Picosecond Laser-produced Plasma Research Articles

Self-absorption of emission lines in picosecond-laser-produced gold plasmas

Ultrashort laser ablation offers several advantages in various applications compared to traditional nanosecond laser ablation techniques. Despite providing a lower damage threshold, cold ablation with high precision, the impact of self-absorption effects in ultrashort laser-produced plasmas (LPPs) significantly affects the assessment of plasma parameters and analytical outcomes in Laser-Induced Breakdown Spectroscopy (LIBS) analysis. This study investigated the impact of laser energy, analyte concentration, and acquisition gate delay on the self-absorption of emission lines from picosecond LPPs (ps-LPPs) of gold targets at atmospheric pressure. We used four gold targets (24 carat gold, 22 carat gold, 18 carat gold, and Hepatizon) with varying concentrations of gold and copper. To analyze the self-absorption effects in ps-LPP, we examined four neutral emission lines of ps-LIBS spectra (Au I: 267.59 and 627.81 nm; Cu I: 327.98 and 510.55 nm). We observed that with an increase in laser pulse energy, there is a corresponding rise in the self-absorption of emission lines under ambient pressure. This increase in analyte species concentration leads to an elevation in the self-absorption of emission lines. Additionally, as the temporal delay of acquisition extends, self-absorption intensifies.

Investigation of thermodynamic properties in picosecond laser-produced plasmas on silicon

The validity of local thermodynamic equilibrium (LTE) in plasma is a long-term concern for laser-induced breakdown spectroscopy applications. In this paper, the spatial and temporal dependence of LTE deviation in picosecond laser-induced plasmas has been proved. A picosecond (∼20 ps) laser operated at 532 nm was used to ablate a silicon target to produce plasma at a pressure of 10−5 mbar. A general electron energy distribution function was used to provide access to the insight into population ratios of different energy levels for the spatially and temporally resolved optical emission intensity of laser-induced plasma. A precise temporal and spatial LTE boundary of plasma generated in picosecond laser ablation was obtained, and the results showed that only at delay times of 180–300 ns, the plasma away from the surface (>4 mm) is in LTE.

Effect of double pulse laser irradiation on the dynamics of picosecond laser-produced plasma

Measurements to control the morphology and characteristics of a picosecond laser produced chromium plasma plume upon double-pulse (DP) irradiation are presented and compared to their single-pulse (SP) counterpart. DP schemes are implemented by employing two geometries where the inter-pulse delay and the spatial separation are the control parameters. The ratio of plume length to plume width decreases upon increasing the inter-pulse delay and/or the energy of the second pulse in the collinear DP scheme. Interestingly, plasmas generated in the DP scheme at lower pressures resemble the expansion features of the plasma generated in the SP scheme at higher pressures. We find that DP schemes are advantageous for applications such as high harmonic generation and the production of quality thin films.

Absolute determination of charge-coupled device quantum detection efficiency using Si K-edge x-ray absorption fine structure

We report a method to determine the quantum detection efficiency and the absorbing layers on a front-illuminated charge-coupled device (CCD). The CCD under study, as part of a crystal spectrometer, measures intense continuum x-ray emission from a picosecond laser-produced plasma and spectrally resolves the Si K-edge x-ray absorption fine structure due to the electrode structure of the device. The CCD response across the Si K-edge shows a large discontinuity as well as a number of oscillations that are identified individually and uniquely from Si, SiO(2), and Si(3)N(4) layers. From the spectral analysis of the structure and K-edge discontinuity, the active layer thickness and the different absorbing layers thickness can be determined precisely. A precise CCD detection model from 0.2 to 10 keV can be deduced from this highly sensitive technique.

Simultaneous high-resolution two-dimensional spatial and one-dimensional picosecond streaked x-ray pinhole imaging

A Kentech x-ray streak camera was run at the LLNL compact multipulse terawatt (COMET) laser to record simultaneous space- and time-resolved measurements of picosecond laser-produced plasmas. Four different x-ray energy channels were monitored using broadband filters to record the time history of Cu targets heated at irradiances of 10(16)-10(19) W∕cm(2). Through the Cu filter channel, a time-resolution below 3 ps was obtained. Additionally, an array of 10 μm diameter pinholes was placed in front of the camera to produce multiple time-resolved x-ray images on the photocathode and time-integrated images on the phosphor with 10 and 15 times magnification, respectively, with spatial resolution of < 13 μm.

Electric field measurements in picosecond laser-produced plasma via X-ray spectroscopy

High resolution, spatially resolved spectroscopy is used to study the interaction of a short 3.4 ps pulse laser at intensities between 10 14 and 10 17 W/cm 2. The detailed lines shapes show a series of regular intensity modulations, or dips, across the spectral line. We present the analysis of the helium-like emission from these aluminium plasmas, and compare the Heβ transition to calculation. Assuming the intensity dips result from the interaction of quasi-monochromatic electric field of a strong plasma wave, we are able to infer the field strengths present and electron density. This suggests that the intensity dips are driven by the strong growth of laser-driven parametric instabilities at quarter critical density. The development of high resolution spectroscopy for the interpretation of parametric driven instabilities in short pulse laser-produced plasma is discussed.

Generation of fluxes of highly charged heavy ions from a picosecond laser-produced plasma

The results of the investigations of ion emission from targets of medium and high atomic numbers irradiated by 1 ps laser pulse of intensity up to 5×1016 W/cm2 are presented. The generation of high energy (up to 1 MeV) highly charged heavy ions (Ta+38, Au+33), as well as Ar-like Ag ions and fully striped Al ions is demonstrated. Some properties of ion fluxes emitted from picosecond laser-produced high-z plasmas are determined.

Enhancement of high-order harmonic generation efficiency from solid-surface plasma by controlling the electron density gradient of picosecond laser-produced plasmas

We have investigated the efficiency of generating the fifth-order harmonic from plasma at the surface of a solid by varying the electron density gradient of the picosecond laser-produced plasmas. In order to modify the plasma scale length, we carried out experiments in which a 2.2 ps prepulse irradiates the solid target 0-20 ps prior to the arrival of the main Nd:glass laser pulse with the same pulse duration. The chirped pulse amplification Nd:glass laser system delivered pulses up to 100 mJ energy at 1054 nm wavelength. Although third-harmonic radiation can also be observed when a single beam is used, a /spl sim/5 ps time delay results in an increase in the harmonic signal by a factor of 2-3, which then decreases after 10 ps. We also have carried out more detailed model-calculations using the simulations. As a result, it is found that it is possible to generate high harmonics efficiency by controlling the density gradient using two laser pulses.

Laser ablation studies of palladium electrolytically loaded with hydrogen and deuterium.

The kinetic-energy distributions (KED's) of ${\mathrm{Pd}}^{+}$, ${\mathrm{H}}_{\mathit{n}}^{+}$, ${\mathrm{D}}_{\mathit{n}}^{+}$ (n=1 to 3), and electrons created in a picosecond laser-produced plasma from palladium metal were studied using a spherical-sector electrostatic energy analyzer followed by a time-of-flight mass spectrometer. High-resolution, Fourier-transform mass spectroscopy showed no ${\mathrm{PdH}}_{\mathit{n}}^{+}$ or ${\mathrm{PdD}}_{\mathit{n}}^{+}$ ions. The total yield of ${\mathrm{Pd}}^{+}$ ions grew linearly with laser power, while the yields of ${\mathrm{H}}^{+}$ and ${\mathrm{D}}^{+}$ increased exponentially. The yields of ${\mathrm{H}}_{2}^{+}$ and ${\mathrm{H}}_{3}^{+}$ increased with laser power density slowly, comparable to the yields of ${\mathrm{H}}^{+}$ and ${\mathrm{D}}^{+}$ at low laser power density only. The average ion kinetic energy was a linear function of laser power for all species, but with a much higher average kinetic energy for ${\mathrm{Pd}}^{+}$ than for other species at a given laser power. The KED's for ${\mathrm{Pd}}^{+}$ were dominated by a plasma component with a Maxwellian shape, accompanied by a thermionic component with a kinetic energy in the range of 0--1 eV. The ${\mathrm{H}}^{+}$, ${\mathrm{H}}_{2}^{+}$, ${\mathrm{H}}_{3}^{+}$, and ${\mathrm{D}}^{+}$ KED's were basically Maxwellian, but the origins were offset to positive kinetic energies, with the offset energy increasing linearly with mass. There was no thermal component for the ${\mathrm{H}}_{\mathit{n}}^{+}$ or ${\mathrm{D}}^{+}$ ions.

Measurement of the expansion of picosecond laser-produced plasmas using resonance absorption profile spectroscopy.

Time-resolved, submicron density scale lengths in picosecond laser-produced plasmas are measured using resonance absorption profile spectroscopy. The density scale length at critical density is inferred from the angle and polarization dependence of the absorption of a picosecond laser pulse in a performed plasma. The early expansion of picosecond laser-produced plasmas produced from an Au target is studied, with measured scale lengths in the range of \ensuremath{\sim}1000\char21{}5000 A\r{}.

Second harmonic emission from a picosecond laser-produced plasma

Several kinds of experiments have been done relating to the second harmonic emission from a plasma produced by a 1 μm laser pulse of 30 psec. Complicated spectra of the second harmonic have been observed above ∼1013 W/cm2. It has been concluded that their production mechanisms are dependent on the self-focusing of the laser beam. In the absence of the self-focusing of the laser beam, it was determined that the complicated spectrum was caused by the parametric decay instability of an electron plasma wave excited by the resonance absorption. In the presence of the self-focusing of the laser beam, on the other hand, other mechanisms have been found to be dominant. It was tentatively concluded that they are the unstable motion of the critical surface of the plasma or the self-phase modulation of the laser pulse.

Effective mass in picosecond laser-produced high-density plasma in silicon

Picosecond laser-produced plasmas in silicon attain carrier densities of about 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and temperatures of up to the melting point of about 1700 K. The optical data indicate only a slight increase of about 20 percent in the optical effective mass from the value at low density and room temperature. A simple model calculation is presented which confirms this behavior.

Important role of dynamical atomic processes on x-ray line emissions from picosecond laser-produced plasmas in the ionizing phase

Transient characteristics of x-ray line radiation from high density plasmas in an ionizing phase are computationally shown in the picosecond region by solving coupled rate equations. Many atomic processes associated with x-ray radiation are found to affect temporal changes of x-ray intensities and, especially, the x-ray intensity ratios. It is pointed out that in the evaluation of electron temperatures in transient plasmas, undesirable errors are apt to be brought about by usual conventional methods based on computations in which steady state equilibrium is assumed.

Development of a flat-field grazing-incidence XUV spectrometer and its application in picosecond XUV spectroscopy

A new type of grazing-incidence spectrometer with a flat focal field is developed, and XUV spectroscopy in the extreme ultraviolet region ranging from 15 to 200 Å is carried out. Soft x-ray line spectra emitted from picosecond laser plasmas of aluminum and iron targets are measured and good resolutions are obtained in the XUV region. The spectral regions of detection are extended to shorter wavelengths (15 Å) using a finer spaced grating. Computational studies on x-ray spectra are also performed taking into account the transient characteristics of picosecond laser-produced plasmas; the importance of the transient treatment is clearly shown. This type of soft x-ray spectrometer should be useful for time-resolved picosecond soft x-ray spectroscopy.

On the distribution of second harmonic emission from a laser-produced plasma

Computational studies of second harmonic generation by resonance absorption in picosecond laser-produced plasmas have shown that by the use of non-planar targets the optimum angle for resonance absorption can be deduced from a single laser shot. Experiments have verified these predictions and demonstrated that this optimum angle increases with increasing laser intensity as expected due to profile modification in the plasmas. At 10 16 W/cm 2 the second harmonic conversion efficiency was measured to be 0.4 ± 0.2%.

X-ray bremsstrahlung and fast-ion measurements from picosecond laser-produced plasmas

Fast-ion and suprathermal electron energies have been measured from plasmas generated by 25 ps duration neodymium laser pulses focussed onto solid targets. The relationship between the fast-ion energy and the energy of the suprathermal electrons in the plasma has been demonstrated and these energies have been found to increase in proportion to I 0.35±0.05 for targets of aluminum and copper, with I the laser intensity.