Laser-induced Plasma Shock Waves Research Articles

The investigation of laser-induced plasma shock wave propagation in liquids

Extending the propagation formula of the attenuated laser-induced plasma shock wave front with spherical symmestry in the air to the non-absolute centric symmetry is described in this paper. According to the analysis of the measured data for the laser-induced plasma shock wave front by using photo shadowgraph, a propagation formula of a rotational ellipsoid shock wave for a point source in the laser-induced plasma in a liquid is presented in this paper.The shock wave attenuation in pure water and ethanol has been investigated by means of the acoustic method. The experimental data are in agreement with the calculated results.

TEA-CO2 laser-induced shock wave plasma modulated by wires and needles placed in front of the target at low pressure.

TEA-CO2 laser-induced shock wave plasma modulated by wires and needles placed in front of the target at low pressure.

Equations of laser‐induced plasma shock wave motion in air

AbstractSimple equations of laser‐induced plasma shock wave motion in air are studied, based on the point blast theory. The theoretical models of the shock wave front evolution characterize two stages of shock wave propagation: the initial formation stage and the free attenuation stage. Due considerations of the boundary conditions, the validity range of the attenuation equation is larger than the one of the t2/5 law of the self‐similar solution. Furthermore, the theoretical results are in good agreement with the experimental data and nuclear explosion results. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 38: 75–79, 2003

Spectrochemical Analysis of Metal Elements Electrodeposited from Water Samples by Laser-Induced Shock Wave Plasma Spectroscopy

We have succeeded in applying laser-induced shockwave plasma spectroscopy (LISPS) to the problem of the detection and analysis of metal elements deposited from water samples by means of electrolysis. It is shown that metal elements are generally deposited in the form of a thin film on the electrode surface, while the electrode also conveniently serves as a subtarget for the relatively soft metal film, thereby providing the necessary conditions for the generation of shockwave plasma, which is favorable for highly sensitive spectrochemical analysis. It is shown that the detection sensitivity of this method reaches its highest value at low surrounding air pressure of around 1 torr. The lowest detection limit attained for various metal elements investigated in this experiment varies from around ten to a few tens of ppb. This limit can be readily improved upon by incorporating an optical multichannel analyzer into the detection system. We have thus presented a promising method for the realization of a compact mobile monitoring system for the accurate control of water and soil quality.

Application of primary plasma standardization to Nd-YAG laser-induced shock wave plasma spectrometry for quantitative analysis of high concentration Au–Ag–Cu alloy

A study was performed on a laser-induced shock wave plasma generated on high concentration Au–Ag–Cu alloys by a Q-switched Nd-YAG laser of 4.8 mJ under reduced air pressure of 2 torr. It was found that the total emission intensity of the secondary plasma is proportional to the intensity of the primary plasma. Assuming linear proportionality between the intensity of the primary plasma and the number of atoms vaporized from the target, it is proposed that quantitative analysis can be applied to the intensities of the analytical emission lines normalized by the total intensity of the primary plasma. This experimental result demonstrated for each metal element shows an excellent linear relationship between the normalized emission line intensity and the content of the corresponding element.

Removal of small particles on silicon wafer by laser-induced airborne plasma shock waves

It has been found that effective removal of small particles from silicon wafer surfaces can be achieved by laser-induced airborne plasma shock waves initiated above the surface. This process has demonstrated successfully the removal of 1 μm sized tungsten particles from the surface. Tungsten is known to be one of the most difficult particles to remove using conventional laser cleaning techniques employing direct laser irradiation onto the surface. The area cleaned by the shock waves was over ten times larger than that achieved by conventional laser cleaning. This provides an advantage in speed for cleaning large areas. The cleaning efficiency was strongly dependent on the gap distance between the laser focus (where laser-induced sparking takes place) and the surface. The particle removal began to occur with a gap of 6 mm and the removal efficiency increased significantly with decreasing the gap resulting in a complete removal at 2 mm. The basic idea behind this new cleaning technique and how to apply it for the removal of small particles from surfaces are described. Theoretical considerations about adhesion forces of small particles on the surface and laser-induced shock wave generation are carried out to understand this process.

Detection of the Density Jump in the Laser-Induced Shock Wave Plasma Using Low Energy Nd:YAG Laser at Low Pressures of Air.

A special interferometric technique with high sensitivity has been devised on the basis of rainbow refractometry for the detection of density jump signal withouthe use of an additional and delicate amplitude-splitting setup.This new technique was used for the characterization of shock wave plasma induced by a low energy of Q-sw Nd: YAG laser (14-64mJ) on zinc samples under reduced air pressure (3-30 Torr). An unmistakable signal of density jump was detected simultaneously with the observation of emission front signal, implying that the emission front and the front of the shock wave coincided and moved together with time at the initial stage of the secondary plasma expansion. The result of measurement also showed that at a later stage, the emission front began to separate from and left behind the blast wave front propagating in the surrounding gas. This result is a direct proof of our shock wave model to explain the excitation process of the laser-induced plasma.

Laser-induced shock wave plasma spectrometry using a small chamber designed for in situ analysis

Direct spectrochemical analyses on large bulk samples such as metal plates have been performed by using a small vacuum chamber, which was attached directly to the sample surface through an o-ring. This technique allowed the in situ generation of laser plasma and hence overcome to a good extent the inconvenient and sometime clumsy sample preparation procedure required in Laser-Induced Shock Wave Plasma Spectrometry. Additionally, the presence of the o-ring near the target surface effectively shielded off the surrounding area from the undesirable continuum emission from the primary plasma, and thereby enhanced the detection sensitivity of this technique. Using zinc plate and Pb glass as samples, it was further demonstrated in this experiment that even the time-integrated spectra, obtained by employing an OMA system, still exhibited a lower background than those obtained by ordinary time-resolved Laser-Induced Breakdown Spectroscopy.

Laser-induced plasma shock wave and cavity on metal surface underwater

The laser-induced plasma shock waves and related cavitation bubbles on an aluminum surface underwater have been experimentally studied with a high-sensitivity optical deflection system. The expansion of the shock wave pulse duration, the positions where shock waves attenuate to acoustic transients, the maximum bubble radii, and the bubble collapse center shifts have been determined experimentally. A high-pressure wall behind the bubble has also been observed with this deflection system. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 25: 307–311, 2000.

Initial formation process of laser-induced plasma shock wave in air

The initial formation process of laser-induced plasma shock wave in air during laser damage process has been studied experimentally with a well-designed optical fiber sensor based on detecting beam deflection. The experimental results indicate that the formation of the laser-induced plasma shock wave is the result of the overlap of vapor- and explosion-shock waves. The plasma explosion shock wave, which generates behind the vapor shock wave but has a higher speed, catches up with the vapor shock wave and overlaps it at a certain distance, giving the result of a strong shock wave in air. The distance is about 3 mm when a 170 mJ laser at 1.06 μm with 30 ns duration irradiates an aluminum target in air.

Selective Vaporization in Q-Switched Nd:YAG Laser Induced Shock Wave Plasma at Low Pressures.

Selective Vaporization in Q-Switched Nd:YAG Laser Induced Shock Wave Plasma at Low Pressures.

Neutral and Ionic Emission in Q-Switched Nd:YAG Laser-Induced Shock Wave Plasma

Excitation mechanisms for copper and zinc atoms as well as their ionic species in the Q-switched (Q-sw) Nd:YAG laser-induced shock wave plasma have been investigated with a time-gated optical multichannel analyzer (OMA) system for various experimental conditions. It is demonstrated that the shock excitation process is the main mechanism responsible for the emission spectra for laser energy ranging from 8 to 86 mJ and surrounding air pressure from 2 to 50 Torr. At air pressure below 2 Torr, collision-induced excitation appears to take over and becomes the operative mechanism. In all cases, the neutral emission dominates over the ionic emission, leading to a high signal-to-background ratio, which tends to decrease, however, at higher laser energy and air pressure.

Instantaneous measurements of laser-induced plasma shock waves with an optical fiber sensor

High power laser-induced plasma shock waves in air were measured instantaneously with an optical fiber sensor based on detection beam deflection. It was proved that the time-of-flight deflection angle directly imaged the changing rate of the refractive index at the measurement position while a shock wave transmitted the detection beam. This method is suitable for measurements of shock waves in transparent flow fields with axisymmetric refractive index distributions.

The Role of Sub-Target in the Transversely Excited Atmospheric Pressure CO 2 Laser-Induced Shock-Wave Plasma

A transversely excited atmospheric pressure (TEA) CO2 laser pulse (50 mJ, 100 ns) was focused on silicon grease which is painted on a copper plate as a subtarget with a power density of 6 GW/cm2 under reduced pressure. The comparison of the characteristics of the induced laser plasma between two cases, with subtarget and without subtarget was made. It is proved that the emission spectrum assigned to the silicon atom can be detected only for the case with the subtarget. It is also proved that in the absence of the subtarget, the gushing speed of the atom is very low, while for the case with subtarget, the gushing speed of atoms becomes very fast. It is shown that the setting of subtarget is very effective for producing laser-induced shock wave plasma and it is very effective for the realize quantitative analysis of a soft material.

The Effect of Selective Vaporization on TEA CO2 Laser Induced Shock Wave Plasma.

TEA CO2 laser (130mJ, 100ns) and Nd-YAG laser (50mJ, 8ns) pulses were employed to generate secondary plasma from brass samples in air under a reduced pressure of 1 Torr. The vaporization effects on Zn and Cu were studied in terms of their emission characteristics for the two cases. In the case of TEA CO2 laser, the emission intensities of copper spectral lines are extremely low, compared to the zinc spectral lines, indicating a highly selective-vaporization effect. The time-profiles of the emission intensities of ZnI 481.0nm and CuI 327.4nm spectral lines observed near the target surface clearly showed that vaporization of Cu atoms occurred later and continued for a long time at a relatively low gushing speed, while the vaporization of Zn atoms took place sooner with a higher gushing speed. It was shown that only Zn atoms form a shock front, and Cu atoms are left behind the shock-wave and are not excited. This selective vaporization process does not occur at a laser power density considerably higher than the threshold for the plasma generation as demonstrated by the use of Nd-YAG laser of higher power density. This phenomenon of selective vaporization described in the present paper also supports our laser-induced shock wave model.

Characteristics of a laser plasma induced by irradiation of a normal-oscillation YAG laser at low pressures

The generation of a laser-induced shock wave plasma in air at reduced pressure was achieved by focusing a normal-oscillation Nd:YAG laser on a brass sheet. The characteristics of the plasma were analysed under three different conditions: free expansion of the plasma, confinement of the plasma by limiting the plasma region using two parallel glass plates and interruption of the plasma by placement of a wedge in front of the plasma, in order to interrupt the expansion of the plasma. Spatially and temporally integrated methods were also developed in order to develop a more detailed understanding of the excitation mechanism of the plasma. Furthermore, a new technique of shadow graphing which involves the use of a He - Ne laser as a probe light was also developed as well as a measurement of the time profile of the temperature. By using this new method the relationship between the density jump and the starting point of atomic emission was detected simultaneously. The results show that the plasma was also generated by the shock wave as for the case of the short-pulse laser (Q switched). The extremely low ion and background emission, as low as compared with neutral emission, makes this plasma potentially useful for spectrochemical analysis.

Laser-Induced Shock Wave Plasma Using Long-Pulse YAG Laser

A laser-induced shock wave plasma was generated by bombarding a brass target with a long-pulse YAG laser at reduced pressure. The characteristics of the laser plasma were studied by examining the reflection and diffraction of the laser plasma when an aluminum wedge block was placed in front of the plasma. A dramatic increase in the emission of the secondary plasma was observed, supporting the shock wave model. The collected data also confirm that the displacement length of the secondary plasma emission front is proportional time raised to the 0.6 power. The temperature of the secondary plasma was found to be 10,000 K at 5 mm above the target, decreasing to 6800 K at 10 mm above the target.

Emission Spectrochemical Analysis of Glass Containing Li and K in High Concentrations Using a XeCl Excimer Laser-Induced Shock Wave Plasma

A XeCl excimer laser (20 ns, 25 mJ) was focused on glass samples containing high concentrations of Li and K (10%, 15%, and 20%) in a surrounding gas of air and He. Plasma characteristics are compared in two cases: at 1 atm and at reduced pressure (1–10 Torr). It is shown that the plasma produced at 1 atm cannot be applied to the elemental analysis of solid samples because of strong background emission and self-absorption in the plasma. The spectrum width of the emission line is very wide due to strong interaction in the plasma. In contrast to this, the shock-wave-induced plasma produced at reduced pressure gives a high S/B (ratio of the intensity of the emission line to that of the background) in the measurement of analytical emission lines, and the slope of the calibration curve is near unity in a log-log plot, thus making it possible for emission spectrochemical analysis.

Laser-Induced Shock Wave Plasma in Glass and its Application to Elemental Analysis

The characteristics of a laser-induced shock wave plasma which was induced by focusing a laser pulse on the surface of glass samples were examined by using radiation from a XeCl excimer laser and a TEA CO2 laser under reduced pressure of around 1 Torr. It was observed that shock wave plasma could not be generated by the TEA CO2 laser on low-melting-point glass because of the lack of expulsion from the sample surface. On the other hand, with the use of an excimer laser, shock wave plasma can be generated, even in low-melting-point glasses, thus making it amenable for spectrochemical analysis. Initial quantitative analysis was performed on a number of glass samples, and a linear calibration curve with a slope of near unity was obtained at a certain pressure. Furthermore, light elements such as Li and B, which are usually difficult to observe by the X-ray fluorescence method, were also successfully detected with a very low detection limit of less than 10 ppm. Other detection limits and background equivalent concentrations of almost all elements usually contained in glass, such as Na, Mg, Al, K, Ca, Ti, Zn, Zr, and Ba, were also presented. These results showed that the detection limit is much lower than those usually required for glass analysis.

Atomic emission spectrometric analysis of steel and glass using a TEA CO 2 laser-induced shock wave plasma

The shock wave plasma induced by the bombardment of laser light coming from a commercial TEA CO 2 laser has been used for the direct elemental analysis of solid samples. By using standard low-alloy steel samples, it is demonstrated that there is a linear relationship between the Cr content and the emission intensity, using an internal standard method. A background equivalent concentration for the Mg 383.8 nm emission line is found to be as low as about 0.005% in glass samples.