Florenta Costache | Fraunhofer - Academia.edu (original) (raw)

Papers by Florenta Costache

Proceedings of SPIE, May 26, 2006

The impact of intense femtosecond laser pulses on dielectric targets results in a non-equilibrium... more The impact of intense femtosecond laser pulses on dielectric targets results in a non-equilibrium state of the surface. We consider the influence of this instability on ablation and surface relaxation phenomena. Important consequences of the laser-material coupling and energy dissipation are addressed such as transient and permanent modification of the surface. From experiments on ablation products kinetics, Coulomb explosion upon

Applied Physics A, Jun 6, 2008

This paper reports the pump and probe experiment for in situ reflectivity measurements in the fem... more This paper reports the pump and probe experiment for in situ reflectivity measurements in the femtosecond laser ablation that brings about nanoscale modification of diamond-like carbon (DLC) film. The characteristic reflectivity changes observed demonstrate that the formation of periodic nanostructure is preceded by a change in bonding structure of DLC in the ablation at low fluences. We have observed a coherent nonlinear wave-mixing signal that can resolve the ultrafast interaction processes for the nanoscale modification on the film surface. Based on the results obtained, a model of the interaction process is proposed.

Proceedings of SPIE, Oct 8, 2004

At the bottom of ablation craters produced in many materials, eg dielectric and silicon crystals,... more At the bottom of ablation craters produced in many materials, eg dielectric and silicon crystals, by the impact of femtosecond laser radiation, regular periodic structures are observed with a feature size at the order of a few 100 nanometers, much smaller than the ...

This thesis addresses fundamental physical processes which take place at the surface region of a ... more This thesis addresses fundamental physical processes which take place at the surface region of a target during and after the interaction with ultra-short laser pulses. The general goal is to bring together different phenomena and discuss the non-equilibrium nature of the interaction of femtosecond laser pulses (τ p < 100 fs) with various materials, in particular dielectrics and semiconductors. Different experiments, using various techniques, are designed to explore the basic mechanisms of laser ionization, defect creation, electron-lattice energetic transfer, charged particles desorption, optical breakdown, phase transformations and surface morphological changes. Such processes are shown to depend strongly on the laser intensity. Thus, they are analyzed for intensities over four orders of magnitude (10 11-10 14 W/cm 2), around the surface optical breakdown (damage) threshold intensity. First, experimental studies using time-of-flight mass spectrometry indicate that non-resonant intense ultra-short laser pulses can efficiently ionize a dielectric (semiconducting) material leading to emission of electrons as well as charged particles, i.e. atomic ions and large clusters, and neutral particles. Under these irradiation conditions, the ionization processes can be at best described by multiphoton ionization and ionization at defects sites. The structural defects provide the means for an increased positive ion desorption rate. A multiple pulse incubation effect in the ion yield can be well related with the reduction of the multi-pulse damage threshold with increasing intensity. Following the initial electron excitation and emission, positive ions are released from the surface in a substantial amount with high ion velocities indicative of a localized microscopic electrostatic expulsion. With increasing intensity, the amount of ions gets larger and larger and their velocity distribution exhibits a bimodal structure. Also, in these conditions, negative ions are detected. The ion desorption can arise from a combination of a localized electrostatic repulsion (macroscopic Coulomb explosion) and a thermal 'explosive' mechanism. The later becomes more important with increasing intensity. The very fast energy input and particle emission result in a transient perturbation and deformation of the target lattice. Using pump-probe experiments the temporal evolution of lattice dynamics can be analyzed upon single-pulse excitation for many different target materials. This deformation is indicated to be a material characteristic. It is associated with the generation of transient defects in dielectrics or fast phase transitions in semiconductors and metals. Therefore, it could well give estimates of lifetime of transient defect states or electron-phonon relaxation times. At last the surface morphology after ablation is analyzed, with emphasis on the laser-induced surface periodic patterns (ripples). The patterns observed appear to be very different from the 'classical' ripples formed after long pulse ablation. They can have periods much smaller than the incident wavelength and are rather insensitive to the variation of the laser wavelength and angle of incidence. We show that control factors are laser beam polarization and the irradiation dose. Additionally, the patterns exhibit features pointing toward a chaotic origin. Their possible formation mechanism is likely linked with the non-equilibrium nature of the interaction. ZUSAMMENFASSUNG Diese Dissertation befasst sich mit den grundlegenden physikalischen Prozessen, die im Oberflächenbereich eines Materials während und nach der Wechselwirkung mit ultrakurzen Laser Pulsen stattfinden. Es ist das wesentliche Ziel der Arbeit unterschiedliche Phänomene zu vereinen und zu zeigen, dass die Wechselwirkung von Femtosekunden-Laserpulsen (τ p < 100 fs) mit unterschiedlichen Materialien, insbesondere Dielektrika und Halbleiter, fern vom thermischen Gleichgewicht stattfindet. Verschiedene Experimente, die unterschiedliche Techniken nutzen, werden entwickelt um die grundlegenden Mechanismen für Laserionisation, Defekterzeugung, Elektron-Gitter Energieaustausch, die Desorption von geladenen Teilchen, den optischen Durchbruch, Phasen-Transformationen und Änderungen der Oberflächenmorphologie zu untersuchen. Es wird gezeigt, dass solche Prozesse sehr stark von der Laserintensität abhängen. Daher werden sie in einem Intensitätsbereich von vier Größenordnungen (10 11-10 14 W/cm 2) um die Schwelle für den optischen Durchbruch an der Oberfläche (Zerstörungsschwelle) studiert. Experimente mit Flugzeit-Massenspektrometrie zeigen, dass nichtresonante, intensive ultra-kurze Laserpulse ein Dielektrikum (Halbleiter) sehr effizient ionisieren können. Neben Elektronen werden auch schwere geladene Teilchen emittiert, d.h. atomare Ionen und große Clusterionen, sowie Neutralteilchen. Der Ionisationsprozess kann hier am besten beschrieben werden als Mehrphotonen-Ionisation und Ionisation von Defekten. Strukturdefekte bewerken eine erhöhte Desorption von positiven Ionen. Daher gibt es einen Effekt der Multi-Puls Inkubation für die Ionenausbeute, der auf einer Reduzierung der Zerstörungsschwelle beruht und stark von der Intensität abhängt. Als Folge der Anregung und Emission von Elektronen verlassen positive Ionen in beträchtlicher Zahl mit hoher Geschwindigkeit der Oberfläche. Dies deutet auf eine lokale mikroskopische elektrostatische Abstoßung hin. Mit zunehmender Intensität wächst die Zahl der Ionen stark an, und ihre Geschwindigkeits-Verteilung entwickelt eine bimodale Struktur. Jetzt können auch negative Ionen nachgewiesen werden. Die Ionen-Desorption kann hier auf einer Kombination von einer lokalen elektrostatischen Abstoßung (Coulomb Explosion) und einem thermischen Explosions-Mechanismus beruhen. Dieser wird mit wachsender Intensität zunehmend wichtiger. Da der Energie-Eintrag und die Teilchen-Emission sehr schnell erfolgen, entsteht eine transiente Störung und Deformation des Gitters der Probe. Mit Pump-Probe Experimenten kann die zeitliche Entwicklung der Gitterdynamik nach Einzelpuls-Anregung für viele unterschiedliche Materialien untersucht werden. Die Gitterdeformation ist offensichtlich Material-spezifisch. Sie ist verknüpft mit der Erzeugung von transienten Defekten in Dielektrika oder schnellen Phasenübergängen in Halbleitern und Metallen. Daher können solche Experimente eine gute Abschätzung liefern für die Lebensdauer von transienten Defektzuständen oder für Elektron-Phonon-Relaxationszeiten. Schließlich wird die Oberflächenmorphologie der Probe nach der Ablation studiert, insbesondere werden Laser-Induzierte Periodische Strukturen (Ripples) beobachtet, die offensichtlich sehr verschieden sind von ‚klassischen' Ripples nach der Ablation mit langen Pulsen. Ihre Periodizität ist viel kleiner als die einfallende Wellenlänge, und es wird kein großer Einfluss von Wellenlänge und Einfallswinkel beobachtet. Wir zeigen, dass eher Laserpolarisation und Bestrahlungsdosis wichtig sind. Außerdem zeigen die Muster Eigenschaften, die auf eine Selbstorganisation hindeuten.

Nanomaterials, Feb 3, 2023

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Integrated Optics: Devices, Materials, and Technologies XXIII, 2019

MOEMS and Miniaturized Systems XVI, 2017

A fluid-filled micro-lens concept with an electrically driven polymer actuator was developed in v... more A fluid-filled micro-lens concept with an electrically driven polymer actuator was developed in view of optimization of its variation in focal length. The high strain electrostrictive terpolymer P(VDF-TrFE-CFE) was first used in the actuator design for this purpose. Our study showed that the electric field-induced strain in polymer thin films could be even further enhanced by mixing high-k BaTiO3 nanoparticles in the terpolymer. A newly developed nanocomposite P(VDF-TrFE-CFE) / BaTiO3 ring-shaped actuator was implemented into the 3 mm aperture, liquid-filled micro-lens concept. The micro-lens was fabricated in a wafer-level process flow, which included micromachining of fluidic chambers on silicon wafers, thin film nanocomposite actuator processing, assembly through wafer bonding and chip filling with liquids. Particular characteristics of the nanocomposite were taken into account such as the homogeneous nanoparticle dispersion into the thin film with impact on thin film dielectric breakdown, electrode adherence as well as nanocomposite film etching. Variable focal length micro-lenses conceps with a single fluidic chamber but also with two fluidic chambers (a design, which can potentially improve the lens membrane stability) were fabricated and characterized. We could demonstrate a wide focal length variation of tens of diopters first for a single chamber plano-convex micro-lens obtained by adjusting the voltage applied on the integrated actuator.

Procedia Engineering, 2016

An electromechanical actuator based on a P(VDF-TrFE-CFE) / BaTiO3 terpolymer nanocomposite of hig... more An electromechanical actuator based on a P(VDF-TrFE-CFE) / BaTiO3 terpolymer nanocomposite of high field-induced strain was used in a variable focal length micro-lens design. The liquid-filled micro-lens consisted of a fluidic chamber, structured on a silicon wafer and sealed by an elastomeric lens membrane with a round-shaped nanocomposite actuator attached to it. A fabrication process was developed taking into account also particular characteristics of the actuator material for thin film parameter optimization. A change in the focal length of the fabricated micro-lens with a nanocomposite actuator was successfully demonstrated. Specifically, a 3 mm aperture, plano-convex variable focal length micro-lens of optical power ranging from 21 dpt to 13 dpt was obtained by varying the electric field applied on the nanocomposite actuator.

Procedia Engineering, 2015

Energy was generated from harvester structures based on various electroactive polymer thin films ... more Energy was generated from harvester structures based on various electroactive polymer thin films of different electro-mechanical properties under applied pressure and frequency characteristics specific to human motion. A self-priming energy harvesting circuit was designed and optimized to collect and store this energy. The circuit enables a gradual increase in voltage each time pressure is applied on the harvester and more energy to be stored with each generation cycle. Thus stored energy of up to 10 μW/cm 2 at 10 Hz was successfully used to power a wireless transmitter. The results revealed that, with increasing number of layers in a fluoropolymer harvester system, the time between signal transmissions could be shortened more and more.

$Research paper thumbnail of Efficient Simulation of 3D Electro-optical Waveguides Using the Effective Refractive Index Method$

3D FEM simulation of millimeter-scale, complex electro-optically induced waveguide based devices ... more 3D FEM simulation of millimeter-scale, complex electro-optically induced waveguide based devices demands the use of grids with more than several million nodes. Hence, the simulation could take substantial time and require large amounts of available memory. This paper presents a computation algorithm based on the conversion of an initial 3D waveguide structure into an analogous 2D structure, where the wave propagation on the 'reduced' dimension is described by an effective refractive index. It is shown that with the proposed algorithm the computing efficiency could be improved. Moreover the algorithm could be successfully used to simulate large-size passive or electro-optically active waveguide structures. For an electro-optical waveguide coupler, we could demonstrate a good agreement between simulated and calculated coupling length.

Proceedings

This paper presents the design and fabrication of a piezoelectric MEMS cantilever harvester of op... more This paper presents the design and fabrication of a piezoelectric MEMS cantilever harvester of optimized shape able to generate a usable amount of power from low frequency vibrations for moderate cantilever deflection. The corresponding harvester design, obtained by means of FEM simulations, comprises a trapezoidal unimorph silicon cantilever beam with a piezoelectric PZT film and a rectangular silicon tip mass. This device was fabricated by wafer-level micro-fabrication processes. The harvester provided 2 μW generated power at 123 Hz resonant frequency.

Optical Fiber Communication Conference, 2016