH radiation hydrodynamic code. The stagnation layer formed is observed to be “hard” in the case of interaction of equal energy sources, S1 = S2. While in the case of unequal energy sources with S2 > S1, the stagnation layer is observed to allow the propagation of energy. For both the cases of equal and unequal energy sources, a maximum increase in temperature at the stagnation layer by ∼1.5 and 1.9 times, respectively, is observed due to the interaction of the expanding plasmas around the stagnation layer. The density and pressure in the unequal interaction case at the stagnation layer is observed to be more than that of the ambient conditions by 4.9 and 19 times, respectively. A clear visualization of shock front of high energy plasma source traversing through that of a lower energy source leads to a channel formation, followed by a jet-let along the laser axis due to density drag. The evolution of interacting plasma is compared with that of the evolution of individual sources. The simulations were validated by the experimental observations giving a one-on-one mapping of the spatiotemporal evolution of plasma and shock front.