Laser-induced Air Plasma Research Articles

Terahertz-Radiation-Enhanced Emission of Fluorescence from Gas Plasma

We report the study of femtosecond laser-induced air plasma fluorescence under the illumination of terahertz (THz) pulses. Semiclassical modeling and experimental verification indicate that time-resolved THz radiation-enhanced emission of fluorescence is dominated by the electron kinetics and the electron-impact excitation of gas molecules or ions. We demonstrate that the temporal waveform of the THz field could be retrieved from the transient enhanced fluorescence, making omnidirectional, coherent detection available for THz time-domain spectroscopy.

A novel normal reflection terahertz spectrometer

A method for normal reflection terahertz (THz) spectrometer is proposed. This novel approach makes use of laser induced air plasmas as THz source and receives the normal reflective THz pulses without any THz transceiver and beam splitter. The signal to noise ratio (SNR) of the system is better than 500 and its effective frequency range is 0.2–2.0 THz. For checking the system's validity, we measure the refractive index of a high-resistivity silicon wafer which agrees with the result of the conventional transmission measurement well. The method is useful for the development of reflection THz spectroscopy and makes special advances for the practicability of THz techniques.

Experimental diagnosis of electron density of laser induced air plasma by interferometry

High quality interference pattern was obtained with a modified Mach-Zehnder interferomenter. An FFT analysis is applied to extracting the phase of the reconstructed interferograms, and then the inverse Abel transformation is used to calculate the three-dimensional electron density distribution. The electron density and the plasma expanding velocity at various delay times were obtained. The results show that the formation of the plasma channel in the early stage of the laser induces air plasma, and the rapid decrease of the plasma expanding velocity accelerates the collapse of the plasma channel. At about 48 nanosecond time, the eccentricity of the plasma shape reaches the maximum value and then evolves to a circular shape.

Terahertz generation in multiple laser-induced air plasmas

An investigation of the terahertz wave generation in multiple laser-induced air plasmas is presented. First, it is demonstrated that the intensity of the terahertz wave increases as the number of air plasmas increases. Second, the physical mechanism of this enhancement effect of the terahertz generation is studied by quantitatively measuring the intensity of the generated terahertz wave as a function of phase difference between adjacent air plasmas. It is found out that the superposition is the main mechanism to cause this enhancement. Thus, the results obtained in this paper not only provide a technique to generate stronger terahertz wave but also enable a better understanding of the mechanism of the terahertz generation in air plasma.

External electric field control of THz pulse generation in ambient air

A theoretical model has been proposed to describe the dependence of the THz wave generated in a laser-induced air plasma on the external electric field. Using this model we predict the following, (i) previously observed results show that the THz pulse enhances linearly with the increase of the external field; (ii) the THz pulse varies as a cosine function with the angle between the direction of the external electric field and the polarization of the incident exciting beam; (iii) and the amplitude is proportional to the square of the intensity of the incident pulse in a low energy region. These predictions are validated by our experiment.

High-intensity nanosecond-pulsed laser-induced plasma in air, water, and vacuum: A comparative study of the early-stage evolution using a physics-based predictive model

A comparative study has been performed for properties (temperature, density, and electron Coulomb coupling constant) of plasma induced by high-intensity (∼GW∕cm2) nanosecond laser-metal interactions in air, water, and vacuum. The study is for early-stage (t≲30ns) plasma evolution, where the above plasma properties are very difficult to measure experimentally and hence a comparative property study has been rarely reported in literature. In this paper a physics-based predictive model is used as the investigation tool. The model was verified based on experimental measurements for the early-stage plasma pressure and front propagation and the late-stage (t≳30ns) plasma temperature and electron number density, which are relatively easy to measure. Therefore, the experimentally verified model can provide reasonably accurate information on the difficult-to-measure plasma temperature and density in the early-stage at least in the semiquantitative sense, and the information will be very useful for the fundamental laser plasma study and relevant laser applications. It has been found that plasma with very different temperatures and densities can be created in different media.

Light focusing from large refractive indices in ionized air

The sheath that surrounds a laser-induced plasma in air was investigated with a light probe. The sheath is a 3-mm-thick halo of ionized gas created by ultraviolet light emitted from the plasma core. A beam of laser light tracked with a streak camera was employed to probe it. It was found that in the first hundreds of a nanosecond after plasma inception, the beam is deflected towards the plasma center. This result points to a high refractive index inside the sheath. The index values obtained (up to 2.0) are due to an increased susceptibility caused by a bulk electric polarization. Using previous results on the sheath’s electric polarizability values, the refractive index was calculated and was found to agree with the observed indices. The application of the electric polarization model to the guiding of laser beams and to plasma lenses is also discussed.

Spatial distribution of hydrogen and other emitters in aluminum laser-induced plasma in air and consequences on spatially integrated Laser-Induced Breakdown Spectroscopy measurements

The Stark broadening of hydrogen Balmer α line is often measured in Laser-Induced Breakdown Spectroscopy (LIBS) to evaluate the electron number density of the plasma. When measurements are carried out in air, hydrogen atoms are supposedly supplied by the atmosphere. This hypothesis casts some doubts on the validity of hydrogen as an indicator of the conditions in the inner part of the plasma. In this experiment, we acquired spectrally resolved images of a Laser-Induced Breakdown Spectroscopy plasma generated in air on an aluminum target by irradiation with the Nd:YAG fundamental emission. The evolution of the plasma was studied by acquiring images at several delay times. In particular, we imaged the spatial distribution of plasma emitters along the direction orthogonal to the target surface. The spatial distribution of hydrogen emission was compared to the one of the species ablated from the sample. Moreover, electron density and temperature were evaluated in the time- and space-resolved spectra extracted from the images, obtaining thus a picture of the propagation and internal structure of the plasma. The fluid dynamics implications of the plasma expansion were discussed. The z-resolved characterization was compared with the picture of the plasma obtained by processing space-integrated spectra.

Effect of Beam Waist on Shock Properties of Laser-Induced Plasmas in Air by the Photoacoustic Probe Beam Deflection Method

Effect of Beam Waist on Shock Properties of Laser-Induced Plasmas in Air by the Photoacoustic Probe Beam Deflection Method

Spatially, temporally, and spectrally resolved measurement of laser-induced plasma in air

Images and emission spectra of sparks produced by laser-induced breakdown in air were investigated with a high degree of spatial and temporal resolution. The laser-induced breakdown was generated by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser. The data were collected using a framing intensified charged coupled device (CCD) camera and a multi-fiber Cassegrain optics system coupled to an intensified CCD spectrometer. The results provided information about the different stages of laser-induced breakdown. The plasma shape and emission spectrum were very reproducible. Different ionization levels in the plasma kernels, which were observed using the high spatial resolution of the multi-fiber Cassegrain optics system, occurred during the plasma formation and cooling and at different locations within the plasma. This was due mainly to the thickness of the plasma relative to the laser wavelength, which created different ionization levels and energy absorption rates throughout. These observations were correlated with the plasma visualizations obtained with the framing ICCD camera. The plasma emission analysis permitted us to study the temperature evolution along the plasma during the laser-induced breakdown process. The analysis demonstrated the validity of a laser-supported wave model during the first stages of laser-induced breakdown and illustrated the weak dependence of the plasma temperature on the input energy.

Radial Variation of Refractive Index, Plasma Frequency and Phase Velocity in Laser Induced Air Plasma

Laser-induced breakdown spectroscopy (LIBS) is a nonintrusive technique that needs no sample preparation and even recently, quantitative measurements were done without the need for calibration standards. Much research has been done on the laser induced air plasma to study the spatial variation of plasma parameters in the axial direction of the laser beam. In this paper, we report investigation on the radial variation of the refractive index, plasma frequency, and phase velocity of a plasma during laser induced breakdown of air. The results show that the radial variations of the plasma frequency and refractive index are caused by the radial electron density gradient of the plasma. The radial variation of the electron density shows a noticeable depletion at the center of the focus which results in plasma phase velocities greater than the speed of light. Further from the plasma core the electron density diminishes to ambient density at the edges of the plasma. The mean full-width at half maximum (FWHM) for these plasma profiles was found to be about 1.2 mm. The results further reveal that the plasma has a concave parabollic electron density and a convex parabollic refractive index profile near the center of the laser axis, i.e., around a diameter of 0.5 mm

Detection of Broadband Terahertz Waves with a Laser-Induced Plasma in Gases

We report the experimental results and theoretical analysis of broadband detection of terahertz (THz) waves via electric-field-induced second-harmonic generation in laser-induced air plasma with ultrashort laser pulses. By introducing the second-harmonic component of the white light in the laser-induced plasma as a local oscillator, coherent detection of broadband THz waves with ambient air is demonstrated for the first time. Our results show that, depending on the probe intensity, detection of THz waves in air can be categorized as incoherent, hybrid, and coherent detection. Coherent detection is achieved only when the tunnel ionization process dominates in gases.

The effect of ambient pressure on laser-induced plasmas in air

A detailed investigation of the effect of ambient pressure in the range of 0.1 to 1.0 atm on the size, temperature, electron number density, and fraction of laser energy absorbed in a laser-induced plasma in air has been conducted. As pressure is reduced the size of the plasma, its electron number density, its peak emission intensity, and the fraction of incident laser energy that is absorbed are all found to decrease significantly. The temporal temperature profile in the plasma and the fraction of the absorbed laser energy that is converted to thermal energy in the plasma remain constant, at least down to 0.2 atm.

Terahertz emission profile from laser-induced air plasma

We report the characterization of the emission profile of terahertz waves generated by four-wave mixing in the presence of laser-induced plasma. Highly directional terahertz waves with a divergence angle smaller than 10° are measured in the presence of long plasmas (>10mm). Frequency-dependent interference structures in the angular distribution of the radiation are observed under tighter focusing conditions and are explained by intense self-phase modulation of the optical pulse in the plasma. This study reveals that terahertz generation from four-wave mixing in air plasmas is a promising source for spectroscopy and imaging in the terahertz range.

Coherent Control of THz Wave Generation in Ambient Air

Our study of THz wave generation in the pulsed laser induced air plasma with individually controlled phase, polarization, and amplitude of the optical fundamental wave (omega) and its second harmonic (2omega) indicates that the third-order nonlinear optical process mixing the omega and 2omega beams in the ionized plasma is the main mechanism of the efficient THz wave generation. The polarity and the strength of the emitted THz field are completely controlled by the relative phase between the omega and 2omega waves. The measured THz field amplitude is proportional to the pulse energy of the fundamental beam and to the square root of the pulse energy of the second-harmonic beam once the total optical pulse energy exceeds the plasma formation threshold. The optimal THz field is achieved when all waves (omega, 2omega, and THz waves) are at the same polarization in the four-wave-mixing process.

Spectroscopic study on the air plasma induced by delayed dual laser pulses

By using a two laser beam method, the spectra of laser-induced air plasmas (LIAP) were studied experimentally. Two laser beams of a laser were used in the experiment. One of the beams was used to create the plasmas, which is called the ionizing-laser-beam (ILB), and the other beam was used to give influence to the plasmas, called action-laser-beam (ALB). This paper reports the experimental results and give a preliminary analysis of the modifications to the spectral characteristics due to the influence of the ALB. The experimental results indicated that the total spectral intensity of laser-induced air plasmas increased obviously under the action of the ALB. The increment of continuous spectra was greater than that of the line spectra; the increment of the short wavelength band was greater than that of the long wavelength band. The longer the delay-time of ALB behind the ILB, the larger the increase of spectra intensities. There is no threshold for the ALB to increase the laser-induced air plasmas spectral intensity. The results also showed that besides increasing the intensity, the decay-time for most of lines would increase under the influence of ALB. The preliminary analyses indicated that the results may be caused by electrons in the plasma which transfer their energy absorbed from the ALB via different routs with different efficiencies. These results provides some new sight into the microscopic mechanisms of the decay of laser-induced air plasmas, and also provides some experimental evidences for prolonging the decay-time of laser-induced air plasmas for some technical applications.

Measurement of the electrical size of a laser-induced plasma in a uniform field

Measurements of the electric field produced by laser-induced plasma in ambient air under an external (bias) are presented. The transient field induced by the plasma was monitored using a D-dot type probe. The plasma field was found to have a dipolar structure with its moment aligned with the bias field. The measurement of the induced dipolar moment gives the electric size of the plasma. These results were compared with fast photography and shadowgraphy which gave sizes of about 2–3 mm in the time interval from 50 to 100 ns. The electric size in the same interval was approximately three times larger than that obtained by the optical techniques. The difference is attributed to an ionized layer 2.6 mm thick which reflects the spatial extent of the air ionized by UV radiation around the plasma.

Characterization of the expansion of an aluminum laser-induced plasma in ambient air by fast photography

Laser-induced aluminum plasma in ambient air at a fluence of 10 J/cm/sup 2/ is observed by means of fast photography. Plasma expansion is found to be nearly hemispherical. The existence of an air plasma at the edge of the Al plasma is suggested by comparing spectrally resolved and unresolved pictures.

Investigation of the State of Local Thermodynamic Equilibrium of a Laser-Produced Aluminum Plasma

In this work, the assumption of local thermodynamic equilibrium (LTE) for a laser-induced plasma in ambient air is examined experimentally using two different laser systems, namely an infrared short-pulse Ti : Sapphire laser and an ultraviolet long-pulse XeCl excimer laser. The LTE assumption is investigated by examining the plasma produced at a laser fluence of 10 J/cm(2) from aluminum targets containing iron and magnesium impurities. The excitation temperature is deduced from Boltzmann diagrams built from a large number of spatially integrated neutral iron lines distributed from 3.21 to 6.56 eV. It is shown that at any time after the end of the laser pulse, the neutral excited states are in excellent Boltzmann equilibrium. Detailed investigation of Boltzmann equilibrium further validates previous temperature measurements using less accurate diagrams. However, observations of ion lines provide some evidence that the ionized species do not obey Saha equilibrium, thereby indicating departure from LTE. This could be explained by the fact that the plasma cannot be considered as stationary for these species.

Space- and time-resolved dynamics of plasmas generated by laser double pulses interacting with metallic samples

Significant improvements to the analytical performance of laser-induced breakdown spectrometry (LIBS) were achieved by the use of laser double pulses to ablate and excite the sample material to be analysed. To clarify the underlying physical phenomena the dynamics of the laser-induced plasma in air is studied using a high-speed electro-optic camera to observe the spatial and temporal development of the plasma geometry. A Mach–Zehnder interferometer was set-up to detect the spatio-temporal changes of the refractive index of the plasma. The velocities of the luminous plasma front and shock waves after the impact of the first laser pulse are consistent with the prediction of Sedov's model for spherical expansion. The dynamics caused by the second laser pulse of the double pulse differ significantly, indicating a modified state of the local atmosphere in the interaction region. Electron densities determined interferometrically amount to about 3 × 1018 cm−3 at the center of the hemispheric plasma geometry at a delay time of 2 µs. Whereas the electron density for single or double pulses is approximately the same, the volume of the plasma is greater by more than a factor of 3 after the interaction with the double pulse of the same total energy.