Shazia Bashir

Effect of laser irradiance on the surface morphology and laser induced plasma parameters of zinc

The effect of laser-irradiance on the surface morphology and laser induced breakdown spectroscopy of zinc has been investigated by employing Nd:YAG laser (wavelength λ= 1064 nm, pulse duration t∼ 10 ns, and repetition rate= 10 Hz) under ambient environment of argon at a pressure of 20 Torr. For this purpose, zinc targets were exposed to various laser irradiances ranging from 13 GW/cm to 100 GW/cm. Scanning electron microscope analysis has been performed to analyze the surface modification of irradiated zinc targets. Scanning electron microscope analysis revealed the formation of various kinds of structures such as ripples, cones, cavities, and wave like ridges at the center and peripheral regions of ablated zinc. In the central ablated region with increasing laser irradiance, the growth of distinct and well defined ripples is observed. Further increase in irradiance makes the appearance of these ripples diffusive and narrow. In order to correlate the plasma parameters with the surface modification, laser induced breakdown spectroscopy analysis has also been performed. The electron temperature and number density of zinc plasma have been evaluated at various laser irradiances. For both plasma parameters, an increasing trend up to a certain value of laser irradiance is observed which is due to enhanced energy deposition. Afterword a decreasing trend is achieved which is attributed to the shielding effect. With further increase in irradiance a saturation stage comes and almost no change in plasma parameters is observed. This saturation is explainable on the basis of the formation of a self-regulating regime near the target surface. A strong correlation between surface modification and plasma parameters is established.

Laser-induced breakdown spectroscopy of tantalum plasma

Laser Induced Breakdown spectroscopy (LIBS) of Tantalum (Ta) plasma has been investigated. For this purpose Q-switched Nd: YAG laser pulses (λ ∼ 1064u2009nm, τ ∼ 10 ns) of maximum pulse energy of 100 mJ have been employed as an ablation source. Ta targets were exposed under the ambient environment of various gases of Ar, mixture (CO2: N2: He), O2, N2, and He under various filling pressure. The emission spectrum of Ta is observed by using LIBS spectrometer. The emission intensity, excitation temperature, and electron number density of Ta plasma have been evaluated as a function of pressure for various gases. Our experimental results reveal that the optical emission intensity, the electron temperature and density are strongly dependent upon the nature and pressure of ambient environment. The SEM analysis of the ablated Ta target has also been carried out to explore the effect of ambient environment on the laser induced grown structures. The growth of grain like structures in case of molecular gases and cone-for...

Mechanical behaviour of excimer laser irradiated polycrystalline zirconium

The effects of laser irradiation on the mechanical response of polycrystalline zirconium (Zr) have been investigated. Zr samples were irradiated with an excimer (KrF) laser (λxa0≈xa0248xa0nm, τxa0≈xa018xa0ns and repetition rate ≈30xa0Hz). The irradiation was performed in the ambient environment of a gas mixture containing (CO2:N2:He) under a filling pressure of 20xa0Torr by varying laser fluences ranging from 3.8 to 5.1xa0Jxa0cm−2. The surface and structural modification of the irradiated targets were investigated using a scanning electron microscope (SEM) and x-ray diffractometer (XRD). In order to explore the mechanical properties of the irradiated Zr, the tensile testing and Vickers microhardness testing techniques were employed. SEM analysis reveals the grain growth on the irradiated Zr surfaces; however, for increasing fluence up to 4.7xa0Jxa0cm−2, the appearance of the grains becomes more distinct with an increase in their number density and decrease in size. For the maximum fluence of 5.1xa0Jxa0cm−2, the grains completely vanish and the surface becomes diffusive. XRD analysis reveals the appearance of new phases of ZrN and ZrO2. The variation in the peak intensity is observed to be anomalous, whereas the decreasing trend in the crystallite size and residual stresses is observed with increasing fluence. The microhardness analysis reveals the increasing trend in surface hardness with increasing fluence. The tensile testing demonstrates the anomalous behaviour of the yield stress and ultimate tensile strength with increasing fluence.

EFFECTS OF SUBSTRATE TEMPERATURE ON STRUCTURAL, OPTICAL AND SURFACE MORPHOLOGICAL PROPERTIES OF PULSED LASER DEPOSITED ZnO THIN FILMS

The effect of substrate temperature on the structural, optical and morphological properties of ZnO thin films has been investigated. ZnO thin films were deposited on quartz substrate for various temperatures ranging from room temperature to 250°C by pulsed laser deposition (PLD) technique. Nd:YAG laser (532 nm, 100 mJ, 6 ns, 10 Hz) with corresponding fluence of 6 J/cm2 was employed for the ablation of ZnO target. Characterization of the thin films was carried out using X-ray diffraction (XRD), high resolution UV-visible spectrometer, atomic force microscope (AFM) and scanning electron microscope (SEM). From XRD analysis, the amorphous behaviors of films at room temperature and crystalline behavior along the preferred orientation of (002) is exhibited for higher substrate temperature. The transmittances of grown films increase with the increasing substrate temperature. The evaluated values of bandgap energies increase with increasing substrate temperature up to the range of 150°C and then monotonically decrease with the further increase in temperature. AFM and SEM analysis illustrates that the density and height of grains for deposited films increase significantly with increasing substrate temperature.

Surface topography of ultrashort laser-irradiated CaF2

A single-crystal CaF2 (111) was irradiated with single and multiple laser (Ti:sapphire, 800 nm, 25 fs) shots at fluences ranging from 0.25 to 1.5 J cm−2. In this fluence regime, a single laser pulse usually leads to typical bump-like features ranging from 200 nm to 1.5 μm in diameter and 10–50 nm in height. These bumps are related to compressive stresses due to a pressure build-up induced by fast laser heating and their subsequent relaxation. When CaF2 is irradiated with successive (in our case 20) shots at a laser fluence of 1.5 J cm−2, nanocavities at the top of the microbumps are observed. The formation of these nanocavities is regarded as an explosion and is attributed to the explosive expansion generated by shock waves due to laser-induced plasma after the nonlinear absorption of the laser energy by the material. Such kinds of surface structures at the nanometre scale could be attractive for nanolithography.

Spectroscopic and morphological study of laser ablated Titanium

The laser-induced breakdown spectroscopy (LIBS) and surface morphology of Titanium (Ti) plasma as a function of laser irradiance have been investigated under ambient environment of argon at fixed pressure of 50 Torr. Ablation was performed by employing Q-switched Nd:YAG laser pulses (λ ≈ 1064 nm, τ ≈ 10 ns, repetition rate ≈ 10 Hz). Ti targets were exposed to various laser intensities ranging from 6 to 50 GW/cm2. LIBS analysis has been employed for the investigation of plasma parameters. Scanning Electron Microscope (SEM) analysis was employed for investigation of surface morphology. Ablation depth was measured by optical microscopy technique. It was observed that both plasma parameters, i.e., excitation temperature and electron density have been significantly influenced by laser irradiance. It is observed that with increasing laser irradiance up to 13 GW/cm2, the electron temperature decreases whereas number density significantly increases and attains its maxima. Afterwards by increasing irradiance electron temperature increases, attains its maxima and a decrease in electron number density is observed at irradiance of 19 GW/cm2. Further increase in irradiance causes saturation with insignificant changes in both electron temperature and electron number density. This saturation in both excitation temperature and electron number density is explainable on the basis of self-sustaining regime. SEM micrographs reveal the ripple and coneformation at the boundaries of ablated region of Ti. The height of cones as well as the ablation depth is maximum at irradiance of 13 GW/cm2 whereas electron number density is also maximum. The maximum electron number density is considered to be responsible for maximum ablation as well as mass removal. A strong correlation between plasma parameters and surface morphology is established.

Laser irradiation effects on the surface, structural and mechanical properties of Al–Cu alloy 2024

Laser irradiation effects on surface, structural and mechanical properties of Al–Cu–Mg alloy (Al–Cu alloy 2024) have been investigated. The specimens were irradiated for various fluences ranging from 3.8 to 5.5 J/cm2 using an Excimer (KrF) laser (248 nm, 18 ns, 30 Hz) under vacuum environment. The surface and structural modifications of the irradiated targets have been investigated by scanning electron microscope (SEM) and X-ray diffractometer (XRD), respectively. SEM analysis reveals the formation of micro-sized craters along the growth of periodic surface structures (ripples) at their peripheries. The size of the craters initially increases and then decreases by increasing the laser fluence. XRD analysis shows an anomalous trend in the peak intensity and crystallite size of the specimen irradiated for various fluences. A universal tensile testing machine and Vickers microhardness tester were employed in order to investigate the mechanical properties of the irradiated targets. The changes in yield strength, ultimate tensile strength and microhardness were found to be anomalous with increasing laser fluences. The changes in the surface and structural properties of Al–Cu alloy 2024 after laser irradiation have been associated with the changes in mechanical properties.

Identification of non-thermal and thermal processes in femtosecond laser-ablated aluminum

Non-thermal and thermal processes due to femtosecond laser ablation of aluminum (Al) at low, moderate, and high-fluence regimes are identified by Atomic Force Microscope (AFM) surface topography investigations. For this purpose, surface modifications of Al by employing 25 fs Ti: sapphire laser pulses at the central wavelength of 800 nm have been performed to explore different nano- and microscale features such as hillocks, bumps, pores, and craters. The mechanism for the formation of these diverse kinds of structures is discussed in the scenario of three ablation regimes. Ultrafast electronic and non-thermal processes are dominant in the lower fluence regime, whereas slow thermal processes are dominant at the higher fluence regime. Therefore, by starting from the ablation threshold three different fluence regimes have been chosen: a lower fluence regime (0.06–0.5 J cm−2 single-shot irradiation under ultrahigh vacuum condition and 0.25–2.5 J cm−2 single-shot irradiation in ambient condition), a moderate-fluence regime (0.25–1.5 J cm−2 multiple-shot irradiation), and a high-fluence regime 2.5–3.5 J cm−2 multiple-shot irradiation. For the lower fluence (gentle ablation) regime, around the ablation threshold, the unique appearance of individual, localized Nano hillocks typically a few nanometers in height and less than 100 nm in diameter are identified. These Nano hillock-like features can be regarded as a nonthermal, electronically induced phase transition process due to localized energy deposition as a result of Coulomb explosion or field ion emission by surface optical rectification. At a moderate-fluence regime, slightly higher than ablation threshold multiple-pulse irradiation produces bump-formation and is attributed to ultrafast melting (plasma formation). The high-fluence regime results in greater rates of material removal with highly disturbed and chaotic surface of Al with an appearance of larger protrusions at laser fluence well above the ablation threshold. These nonsymmetrical shapes due to inhomogeneous nucleation, cluster formation, and resolidification of a metallic surface after melting are attributable to slow thermal processes (ps time scale).

Optical emission spectroscopy of magnetically confined laser induced vanadium pentoxide (V2O5) plasma

Optical emission spectra of a laser induced plasma of vanadium pentoxide (V2O5) using a Nd:YAG laser (1064u2009nm, 10u2009ns) in the presence and absence of the magnetic field of 0.45u2009T have been investigated. The effect of the magnetic field (B) on the V2O5 plasma at various laser irradiances ranging from 0.64 GW cm−2 to 2.56 GW cm−2 is investigated while keeping the pressure of environmental gases of Ar and Ne constant at 100u2009Torr. The magnetic field effect on plasma parameters of V2O5 is also explored at different delay times ranging from 0 μs to 10 μs for both environmental gases of Ar and Ne at the laser irradiance of 1.28 GW cm−2. It is revealed that both the emission intensity and electron temperature of the vanadium pentoxide plasma initially increase with increasing irradiance due to the enhanced energy deposition and mass ablation rate. After achieving a certain maximum, both exhibit a decreasing trend or saturation which is attributable to the plasma shielding effect. However, the electron density shows a decreasing trend with increasing laser irradiance. This trend remains the same for both cases, i.e., in the presence and in the absence of magnetic field and for both background gases of Ar and Ne. However, it is revealed that both the electron temperature and electron density of the V2O5 plasma are significantly enhanced in the presence of the magnetic field for both environments at all laser irradiances and delay times, and more pronounced effects are observed at higher irradiances. The enhancement in plasma parameters is attributed to the confinement as well as Joule heating effects caused by magnetic field employment. The confinement of the plasma is also confirmed by the analytically calculated value of magnetic pressure β, which is smaller than plasma pressure at all irradiances and delay times, and therefore confirms the validity of magnetic confinement of the V2O5 plasma.Optical emission spectra of a laser induced plasma of vanadium pentoxide (V2O5) using a Nd:YAG laser (1064u2009nm, 10u2009ns) in the presence and absence of the magnetic field of 0.45u2009T have been investigated. The effect of the magnetic field (B) on the V2O5 plasma at various laser irradiances ranging from 0.64 GW cm−2 to 2.56 GW cm−2 is investigated while keeping the pressure of environmental gases of Ar and Ne constant at 100u2009Torr. The magnetic field effect on plasma parameters of V2O5 is also explored at different delay times ranging from 0 μs to 10 μs for both environmental gases of Ar and Ne at the laser irradiance of 1.28 GW cm−2. It is revealed that both the emission intensity and electron temperature of the vanadium pentoxide plasma initially increase with increasing irradiance due to the enhanced energy deposition and mass ablation rate. After achieving a certain maximum, both exhibit a decreasing trend or saturation which is attributable to the plasma shielding effect. However, the electron density show...

Effect of excimer laser fluence on the surface structuring of Ti under vacuum condition

The effect of variation of excimer laser fluences on the surface structuring of titanium (Ti) targets has been investigated. The KrF excimer laser (λu2009=u2009248u2009nm, tu2009=u200920u2009ns, repetition rate 20u2009Hz) has been employed for this purpose. The targets were irradiated for various laser fluences ranging from 0.86u2009J cm−2 to 1.27u2009J cm−2 under vacuum condition. Various diagnostic techniques like scanning electron microscope (SEM), atomic force microscope (AFM), and X-ray diffraction (XRD) have been utilized to investigate the surface topography and structural changes of laser ablated targets. SEM analysis reveals the formation of laser-induced periodic surface structures (LIPSS) at the central ablated region. The periodicity of LIPSS increases from 5u2009μm to 88u2009μm with the increase of fluence from 0.86u2009J cm−2 to 1.27u2009J cm−2. The formation of grains is observed at the peripheral ablated region for all laser fluences. Reduction in grain size from 7.7u2009μm to 3.8u2009μm is observed with increasing fluence from minimum to maximum v...