Ashwini K. Sharma

Reactive pulsed laser ablation: Plasma studies

We report on the pulsed laser ablation of aluminum (Al) plasma in presence of ambient nitrogen to understand the formation of aluminum nitride (AlN). Formation of carbon nitride (CN) and titanium oxide (TiO) by pulsed laser-ablation of graphite and titanium targets in presence of ambient nitrogen and oxygen is also compared. We discuss the dynamics of plasma expansion based on existing models, shock and drag models, and the plasma gas interface distortion, Rayleigh-Taylor instability at various ambient pressures of nitrogen. Since Rayleigh-Taylor instability may give rise to self-generated magnetic field in the plasma, an attempt is made to understand the mechanism of generation as well as the estimation of this field near the focal spot using the information from the images of the expanding plasma. This is the first time images of the expanding plume are used to estimate self generated magnetic fields. At the irradiance level used in the experiment the field is high very close to the target surface therefore we expect splitting of the energy levels thus giving rise to emissions that may be anisotropic in nature. We discuss the extent of anisotropy by measuring the degree of polarization using emission intensity in optical emission spectrum of selected Al III transition 4s 2 S 1/2 –4p 2 P 3/2 o at 569.6 nm using both nanosecond and picosecond pulses.

Pulsed laser ablation of aluminum in the presence of nitrogen: Formation of aluminum nitride

We report on the pulsed laser ablation of aluminum in the presence of nitrogen gas using a 1.06 μm wavelength of Nd:YAG laser. A prominent band of aluminum nitride corresponding to the (0-0) band of the system belonging to a 3π–3π transition was observed at 507.8 nm. An attempt is made to identify the ionized states of aluminum and nitrogen contributing to formation of the AlN band. AlN films were deposited at room temperature and characterized using x-ray diffraction. A direct correlation between the laser ablated aluminum plasma and the deposited AlN film is reported.

Spectroscopic investigations of carious tooth decay

We report on the elemental composition of healthy and infected part of human tooth using laser induced breakdown spectroscopy (LIBS). We have used prominent constituent transitions in laser-excited tooth to diagnose the state of the tooth. A nanosecond laser pulse (355nm, 5ns) was used as an ablating pulse and the sodium (3s2S-3p2P) at 588.99 and (3s2S-3p2P) at 589.99nm, strontium (5s21S-1s5P) at 460.55nm, and calcium (3d3D-4f 3F0) at 452.55nm transitions for spectroscopic analysis. The spectroscopic observations in conjunction with discriminate analysis showed that calcium attached to the hydroxyapatite structure of the tooth was affected severely at the infected part of the tooth. The position-time plots generated from two-dimensional (2D) images conclusively showed a decrease in calcium concentration in the infected region of the irradiated tooth. Using the technique, we could distinguish between the healthy and carious parts of the tooth with significant accuracy.

Characterization of laser-produced aluminum plasma in ambient atmosphere of nitrogen using fast photography

We report on the pulsed-laser ablation of aluminum in ambient pressure of nitrogen varying from 0.01 to 70 Torr using images of the expanding plasma plume. At pressures ⩾1 Torr plasma–gas interface showed severe distortion in the front of the expanding plume. The plasma expansion velocity showed oscillatory behavior with delay time beyond 260 ns and is attributed to Rayleigh–Taylor instability. The effect of background gas on inducing polarization in the ablated plasma is also reported. At low pressure of 0.1 Torr the degree of polarization of Al III transition 4s 2S1/2–4p 2P3/20 at 569.6 nm increased with delay time. At pressures ⩾1 Torr it showed an oscillatory behavior. The observed steep pressure gradient at the plasma–gas interface may result in strong self-generated magnetic field due to Rayleigh–Taylor instability.

Multi-structured temporal behavior of neutral copper transitions in laser-produced plasma in the presence of variable transverse static magnetic field

We report on the effect of variable magnetic field on temporal behavior of neutral copper (Cu I) transitions in laser-produced copper plasma at atmospheric pressure using optical emission spectroscopy. In the presence of magnetic field, the intensity of copper atomic lines at 510.5, 515.3, and 521.8 nm gets enhanced due to increase in electron-impact excitation rate. The enhancement factor of the neutral lines is different due to different electron-impact excitation rates. We observed that the Cu I profile consists of two components recorded in the absence of magnetic field and at 0.1 T. At magnetic field of 0.3 T, the appearance of third slow component at delayed time, i.e., 122, 130, and 140 ns for Cu I (521.8, 515.3, and 510.5 nm) is also observed. We demonstrate that the generation of slow component is related to electron-impact excitation of Cu I atom rather than backflow particles and instabilities at atmospheric pressure. The instabilities generated during the plasma deceleration by magnetic field ...

Spatially resolved behavior of laser-produced copper plasma along expansion direction in the presence of static uniform magnetic field

We report on the spatially resolved optical emission spectroscopic study of laser-produced copper plasma in the presence of static uniform magnetic field in air ambient at atmospheric pressure. The response of copper atomic/ionic lines to magnetic field along the axial direction of plasma is different. It is attributed to the difference in populating process (electron impact excitation and recombination) of each transition. In the present work, we introduced air pressure to calculate the stopping radius and found it to be around the distance at which the intensity is pronounced. The electron density varied as ne = 9.2z−0.33 without magnetic field and in the presence of 0.3 T magnetic field, it varied as ne = 7.9z−0.27. The electron temperature variation with distance from the target in the absence and presence of magnetic field is found to be Te = 1.1z−0.23 and Te = 0.9z−0.18. The electron density and temperature decay slowly along the plasma expansion direction in the presence of magnetic field. It is du...

Effect of variation of magnetic field on laser ablation depth of copper and aluminum targets in air atmosphere

We report on the effect of transverse magnetic field on laser ablation of copper and aluminum targets both experimentally and numerically. The ablation depth is found to increase with magnetic field from 0 to 0.3 T and decreases at a higher magnetic field (0.5 T). It is demonstrated that the nanosecond laser ablation is mainly due to melt ejection and it solely depends on the thermo-physical parameters of the material. The increase in ablation depth with magnetic field is attributed to the increase in heat transfer from the plasma to the target, vapor pressure, and shock pressure. The ablation due to melt ejection is also calculated using vapor pressure through simulation and compared with the experimentally measured depth. In the presence of magnetic field, we introduce the magnetic pressure in Clausius–Clapeyron vapor pressure equation to account for the combined effect of magnetic field and atmospheric pressure on the vapor pressure of plasma. The ratio of calculated ablation depth at 0.3 T with respec...

Effect of lens focusing distance on laser-produced copper plasma in air in the presence of static transverse magnetic field

We report on the role of lens focusing conditions on laser-produced copper plasma in air at atmospheric pressure in the presence of magnetic field using optical emission spectroscopy. From the time integrated and temporal optical emission spectroscopy, we observed that the copper neutral/ionic line showed a higher intensity when the focal point of the lens was below the target surface in the absence of the magnetic field. In the presence of magnetic field, significant intensity enhancement was observed when the focal point was below the target surface which is attributed to an increase in the plasma-magnetic field interaction at this focusing condition. When the focal point was above the target surface, the intensity of neutral line was quite low and the effect of magnetic field was insignificant. This is because of air breakdown which caused lesser laser-matter interaction and eventually plasma-magnetic field interaction. Based on heat conduction equation, we also simulated the laser heating of copper ta...

Polarization-resolved measurements of picosecond laser-ablated plumes

We discuss the ablation of aluminum plasma using picosecond pulsed laser in vacuum and in ambient atmosphere of nitrogen. The plume dynamics of picosecond and nanosecond laser-ablated plumes in ambient atmosphere is discussed. The degree of polarization is measured using optical emission spectroscopy for the AlIII transition 4sS1∕22−4pP3∕2o2 at 569.6nm. Strong anisotropy is observed using picosecond laser pulse as compared to nanosecond laser pulse.

Highly c -Axis Oriented Growth and Optical Characterization of ZnO Pore-Like Structures Surrounded by Craters via Pulsed Laser Deposition

This research deals with the deposition and characterization of ZnO nanostructures by the pulsed laser deposition technique at various ambient pressures of oxygen on a silicon substrate at 600 ∘C temperature. X-ray diffraction study shows that the ZnO nanostructure is single crystal in nature and has strong c-axis orientation. The field emission scanning electron microscopic image shows that the formation of pores within the craters is highly sensitive to the oxygen pressure. The atomic force microscopy image reveals the reduction in surface roughness of the nanostructure indicating transformation of pores to particles. A near-band-edge emission peak along with the defect peaks arising from oxygen defects and zinc interstitials are observed in the photoluminescence spectra. The defect related emission due to O 2 vacancy decreases with increasing O 2 pressure due to more incorporation of O 2 during pulsed laser deposition.