Laser-supported Absorption Wave Research Articles

Dynamics of laser-induced phase transformation in bulk of monocrystalline sapphire

Transient effects during and after the optical breakdown initiated by an intense sub-nanosecond laser pulse in bulk of monocrystalline sapphire were investigated using ultrafast time-resolved microscopy. The method provided direct quantitative information about temporal evolution of the laser-induced plasma channel as well as spatio-temporal characterization of the shock-waves. It was found that plasma channel evolved according to the classical model of moving breakdown, but in the initial phase there were signs of the laser-supported absorption wave (LSAW). The estimated maximum of the of absorption wave velocity in sapphire reached 340 km/s to decrease quickly to 53 km/s in the moment when the shock wave was emerged for the first time. At the final state of propagation, the front of the shock wave has converted to a spherical shape moving with a speed very close to the speed of acoustic wave in sapphire. The evolution of radii of the plasma channel and shockwave front were reasonably well fitted with the Sedov’s scaling rule. The structural analysis of the post-irradiated site showed phase-transitions from single crystal to a polycrystalline or amorphous phase but only very close to the void walls.

The acceleration mechanism of shock wave induced by millisecond-nanosecond combined-pulse laser on silicon

The velocity variation law of shock wave induced by millisecond-nanosecond combined-pulse laser has been investigated experimentally. The pulse delay and laser energy are important experimental variables. The method of laser shadowgraphy is used in the experiment. Experimental results show that when the pulse delay is 2.4 ms, the ms and ns laser energy density is 301 J cm−2 and 12 J cm−2, respectively, the velocity of shock wave is 1.09 times faster than that induced by single ns pulse laser. It is inferred that the shock wave propagates in the plasma is faster than that in air. When the ms and ns laser energy density is 414.58 and 24 J cm−2, the velocity of shock wave shows rising trend with pulse delay in a range of 1.4 ms > Δt > 0.8 ms. It is indicated that with the increase of ns laser energy, the laser energy absorbed by laser-supported absorption wave increases. The mechanism of inverse bremsstrahlung absorption acts with target surface absorption simultaneously during the ns laser irradiation. Thus, the phenomenon of the double shock wave is induced. The numerical results of the phenomenon were accordance with experiment. The results of this research can provide a reference for the field of laser propulsion.

Experimental study of different pulse delays on the phenomenon of double shock waves induced by a millisecond-nanosecond combined-pulse laser.

We study the motion morphology, distance, and velocity of plasma and laser-induced shock waves induced by a millisecond-nanosecond (ms-ns) combined-pulse laser with different pulse delays on silicon. The laser shadowgraph method is used, and the phenomenon of double laser-induced shock waves has been found while the pulse delay is 1.2-1.8 ms. The controlling variable method is used to study this phenomenon, and it is found that it is mainly related to the ignition of the laser-supported absorption wave induced by the ms laser. Moreover, the plasma expansion velocity increases with the increase of pulse delay, the axial propagation distance of laser-induced shock waves increases monotonically with pulse delay, and the velocity of laser-induced shock waves decreases with the increase of pulse delay.

Species distribution in laser-induced plasma on the surface of binary immiscible alloy

Investigation on species distribution of vapor plasma is of great significance for understanding the laser-target interaction, the plasma dynamics and verifying theoretical models. It is also helpful for optimizing the spectral acquisition to improve the quantitative performance of spatial-resolved laser-induced breakdown spectroscopy (LIBS). In this work, the laser-induced plasma on the surface of binary alloy is analyzed by using spectrally, spatially and temporally resolved dual-wavelength differential imaging to explore the dependence of species distribution in plasma on laser-supported absorption (LSA) wave regime and sample composition. The laser-supported combustion (LSC) and laser-supported detonation (LSD) wave dominated plasmas with different elemental mass ratios are induced from a series of binary immiscible alloys by using low and high irradiance laser pulses, respectively. By comparing the spatial-temporal emissivity images of species, the following conclusions are drawn: 1) laser irradiance can change the species distribution of plasma, while the sample composition cannot; 2) for LSC-wave dominated plasma, the species distribution mainly depends on the melting points of constituted elements in the sample, while for LSD-wave dominated plasma, it mainly depends on the atomic masses of elements. The above behaviours on species distribution in plasma produced from binary immiscible alloys are expected to be suitable for other elements or even multi-element plasmas.

Shielding and diagnostics of laser-induced air plasmas generated in collinear double pulse configuration

Plasma shielding in collinear double-pulse laser-induced breakdown spectroscopy was characterized in terms of its time-resolved absorptivity, photographic images, and emission spectra. In experiments performed with the double pulse beam configuration, one to generate plasma and another a probe beam, the transmission of the probe beam was measured at various inter-pulse delay times up to 1500 ns as the plasma opacity evolves rapidly with time. Photographic images obtained at the same time delay indicate that the plasma absorptivity becomes weaker as plasma evolves. At times up to 500 ns after plasma initiation, plasma absorptivity rose above 70% but later decreased markedly. When viewing the transmission and photographic images, the beam-plasma interaction realized with the double-pulse configuration is explainable as laser-supported absorption waves. To clarify the phenomenon and plasma properties when the probe beam and plasma are coupled, Stark broadening was used to obtain the electron number density, which is of the order of 1018 cm−3. Also, emission spectra under single- and double-pulse experimental configuration were obtained and analyzed using the Boltzmann-plot method to provide the plasma temperatures. The intensities from the double-pulse experiments were slightly stronger, in agreement with the laser energy absorption data. Moreover, from emission signal enhancements obtained from the double-pulse experiments, the increase in the intensity of the ionic emission was more than that obtained from the atomic emission for which a larger fraction of the probe beam energy was absorbed.

Morphology and characteristics of laser-induced aluminum plasma in argon and in air: A comparative study

In laser-induced breakdown spectroscopy (LIBS), ablation takes place in general in an ambient gas of the atmospheric pressure, often in air but also in noble gas such as argon or helium. The use of noble gas is known to significantly improve the performance of the technique. We investigate in this work the morphology and the characteristics of induced plasma in argon and in air. The purpose is to understand the mechanism of the analytical performance improvement by the use of argon ambient with respective to air ambient and the dependence on the other experimental parameters such as the laser fluence. The observation of plasma morphology in different ambient gases provides also information for better design of the detection system which optimizes the signal collection according to the used ambient gases. More specifically, the expansion of the plasma induced on an aluminum target with nanosecond infrared (1064nm) laser pulse in two ambient gases, argon and the atmospheric air, has been studied with spectroscopic imaging at short delays and with emission spectroscopy at longer delays. With relatively low ablation laser fluence (65J/cm2), similar morphologies have been observed in argon and in air over the early stage of plasma expansion, while diagnostics at longer delay shows stronger emission, higher electron density and temperature for plasma induced in argon. With higher ablation laser fluence (160J/cm2) however, different expansion behaviors have been observed, with a stagnating aluminum vapor near the target surface in air while a propagating plume away from the target in argon. The craters left on the target surface show as well corresponding difference: in air, the crater is very shallow with a target surface chaotically affected by the laser pulse, indicating an effective re-deposition of the ablated material back to the crater; while in Ar a deeper crater is observed, indicating an efficient mass removal by laser ablation. At longer delays, a brighter, denser and hotter plasma is always observed in argon than in air as with lower ablation laser fluences. The observed different influences of the ambient gas on the plasma expansion behavior for different laser fluences are related to the different modes of laser-supported absorption waves, namely laser-supported combustion (LSC) wave and laser-supported detonation (LSD) wave.

Numerical study on impulse characteristics of laser-supported air-breathing pulsed detonation thrusters

A laser-supported air-breathing thruster utilizes the remote laser energy and atmospheric air to boost a vehicle. To calculate the impulse induced by a laser pulse, the operational process was divided into two phases. First, one dimension (1D) laser-supported absorption waves in the air were simulated by an implicit dual-time method, and laser absorption efficiencies were predicted, based on a more accurate absorption model and three temperatures thermal nonequilibrium. Sequentially, impulses for different parabolic thrusters and pulse energies were computed, considering the high-temperature real gas effect. Then experiments were conducted with a ballistic pendulum apparatus. The calculations of 1D absorption waves show that as laser intensity increases, the electron number density would reach the critical value, resulting in a laser reflection and decrease of absorption efficiency. Further calculations for thrusters imply the thrust oscillation due to air-refilling has an evident influence on the total impulse received, and because of a higher thrust peak and longer positive phase time, the flat top and longer configuration would significantly enhance the performance. Experimental results show that the errors of the impulse calculations for two thrusters are 4.2% and 9.4%, respectively, which verifies the calculation model.

Convoluted effect of laser fluence and pulse duration on the property of a nanosecond laser-induced plasma into an argon ambient gas at the atmospheric pressure

We studied the behavior of the plasma induced by a nanosecond infrared (1064 nm) laser pulse on a metallic target (Al) during its propagation into argon ambient gas at the atmospheric pressure and especially over the delay interval ranging from several hundred nanoseconds to several microseconds. In such interval, the plasma is particularly interesting as a spectroscopic emission source for laser-induced plasma spectroscopy (LIBS). We show a convoluted effect between laser fluence and pulse duration on the structure and the emission property of the plasma. With a relatively high fluence of about 160 J/cm2 where a strong plasma shielding effect is observed, a short pulse of about 4 ns duration is shown to be significantly more efficient to excite the optical emission from the ablation vapor than a long pulse of about 25 ns duration. While with a lower fluence of about 65 J/cm2, a significantly more efficient excitation is observed with the long pulse. We interpret our observations by considering the post-ablation interaction between the generated plume and the tailing part of the laser pulse. We demonstrate that the ionization of the layer of ambient gas surrounding the ablation vapor plays an important role in plasma shielding. Such ionization is the consequence of laser-supported absorption wave and directly dependent on the laser fluence and the pulse duration. Further observations of the structure of the generated plume in its early stage of expansion support our explanations.

Ultraviolet versus infrared: Effects of ablation laser wavelength on the expansion of laser-induced plasma into one-atmosphere argon gas

Laser-induced plasma from an aluminum target in one-atmosphere argon background has been investigated with ablation using nanosecond ultraviolet (UV: 355 nm) or infrared (IR: 1064 nm) laser pulses. Time- and space-resolved emission spectroscopy was used as a diagnostics tool to have access to the plasma parameters during its propagation into the background, such as optical emission intensity, electron density, and temperature. The specific feature of nanosecond laser ablation is that the pulse duration is significantly longer than the initiation time of the plasma. Laser-supported absorption wave due to post-ablation absorption of the laser radiation by the vapor plume and the shocked background gas plays a dominant role in the propagation and subsequently the behavior of the plasma. We demonstrate that the difference in absorption rate between UV and IR radiations leads to different propagation behaviors of the plasma produced with these radiations. The consequence is that higher electron density and temperature are observed for UV ablation. While for IR ablation, the plasma is found with lower electron density and temperature in a larger and more homogenous axial profile. The difference is also that for UV ablation, the background gas is principally evacuated by the expansion of the vapor plume as predicted by the standard piston model. While for IR ablation, the background gas is effectively mixed to the ejected vapor at least hundreds of nanoseconds after the initiation of the plasma. Our observations suggest a description by laser-supported combustion wave for the propagation of the plasma produced by UV laser, while that by laser-supported detonation wave for the propagation of the plasma produced by IR laser. Finally, practical consequences of specific expansion behavior for UV or IR ablation are discussed in terms of analytical performance promised by corresponding plasmas for application with laser-induced breakdown spectroscopy.

Pulse Profile Rule in Laser Heating of Opaque Targets in Air

A theoretical model is developed to determine time evolution of temperature at the surface of an opaque target placed in air for cases characterized by the formation of laser supported absorption waves (LSAW) plasmas. The model takes into account the power temporal variation throughout an incident laser pulse, (i.e. pulse shape, or simply: pulse profile).Three proposed profiles are employed and results are compared with the square pulse approximation of a constant power.

Pulse Profile Rule in Laser Heating of Opaque Targets in Air

A theoretical model is developed to determine time evolution of temperature at the surface of an opaque target placed in air for cases characterized by the formation of laser supported absorption waves (LSAW) plasmas. The model takes into account the power temporal variation throughout an incident laser pulse, (i.e. pulse shape, or simply: pulse profile).Three proposed profiles are employed and results are compared with the square pulse approximation of a constant power.

Dynamics of double plume emissions in TEA-Co/sub 2/ laser ablation of graphite

Streak photography of TEA-CO/sub 2/ laser ablation of graphite in helium (He) and argon (Ar) background gases shows two luminous fronts, that are spatially and temporally separated, lifting off the graphite target. The velocity profile of the second front appears to follow closely that of the first luminous front. The second front is absent if the laser-supported absorption wave sets in when the background gas pressure is increased to 50-mbar Ar. The optical emission spectroscopy display many of triply, doubly, and singly ionized carbon species in the expanding plume at <0.3 mbar He, while the excited and singly ionized helium species are found in (1-30) mbar He which coincides with streak observations of strong luminous fronts. Computer simulations of the propagation trajectories of luminous fronts are carried out using the blast-wave, drag, and the snowplow models.

Modeling of Momentum Transfer to a Surface by Laser-Supported Absorption Waves

A simplified parametric model of the flowfield produced by impingement of a high-energy laser pulse onto a nonablative surface in a static environment is described. An attempt has been made to account for the twodimensional effects arising out of the rarefaction structure within the laser-heated gases. An optimization model is developed to indicate the laser spot size, pulse width, and intensity dependencies of the efficiency of delivery of both impulse and mechanical energy to the target surface.

On pulsed-laser heating of thin targets in air

A simple analytical model has been developed which describes the heating of a thin opaque target in air when subjected to irradiation from a high-power pulse laser. Effects due to the formation of a laser-supported absorption wave (LSAW) above the target surface are explicitly included in this model. The effective on-target flux distribution is assumed to be described by a radially expanding Gaussian profile in order to simulate the effects of enhanced coupling dominated by uv emission from the hot expanding LSAW plasma. Target heating inside as well as outside the incident beam-spot region is described. This model is an improvement over the previous approach [J.E. Robin, Appl. Phys. Lett. 30, 401 (1977)] which employed a radially expanding uniform flux distribution that was based on the assumption of a thermal-conduction-dominated enhanced-coupling mechanism.

Two-dimensional modeling of aerosol-induced breakdown in air

The dynamics of aerosol-induced breakdown in air is described using a three-temperature nonequilibrium thermodynamic model of the air plasma and two-dimensional Lagrangian hydrodynamics. From a heated sphere of air used as the initial condition an expanding laser-supported absorption wave develops, provided a maintenance intensity-threshold criterion is met. The maintenance threshold is close to 6×106 W/cm2 for 10.6-μm radiation. Although the absorption wave develops a shock, it differs in important ways from a one-dimensional laser-supported detonation wave due to its two-dimensional character.

Pulsed laser heating of thick opaque targets in air

An analytical model has been developed which describes the surface heating of a thermally thick opaque target in air when subjected to irradiation from a high-power pulsed laser. Both single pulsed and repetitively pulsed irradiation effects were examined. Effects due to the formation of a laser-supported absorption wave (LSAW) above the target surface are explicitly included in this model. The results show that despite the increase in thermal coupling due to the presence of the LSAW, the retarded heating effects experienced by tightly focused beams due to energy spreading as the LSAW expands over the target surface can be quite severe. This model shows that for small incident beam diameters the rate of temperature rise at the target surface can become negative during the pulse deposition time due to the combined effects of energy spreading and axial heat conduction.

Gas breakdown initiated by laser radiation interaction with aerosols and solid surfaces

The threshold for generation of high-density plasma caused by the interaction of laser radiation with solids is examined theoretically. The developed semiquantitative model is applicable for predicting the threshold for generating laser-supported absorption waves, absorbing plasma produced on surfaces with high-intensity laser radiation, and also for predicting the threshold of aerosol-induced gas breakdown, commonly referred to as dirty air breakdown. In developing the model, the vapor density generated by laser interaction with the solid is calculated and then the intensity required to cascade ionize the vapor to full ionization is calculated. Analytic expressions are derived for the breakdown intensity which show the parametric dependence of the threshold. Since the vapor density is a function of laser intensity, and characteristics of the solid, different parametric dependence of the threshold on wavelength, pulse length, and particle characteristics is predicted compared to those determined by previous theoretical models of gas breakdown. Comparison of the threshold predicted by the model agree within a factor of 2 of experimental data for a wide range of experimental conditions including laser pulse duration from 10 nsec to cw, particle sizes from 0.1 μ to laser spot sizes of 1 cm, and wavelength scaling of 1.06–10.6 μ wavelength. Based on the good agreement between theoretical predictions and experimental data, the model can be used to predict thresholds for plasma production for a wide variety of atmospheric and solid-surface conditions.

Pulsed laser heating of thin opaque targets in air

A simple analytical model has been developed which describes the heating of a thin opaque target in air when subjected toirradiation from a high-power pulse laser. Effects due to the formation of a laser supported absorption wave (LSAW) above the target surface are explicitly included in this model. The results shows that despite the increased coupling due to the presence of the LSAW, the resultant target heating from a tightly focused beam (small spot diameter) can be severely retarded by energy spreading as the LSAW expands over the target surface.

Spectroscopic diagnostics of laser-supported absorption (LSA) waves produced by a HF laser

LSA waves have been generated by a pulsed chemical HF laser beam (2.8 μ, 45 J, 7 μsec) when focused on solid surfaces in air. The emission spectrum is found to contain mainly N+ and O+ lines broadened by the Stark effect and superimposed on a strong continuum. The LSA wavefront travels at an average speed of 0.72 cm/μsec as measured by the time-of-flight method. Target species such as atomic Al emit light for 600 μsec after the laser pulse. In contrast with the results from a CO2 laser, however, no forbidden transition of AlI is observed.

Recent experiments in laser supported absorption waves

Recent experimental studies with laser supported absorption waves, arising from intense laser radiation incident upon various surface materials, are discussed. The propagation characteristics of both subsonic (laser supported combustion), intensity about 10 5 W/cm 2, and supersonic (laser supported detonation), intensity greater than 10 6 W/cm 2, waves are described, including the dependence of wave speed on laser intensity, beam diameter, gas density and laser wave length. Measurements of the plasma properties in such waves are described, and the analysis of these data allows conclusions identifying the dominant transport mechanism in such waves. It is also concluded that in the LSC (subsonic) case, radial flow ahead of and within the wave are dominant features of the wave structure. The equipment and laser facilities used in the experiments are also discussed.