Surface Morphology In Laser Ablation Of Tin Biology Essay

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Abstract- Plume imaging and surface morphology was carried out to analyze the deformations and intensities in different irradiated zones of the ablated metal surface. A Q switched 10mJ, 1.064 ?m, Nd: YAG laser was used to irradiate through air to a Tin target which was 99.9% pure to generate ions from laser induced plasma. Laser pulses ranging from 1 to 50 were shot on the target and images of the laser plume were captured with the help of digital camera. These were later analyzed using Image-J software. The samples after laser irradiation were analyzed using optical microscopy. This gave the depth of the crater formed and focused images of different zones showing strong modifications in surface producing roughness, ripples and different structures on the metal surfaces. Damage on the sample was found to be increasing and was predominant at the center. Different intensity zones within the plume were observed through out the experiment. Inhomogeneous distribution of energy on tin surface is responsible for the growth of ripples. Research is recommended for next level to analyze the behavior of perfect natural micro grating on the surface.

Key words: Surface Morphology, Laser ablation, Nd: YAG, Coherent structures, Knudsen layer.

1.1 Introduction

The word LASER is an acronym for 'Light Amplification by Stimulated Emission of Radiation'. A laser is a device that amplifies and produces a highly directional, high intensity beam that most often has a very pure frequency and wavelength. It comes in sizes ranging from approximately one tenth the diameter of human hair to the size of a very large building, in powers ranging from 10-9 to 1020 W and in wavelengths ranging from the microwave to the soft X-ray spectral regions with corresponding frequencies from 1011 to 1017 Hz. Lasers have pulse energy as high as 104 J and pulse duration as short as 6'10-15 s [1]. They can easily drill holes in the most durable of materials and can weld detached retinas within the human eye. There is nothing magical about laser. When strongly focused high power laser interacts with the solid target, the plasma formation takes place. Laser induced plasmas are transient in nature and their properties depend upon laser parameters, target composition, ambient atmosphere and surface morphology of target material [2]. Laser induced plasma exploits many applications such as pulsed laser deposition, laser induced breakdown spectroscopy, ions generation, soft and hard X-ray emissions etc. In many applications like pulsed laser deposition of high quality thin films or nano-cluster formation, the studies of plasma plume dynamics and expansion are of extreme importance. When laser light intensity is high enough to induce significant material vaporization a, a dense vapor plume is formed. The vapor consists of clusters, molecules, atoms, ions, and electrons. Thermalization of species leaving the surface is mediated via collisions within a few mean free paths from the surface. This region is called the Knudsen layer [3]. In any case the species leaving the surface generate recoil pressure on the substrate. In the presence of molten surface layer and with focused laser beam irradiation, the recoil pressure expels the liquid in part. The ablated material may also generate a shock wave. The vapor plume will absorb and scatter the incident laser radiation.

1.2 Laser Ablation

Laser ablation is the process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam. It a means of depositing thin coatings, of a wide range of target materials, on a wide range of substrates, at room temperature. The process is often visualized as a sequence of steps, initiated by the laser radiation interacting with the solid target, absorption of energy and localized heating of the surface, and subsequent material evaporation. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser flux, the material is typically converted to plasma. The properties and composition of the resulting ablation plume may evolve, both as a result of collisions between particles in the plume and through plume-laser radiation interactions. Finally the plume impinges on the substrate to be coated; incident material may be accommodated, rebound back into the gas phase, or induce surface modification (via sputtering, compaction, sub-implantation, etc.). Usually, laser ablation refers to removing material with a pulsed laser, but it is possible to ablate material with a continuous wave laser beam if the laser intensity is high enough. The depth over which the laser energy is absorbed, and thus the amount of material removed by a single laser pulse depends on the material's optical properties and the laser wavelength. Laser pulses can vary over a very wide range of duration (milliseconds to femtoseconds) and fluxes, and can be precisely controlled. This makes laser ablation very valuable for both research and industrial applications. Furthermore, the laser-target interactions will be sensitively dependent both on the nature and condition of the target material, and on the laser pulse parameters (wavelength, intensity, fluences, pulse duration, etc.). Techniques available for detailed analysis of the resulting films (which are deposited on a range of substrate materials, at substrate temperatures ranging from room temperature to ~500'C) include laser Raman and IR transmission spectroscopy, SEM and TEM, XPS and, when appropriate, SIMS.

1.3 Pulse Laser Deposition (PLD)

Pulsed laser deposition is a physical vapor deposition process, carried out in a vacuum system that shares some process characteristics common with molecular beam epitaxy and some with sputter deposition. For sufficiently high laser energy density, each laser pulse vaporizes or ablates a small amount of the material creating a plasma plume. The ablated material is ejected from the target in a highly forward-directed plume. The ablation plume provides the material flux for film growth. For multi component inorganics, PLD has proven remarkably effective at yielding epitaxial films. In this case, ablation conditions are chosen such that the ablation plume consists primarily of atomic, diatomic, and other low-mass species. This is typically achieved by selecting an ultraviolet (UV) laser wavelength and nanosecond pulse width that is strongly absorbed by a small volume of the target material. Laser absorption by the ejected material creates plasma. Several features make PLD particularly attractive for complex material film growth. These include stoichiometric transfer of material from the target, generation of energetic species, hyperthermal reaction between the ablated cations and the background gas in the ablation plasma, and compatibility with background pressures ranging from ultrahigh vacuum (UHV) to 1 Torr. Multication films can be deposited with PLD using single, stoichiometric targets of the material of interest, or with multiple targets for each element. [4]. One of the most important and enabling characteristics in PLD is the ability to realize stoichiometric transfer of ablated material from multication targets for many materials. This arises from the non-equilibrium nature of the ablation process itself due to absorption of high laser energy density by a small volume of material. For low laser fluence or low absorption at the laser wavelength, the laser pulse would simply heat the target, with ejected flux due to thermal evaporation of target species. In this case, the evaporative flux from a multi component target would be determined by the vapor pressures of the constituents. Given the attractive characteristics of pulsed laser deposition in the synthesis of multi component thin-film materials, a number of applications are being actively pursued using this technique. In some cases, the application focuses on the synthesis of a thin-film material or structure. In other cases, the research has targeted the development of specific devices. [5].

1.4 Plasma and its Formation

Plasma is sometimes called the fourth state of matter (the first three being solid, liquid, and gas). It is unique in the way it interacts with itself, with electric and magnetic fields, and with its environment. Plasmas are conductive assemblies of charged particles, neutrals and fields that exhibit collective effects. Further, plasmas carry electrical currents and generate magnetic fields. It can be accelerated by electric and magnetic fields, which allows it to be controlled and applied. Plasmas are the most common form of matter, comprising more than 99% of the visible universe, and permeate the solar system, interstellar and intergalactic environments. Energy is needed to strip electrons from atoms to make plasma. The energy can be of various origins: thermal, electrical, or light (ultraviolet light or intense visible light from a laser). With insufficient sustaining power, plasmas recombine into neutral gas. When the laser beam interacts with the substrate then as the intensity (I) of laser beam is increased, an increasing fraction of atoms and molecules becomes ionized. When the vapor becomes substantially ionized, it is more appropriately described as plasma. The laser-produced plasmas are usually smaller in volume, normally produced in shorter time and have a rather different spatial symmetry.

1.5 Ablation Mechanism

An atom (ion) can be removed from a solid (ablated) if its total energy exceeds the binding energy (i.e., the energy of vaporization per particle), Etotal > Eb. The kinetic energy of a free particle should be Ekin = (Etotal - Eb) > 0 allowing the atom (ion) to leave the solid. This is a general case of non-equilibrium ablation by ultra-short laser pulses. Two non-equilibrium ablation mechanisms occur depending on the relation between pulse duration and relaxation times and on the absorbed energy electrostatic ablation and non-equilibrium ablation at T ? Eb. Ablation can also proceed under equilibrium conditions at T < Eb when the energy distribution is Maxwellian. This is the case of conventional thermal evaporation when only a few particles with energy ?Eb from the high-energy tail of the equilibrium distribution can be removed from a solid. In either case the removal of atoms requires the atom (ion) to acquire energy equal to the binding energy. [6].

1.6 Photophysical and photochemical processes

A laser-induced process as thermally activated if the thermalization of the excitation energy is fast compared to both the excitation rate and the initial processing step. The term photochemical is used if laser induced process proceeds mainly non-thermally. This means if the overall thermalization of the excitation energy is slow, i.e. if TT ? TR. This condition frequently holds for chemical reactions of excited molecules among themselves or with the substrate surface, for photoelectron transfer and subsequent chemisorptions of species on solid surfaces, for photochemical desorption of species from surfaces etc. If both thermal and non-thermal mechanisms are significant, we denote the process as photo-physical. Thermal and photochemical processes can be considered as limiting cases of photophysical processes [7]. We denote a process as photophysical if both thermal and non-thermal mechanism directly contributes to the overall processing rate. The degree of thermal and non-thermal contributions depends on the relative yield of the respective reaction channels. Particularly in connection with photo-decomposition processes, we frequently use, instead of photo-thermal and photochemical, the terms pyrolytic and photolytic, respectively.

1.7 Coherent and Non-Coherent Structures

Structures that develop on solid or liquid surfaces under the action of laser light can be classified into coherent structures and non-coherent structures. Coherent structures are directly related to the coherence, the wavelength, and the polarization of the laser light. For non-coherent structures such a direct relation to these laser parameters is absent. The feedback that causes coherent or non-coherent structure formation can originate from different mechanisms such as local thermal expansion, changes in optical or thermal properties, surface tension effects, and surface acoustic waves (SAW), capillary waves, melting, vaporization, transformation energies, chemical reactions etc [8]. Coherent structures have a common origin: the oscillating radiation field on the material surface which is generated by the interference between the incident laser beam and scattered /excited surface waves. The spatial periods of such structures are therefore proportional to the laser wavelength. Non-coherent structures are not directly related to any periodicity of the energy input caused interference phenomena. Here the feedback results in either spontaneous symmetry breaking or a non-trivial spatiotemporal ordering of the whole system.

1.9 Literature Review

A great deal of work has already been done on tin and its compounds. For decades, scientists are involved in consistent research on laser interaction with plasma plume generated once it strikes the target and then analyzing the surface changes and different phenomena that takes place.

Different investigations on pulse laser ablation of Sn at 1064 nm wavelength were made by L Torrisi et. al. [10]. A Nd:Yag laser operating at 1064 nm, 900 mJ maximum pulse energy and 9 ns pulse duration, is employed to irradiate solid tin targets placed in a high vacuum (10?7 mbar). The Sn plasma produced on the target surface is investigated with different analysis techniques, such as ion collectors, mass quadrupole spectrometry, electron microscopy and surface profilers. Measurements of ablation threshold, ablation yield, atomic and molecular emission, ion and neutral emission were reported. A time-of-flight technique was employed to calculate the velocity and the kinetic energy of the ion emission from the plasma. The angular distributions of the ejected ion species and of their kinetic energy are strongly peaked along the normal to the target surface. A valuation of the electric field generated inside the non-equilibrium plasma is given and discussed.

Pulsed Laser Ablation of Sn and SnO2 Targets: neutral Composition, energetics and wavelength dependence were explored by Scott A. Reid et. al. [11]. They reported time-of-flight mass spectrometric studies of neutral gas-phase species generated in 532 and 355 nm laser ablation of Sn and SnO2 targets at intensities of approximately 108 W cm-2, below the plasma threshold. A wavelength-dependent yield of Sn : SnxOy species is observed for the oxide target, with SnxOx (x = 1-3) the primary products at 532 nm and atomic Sn dominant at 355 nm. Sn and Sn2 are the primary products of Sn metal ablation, and the relative Sn : Sn2 yield increases at the shorter wavelength. The speed distributions of neutrals ejected from the oxide target are well represented by un-shifted time-transformed Maxwell-Boltzmann (MB) distributions, while those ejected from the metal target exhibit bimodal MB distributions. Typical most probable speeds are (1-2) ' 105 cm s-1, with peak kinetic energies (KEs) of 1-2 eV.

Extensive work has been carried out on the synthesis of SnO2 thin films or nanoparticles and exploration of their novel properties. Wide and long ribbons of single crystalline SnO2 have been achieved via laser ablation of SnO2 target. This was done by Junqing Hu et. al. [12]. Transmission electron microscopy (TEM) showed that the grown ribbons of SnO2 were structurally perfect and uniform. X-ray diffraction (XRD) pattern and energy dispersive X-ray spectroscopy (EDS) spectral analysis indicated that the ribbons have the phase structure and chemical composition of the rutile form of SnO2.

Spectroscopic studies of tin plasma using laser induced breakdown spectroscopy were made by Nek M Shaikh et. al. [13]. Laser-induced Sn plasma generated at different laser intensities has been characterized using visible emission spectroscopy. A CO2 laser pulse 85 ns in duration is used to generate plasma from a planar Sn sample in a vacuum of 10-5 torr. The plasma electron temperature is inferred by the Boltzmann plot method from singly ionized Sn emission lines, and plasma electron density is inferred using stark broadened profiles. Electron temperature is measured in the range of (0.53 - 1.28) eV, and electron density is measured in the range of (9.19'1015 - 7.45'1016) cm-3, as the laser intensity is varied from (1'1010 to 2.5'1010) W/cm2. The plasma shielding effect has been observed within the laser intensities of (2'1010 '2.5'1010) W/cm2.

Spectroscopic characterization of laser-induced tin plasma was explored by S. S. Harilal et. al. [14]. Optical emission spectroscopic studies were carried out on tin plasma generated using 1064-nm, 8-ns pulses from a Nd: yttrium aluminum garnet laser. Temperature and density were estimated from the analysis of spectral data. The temperature measurements have been performed by Boltzmann diagram method using singly ionized Sn lines, while density measurements were made using the stark broadening method. An initial temperature of 3.2 eV and density of 7.7 '1017 cm?3 were measured. Temporal and spatial behaviors of electron temperature and density in the laser-generated tin plasma have been analyzed. Time evolutions of density and temperature are found to decay adiabatically at early times. The spatial variation of density shows approximately 1/z dependence. The time-integrated temperature exhibited an appreciable rise at distances greater than 7 mm. This may be caused by the deviation from local thermodynamic equilibrium at larger distances from the target surface.

Pulsed Laser Ablation for Tin Dioxide: Nucleation,Growth, and Microstructures were studied by Z.W. Chen et. al. [15]. In this review, SnO2 thin films of various microstructures have been made using the pulsed-laser deposition method. The micro-structural aspects include tetragonal, porous, and orthorhombic structure characteristics. The quantum-dots and dynamic simulations of SnO2 nano-crystals have blossomed into a sub mono layer regime devoted to the nucleation and growth for these functional films. SnO2 thin films with some of the micro-structural features have great implications for the development of novel prototype gas sensors and transparent conduction electrodes.

Pulsed laser ablation of indium tin oxide in the nano and femtosecond regime: 'Characterization of transient species' has been done by A. De Bonis et. al. [16]. Pulsed laser ablation of indium tin oxide in the nano and femtosecond regime has been performed. Plume diagnostics has been carried out by means of a fast Intensified Coupled Charge Device (ICCD) camera. Optical emission spectroscopy has been applied to characterize the transient species produced in the nano and femtosecond regime. The time evolution of emission lines, in the femto and nanosecond regime, have been compared and discussed. In the mass spectrometry, of the ionized species, the presence of mixed metal oxide clusters has been detected. This fact is an indication that chemical reactions can occur during the plasma expansion or on the ITO surface.

Chapter - 2


2.1 Experimental Details

The setup for experiment was pretty much straight forward. It was performed in air rather than vacuum. Following apparatuses and materials were used:

Four samples of tin having thickness 1 mm and purity 99.9%

Abrasive paper of grating (600, 800, 1000, 1200, 1500, 2000)


Ultra sonic bath

Focusing mirror

Nd:YAG Laser

Optical microscope

Digital camera for photography and video shot

2.1.7 Neodymium YAG (Nd:YAG)Laser

The Neodymium (Nd) ion when doped into a solid-state host crystal produces the strongest emission at a wavelength just beyond 1 micro-meter. The two host materials most commonly used for this laser ion are yttrium aluminium garnate (YAG) and glass. When doped in YAG, the Nd:YAG crystal produces laser output primarily at 1.064 micro-meter, when doped in glass, the Nd:glass medium lases at wavelengths ranging from 1.054 to 1.062 micro-meter, depending upon the type of glass used. The Nd:YAG crystal has good optical quality and high thermal conductivity, making it possible to provide pulsed laser output at repetition rates of up to 100 Hz [20]. The laser which, I used to perform this particular experiment was a passive Q-switched pulsed Nd: YAG laser having wavelength 1064nm, energy 10 mJ and time duration 9-14ns.

2.2 Preparation of samples

Firstly, Tin strips were melted so that it can be solidified again according to the desired thickness. With this four samples were prepared. They were all in circular shape. The next job was to attain a surface as shiny as a mirror. It was very arduous task. For this each sample was supposed to be rubbed with sand paper of different gratings. Starting with the lowest grating, I spent approximately one hour with each sand paper on each sample. The rubbing was not done in random fashion rather it was done in a very systematic way. It goes like this, holding the sample and rubbing against let's say a paper of 600 grating, moving from lower end of paper to its upper end then lifting the sample as it reaches the upper end and took it back to the lower end again and continue in the same fashion. Meaning by this is that, sample was not rubbed in multiple directions rather one specified direction is followed. Now, when it moves from one grating paper to the next, I needed to change the angle of the sample by 90 degree and continued the rubbing in the same manner. Once all samples were ready i.e. their surfaces were shining like mirror, I was supposed to wash them with acetone first. Now, I took some cotton and dipped in a jar containing acetone. This wet cotton was then used to clean the surfaces of samples. It was rubbed very softly and smoothly on the surface of the samples to avoid any unintentional scratches and scars. After cleaning the surfaces with acetone, samples were washed in ultra sonic bath for 10 minutes. Once it was done, they were collected and were contained in air tight jar to avoid any sort of oxidation.

2.3 Experimental set-up

This was a bit hectic task because as per requirement of experiment, the laser should strike the target perpendicularly and the camera capturing the video should have been placed perpendicular to the target holder. The purpose of this is to capture the plume formation whose images could later be used for plume study. The toughest part of the setup was to obtain the exact focus length of the lens so that the maximum use of laser intensity could be made. This was obtained by moving the lens in between the dummy target and the laser. Moving it by little distance and then striking the laser to check the maximum focus. The strike on which the brightest spot was attained was the required focal length against maximum focusing. The next step was now to make the area around the setup dark that was done by covering the setup from all sides by black thermocouple sheets wrapped in black paper. To make it much more efficient the lights of the room were switched off. After this the dummy sample was tested with couple of shots to verify the setup. Getting it done satisfactorily, original samples were used to proceed with the experiment. The whole experiment is summarized in the table below.

Table 2.5: Summary of Laser pulses for samples of 'Tin' used



3.1 Plasma Plume Imaging

In this work, the intensity profiles of tin plasma plume images were analyzed using image-J software. Fig. 3.1 (a) is an original image of a plasma plume induced by a single laser pulse captured by digital camera. The intensity profile of the image along the line in (3a) is shown in (3b) where one pixel represents the distance of 0.29 mm. The captured images clearly describe the overall view of plasma plume.

Variation in the intensities signifies different zones subjected to different density and temperature gradients. In general, the plume intensity is uniform at the center and decreases abruptly at the edges. Similarly, 50 such images were captured and analyzed with the help of Image-J software. Then for every pulse of laser, intensity was calculated. For this particular shot the results were as given in table 3.1.

Further, 50 such values of intensities were combined and plot in new profile. In this profile, it was the total integrated intensity plotted against the number of pulses as shown in Fig 3.2. High intensity laser irradiation on target surface creates a large population of excited non-equilibrium electrons leading to bond breaking of the sample and subsequently causes atomic size particulates ejection via non-thermal ablation process. Another channel of material ablation is thermal process where laser excites the electrons which transfer energy to phonons during electron'phonon relaxation through lattice vibrations and consequently heat is conducted through the sample. This heating leads to local melting and then vaporization takes place. These vapours exert a recoil pressure on the melt surface and molten mass is pushed outwards forming plasma plume. The plasma plume consists of ions, electrons, neutrals, clusters, micron sized particles, molten globules and electromagnetic radiations. Plasma near the surface has maximum density of ions, electrons and atoms, etc. forming Knudsen layer. The species produce more ionization within this layer due to more collision. As a result bright luminous plasma plume is observed due to photo-ionization, Bremsstrahlung, recombination and de-excitation processes [].

From this the profile is pretty much straight forward i.e. it is a linear graph with a negative slope. This negative slope can be explained by considering the fact that as the number of pulses were kept increasing from 1-50, their formed a carter on the tin surface by the laser interaction i.e. the surface was ablated. Now, at the very early pulses like 1-5, there was no evident loss of energy except to create incubation centers, ablate material, thermal stresses, cracks, surface roughness, etc. As the surface starts getting shallower, carter formation was started, whereby; the plasma plume started getting shorter. This is because of the multiple exposures of the target to laser, made the carter deeper Owing to this fact, the direction of plume ejection is no more along the normal to the surface but a bit tilted and it appears shorter to digital camera. This led to the intensity of laser pulses being got wasted in different manners instead of being fully utilized for plasma formation e.g. making the carter deeper, against the absorption by the walls of the carter, interaction with the plasma plume and the conduction of heat to the surroundings.

3.2 Surface Morphology

Laser induced structures on solids and liquids are generally classified into coherent and non-coherent structures, discussed in section 1.8. Ripples are such spatially periodic coherent structures that are most frequently observed in laser surface interaction. In present work optical microscopy of all the spots on tin samples were done. The purpose was to analyze the surface changes and to measure the depth of the carter formed on successive laser pulses. The spectroscopy is done at different zoom strength like 100X, 200X, 500X etc. and the scale for the spectroscopy was 100 'm. Following are the images produced by optical spectroscopy at 5,10,15,20,25,30,35,40,45,50 laser pulses. In these the central region of the samples is focused at 200X.

The figures above clearly show that as the number of laser pulses increased the extent of damage at the surface also increased. Figures for the lesser number of pulses show that initially small crater was formed upon laser irradiation which is the indication of mass ejection from the surface. Figures for the higher number of pulses indicate that the damage at the center was much more dominant and significant heat affected zone (HAZ) at the boundary of irradiated surface. Following is the sequence for different optical zoom power for the pulse number 25. This sequence clearly indicates two facts i.e. the damage at the center with increased number of pulses got noteworthy and it was much more prevailing at the center than at the outer edges.

The basic sequence is that the material melts, undergoes deformation, and finally after irradiation re-solidifies making the deformation permanent. This surface deformations or irregularities act to scatter a small amount of light from the incident laser beam. This scattered light may propagate as a surface wave above or within the irradiated material, and interferes with the incident beam producing an intensity distribution across the surface. This also formed ripples on the surface. The intensity distribution acts as a diffraction grating, scattering additional light into the surface wave, therefore creating a positive feedback effect [20]. The rippling can also be attributed to scattering from the surface roughness and to re-radiation from surface defect sizes [21]. Besides, the measurement of damage, the depth of carter formed for different number of pulses was also analyzed. This was done with the help of Microcal Origin software. Firstly, the data was recorded with the help of optical spectroscope and later and was studied with the help of above mentioned software. In this, the depth of the carter formed was plotted against the number of laser pulses. The data for this analysis is summarized in the table below.

Table 3.2: Depth of carter formed for different number of laser pulses

It is very much evident from the data that the trend of depth is increasing one i.e. with laser pulses increment the depth of carter formed is increasing. The deviation from general trend at 40 and 45 laser pulses can be attributed towards experimental error. The other possible explanation can be the re-solidification of the ablated material right on the carter surface thus decreasing the depth when analyzed with optical microscopy. The graph generated is shown in figure 3.20 below. To decide whether a graph is linear fit or not, its R-value with the help of Micoocal Orgin software is calculated. If the value is less than 0.5 then the graph is considered non-linear but if it's more than 0.5 then the graph is regarded as linear fit. The experimental result has produced a linear graph as the R-value is equal to 0.79398. This linear relation signifies the fact that as the number of laser pulses increases the more material from the surface is ablated resulting in a deeper carter which is in agreement with the optical spectroscopy results.

Figure 3.20: Graph of ablated depth versus number of pulses


It is concluded from the plume images that there is shot-to-shot variation in both maximum and integrated intensities. Various intensity regions are present within the plume, regardless of the number of shots. Generally, at the center it's the highest temperature zone which gets milder away from the center towards edges. The central portion of the plume is more intense whereas the intensity drastically decreases at the edges. The plume length appears larger for first few shots but decreases with subsequent laser pulses due to crater formation on the target surface. As the number of pulses increased from 1-50, initially the carter formed was shallower so the laser energy was solely used to create it but as the pulses increased the laser energy is being wasted within the carter and in interaction with the plasma plume.

Surface morphology exhibits that mechanism like thermal or non-thermal ablation depending on the number of pulses as explained in section 1.6. This lead towards the formation of ripples on tin surface. Inhomogeneous distribution of energy on the surface due to interaction between the incident light and surface scattered waves is responsible for the growth of these ripples. Besides, it showed that the damage at the center was much more prominent than at the outer edges. With increased number of pulses when laser light interacted with the deformed tin surface it was scattered in multiple directions which then interacted with the oncoming laser beam and thus produced intensity distribution across the surface.