This thesis presents the study of x-ray emission from femtosecond laser micromachining and laser ablation of semiconductors. Prior to femtosecond machining experiments we investigated the nature of radiation emitted during the irradiation of solid targets with 120 femtosecond pulses with energies between 500 nJ and 0.3 mJ at a 1 kHz repetition rate. We have shown that the majority of the radiation was emitted below 10 keV with the high energy edge extending up to 25 keV. Under our experimental conditions K line emission was observed from materials with Z<32. We have also measured the x-ray dose rates during laser machining of various targets on the order of 10 mSv/h at a distance of 13 cm from the target. The implications for work pace safety, micromachining control, and potential for pulsed x-ray line sources for spectroscopic and imaging applications are discussed. In our studies of single shot femtosecond ablation of selected semiconductors: Si, GaAs, and InP, we have concentrated on the studies of microstructure and composition of the material after irradiation with 120 femtosecond pulses with energies between 2 nJ and 2 µJ. The resulting surface morphology, structure and composition of the micron scale ablation features on the semiconductors were studied by electron microscopy and atomic force microscopy. We found that no sharp threshold in the surface morphology was observed with increasing pulse power; however three ablation stages were identified based on the characteristic features of the ablation craters. TEM analysis revealed essentially no crystal damage beneath and in the vicinity of the ablation craters. In case of the binary semiconductors 5-30 nm polycrystalline grains were found over the ablated surfaces. The results were discussed in terms of the existing state of knowledge of ablation dynamics. The implications for practical micromachining applications are also discussed. / Thesis / Master of Engineering (ME)

Ti: sapphire femtosecond laser ablation of dielectrics (fused silica and BK7 glass) and metals (Cu, Fe, Al) is presented. Results of laser -induced breakdown experiments in fused silica and BK7 glass employing 130 fs -1.7 ps, 790 nm laser pulses are reported. The fluence ablation threshold does not follow the scaling of 4>th ~ ^/2 when pulses are shorter than 1 ps. Single-shot and multi-shot (130 fs pulse) ablation of selected materials are investigated with laser wavelengths of 395 nm, 790 nm, and 1300 nm. The ablation threshold is almost independent of the laser wavelength. The surface morphologies in metals after ultrashort pulse ablation are very different from dielectrics and semiconductors. The roughness of the ablated surface depends on the thermal properties of the metal target. The preliminary TEM result from Cu single crystal that was irradiated by single laser pulses shows few defects in the center region of the ablated crater. Single-shot ablation of single-crystal Fe induces much different surface features than on selected samples of poly-crystal Fe metal. / Thesis / Master of Engineering (ME)

In this work, laser ablation is investigated as a possible propulsion technique for space applications.

ITC/USA 2007 Conference Proceedings / The Forty-Third Annual International Telemetering Conference and Technical Exhibition / October 22-25, 2007 / Riviera Hotel & Convention Center, Las Vegas, Nevada / Real-time transmission of airborne images to a ground station is highly desirable in many telemetering applications. Such transmission is often through an error prone, time varying wireless channel, possibly under jamming conditions. Hence, a fast, efficient, scalable, and error resilient image compression scheme is vital to realize the full potential of airborne reconnaisance. JPEG2000, the current international standard for image compression, offers most of these features. However, the computational complexity of JPEG2000 limits its use in some applications. Thus, we present a scalable low complexity coder (SLCC) that possesses many desirable features of JPEG2000, yet having high throughput. Continuous radio-wave telemetry is required during planned tests of directed-energy weapons systems in order to characterize in situ the effects of laser irradiation on different target materials. Unfortunately, the incident radiation can cause disruption of the radio signal during the directed-energy testing. Several phenomena associated with directed-energy impact can lead to communication path losses, such as ablation, charged particle emission, charring, and chemical changes in the target materials. Directed-energy impact on the target material leads to target heating and consequent ablation. In this paper, a numerical model has been developed to describe the laser induced ablation of metal surfaces. The model describes the absorption of the laser energy by the metal and the resulting temperature rise in the surface. This temperature rise then induces ablation of the target material. Results for an aluminum target irradiated with a KrF laser were obtained. Temperature profiles in the target material and surface temperature changes are presented along with the ablation rate as a function of time as the aluminum target is irradiated. This report presents results for cases when laser energy absorption by the plasma plume created above the surface is not significant.

Current research in the field of nano-electronics is directed towards device miniaturization in order to find ways to increase the speed of electronic devices. The work presented in this dissertation is on the electronic transport properties of single walled carbon nanotube (SWNT) ropes synthesized by laser ablation. The measurements were performed on devices with different geometries; namely SWNT mats, metal incorporated (aligned individual and bundled) SWNTs and lastly on aligned pure SWNTs from low temperatures up to room temperature. The work was performed so as to gain an understanding on how best to utilize SWNTs in the semiconductor industry towards miniaturization. Such an understanding would ultimately highlight if SWNTs can be considered as a viable alternative to the current silicon-based technology, which seems to be approaching its physical limit. For a mat of SWNTs, 3D-Variable range hopping is the principal conduction mechanism from 2 K – 300 K. The magneto-resistance was found to be predominantly negative with a parabolic nature which converts to a linear nature as the temperature is increased. The negative MR is a consequence of quantum interference and the positive upturn is attributed to wave function shrinkage at low temperatures as described by the Efros-Shklovskii model. The hopping ranges of the electrons for a SWNT mat increases as the temperature decreases due to manifestation of quantum effects and reduced scattering. It was also found that metal incorporation does not alter the properties of the SWNT significantly. SWNT ropes aligned by di-electrophoresis across a 1 micron gap between gold micro-electrodes, exhibit Tomonaga-Luttinger liquid (TLL) like behaviour, within the 80 K – 300 K temperature range. The effects of confinement and electron-electron interaction unique to one dimension were identified in electronic transport as a non-universal power law dependence of the differential conductance on temperature and source-drain voltage. Ballistic conductance at room temperature was confirmed from the high frequency transport of the SWNT devices. The complex impedance showed some oscillatory behaviour in the frequency range 6 to 30 GHz, as has been predicted theoretically in the Tomonaga-Luttinger Liquid model. The observation of Luttinger Liquid behaviour demonstrates the outstanding nature of these one-dimensional molecular systems. In these devices the charging Coulomb energy of a single particle played a critical role in the overall device performance. This study can be used to understand the nature of dynamics of plasmons which are the charge carriers in a TLL system and how Coulomb interactions can be used to design highly tuneable systems for fabrication of single molecule devices. The incorporation of metal onto individual SWNT ropes does not alter its electronic properties significantly but the properties of the bundled metal incorporated SWNT ropes are altered. This study has found that under optimized conditions SWNTs might be a viable option for incorporation in nano electronics devices. Individual SWNT ropes promise better devices compared to SWNT mats and further work should be done on individual SWNTs.

A research report submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science. Johannesburg, 2015. / In this thesis electron transport properties of silicon nanowires are studied. The devices are synthesized using a laser ablation technique. The catalysts used in the synthesis are nickel nanoparticles. The silicon nanowires are characterized by scanning electron microscopy, transmission electron microscopy, atomic force microscopy and Raman spectroscopy. Dielectrophoresis is used to align and contact nanowires across two electrodes to create two-terminal devices. In addition four-terminal devices are fabricated using PMMA lift-off based electron beam lithography. Electron transport properties of the fabricated devices have been studied using dc measurement techniques. Resistance of the silicon nanowires has been measured as a function of temperature and magnetic field. These measurements have been accomplished using a Cryogenics system at low temperature, and high magnetic field. Temperature dependent studies reveal that Arrhenius type thermally activated transport behavior is the dominant transport mechanism in measurements at zero magnetic field. Magnetic field dependent measurements show a weak positive linear magnetoresistance. There are also strong oscillations in magnetoresistance curves. The temperature and field independence of the oscillations has been attributed to quantum interference effects.

by Fei Yang. / Title also in Chinese characters. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1993. / Includes bibliographical references (leaves 92-96). / ACKNOWLEDGEMENTS --- p.i / ABSTRACT --- p.ii / TABLE OF CONTENT --- p.iii / Chapter CHAPTER I. --- INTRODUCTION --- p.1 / Chapter §1.1 --- The Background of Pulsed Laser Ablation --- p.1 / Chapter §1.2 --- Properties of PZT --- p.3 / Chapter §1.3 --- Objectives of This Thesis --- p.7 / Chapter CHAPTER II. --- INTERACTION OF PULSED LASER RADIATION WITH A SOLID TARGET --- p.8 / Chapter §2.1 --- Introduction --- p.8 / Chapter §2.2 --- Absorption of Laser Radiation by the Target --- p.9 / Chapter §2.3 --- Absorption of Laser Radiation by the Plume --- p.11 / Chapter §2.4 --- Ablation Depth and Ablation Rate --- p.12 / Chapter §2.5 --- Formation and Evolution of the Plume --- p.14 / Chapter (a) --- Small-time Solution (c´Øt ζ) --- p.15 / Chapter (b) --- Large-time Solution (c´Øt ζ) --- p.15 / Chapter §2.6 --- Piezoelectric Signal --- p.17 / Chapter §2.7 --- Angular Distributions --- p.21 / Chapter §2.8 --- Target Surface Temperature --- p.26 / Chapter CHAPTER III. --- EXPERIMENTAL TECHNIQUES --- p.27 / Chapter §3.1 --- Pulsed Laser System --- p.27 / Chapter §3.2 --- Setup for Piezoelectric Detection of Laser Pulse --- p.28 / Chapter §3.3 --- Setup for Optical Measurement of Laser Plasma --- p.30 / Chapter §3.4 --- Setup for Ion Detection --- p.32 / Chapter §3.5 --- Sample Preparation --- p.35 / Chapter CHAPTER IV. --- PIEZOELECTRIC DETECTION OF LASER ABLATION --- p.38 / Chapter §4.1 --- Laser Ablation of a Piezoelectric Target --- p.38 / Chapter §4.1.1 --- Pyroelectric Effect --- p.38 / Chapter §4.1.2 --- Laser-induced Piezoelectric Signal (LIPS) --- p.39 / Chapter §4.2 --- Propagation of LIPS --- p.41 / Chapter §4.3 --- Laser Ablation Threshold --- p.45 / Chapter §4.4 --- Ablation Rate Measurement by LIPS --- p.47 / Chapter §4.5 --- Temperature Dependence of LIPS --- p.53 / Chapter §4.5.1 --- The Polarization Measurement --- p.53 / Chapter §4.5.2 --- The Piezoelectric Coefficient --- p.56 / Chapter CHAPTER V. --- STUDIES OF LASER PLASMA BY FARADAY PROBE METHOD --- p.59 / Chapter §5.1 --- Angular Distribution of The Plume Density --- p.59 / Chapter §5.2 --- The Plume Ionization Fraction --- p.64 / Chapter §5.3 --- The Plume Velocity Distribution and The Plume Temperature --- p.72 / Chapter §5.4 --- The Target Surface Temperature --- p.82 / Chapter §5.5 --- Plume Orientation Determination --- p.85 / Chapter CHAPTER VI. --- OVERALL CONCLUSION --- p.90 / REFERENCES --- p.92

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