Raj K. Thareja

Photoluminescence and ultraviolet laser emission from nanocrystalline ZnO thin films

We report on photoluminescence and ultraviolet laser emission from ZnO pellets and thin films of ZnO. Laser emission from disordered polycrystalline thin films and pellets was observed in all directions. ZnO films were deposited on glass substrate at room temperature in various ambient gas pressures of oxygen using the pulsed laser deposition technique. The dependence of laser emission on the size of nanocrystallites observed at different pressures of ambient gas is discussed. Photoluminescence spectra depend on the stoichiometry and the microstructure of the film.

Synthesis and characterization of ZnO thin films for UV laser

We report on deposition of zinc oxide thin films on glass substrate at room temperature in ambient oxygen pressure ranging from 10 mTorr to 1 Torr by pulsed laser deposition. As grown polycrystalline thin films were optically pumped to investigate the dependence of laser action and photoluminescence (PL) on stoichiometry and microstrucure of the films. The intensity of the laser emission increases with thickness of the film and depends on the size of nano-crystallites. Films deposited at pressures lower than 300 mTorr showed better morphology, stoichiometry and adhesion quality. X-ray diffraction, atomic force microscopy, Rutherford back scattering and PL techniques were used to characterize as grown films.

Laser‐induced breakdown studies of laboratory air at 0.266, 0.355, 0.532, and 1.06 μm

We report laser‐induced breakdown studies of laboratory air using 0.266, 0.355, 0.532, and 1.06 μm radiation for focal spot varying from 30 to 100 μm. The breakdown intensities were measured for two pulse widths for all four wavelengths. We observe a t−0.5p dependence of pulse width on threshold field at 0.532, 0.355, and 0.266 μm while for 1.06 μm the pulse width dependence of t−0.35p is observed.

Synthesis of nanoparticles in laser ablation of aluminum in liquid

We report the synthesis of aluminum nanoparticles using pulsed laser ablation in water confined plasma. Nanoparticles have spherical shape and size distribution depends on laser fluence. Strong blue photoluminescence peaks at 405 nm (3.06 eV) and 430 nm (2.89 eV) due to oxygen deficient defects (F, F+, and F++ centers) is reported with different UV excitations. A comparative study of plasma in deionized water and air ambient reveals enhanced line broadening and higher electron density in water confined plasma compared to that in air, in agreement with radiative recombination model. The temporal dependence of spectral radiant energy density of plasma is also discussed.

Synthesis of p-type ZnO thin films using co-doping techniques based on KrF excimer laser deposition

Preparation of N-doped ZnO thin films was attempted using various co-doping methods. A ZnO:Ga (Ga 2 O 3 of 5 wt.%) target was ablated in NO gas by pulsed laser deposition (PLD). In addition, a nitrogen ion gun and an ECR nitrogen plasma source were used as post-N-doping treatment of undoped ZnO films. Optical emission from elemental Zn I, Ga I and O I, as well as from N 2 molecules, was identified in the plasma plume. The structural, optical and electrical properties of these synthesized films were investigated. All films show n-type conduction, with resistivity in the range 10 -3 -10 -2 Ω cm and carrier density from 10 17 to 10 20 cm -3 .

Laser-produced carbon plasma expanding in vacuum, low pressure ambient gas and nonuniform magnetic field

Laser-produced carbon plasma expanding in vacuum, a low pressure ambient gas (5 mTorr and 100 mTorr) and a nonuniform magnetic field is studied using fast photography. The temporal behavior of various plasma parameters, like velocity, electron density, electron temperature and intensity, is reported. In 5 mTorr air, the plasma breaks into slow and fast components. In the magnetic field, oscillations are observed in the plasma parameters.

Dynamics of laser-ablated carbon plasma: formation of C2 and CN.

We report time-resolved imaging of a laser-ablated carbon plasma plume to investigate the expansion dynamics of C(2) and CN in an ambient atmosphere of nitrogen gas at various pressures. An attempt is made to locate C(2) and CN species in the carbon plasma plume and correlate them with the results of spectroscopic observations. The ablated C(2) and CN species decelerate due to collisions with nitrogen gas and are localized in the slower part (approximately 300 ns) of the expanding plume. Further expansion (<700 ns) of the plasma reveals the concentration of C(2) species on the periphery of the plume, whereas CN dominates at the core of the plume. However, at times greater than 700 ns, the collisions and recombination processes dominate in the plume and C(2) expands slower than CN. The plume dynamics is studied in terms of shock-wave and drag models.

DIAGNOSTICS OF LASER ABLATED PLASMAS USING FAST PHOTOGRAPHY

We report on dynamics of the laser ablated plumes in an ambient atmosphere using fast photography. The expanding plasma is studied at different delay times with respect to the ablating pulse. Dependence of plasma parameters such as velocity, temperature, density, and pressure on time and ambient atmosphere is presented. The images of the expanding plume are used to estimate the size of the particles in the plume.

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.

DYNAMICS OF LASER PRODUCED CARBON PLASMA EXPANDING IN A NONUNIFORM MAGNETIC FIELD

The laser produced carbon plasma expanding in a nonuniform magnetic field in vacuum (P<10−3 Torr) has been studied by emission spectroscopy and fast photography using intensified charged coupled device. The laser ablated plume is observed to break into two lobes along the vertical direction. The temporal profiles of C I transition at 399.7 nm, C II transition at 426.7 nm, C III transition at 465.0 nm, and C IV transition at 580.1 nm show three peaks (fast, intermediate, and slow) for each of these two lobes. The temporal profile of the species at the outermost boundary of the plume shows oscillations.