The dynamics of melting and ablation of thin films using nanosecond laser pulses is of significant importance when considering fundamentalinterests and technological aspects. Such aspects include the cleaning of optical components and mirrors from unwantedcoatings. The present study investigates the dynamics of melting and ablation using time-resolved probe measurements. The focusis on the front- and rear-side ablations of aluminium thin films with nanosecond pulses. The study revealed fundamentally differentablation behavior for the two geometries, and in order to enhance understanding, a comprehensive experimental investigation wasconducted with variation in background pressure, ablation and probe geometry, ablation fluence and film thickness. The results ofthis study are of significant interest towards fundamental aspects, as well as for the broader scientific community engaged in thefield of laser-based cleaning of windows and mirrors.

We present a systematic spectroscopic investigation of laser-produced aluminum plasma to address inconsistencies in Stark broadening parameters and establish a self-consistent reference data sets for electron density diagnostics. Optical line emissions of Al II and Al III in the visible wavelength range were recorded from plasmas having different electron densities and temperatures, however, with the same experimental configuration, only by varying the background pressure, spatial position, and delay time. The Stark width parameter of Al III lines, which shows consistency across different earlier studies, is used for standardizing the Al II transition from the highest energy level, which is abundant in the emission spectra. This reference spectrum is then used to estimate the Stark parameters of other Al II transitions to obtain a self-consistent database for Al II transitions. This approach significantly reduces the uncertainty in the estimated plasma electron density using Stark parameters of multiple emission lines. We also report the spatial and temporal evolution of plasma density and Stark shift as well as asymmetry in spectral lines. This work addresses the uncertainty in Stark parameters of Al II transitions in the visible range through a unified approach in estimating these parameters simultaneously.

The present study investigates the observation of the hydrogen Balmer series emission in a capacitively coupled radio frequency (CCRF) helium plasma using optical emission spectroscopy (OES). Experimental evidence suggests that helium metastables transfer energy to trace amounts of water vapor present in the vacuum chamber. The plausible mechanism is the energy exchange between metastable helium and water molecules via Penning ionization. This occurs because the energy of helium metastables exceeds the dissociation energy of a water molecule and the ionization energy of hydrogen. The hydrogen ions that are formed subsequently combine with electrons in the plasma, resulting in the emission of the Balmer series during the subsequent relaxation process. The intensity observed in the Balmer lines indicates unconventional ratios between them, demonstrating a strong dependence on neutral pressure and RF power. This phenomenon is indicative of the role of the energetic tail population. The observation reveals a direct correlation between the intensity of Balmer line emission and the electron density. In order to comprehend the dependence of helium metastables on the Balmer series intensity, the collisional radiative (CR) model is employed to estimate the metastable population enhancement under the present experimental conditions. The observed enhancement can be attributed to the increase in metastable densities and the radiative recombination processes. The results of this study hold significant potential for applications, in addition to providing a fundamental understanding of energy transfer between the metastables of helium and water vapor.

In this work, we conducted experiments using a nanosecond Nd:YAG laser to generate aluminium plasma under ambient argon pressure of 10 mbar. The spatial and temporal evolution of the line emission of six Al II lines in the visible region is recorded with a high resolution spectrograph. To accurately estimate the electron temperature and density of the plasma, a comprehensive collisional radiative (CR) model for Al plasma is developed, using the Al II emissions by incorporating extensive atomic and collisional data to enhance the accuracy of interpretation of diagnostics data. Using the multiconfiguration Dirac-Hartree-Fock method within the GRASP2018 framework, we computed radiative decay rates among 44 fine-structure levels of Al II. Electron impact excitation (EIE) cross-sections were calculated for incident electron energies up to 600 eV using the relativistic distorted wave theory, covering transitions from the lower to all upper states among 44 fine-structure levels considered for Al II. Our CR model includes key processes such as EIE, de-excitation, ionization, three-body recombination, and radiative decay. The plasma diagnostics is done by coupling the CR model with the laser-induced breakdown spectroscopy measurements of the six emission lines and the electron temperature and electron density are extracted. A reasonable agreement between the CR model and experimental intensities is found, which confirms to the reliability of our model. Also the obtained electron temperatures from CR model at different delay times are compared with the temperature estimated using the Boltzmann plot method. This study emphasizes the use of comprehensive CR modeling for accurately estimating plasma parameters, demonstrating its effectiveness and appropriateness in plasma diagnostics.

The present study investigates the observation and enhancement in the intensity of the hydrogen Balmer series emission in a He CCRF plasma using optical emission spectroscopy (OES). In addition to the characteristic line emission of He atoms, the Balmer series of hydrogen and the molecular emission of N2 are also observed in the He discharge. These emissions were primarily attributed to the presence of water vapor in the chamber. In order to confirm the role of He, the study is also performed using air and Ar where no such Balmer series emissions is seen. Experimental evidence suggests that He metastables transfer energy to trace amount of water content present in the vacuum chamber. The results point towards the hypothesis that energy exchange between metastable He and water molecules could be the underlying mechanism. Since the energy of He metastables exceeds the ionization energy of H or H2O molecule, Penning ionization is expected to occur upon their interaction. The H ions formed as a result, consequently recombine with electrons in the plasma, emitting the Balmer series. Furthermore, the emission intensity of the Balmer series of H depends on the electron density of the He plasma. Experiments also show significant deviations in the intensity ratios of the Balmer series from conventional discharges, indicating difference in the underlying population mechanism. A Collisional Radiative (CR) model for measured plasma parameters was used to estimate the metastable population density to understand the mechanism behind the enhancement of the emission intensity. The increase in the metastable densities as well as the radiative recombination cross section appear to be responsible for the observed enhancement. We believe these results will be significant in terms of applications, in addition to providing a fundamental understanding of energy transfer between metastables of He and H2O.

In this work, we report an innovative pump-probe based experimental setup, to study the melting, subsequent evaporation, plasma formation and re-deposition in a thin film coated on a glass substrate under different ambient conditions and laser fluences. The ambient conditions restrict the expansion of the plasma plume. At high ambient pressure, plume expansion stops closer to the substrate and gets re-deposited at the site of the ablation. This helps in the identification of multiple processes and their temporal evolutions during the melting, expansion and re-deposition stages. The ambient conditions affect the plasma plume formed upon ablation, thus modulating the transmission of probe laser pulses, which provides information about the plume dynamics. Further, the study offers valuable insights into the laser-based ablation of thin film coatings, which will have implications in situ cleaning of view ports on large experimental facilities such as tokamaks and other systems e.g. coating units, pulsed laser deposition, Laser induced forward transfer, Laser surface structuring, etc.

Microwave interferometer is one of the indispensable diagnostics tools for measuring electron density in a Tokamak. The interferometer system requires a stable microwave frequency generator to probe the plasma. Short term, random fluctuations in the phase of the microwave source like Voltage Controlled Oscillator (VCO) can significantly affect the frequency stability of the interferometer. These phase noise can degrade the performance of the interferometer system and cause errors in electron density measurements. Operating the VCO in a phase locked loop (PLL) configuration can significantly improve the frequency stability of the interferometer which can reduce the measurement errors of plasma density. The implementation of PLL configuration in interferometer generates a stable intermediate frequency ($I F$), to ensure that the changes in the $I F$ frequency during the plasma discharges are only due to the plasma density variations.

We report nanosecond laser texturing of crystalline silicon with subsequent superhydrophilic and heat resistive properties. The samples are laser processed in ambient conditions in normal atmospheric pressure. Micro structures with micro/nano channels are evolved after processing which contributes to the wettability change of the silicon surface. The samples are characterized by XRD, FESEM, EDX and Raman spectroscopy. The contact angles are measured using an indigenously built contact angle goniometer. The samples show superhydrophilic nature after laser processing. Interestingly, the samples retain their inherent properties and super-wettability even after annealing at 200 degrees C for 8 hours. Such architectures could find applications in devices working in high temperature environments.

In this review we attempt to provide a brief account of laser induced breakdown spectroscopy (LIBS) methodology and technological developments. We also summarise various methods adopted for exploiting LIBS. Besides, a brief overview of combination of LIBS in conjunction with other methods is also given.

Laser-induced breakdown spectroscopy (LIBS) is a well-established technique widely used in fundamental research and diverse practical fields. Polarization-resolved LIBS, a variant of this technique, aims to improve the sensitivity, which is a critical aspect in numerous scientific domains. In our recent work we demonstrated that the degree of polarization (DOP) in the emission depends on the spatial location and time in a nano second laser generated aluminium plasma1. Present study investigates the effect of polarized emission on the estimation of plasma parameters. The plasma parameters are estimated using the conventional spectroscopic methods such as Boltzmann plot and line intensity ratio for the estimation of electron temperature and Stark broadening for estimating the electron density. The estimated plasma temperature using Boltzmann plot method shows large errors in electron temperature for the locations where DOP is higher. However, the electron density estimated using the Stark width does not show such variation. The observed ambiguity in temperature estimation using the Boltzmann plot method appears to be a consequence of deviation from expected Maxwell Boltzmann distribution of population of the involved energy levels. These findings highlight the need of assessing the DOP of the plasma before selecting the polarization for PRLIBS or temperature estimation using Boltzmann plots in elemental analysis.

In this paper, we report experimental investigation to improve the shape of a supersonic nozzle for rarefied flows for high axial density at extended distances from the nozzle. The reported work is significant for molecular jet/beam applications that require high center-line density and narrow jet profile. We investigate a parabolic nozzle whose profile is generated using the virtual source model of free expansion and compare its performance with a set of conical nozzles having different cone angles using simulations as well as experiments. Axial density and lateral spread of the jets are measured using a pitot tube assembly. The accuracy and operational limit of the pitot tube for rarefied flow is quantified by using established mathematical and empirical models for a sonic nozzle. The study demonstrates that the performance of the parabolic nozzle is comparable or slightly better as compared to conical counterparts. Moreover, the parabolic profile can be used for optimizing the opening angle for conical nozzles of various lengths.

In this work, we report evolution of atomic clusters in a highly under-expanded supersonic jet of Argon. A high resolution and sensitive Rayleigh scattering based experimental set-up is designed to overcome the limitations encountered in conventional set-ups. Further, the measurement range could be extended from a few nozzle diameters to 50 nozzle diameters. Simultaneously, we had been able to generate 2D profiles of the distribution of clusters inside the jet. This paves the way to track the growth of clusters along the flow direction experimentally, which until now was limited to few nozzle diameters. The results show that spatial distribution of clusters inside the supersonic core deviates considerably from the prediction of the free expansion model. We exploit this to estimate cluster growth along the expansion direction. Further, it is observed that the growth of the clusters gets saturated after a certain distance from the nozzle. At the jet boundary, we see substantial cluster strengthening immediately upstream of barrel shock while the normal shock exhibits disintegration of clusters. These observations are noticed for the first time, which, we believe will further the understanding of cluster dynamics in a supersonic jet.

Intense pulsed supersonic molecular beams are used in many applications such as tokamak fueling, edge plasma diagnostics, ion beam profile monitors, laser cluster experiments, chemical kinetics, etc. Measurement of absolute density is required to optimize beam sources used in these experiments. Absolute density measurement of a continuous molecular beam is challenging due to its small size and rarefied flow, which makes it even more difficult for a pulsed molecular beam due to its transient nature. In this work, we demonstrate a novel probe to measure the spatiotemporal evolution of the absolute number density of a pulsed supersonic molecular beam. The probe is named the Shielded Ionization Discharge probe. It measures density using localized discharge within the molecular beam created by the thermionic emission of electrons from a hot filament. We describe the design, calibration, and characterization of the developed probe. The performance of the probe is demonstrated by measuring the spatial and temporal profiles of a pulsed supersonic molecular beam of 1.5 ms duration.

Polarized emission carries captivating information and can help understand various elementary processes involving collisions within the plasma as well as in radiative transitions. In this work, we investigate the spatio-temporal dependence of the emission anisotropy of a nanosecond laser produced aluminium plasma at 100 mbar background pressure. We observe that the anisotropy of the emission spectra exhibits interesting spatio-temporal characteristics which in turn depend on the charge state of the emitting species. The degree of polarization (DOP) is found to reverse its sign along the plume propagation direction. Observed behaviour in DOP appears to be due to the contribution from various involved atomic processes. However, closer to the sample the contribution from the self-generated magnetic field predominantly affect the polarization. On the other hand, the effect of the self generated magnetic field on the observed polarized emission is insignificant as the plume propagates away from the sample. This is of particular interest in polarization resolved laser induced breakdown spectroscopy as spatio-temporal profile of the degree of polarization has to be properly taken into account prior to the spectral analysis.

In this work, dynamics of multi-charged ions emitted from an aluminum plasma produced by Q switched Nd: Yag laser is studied using time of flight (TOF) measurements from Langmuir probe and spectroscopy under Ar ambient of 0.02 mbar. The temporal evolution of multi-charged ions, background neutrals, and ions is systematically studied for varying laser intensities. The temporal evolution shows all species have double-peak structure for all laser intensities considered in the study. The fast peak is sharp whereas the slow peak is broadly similar to that observed in previous studies. Moreover, higher charged ions have higher velocity, indicating acceleration from the transient electric field produced at the very initial temporal stages of expansion. Interestingly, the fast peak gets delayed, whereas the slow peak advances in time with increased laser intensity, which has not been reported in earlier studies. The observations point toward the possible role of ambipolar electric fields in the unexpected observed behavior of the TOF profiles.

We present Raman analysis of nanosecond laser textured silicon. The samples have also been characterized by field emission scanning electron microscopy (FESEM) and x ray diffraction. Contact angles (CAs) are measured to trace the hydrophilic nature. Characterization of the textured samples in argon and air shows that cleavage cracks are developed during texturing. CA measurements reveal the superhydrophilic nature of textured samples obtained in the presence of ambient oxygen and argon. In vacuum, however, the hydrophilicity is decreased. Micro-Raman analysis indicates the formation of nano-sized cleavage cracks that impart stable superhydrophilic properties to textured silicon is supported from FESEM images also. On the other hand, in vacuum textured silicon, evidence of such cracks is not noticed, which is also supported by Raman analysis. Further, the hydrophilicity is decreased. A definitive trend appears to exist between Raman signatures and hydrophilicity. We believe that the study will further the understanding of the mechanistic aspect in designing textured silicon with a high degree of self-cleaning capability.

Plasma diagnostics in a tokamak is an important aspect for improving the plasma parameters for nuclear fusion. Supersonic Helium Beam Injection (SHBI) diagnostic is used to measure the density and temperature evolution of edge plasma using CR-model of neutral helium emission lines. SHBI system is being developed for tokamak SST-1. It requires directional intense molecular beam for good spatial and temporal resolutions. Typically, this involves the generation of highly under-expanded supersonic helium jet and extracting the central flow using a skimmer.

In this paper, we report experimental investigation to improve the shape of a supersonic nozzle for rarefied flows to generate high axial density at extended distances from the nozzle. The reported work is significant for molecular jet/beam applications that require high center-line density and narrow jet profile. We investigate a parabolic nozzle whose profile is generated using the virtual source model of free expansion and compare its performance with a set of conical nozzles having different cone angles using simulations as well as experiments. All nozzles are made by additive manufacturing using ABS and performance is found to be satisfactory. Axial density and lateral spread of the jets are measured using a pitot tube assembly. The accuracy and operational limit of the pitot tube for rarefied flow is quantified by using established mathematical and empirical models for a sonic nozzle. The study demonstrates that the performance of the parabolic nozzle is comparable or slightly better as compared to conical counterparts. Moreover, the parabolic profile can be used for optimizing the opening angle for conical nozzles of various lengths.

We report long time (250 μs) emission from Al I in laser produced plasma in argon ambient and its dependence on laser power density, background pressure and distance from the target surface. Role of argon metastables as energy reservoirs is discussed.

Cu2O and CuO films are deposited on a glass substrate by pulsed laser deposition (PLD) technique under different ambient environments and pressures. X-ray Diffraction and micro-Raman spectroscopy revealed that the films, synthesized at 2 Pa of N-2 ambient have Cu2O phase whereas the films synthesized at 2 Pa of O-2 ambient are of CuO. Field emission scanning electron microscopy shows uniform surface morphology of the samples with a film thickness of similar to 200 nm. Optical band gaps of the samples are measured from optical absorption spectra in the UV and visible region. The direct band gaps for the deposited films are estimated to be 2.45 eV and 2.25 eV corresponding to Cu2O and CuO phases respectively. Using these films, photo-catalysis of methylene blue (MB) dye was performed under direct sunlight. Remarkable efficiency having reaction rate constant of 0.020 min(-1) with good recycling ability is exhibited by the samples.