M. Polek

Dynamics of C2 formation in laser-produced carbon plasma in helium environment

We investigated the role of helium ambient gas on the dynamics of C2 species formation in laser-produced carbon plasma. The plasma was produced by focusing 1064 nm pulses from an Nd:YAG laser onto a carbon target. The emission from the C2 species was studied using optical emission spectroscopy, and spectrally resolved and integrated fast imaging. Our results indicate that the formation of C2 in the plasma plume is strongly affected by the pressure of the He gas. In vacuum, the C2 emission zone was located near the target and C2 intensity oscillations were observed both in axial and radial directions with increasing the He pressure. The oscillations in C2 intensity at higher pressures in the expanding plume could be caused by various formation zones of carbon dimers.

Multidiagnostic analysis of ion dynamics in ultrafast laser ablation of metals over a large fluence range

The dynamics of ions in ultrafast laser ablation of metals is studied over fluences ranging from the ablation threshold up to ≈75 J/cm2 by means of three well-established diagnostic techniques. Langmuir probe, Faraday cup, and spectrally resolved intensified charge coupled device imaging simultaneously monitored the ions produced during ultrafast laser ablation of a pure copper target with 800 nm, ≈50 fs, Ti: Sapphire laser pulses. The fluence dependence of ion yield is analyzed, resulting in the observance of three different regimes. The specific ion yield shows a maximum at about 4–5 J/cm2, followed by a gradual reduction and a transition to a high-fluence regime above ≈50 J/cm2. The fluence dependence of the copper ions angular distribution is also analyzed, observing a gradual increase in forward-peaking of Cu ions for fluences up to ≈10 J/cm2. A broader ion component is observed at larger angles for fluences larger than ≈10 J/cm2. Finally, an experimental characterization of the ionic angular distrib...

Morphological changes in ultrafast laser ablation plumes with varying spot size.

We investigated the role of spot size on plume morphology during ultrafast laser ablation of metal targets. Our results show that the spatial features of fs LA plumes are strongly dependent on the focal spot size. Two-dimensional self-emission images showed that the shape of the ultrafast laser ablation plumes changes from spherical to cylindrical with an increasing spot size from 100 to 600 μm. The changes in plume morphology and internal structures are related to ion emission dynamics from the plasma, where broader angular ion distribution and faster ions are noticed for the smallest spot size used. The present results clearly show that the morphological changes in the plume with spot size are independent of laser pulse width.

Late-time particle emission from laser-produced graphite plasma

We report a late-time “fireworks-like” particle emission from laser-produced graphite plasma during its evolution. Plasmas were produced using graphite targets excited with 1064 nm Nd: yttrium aluminum garnet (YAG) laser in vacuum. The time evolution of graphite plasma was investigated using fast gated imaging and visible emission spectroscopy. The emission dynamics of plasma is rapidly changing with time and the delayed firework-like emission from the graphite target followed a black-body curve. Our studies indicated that such firework-like emission is strongly depended on target material properties and explained due to material spallation caused by overheating the trapped gases through thermal diffusion along the layer structures of graphite.

Influence of laser pulse duration on extreme ultraviolet and ion emission features from tin plasmas

We investigated the role of laser pulse duration and intensity on extreme ultraviolet (EUV) generation and ion emission from a laser produced Sn plasma. For producing plasmas, planar slabs of pure Sn were irradiated with 1064 nm Nd:YAG laser pulses with varying pulse duration (5–20 ns) and intensity. Experimental results performed at CMUXE indicate that the conversion efficiency (CE) of the EUV radiation strongly depend on laser pulse width and intensity, with a maximum CE of ∼2.0% measured for the shortest laser pulse width used (5 ns). Faraday Cup ion analysis of Sn plasma showed that the ion flux kinetic profiles are shifted to higher energy side with the reduction in laser pulse duration and narrower ion kinetic profiles are obtained for the longest pulse width used. However, our initial results showed that at a constant laser energy, the ion flux is more or less constant regardless of the excitation laser pulse width. The enhanced EUV emission obtained at shortest laser pulse duration studied is related to efficient laser-plasma reheating supported by presence of higher energy ions at these pulse durations.

Jetlike Emission From Colliding Laser-Produced Plasmas

We report a large jetlike collimated emission feature from colliding laser-produced plasmas (LPPs). The plasmas are produced by focusing two parallel laser beams on a planar copper target. Time-resolved spectrally integrated emission features in the visible region are recorded using an intensified charge-coupled device camera. Both laser plasmas expanded perpendicular to the target and collided side-on. The jetlike emission has a spatial extension of more than 3 cm and exists in a prolonged time compared with single LPPs.

Dynamics of low- and high-Z metal ions emitted during nanosecond laser-produced plasmas

Dynamics of metal ions during laser-produced plasmas was studied. A 1064 nm, Nd: YAG laser pulse was used to ablate pure Al, Fe, Co, Mo, and Sn samples. Ion flux and velocity were measured using Faraday cup ion collector. Time-of-flight measurements showed decreasing ion flux and ion velocity with increasing atomic weight, and heavy metal ion flux profile exhibited multiple peaks that was not observed in lighter metals. Slow peak was found to follow shifted Maxwell Boltzmann distribution, while the fast peak was found to follow Gaussian distribution. Ion flux angular distribution that was carried out on Mo and Al using fixed laser intensity 2.5 × 1010 W/cm2 revealed that the slow ion flux peaks at small angles, that is, close to normal to the target ∼0° independent of targets atomic weight, and fast ion flux for Mo peaks at large angles ∼40° measured from the target normal, while it completely absents for Al. This difference in spatial and temporal distribution reveals that the emission mechanism of the ...

Two-dimensional mapping of the electron density in laser-produced plasmas

We performed two-dimensional (2D) mapping of the electron density in a laser-produced plasma with high spatial and temporal resolution. The plasma was produced by irradiating an aluminum target with 1064 nm, 6 ns pulses from a Nd:YAG laser under vacuum conditions. Stark broadening of the lines was used to estimate the electron density at various locations inside the plasma. The 2D spectral images were captured at different spatial points in the plasma using an imaging spectrograph coupled to an intensified CCD at various times during the plasma expansion. A comparison between radially averaged and radially resolved electron density profiles showed differences in the estimated values at the earlier times of plume evolution and closer distances to the target. However, the measured radially averaged values are consistent with 2D radial profiles at later times and/or farther distances from the target surface.

Ion emission dynamics in ultrafast laser ablated plasmas

There has been intensive interest in the studies of ultrafast laser ablation dynamics due to its wide application in the fields of micromachining, laser surgery, surface smoothing, nanoparticle production etc.1,2 Compared to long-pulse nanosecond (ns) laser ablation, the thermal processes are less pronounced in plasmas produced by femtosecond (fs) ultrafast lasers. Ultrafast lasers are better suited for micromachining, pulsed laser deposition and various other applications. Ultra-intense laser produced plasmas can also be used for producing collimated proton, electron and neutron beams. Moreover, the laser plasmas provide intense ion beams with a high charge state, the so called laser ion source (LIS). However, the lack of control of the ion pulse from laser-produced plasmas limits its applications and further studies are necessary to improve basic understanding of ion emission dynamics from laser-produced plasmas. We investigated the dynamics of ion emission from ultrafast laser ablated plasmas in vacuum from metal targets. The angular features of ion emission features are studied for ultrafast laser ablation and compared with ion emission features of long-pulse ns laser-produced plasmas.

Laser-produced carbon plasma evolution and lifecycle

Laser produced plasmas (LPPs) have been the focus of extensive basic research which have led to a diverse range of applications in many domains including extreme ultraviolet lithography, cellular microsurgery, laser-induced breakdown spectroscopy, nanocluster and nanotube production, laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS) etc. [1,2]. Studies of LPPs have been necessary to understand the physical and chemical processes associated with the ablation of a target material and the hydrodynamic target response to the intense heating induced by nanosecond and femtosecond lasers. While numerous materials have been studied using various laser beams, carbon has received particular attention due to its applications in fullerene production, deposition of diamond-like thin films, and choice for the design of plasma facing components in magnetic and inertial fusion reactors. However, though much effort has been expended to characterize carbon plasmas with laser ablation through the investigation of emission spectra, a thorough understanding of the entire cycle of carbon produced plasmas is lacking.