Tailored Laser-Droplet Interaction:for Target Formation in Extreme Ultraviolet Sources

This thesis investigates the interaction between laser pulses and metallic microdroplets, and the ensuing dynamics, with a specific focus on exploiting the tunability of laser pulses and pulse sequences. In Chapter two, we present the laser system that made this possible. The system combines arbitrary pulse shaping, enabled by the use of high-speed electro-optic modulators, with amplification to pulse energies of several hundred millijoules. Each following chapter presents experimental work in which we employed this system and which profited from its unique capabilities. In Chapter three, we study the fluid dynamic deformation of tin microdroplets resulting from impact of short pulses. If intense enough, these pulses will launch shock waves into the droplet interior that subsequently induce cavitation and spallation. We quantify the resulting deformation velocities for a large range of laser pulse energies and droplet sizes by combining data from multiple experiments. Chapter four presents a study on the laser-induced droplet deformation for pulses of varying duration and Gaussian and square temporal shapes. The range of pulse durations studied bridges the transition from shock-wave-dominated deformation, i.e. cavitation and spallation, as presented in Chap. 3 to sheet-type expansion. We quantitatively study the pulse-duration dependence of the center-of-mass propulsion, and expansion and spall-debris velocities. In Chapter five, we study the deformation following impact of two pulses of 0.4 ns each. The time delay between the two pulses is scanned from 1 ns to 100 ns and repeated for various droplet sizes. These scans reveal a reduction of the spall velocity which takes place at increasing interpulse spacing for increasing droplet size. In Chapter six, we measure the time-resolved reflection from the droplet to study the moment of plasma onset and the plasma-formation fluence threshold. In Chapter seven, we study the mass distribution of a stretching sheet of liquid tin formed after ns-pre-pulse impact by using an auxiliary, low-intensity vaporization pulse (VP). This VP gradually vaporizes the sheet enabling an investigation of the sheet thickness and mass. A supplementary shadowgram gallery presents a selection of captivating and intriguing shadowgrams that did not make it into any of the main chapters

A cold atom electron source

Pulsed bright electron sources offer the possibility to study the structure of matter in great spatial and temporal detail. An example of an indirect method is to generate hard X-ray °ashes with high brilliance, a new Free Electron Laser facility is under construction. It requires an electron source with a very high quality. Electron beams may also be used directly to study matter with, e.g., ultrafast electron diffraction. This also requires a pulsed electron source with high brightness. An overview of experiments that require a bright electron source is presented in Chapter 1. Also the pulsed electron sources used at this moment, i.e., photo-emitters and field-emitters, are described in Chapter 1. Brightness is an important figure of merit for electron source quality. It is expressed in its most general form as the current density per unit solid angle and unit energy spread. Recent brightness improvements are based on increasing the current density at the source, but this is not sufficient for all types of experiments. A new type of source, based on ultracold plasma, is described in Chapter 2. Contrary to the usual approach to increase the current density at the source to obtain a higher brightness, the new method tries to increase the angular intensity for moderate values of the emission area. For the field-emitters and photo-emitters the effective electron temperature of the source is typically 10 3 – 10 4 K. If one is able to lower these temperatures at the source, then a gain in brightness proportional to the reduction of the temperature can be achieved for the same current density. The new source concept based on this idea proposes pulsed extraction of electrons from an ultracold plasma, that is created from a laser-cooled cloud of neutral atoms by photoionization just above threshold. These plasmas are characterized by electron temperatures of 10 K. A simple estimate serves to illustrate the possible performance of such a source. Laser-cooled atomic clouds can have central densities up to n = 1018 m¡3 and contain up to 1010 atoms, requiring a cloud with rms (root-mean-square) size ¾ = 1 mm. If all these atoms could be ionized to form a UCP (ultracold plasma) with an electron temperature T = 10 K, then an electron bunch with a charge Q = 1 nC and an emittance " = 0:04 mm mrad could be extracted. If, in addition, all of these electrons can be packed into a temporal bunch length on the order of ¾t = 100 fs, the transverse brightness of the resulting electron bunch would be B? = 6£1016 A/(m2 sr). This is a few orders of magnitude higher than what has been achieved so far in the regime of (sub)-ps electron bunches. A four-step procedure is used to realize a UCP-based electron source in practice. First, atoms are cooled and trapped in a magneto-optical trap. Second, part of the cold atom cloud is excited to an intermediate state with a quasi-continuous laser pulse. Third, a pulsed laser beam propagating at right angles to the excitation laser ionizes the excited atoms only within the volume irradiated by both lasers. Subsequently, a UCP is formed. Finally, the electrons of the UCP are extracted by an externally applied electric field pulse. Each step toward the ultracold plasma is explained in detail in Chapter 2. Subsequently, with the help of simulations with a particle tracking program, the expectations from a more realistic situation are investigated. Two geometries are discussed. First, an initial charge distribution called "pancake" (bunch length much smaller than its transversal size) with a half-circle radial charge density distribution offers for a beam transverse size of 2 mm an emittance of 0:1 mm mrad and a temporal bunch length of 150 fs. This results in a transverse brightness of 6 £ 1013 A/(m2 sr). Second, a "cigar" geometry (transverse size much smaller than bunch length) with a parabolic longitudinal charge density distribution offers for a beam transverse size of 1 mm an emittance of 0:07 mm mrad and a temporal bunch length of 20 fs. This results in a transverse brightness of 1 £ 1014 A/(m2 sr). In this Thesis the first practical steps are reported towards this new concept. In Chap- ter 3, a specially designed accelerator structure and a pulsed power supply are described. They are essential parts of a high brightness cold atoms-based electron source. The acce- lerator structure allows a magneto-optical atom trap to be operated inside of it, and also transmits sub-nanosecond electric field pulses. The power supply produces high voltage pulses up to 30 kV, with a rise time of up to 30 ns. The resulting electric field inside the structure is characterized with an electro-optic measurement and with an ion time-of-fight experiment. In Chapter 4 measurements of the transverse momentum spread of pulsed electron beams are presented. Rubidium atoms are cooled and trapped in a magneto-optical trap. A small cylinder of these atoms is photoionized, resulting in free electrons. The electrons are extracted by a DC electric field. Images of the cylinder-like electron beam are obtained on the detector. On the path that they travel to a phosphor screen, they interact with an electromagnetic beam transport system, composed of an electrostatic lens (the accelerator itself) and a magnetic lens (the trapping coils). Due to the magnetic lens, this optical system is energy dependent. A dependence between the size of the small axis of the cylin- der at the detector and the beam kinetic energy is obtained. With the help of an optical matrix that describes this electromagnetic system, the size of the cylinder is related to the initial electron temperature, which is the parameter that we are actually interested in. Transverse electron temperatures ranging from 200 K down to 15 K are demonstrated, ea- sily controllable with the wavelength of the ionization laser. The temperature is influenced due to the Stark effect by the presence of the accelerating electric field. In this experiment the temporal length of the bunch is limited by the length of the ionization laser pulse to 4:7 ns. A typical bunch contains a charge of 10 fC. To lower the bunch length, another experiment was carried out. The results are presen- ted in Chapter 5. This time, excited Rydberg states of rubidium atoms are field ionized. The atoms are first magneto-optically trapped at the center of the accelerator structure. Subsequently they are excited to a Rydberg state (here between 26 and 35) and then field ionized by a pulsed electric field with a slew rate of 58 (V/cm)/ns. Electron temperatures at the source on the order of 10 K are measured. In the same way as in Chapter 4, the temperature is deduced from images of the electron pulses on the phosphor screen, using a model of the beam transport system. An advantage of this method is that sub-ns temporal bunch lengths might be reached. Here, the length is measured to be 2 ns FWHM, which is limited by instrumental resolution; particle tracking simulations show that it might be on the order of tens of ps. As a continuation of the experiments presented in Chapter 4 and 5, a method to pro- duce electron bunches with high energy and low temperature at the source is presented in Chapter 6. Rydberg atoms with the principal quantum number n between 15 and 25 are employed. It is shown that energies up to 14 keV can be produced. An Einzel lens is employed to focus the beam on the detector. An optical model including the Einzel lens is built, this time with the transverse beam size at the detector being dependent on the voltage applied on the Einzel lens. It does not fit very well the expectations due to the work of the Einzel lens, but this is a new model that can be further used to describe the behavior of a bunch in an optical system. Source temperatures of about 15 K are expected, but an upper limit of 1000 K is estimated using the optical model. In conclusion, in this Thesis an electron source with a 30 ¹m rms size, temperature of 10 K, and normalized transverse emittance of 0:001 mm mrad has been produced. Electron bunches with charges up to 10 fC and kinetic energies up to 14 keV have been produced. An upper limit for the FWHM length of 2 ns has been established. On the basis of these numbers, a transverse brightness B? = 8 £ 1010 A/(m2 sr) can be calculated. To further improve the brightness of this source, the source parameters as the charge column density Q=(¾x¾y), the bunch length ¾t, and the source temperature T should be improved. Together they may improve the brightness a few orders of magnitude. This project provides a solid basis for the next generation of cold electron sources that combines the present source based on cold atoms with radio-frequency technology. In addition, with this technique new research directions have been opened, as illustrated by an experiment with cold ions

Portable High Throughput Digital Microfluidics and On-Chip Bacteria Cultures

An intelligent, portable, and high throughput digital microfluidic (DMF) system is developed. Chapter 1 introduces microfluidics and DMF systems. In Chapter 2, a low-cost and high resolution capacitive-to-digital converter integrated circuit is used for droplet position detection. A field-programmable gate array FPGA is used as the integrated logic hub of the system for highly reliable and efficient control of the circuit. In this chapter a fast-fabricating PCB (printed circuit board) substrate microfluidic system is proposed. Smaller actuation threshold voltages than those previously reported are obtained. Droplets (3 µL) are actuated using 200 V, 500 Hz DC pulses. Droplet positions can be detected and displayed on a PC-based 3D animation in real time. The actuators and the capacitance sensing circuits are implemented on one PCB to reduce the size of the system. In Chapter 3, an intelligent EWOD (electrowetting on dielectric) top plate control system is proposed. The dynamic top plate is controlled by a piezoelectric (PZT) cantilever structure. A high resolution laser displacement sensor is used to monitor the deflection of the top plate. The gap height optimization and the harmonic vibration significantly improve the droplet velocity and decrease the droplet minimum threshold actuation voltage. The top plate vibration induced actuation improvement is magnitude and frequency dependent. 100 µm and 200 µm vibrations are tested at 25 Hz. Vibration frequencies at 5 Hz, 10 Hz, and 20 Hz are tested while the magnitude is 200 µm. Results show greater improvements are achieved at larger vibration magnitudes and higher vibration frequencies. With a vibrated top plate, the largest reduction of the actuation voltage is 76 VRMS for a 2.0 µl DI water droplet. The maximum droplet instantaneous velocity is around 9.3 mm/s, which is almost 3 times faster than the droplet velocity without top plate vibration. Liquid that has different hysteresis such as acetonitrile with various concentrations are used as a control to show its compatibility with the proposed DMF chip. Contact line depinning under top plate vibration is observed, which indicates the underlying mechanism for the improvements in actuation velocity and threshold voltage. The top plate control technique reported in this study makes EWOD DMF chips more reliable for point of care diagnostics. In Chapter 4, the mechanisms of the improvements were investigated by observing the detailed changes in the contact angle hysteresis using both parallel and nonparallel top plates. In Chapter 5, on-chip cell cultures are used for anti-biotic resistant bacteria detection. The passively dispensed on-chip cell cultures realize the isolated micro environment electrochemistry measurement, shorten the culturing time, and reduce the required sample volume. The design of the next generation ultra-portable DMF system is covered in the Appendix. Detailed technical notes and hardware design is covered in the Appendix. The proposed portable and high throughput DMF system with on-chip cell cultures have a great potential to change the standards for micro-environment culturing technologies, which will significantly improve the efficiency of actuation, sensing, and detecting performance of the DMF systems

A High-Resolution Polarizing Microscope for Cryogenic Imaging: Development and Application to Investigations on Twin Walls in SrTiO3 and the Metal-Insulator Transition in V2O3

The present work comprises the development and testing of a combined scanning laser- and widefield polarizing microscope and its application to research on the properties of two material systems that have received extensive attention over the past years: the two-dimensional electron-gas at the LaAlO3 / SrTiO3 interface and the metal-insulator transition in the correlated oxide V2O3. The microscope combines two imaging modes: a scanning polarizing microscope achieving a spatial resolution of ~ 240 nm and a sensitivity for the orientation of the polarization of 5.0 × 10^-6 rad /√Hz, and a widefield polarizing microscope providing a resolution of ~ 480 nm and a sensitivity for the orientation of the polarization of 1.0 × 10^-4 rad /√Hz. To enable low-temperature imaging, the sample is mounted on a 4He continuous flow cryostat providing a temperature range between 4 K and 300 K. Electromagnets are used to apply magnetic fields with variable orientation and a maximum strength of up to 800 mT. The scanning laser microscope offers an additional imaging mechanism. By locally perturbing the sample using the focused laser beam, and detecting the beam-induced voltage change across a current-biased sample, information on the local electric transport properties can be extracted. The instruments polarization-sensitive detectors enable the imaging of a wide variety of effects that affect the polarization of light, as for example magneto- optical effects and birefringence. Altogether, this makes the microscope a versatile tool that offers the possibility to image magnetic, structural, and electric features of a sample. The microscope is applied to investigations on twin walls between ferroelastic domains in tetragonal SrTiO3 and their effect on the transport properties of the two-dimensional electron gas at the interface between LaAlO3 and SrTiO3. The influence of twin walls on the electric transport at the LaAlO3 / SrTiO3 interface has been studied using low-temperature scanning electron microscopy, low-temperature scanning laser microscopy, and the widefield polarizing microscope. The findings of this study confirm the presence of twin walls at angles with respect to the crystallographic axes of the SrTiO3 substrates that match the predictions obtained from the tiling rules of tetragonal domains. It is further shown that electric order within the twin walls can be induced by applying an electric field that exceeds a threshold field of ~ 1.5 kV / cm. Furthermore, the low-temperature widefield polarizing microscope was applied to study the metal-insulator transition in V2O3. Within this study it was possible to confirm the presence of a phase separation into metallic and insulating domains at the metal-insulator transition. In addition the microscope was used to study the electrical breakdown of the insulating phase of V2O3 . It was found that the breakdown occurs through the formation of metallic filaments and domains. Complementary numerical simulations confirmed that the metallic filaments are formed by self-reinforced current focusing due to Joule heating and the negative temperature coefficient of the resistivity.Die vorliegende Arbeit behandelt die Entwicklung und Inbetriebnahme eines kombinierten Rasterlaser-Polarisations- und Weitfeld-Polarisationsmikroskops, sowie dessen Anwendung zur Untersuchung der Eigenschaften zweier Materialsysteme die im Zentrum aktueller Forschung stehen: das zweidimensionale Elektronengas an der Grenzfläche zwischen LaAlO3 und SrTiO3 und der Metall-Isolator Übergang in V2O3. Das Mikroskop vereinigt zwei Abbildungsverfahren: ein Rasterlaser- Polarisationsmikroskop das eine räumliche Auflösung von ~ 240 nm, sowie eine Empfindlichkeit für die Ausrichtung der Polarisation von 5.0 ×10^-6 rad /√Hz erreicht und ein Weitfeld-Polarisationsmikroskop mit einer räumlichen Auflösung von ~ 480 nm und einer Polarisationsempfindlichkeit von 1.0 × 10^-4 rad /√Hz. Um Abbildungen bei tiefen Temperaturen zu ermöglichen befindet sich die Probe in einem 4He-Durchfluss-Kryostaten, der es erlaubt die Probentemperatur zwischen 4 K und 300 K zu variieren. Mithilfe von Elektromagneten können Magnetfelder mit wählbarer Richtung und einer Feldstärke von bis zu 800 mT an der Probe angelegt werden. Das Rasterlaser-Mikroskop bietet einen weiteren Abbildungsmechanismus. Durch lokale Beeinflussung der Probe mit dem fokussierten Laserstrahl ist es möglich, Informationen über die lokalen elektrischen Transporteigenschaften zu gewinnen, indem man die strahlinduzierte Spannungsänderung über einer, mit einem konstanten Strom beaufschlagten, Probe misst. Die polarisationsempfindlichen Detektoren des Mikroskops bieten die Möglichkeit eine breite Palette an Effekten, welche die Polarisation beeinflussen, abzubilden. Beispiele hierfür sind magnetooptische Effekte und Doppelbrechung. Insgesamt macht dies das Mikroskop zu einem vielseitigen Werkzeug, das die Abbildung von magnetischen, strukturellen und elektrischen Eigenschaften einer Probe ermöglicht. Das Mikroskop wurde zur Untersuchung von Domänenwänden zwischen ferroelastischen Domänen in der tetragonalen Phase von SrTiO3 und deren Einfluss auf die elektrischen Transporteigenschaften des zweidimensionalen Elektronengases an der Grenzfläche zwischen LaAlO3 und SrTiO3 eingesetzt. Die Untersuchung der Beeinflussung des elektrischen Transports an der LaAlO3 / SrTiO3 -Grenzfläche erfolgte mittels Tieftemperatur-Rasterelektronenmikroskopie, Tieftemperatur-Rasterlasermikroskopie und dem Weitfeld-Polarisationsmikroskop. Die Ergebnisse dieser Studie bestätigten das Auftreten von Domänenwänden unter Winkeln bezüglich der kristallo- graphischen Achsen des SrTiO3 - Substrates, die mit den Voraussagen für tetragonale Domänen übereinstimmen. Des Weiteren wurde gezeigt, dass durch Anlegen eines elektrischen Feldes, welches einen Schwellwert von ~ 1,5 kV / cm übersteigt, elektrische Ordnung innerhalb der Domänenwände erzeugt werden kann. Darüber hinaus wurde das Tieftemperatur-Weitfeld-Polarisationsmikroskop eingesetzt, um den Metall-Isolator-Übergang in V2O3 zu untersuchen. Diese Studie bestätigte das Auftreten einer Phasentrennung in metallische und isolierende Domänen am Metall-Isolator-Übergang. Zudem wurde der elektrische Durchbruch in der isolierenden Phase von V2O3 mit dem Mikroskop untersucht. Es zeigte sich, dass der Durchbruch durch Bil- dung elektrisch leitender Filamente und Domänen auftritt. Ergänzende numerische Simulationen zeigten, dass sich die metallischen Filamente durch selbstverstärkte Strombündelung, aufgrund von Jouleschem Heizen und dem negativen Temperaturkoeffizienten des spezifischen Widerstands, bilden

Enhance ZDDP tribofilm growth and fatigue lifetime of rolling bearings by Laser Surface Texturing

Laser surface texturing (LST) is a fast and precise surface engineering method capable of improving the frictional performances of machine elements, thus contributing to an increase in energy efficiency. However, not many studies have carried out research of LST in the boundary lubrication regime, likely due to concerns of higher contact stresses that can occur with the increasing surface roughness. This study aims to improve the fatigue lifetime of rolling bearings in the boundary lubrication condition by LST combined with ZDDP-added lubrication. Firstly, the ZDDP antiwear tribofilm was characterized. The composition of the tribofilm and the difference between the blue- and the brown-colored region was revealed. Moreover, the analyses by high resolution methods indicated a sulfur enrichment at the interface between the tribofilm and the steel substrate. Secondly, the LST patterns were verified to reduce wear due to the capacity of lubricant storage and the enhanced growth of ZDDP tribofilm. The pressure distribution on the textured surface was calculated by a contact simulation, and the increase of normal stress led to a promotion of the tribofilm. Finally, the rolling bearings with the LST patterns demonstrated an increase in fatigue life.Die Laseroberflächenstrukturierung (eng.: Laser Surface Texturing (LST)) ist ein schnelles und präzises oberflächentechnisches Verfahren, welches es erlaubt die Reibung von Maschinenelementen zu reduzieren und somit zur Steigerung der Energieeffizienz beiträgt. Außerdem können die LST-Strukturen die Verwendung eines üblichen Verschleißschutzadditivs wie Zinkdialkyldithiophosphat (ZDDP) unterstützen. Allerdings ist der Einsatz von LST im Grenzreibungsregime noch nicht etabliert. Ziel dieser Arbeit war es, die Lebensdauer von Wälzlagern unter Grenzreibung durch eine Kombination aus LST und ZDDP-Einsatz zu verlängern. Zunächst wurde die chemische Beschaffenheit der Verschleißschutz-Triboschicht charakterisiert. Messungen mittels Atomsonden-Tomographie zeigen eine Anreicherung von Schwefel in der Grenzschicht zwischen Grund und Gegenkörper. Darüber hinaus wurde nachgewiesen, dass die LST-Strukturen den Verschleiß aufgrund von Schmiermittelspeicherung und einer verstärkten Triboschichtbildung verringern. Die Druckverteilung auf einer strukturierten Oberfläche wurde durch eine Kontaktsimulation berechnet, und die Zunahme der Normalspannung führt zu einer Förderung des Triboschichtbildung. Schließlich zeigten die Wälzlager mit den LST-Strukturen eine dreifache Ermüdungslebensdauer

Numerical Simulations

This book will interest researchers, scientists, engineers and graduate students in many disciplines, who make use of mathematical modeling and computer simulation. Although it represents only a small sample of the research activity on numerical simulations, the book will certainly serve as a valuable tool for researchers interested in getting involved in this multidisciplinary field. It will be useful to encourage further experimental and theoretical researches in the above mentioned areas of numerical simulation

Optical tweezers for signal detection and micromanipulation

The work presented in this thesis explores new multi-disciplinary applications of optical tweezers in the physical and biological sciences. Firstly, the three dimensional trapping of partially silvered sphere in a standard TEM00 optical trap was characterised. These spheres were then coated with an azo dye such that surface-enhanced resonance Raman (SERRS) measurements could be made on a single bead whilst it was simultaneously trapped in 532 nm optical tweezers. The length of time over which the SERRS signal could be recorded was increased, from milli-seconds to minutes, by using 1064 nm optical tweezers and introducing second harmonic light, generated via a frequency doubling crystal, for the excitation of the SERRS signal. In addition to trapping single particles, a spatial light modulator (SLM) was introduced into the optical tweezers to produce holographic optical tweezers. The SLM allowed the creation and manipulation of several optical beams both simultaneously and independently of each other. Three dimensional trapping and manipulation of multiple micron-sized spheres were achieved using the SLM in the Fourier plane of the traps. This ability to trap and manipulate objects, such as fluorescent spheres and E. coli, in 3D was extended to create permanent 3D structures that were set within a polymer matrix. These objects could be created, permanently set and imaged ex-situ. A summary of conclusions and ideas for future work are included