Дисертації з теми "Ultra-Fast optic"

(1) Nous aVO₂vons étudié la transition de phase dans le VO₂ amorphe ultrafin et son mécanisme physique : Nous avons préparé avec succès des films amorphes ultraminces (à l'échelle nanométrique) de VO₂ avec une transition de phase significative par pulvérisation magnétron et démontré la transition de phase du VO₂ amorphe - EGT. En outre, nous avons modélisé quantitativement la transition de phase du VO₂ amorphe et classé différentes épaisseurs de VO₂ en "système fort" (>5 nm) et "système fragile" (0-2 nm). Pour le système fort, les propriétés du matériau sont moins affectées par la température, et le modèle d'Arrhenius est utilisé pour décrire le transport d'électrons de la transition de phase du VO₂. Alors que pour le système fragile, les propriétés du matériau sont plus affectées par les fluctuations de température, et le modèle de Vogel-Tammann-Fulcher peut être utilisé pour l'analyse. Les résultats démontrent le mécanisme de transition de phase des matériaux amorphes et fournissent une nouvelle idée pour comprendre la transition de phase. En outre, cette méthode directe de croissance de VO₂ ultra-mince par pulvérisation magnétron est pratique et rapide, et il peut être cultivé dans le même lot avec d'autres matériaux dans l'hétérostructure, ce qui devrait promouvoir l'application de matériaux à transition de phase dans des dispositifs pratiques. (2) Nous avons exploré une méthode permettant de réguler dynamiquement le couplage d'échange entre couches par transition de phase : nous avons introduit le VO₂ dans la couche ferromagnétique/non magnétique et nous avons réussi à réaliser la transformation réversible du couplage antiferromagnétique et du couplage ferromagnétique en régulant les électrons de conduction par le MIT de VO₂. En même temps, à partir de l'analyse du changement des propriétés magnétiques, nous clarifions que le CEI induit par le VO₂ dans différents états électroniques est dominé par le RKKY et l'effet tunnel dépendant du spin. En outre, nous étudions en détail la racine physique derrière la régulation de l'IEC par le VO₂, et nous révélons le mécanisme de régulation de l'effet de spin de l'interface par la régulation des états électroniques de l'espaceur non magnétique. Cette partie du travail propose une nouvelle approche de la régulation dynamique de l'IEC, qui fournit de nouvelles idées pour l'application de l'IEC dans les dispositifs spintroniques. (3) Nous étudions la régulation dynamique du transport d'électrons chauds polarisés en spin par transition de phase : Dans une hétérostructure ferrimagnétique/non magnétique à canal de diffusion/ferromagnétique, nous introduisons du VO₂ dans le canal de diffusion pour contrôler les propriétés électriques du canal par MIT, puis nous régulons dynamiquement le transport des électrons chauds polarisés en spin générés par la désaimantation ultrarapide de GdCo. En régulant l'activation et la désactivation des électrons chauds dans le canal, nous obtenons une régulation dynamique de l'aimantation des couches ferromagnétiques adjacentes. Parallèlement, grâce aux changements de propriétés optiques introduits par le VO₂, nous avons réussi à commuter l'aimantation de matériaux ferromagnétiques sans AOS en ferrimagnétisme excité par un laser femtoseconde à impulsion unique. De plus, nous avons vérifié et analysé le mécanisme de cette modulation ultrarapide. Dans ce travail, nous utilisons le matériau de transition de phase VO₂ comme canal de diffusion avec des propriétés électriques contrôlables pour contrôler le transport des électrons chauds à travers le MIT. Les résultats montrent que les matériaux non magnétiques jouent un rôle important dans différents types d'hétérostructures
(1) We have investigated the phase transition in ultrathin amorphous VO₂ and its physical mechanism: We have successfully prepared ultrathin (nano-scale) amorphous VO₂ films with significant phase transition by magnetron sputtering and demonstrated the phase transition of amorphous VO₂ - EGT. In addition, we quantitatively modeled the phase transition of amorphous VO₂ and classified different thicknesses of VO₂ into "strong system" (>5 nm) and "fragile system" (0-2 nm). For the strong system, the material properties are less affected by temperature, and the Arrhenius model is used to describe the electron transport of VO₂ phase transition. While for the fragile system, the material properties are more affected by temperature fluctuations, and the Vogel-Tammann-Fulcher model can be used for analysis. The results demonstrate the phase transition mechanism of amorphous materials and provide a new idea for understanding phase transition. In addition, this direct method of growing ultrathin VO₂ using magnetron sputtering is convenient and fast, and it can be grown in the same batch with other materials within the heterostructure, which is expected to promote the application of phase transition materials in practical devices.(2) We explored a method to dynamically regulate the interlayer exchange coupling by phase transition: we introduced the VO₂ into the ferromagnetic/nonmagneticspacer/ferromagnetic heterostructure, and successfully realized the reversible transformation of the antiferromagnetic coupling and ferromagnetic coupling through regulating conduction electrons by MIT of VO₂. At the same time, from the analysis of the change of magnetic properties, we clarify that the IEC induced by VO₂ in different electronic states is dominated by the RKKY and spin dependent tunneling. Furthermore, we fully investigate the physical root behind the regulation of IEC by the VO₂, and reveal the regulation mechanism of the interface spin effect by the regulation of electronic states of non-magnetic spacer. This part of the work proposes a novel approach to the dynamic regulation of IEC, which provides new ideas for the application of IEC in spintronic devices.(3) We study the dynamic regulation of spin-polarized hot electron transport by phase transition: In a ferrimagnetic/nonmagnetic diffusion channel/ferromagnetic heterostructure, we introduce VO₂ into the diffusion channel to control the electrical properties of the channel by MIT, and then dynamically regulate the transport of spin-polarized hot electrons generated by the ultrafast demagnetization of GdCo. By regulating the on/off of hot electrons in the channel, we achieve dynamic regulation of the magnetization of adjacent ferromagnetic layers. Meanwhile, with the optical property changes introduced by VO₂, we have successfully achieved the switching of the magnetization of ferromagnetic materials without AOS in ferrimagnetism excited by a single-pulse femtosecond laser. Furthermore, we have verified and analyzed the mechanism of this ultrafast modulation. In this work, we use the phase transition material VO₂ as a diffusion channel with controllable electrical properties to control the hot electron transport through MIT. The results show that the non-magnetic materials play an important role in various types of heterostructures

Terahertz time-domain spectroscopy systems were developed and used for the anaylsis and characterization of various materials. By using ultra-fast Ti:Sapphire and Er-doped fiber lasers, terahertz time-domain spectrometers of different configurations were constructed and tested. To increase the accuracy and sensitivity of the measurements, the systems were optimized for spectroscopic analysis. MBE grown nominally undoped epitaxial GaAs samples were used for the spectroscopic measurements. These samples were first charactrized by electrical measurements in order to check the accuracy of the terahertz time-domain experiments. We have shown that the terahertz time-domin spectroscopic techniques provides a quick way of the determining the real ( ) and complex () components of the refractive index of material. In addition, we have investigated the photoexcitation dynamics of these GaAs samples. We have demonstrated that direct and photoexcited terahertz time-domain measurements give an estimate of the carrier densities and both the hole and electron mobility values with good precision. rnin An algorithm is developed to prevent the unwanted Fabry-Perot reflections which is commonly encountered in Terahertz Spectroscopy systems. We have performed terahertz time-domain transmission measurements on ZnTe <
110>
crystals of various thicknesses to test the applicability of this algorithm. We have shown that the algorithm developed provides a quick way of eliminating the &ldquo
etalon&rdquo
reflections from the data. In addition, it is also shown that these &ldquo
etalon&rdquo
effects can be used for the frequency calibration of terahertz time-domain spectrometers.

This document presents the design, fabrication, characterization and packaging of a stand-alone all-photonic 1X4 switch. All of the designs pertaining to the implementation of the device including those of required custom components are given. The core of the switch is comprised of a "2-bit" electro-optic deflector capable of sub-microsecond deflection times. The packaged switch operates at 1100V and boasts a worst-case fiber-to-fiber insertion loss and crosstalk reading of 5.3dB and -23dB, respectively. Furthermore, a worst-case deflection time of 86ns has been achieved.

This dissertation is devoted to the study of third-harmonic generation (THG) in push-pull chromophore-doped polymer films. This kind of films, with amorphous structure, exhibit null second harmonic generation but strong THG when pumped at the fundamental wavelengths within the telecommunication range (1.4-1.6 μm). It is demonstrated that at 1550 nm, micrometer-thick samples generate up to 17 muW of green light with an input power of 250 mW delivered by an optical parametric oscillator. This high conversion efficiency is achieved without the use of phase matching or cascading of quadratic nonlinear effects and it is due to high values of the third-order nonlinear susceptibility combined with weak film absorption at the third harmonic wavelength. The efficient THG process opens the doors to low cost and sensitive third-order optical autocorrelation and cross-correlation applications. So, in addition to the basic research performed about the characterization of the THG in push-pull chromophore-doped polymer films, two applications are demonstrated. The first is the complete diagnostic of femtosecond pulses by THG-Interferometric Autocorrelation and by THG Frequency-Resolved Optical Gating. The second is the THG-Cross-correlation Time-Gated Imaging of objects embedded in highly scattering conditions.

Solid-state lasers are capable of providing versatile output characteristics with greater flexibility compared to other popular laser systems. Lasing action has been achieved in many hundreds of solid-state media, but Nd-ion doped gain media are widely used to reach high power levels with short pulses. In this work, commercially available Nd:KGW crystal served as a gain medium to achieve pulsed operation at 1067 nm. This laser crystal offers large stimulated emission crosssection and gain bandwidth which facilitates generation of high peak power pulses in the picosecond regime. The KGW crystal is monoclinic and biaxial in structure, and anisotropic in its optical and thermal properties. Due to poor thermal conductivity, this crystal can be operated within a limited power range before crystal fracture takes place. To reduce the amount of heat deposited in the gain media, we introduced a new pumping wavelength of 910 nm which reduces the quantum defect by more than 45%. Continuous-wave laser operation was optimized to operate in mode-locked regime. In order to achieve short light pulses from the continuous-wave laser, one of the end mirrors was replaced by a semiconductor saturable absorber mirror (SESAM) to generate 2.4 ps pulses at a repetition rate of 83.8 MHz. An average output power of 87 mW was obtained at lasing wavelength of 1067 nm and the beam was nearly diffraction limited with M^2 < 1.18. The peak power of the generated pulses was 427 W and energy of each pulse was >1 nJ. Pumping the crystal at longer wavelength (910 nm) reduced the thermal lensing of the crystal by half when compared to conventional pumping at shorter wavelength (808 nm). To the best of our knowledge, this is the first time passive mode-locking of a Nd:KGW laser was explored using the pump wavelength at 910 nm.
February 2017

Optical Scanning Acoustic Microscopy (OSAM) is a non-contacting method of investigating the properties and hidden faults of solid materials. This thesis presents an ultra-fast data acquisition system (DAQ) which samples and digitises the output signal of OSAM. The author's work includes the design of the clock source and the sampler, and integration of the whole system. The clock source is a unique pulse generator based on a 2.624GHz PLL with a Quadrature VCO (QVCO), which is able to generate 4 clock signals in accurate quadrature phase difference. The pulse generator used the 4-phase clocks to provide control pulses to the sampler. The pulses were carefully aligned to the clock edges by digital logic, so that jitters were reduced as much as possible. The required short time delay for the sampler was also provided by the pulse generator, and this was implemented by a smartly-controlled switch box which re-shuffles the 4-phase clocks. The presented sampler is a novel 10.496GSample/s Sub-Sampling Sample-and-Hold Amplifier (SHA). The SHA sampled the input, and transformed its spectrum down to a low-frequency range so that it can be digitised. Charge-domain sampling strategy and double differential switches were both developed in this circuit to significantly improve the sampling speed. The periodicity of the system input was exploited in repetitive sampling to reduce the noise. These designed modules were integrated into a DAQ for a 2x8 sensor array. A pseudo-parallel scanning strategy was presented to minimise the power consumption, and a current-based buffer was applied to deliver the control pulses into the array. The DAQ was implemented on-chip in a low-cost 0.35um standard CMOS process. The measurement results showed that the DAQ successfully achieved a sampling rate more than 10GS/s, with a maximum output resolution of approximately 6 bits.

Les lasers de haute puissance permettent d'atteindre des intensités très importantes (jusqu'à 10²²W.cm⁻²). Parvenir à ce niveau d'intensité nécessite de concentrer une quantité modérée d'énergie (de l'ordre du joule) dans un temps très court (de l'ordre de la dizaine de femtosecondes) sur une surface réduite (de l'ordre du μm²). Ces faisceaux sont donc ultra-courts et focalisés à l'aide d'une optique à grande ouverture. Ces caractéristiques signifient que leur diamètre avant focalisation est grand et leur largeur spectrale est importante. Pour cette raison, ces faisceaux sont à même de présenter des distorsions spatio-spectrales (ou couplages spatio-temporels). Après focalisation, ces distorsions ont pour effet une diminution drastique de l'intensité pic. Ceci est d'autant plus vrai que le système laser est puissant et donc que son diamètre et sa largeur spectrale sont grands. En dépit de cet effet néfaste, les couplages spatio-temporels présentent aussi un intérêt lorsqu'ils sont maitrisés. On peut en effet introduire des couplages spatio-temporels de faible amplitude à des fins expérimentales. Dans les années 1990 et 2000, un effort important a été fourni pour permettre la caractérisation et l'optimisation du profil temporel des lasers femtoseconde. Dans le même temps, des solutions d'optique adaptative ont été développées pour contrôler le profil spatial des faisceaux ultra-intenses et obtenir la meilleure tache focale possible. Les systèmes laser de haute-puissance actuels sont maintenant caractérisés et optimisés indépendamment par ces deux types de diagnostics. Par essence, cette approche est aveugle aux couplages spatio-temporels. Seule une caractérisation spatio-temporelle permettrait de mesurer ces distorsions. Il existait déjà des méthodes de caractérisation spatio-temporelle avant le début de cette thèse. Aucun de ces dispositifs n'avait cependant été adapté à la mesure de faisceaux ultra-intenses. Lors de cette thèse, nous avons développé une nouvelle technique de caractérisation spatio-temporelle appelée TERMITES. Cette technique est basée sur un schéma de spectroscopie par transformée de Fourier auto-référencée. TERMITES nous a permis d'effectuer la première caractérisation spatio-temporelle totale d'un laser 100 TW (le laser UHI-100 du CEA Saclay). Les distorsions spatio-temporelles détectées à l'aide de ces mesures ont confirmé la nécessité d'une généralisation de la métrologie spatio-temporelle des lasers de haute puissance
High power laser make it possible to reach very high intensities (up to 10²²W.cm⁻²). In order to get to this level of intensity, a moderate quantity of energy (on the order of the Joule) is concentrated in a very short time (on the order of tens of femtoseconds) onto a small surface (on the order of 1 μm²). These beams are therefore ultra-short and focused with a high aperture optic. These features mean that their diameter prior to focus is large and their spectral width is big. As a result, these beams are subject to spatio-spectral distorsions (of spatio-temporal couplings). After focus, these distorsions induce a dramatic reduction of the peak intensity. This situation is all the more true when the laser is more intense and its diameter and spectral width are therefore bigger. Despite their detrimental effects, spatio-temporal couplings can be of great interest when controlled. One can indeed introduce weak spatio-temporal couplings for experimental purposes. In the 1990s and 2000s, a big effort was put in order to characterize dans optimize the temporal profile of femtosecond lasers. Meanwhile, adaptative optics solutions were developed to control the spatial profil of ultra intense laser beams and provide the best focal spot achievable. By nature, this approach is blind to spatio-temporal couplings. Measuring these distorsions requires a spatio-temporal characterization. Before the start of this Phd thesis, spatio-temporal characterization methods already existed. Although none of these devices were ever adapted to the measurement of ultra-intense laser beams. During this Phd Thesis, we developped a new spatio-temporal characterization technique which we called TERMITES. This technique is based on a self-referenced Fourier transform spectroscopy scheme. TERMITES made it possible for us to perform the first total spatio-temporal characterization of a 100 TW laser (UHI-100 at CEA Saclay, France). The detection of spatio-temporal distorsions with the help of these measurements confirmed the need for a generalization of spatio-temporal characterization of ultra-high power lasers

La matrice de transmission permet de décrire les effets produit par un milieu multi-diffusant sur une onde monochromatique incidente. L'objectif des travaux présentés dans cette thèse est de développer le concept de matrice de transmission d'un milieu multi-diffusant au cas plus général d'une onde polychromatique impulsionnelle ultra-brève. Dans ce manuscrit nous présentons et mesurons la matrice de transmission multi-spectrale d'un milieu complexe. Cette nouvelle matrice nous donne l'information fondamentale sur le couplage spatio-temporel et spatio-spectral que le milieu engendre au passage d'une onde ultra-brève. Elle permet aussi de contrôler une source monochromatique et polychromatique, après avoir traversé un milieu complexe, de manière déterministe. Nous exploitons ainsi cette connaissance du milieu pour compenser les distorsions du champs en focalisant, façonnant et contrôlant spatialement, spectralement et temporellement un laser ultra-bref grâce à la seule mesure d'une matrice de transmission multi-spectrale. Cette méthode ouvre les portes de plusieurs applications d'imagerie à travers des milieux complexes, ainsi que pour l'interaction lumière-matière en milieux diffusants
The transmission matrix allows to describe the effects generated by a multiple scattering medium on an incident monochromatic wave. The aim of the work presented in this dissertation is to develop the concept of transmission matrix of a multiple scattering medium to the more general case of a polychromatic ultra-fast pulsed light. In this dissertation we present and measure the multi-spectral transmission matrix of a complex medium. This new matrix describes the spatio-temporal coupling and the spatio-spectral coupling induced by the medium on a polycrhomatic illumination passing through it. The measurement of the multi-spectral transmission matrix allows us to control a monochromatic as well as a polychromatic source, after being scattered by the medium, in a deterministic way. We exploit this knowledge about the medium to compensate the distortions of the optical field by focusing, shaping and controlling spatially, spectrally and temporally an ultra-fast laser, thanks to the knowledge of the multi-spectral transmission matrix. This method paves the way towards many applications in the domain of imaging and light-matter interaction of light through complex media

Femtosecond laser filamentation is a highly nonlinear propagation mode. When a laser pulse propagates with a peak power exceeding a critical value Pcr (5 GW at 800 nm in air), the Kerr effect tends to collapse the beam until the intensity is high enough to ionize the medium, giving rise to plasma defocusing. A dynamic competition between these two effects takes place leaving a thin and weakly ionized plasma channel in the trail of the pulse. When an ultrafast laser pulse interacts with molecules, it will align them, spinning them about their axis of polarization. As the quantum rotational wave packet relaxes, the molecules will experience periodic field-free alignment. Recent work has demonstrated the effect of molecular alignment on laser filamentation of ultra-short pulses. Revival of the molecular alignment can modify filamentation parameters as it can locally modify the refractive index and the ionization rate. In this thesis, we demonstrate with simulations and experiments that these changes in the filament parameters (collapse distance and filament plasma length) can be used to probe molecular alignment in CO2.
M.S.
Masters
Optics and Photonics
Optics and Photonics
Optics

A técnica de eco de fótons com luz temporalmente incoerente (EFLI) foi utilizada neste trabalho para a medida do tempo de relaxação transversal T2 do Iodeto de 3-3´-Dietiloxadicarbonocianina (DODCI) como função da temperatura. Nestes experimentos foi utilizado um laser de corante de banda larga, bombeado pelo 2° harmônico de um laser de Nd+3 : YAG Q-switched. Este laser operou com os corantes Kiton Red 620 e rodamina 640, cujos máximos do espectro de emissão estão respectivamente em 598 e 610 nm. O tempo de relaxação T2, que é proporcional ao inverso da largura de linha homogênea, segue uma dependência funcional com a temperatura do tipo T-1,9. Encontramos o valor de T2 entre 0 e 30fs para λ = 598nm e entre 30 e 590fs para λ = 610nm, no intervalo de temperatura entre 300 e 60k. Os perfis das medidas de EFLI podem ser descritos por um modelo baseado num sistema quântico de dois níveis
The photon echo with incoherent light technique (EFLI) has been used in this work for the measurement of the transverse relaxation time T2 in 3-3´-Dietiloxadicarboncyanine Iodide (DODCI) as a function of the temperature. A broad-band dye laser, pumped by the second harmonic of a Q-switched Nd+3 : YAG laser, was used in this experiment. The laser used Kiton Red 620 and rodamine 640 dyes, whose maxima output power are respectively around 598 and 610nm. The relaxation time T2 , which is inversely proportional to the homogeneous linewidth, depends on the temperature according to a T-1,9 Law. We found the value of T2 ranging from 0 to 30fs at 598nm and from 30 to 590fs at 610nm in the temperature range between 300 and 60K. The EFLI profiles can be described by means of a two-level quantum system model

Short optical pulses and engineered nonlinear media is a powerful combination. Mode locked pulses exhibit high peak powers and short pulse duration and the engineered ferro-electric KTiOPO4 facilitates several different nonlinear processes. In this work we investigate the use of structured, second-order materials for generation, characterization and frequency conversion of short optical pulses. By cascading second harmonic generation and difference frequency generation the optical Kerr effect was emulated and two different Nd-based laser cavities were mode locked by the cascaded Kerr lensing effect. In one of the cavities 2.8 ps short pulses were generated and a strong pulse shortening took place through the interplay of the cavity design and the group velocity mismatch in the nonlinear crystal. The other laser had a hybrid mode locking scheme with active electro-optic modulation and passive cascaded Kerr lensing incorporated in a single partially poled KTP crystal. The long pulses from the active modulation were shortened when the passive mode locking started and 6.9 ps short pulses were generated. High-efficiency frequency conversion is not a trivial task in periodically poled materials for short pulses due to the large group velocity mismatch. Optimization of parameters such as the focussing condition and the crystal temperature allowed us to demonstrate 64% conversion efficiency by frequency doubling the fs pulses from a Yb:KYW laser in a single pass configuration. Quasi phase matching also offers new possibilities for nonlinear interactions. We demonstrated that it is possible to simultaneously utilize several phase matched second harmonic interactions, resulting in a dual-polarization second harmonic beam. Short pulse duration of the fundamental wave is a key parameter in the novel method that we demonstrated for characterization of the nonlinearity of periodically poled crystals. The method utilizes the group velocity mismatch between the two polarizations in a type II second harmonic generation configuration. The domain walls of PPKTP exhibit second order nonlinearities that are forbidden in the bulk material. This we used in a single shot frequency resolved optical gating arrangement. The spectral resolution came from Čerenkov phase matching, a non-collinear phase matching scheme that exhibits a substantial angular dispersion. The second harmonic light was imaged upon a CCD camera and with the spectral distribution on one axis and the temporal autocorrelation on the other. From this image we retrieved the full temporal profile of the fundamental pulse, as well as the phase. The spectral dispersion provided by the Čerenkov phase matching was large enough to characterize optical pulses as long as ~200 fs in a compact setup. The Čerenkov frequency resolved optical gating method samples a thin stripe of the beam, i.e. the area close to the domain wall. This provides the means for high spatial resolution measurements of the spectral-temporal characteristics of ultrafast optical fields.
QC 20100831

En passant de l’état massif à la nanoparticule les matériaux métalliques voient certaines de leurs caractéristiques modifiées de manière notable comme par exemple les propriétés optiques avec l’apparition d’une résonance dans le spectre optique, la Résonance Plasmon de Surface Localisée (RPSL) responsable du changement de couleur des nanoparticules métalliques. Les propriétés vibrationnelles et thermiques de nanoparticules métalliques ont été étudiées à l’aide d’une technique de Spectroscopie Femtoseconde. Nous avons montré qu’il était possible d’exciter et de détecter optiquement des fréquences de vibrations mécaniques dans le domaine térahertz pour des nanoparticules de platine composées de moins de cent atomes. D’autre part l’augmentation des effets dus aux interfaces a été mis en évidence sur les propriétés thermiques de nanoparticules d’or et d’argent. La résistance thermique à l’interface, résistance de Kapitza, voit son rôle augmenter lors du transfert thermique à l’échelle nanométrique. Une corrélation entre les valeurs mesurées et les impédances acoustiques des matériaux composants les interfaces a été mise en évidence. Nous avons aussi montré qu’elle augmente quand la température diminue de 300K à 70K. Les propriétés optiques de nanoparticules non sphériques ont été étudiées à l’aide de la Spectroscopie à Modulation Spatiale. Cette technique a permis de repérer puis de caractériser des nano-bâtonnets d’or individuels. Nous avons montré que la largeur spectrale de la RPSL est fortement dépendante de la géométrie des nanoparticules (diamètre et longueur). Cette double dépendance n’est pas prédite par les modèles classiques ou quantique existants
The size reduction of metals, from bulk to nanoparticles, induces significant modifications of their properties. For instance, the optical properties evolve and a new resonance, the localized surface plasmon resonance, appears in the optical spectrum and is responsible for the change of colors of metallic nanoparticles. This work is focused on studies of metals’ properties at the nanometric scale. In the first part, the vibrational and thermal properties are studied with a femtosecond spectroscopy technique. It is shown that it is possible to excite and detect optically vibrational frequencies in the terahertz domain by studying platinum nanoparticles formed by less than 100 atoms. The study of the thermal properties of the metallic nanoparticles (gold and silver) has shown that the boundary effect increases. This thermal boundary resistance, known as the Kapitza resistance, plays a dominant role in the heat transfer at the nanometric scale. A correlation between the experimental values of the thermal boundary resistance and the acoustic impedances of the boundary’s materials has been found. We have also shown that the Kapitza resistance is a decreasing function of the temperature in the 70-300K range. In the second part, the effect of the size reduction on the optical properties of non-spherical nanoparticles is observed. The Spatial Modulation Spectroscopy technique is used in order to locate and study individual gold nanorods. It is shown that the two geometrical parameters (the length and the diameter) of the nanorods influence the spectral linewidth of the localized surface plasmon resonance. This effect is not predicted by existing classical or quantum models

Up to 20% of breast cancer patients who undergo presurgical (neoadjuvant) chemotherapy have no response to treatment. Standard-of-care imaging modalities, including MRI, CT, mammography, and ultrasound, measure anatomical features and tumor size that reveal response only after months of treatment. Recently, non-invasive, near-infrared optical markers have shown promise in indicating the efficacy of treatment at the outset of the chemotherapy treatment. For example, frequency domain Diffuse Optical Spectroscopic Imaging (DOSI) can be used to characterize the optical scattering and absorption properties of thick tissue, including breast tumors. These parameters can then be used to calculate tissue concentrations of chromophores, including oxyhemoglobin, deoxyhemoglobin, water, and lipids. Tumors differ in hemoglobin concentration, as compared with healthy background tissue, and changes in hemoglobin concentration during neoadjuvant chemotherapy have been shown to correlate with efficacy of treatment. Using DOSI early in treatment to measure chromophore concentrations may be a powerful tool for guiding neoadjuvant chemotherapy treatment. Previous frequency-domain DOSI systems have been limited by large device footprints, complex electronics, high costs, and slow acquisition speeds, all of which complicate access to patients in the clinical setting. In this work a new digital DOSI (dDOSI) system has been developed, which is relatively inexpensive and compact, allowing for use at the bedside, while providing unprecedented measurement speeds. The system builds on, and significantly advances, previous dDOSI setups developed by our group and, for the first time, utilizes hardware-integrated custom board-level direct digital synthesizers (DDS) and analog to digital converters (ADC) to generate and directly measure signals utilizing undersampling techniques. The dDOSI system takes high-speed optical measurements by utilizing wavelength multiplexing while sweeping through hundreds of modulation frequencies in tens of milliseconds. The new dDOSI system is fast, inexpensive, and compact without compromising accuracy and precision.

Exciton dynamics in semiconductor nanostructures are dominated by the effects of many-body physics. The application of coherent spectroscopic tools, such as two-dimensional Fourier transform spectroscopy (2dFTS), to the study of these systems can reveal signatures of these effects, and in combination with sophisticated theoretical modeling, can lead to more complete understanding of the behaviour of these systems. 2dFTS has previously been applied to the study of GaAs quantum well samples. In this thesis, we outline a precis of the technique before describing our own experiments using 2dFTS in a partially collinear geometry. This geometry has previously been used to study chemical systems, but we believe these experiments to be the first such performed on semiconductor samples. We extend this technique to a reflection mode 2dFTS experiment, which we believe to be the first such measurement. In order to extend the techniques of coherent spectroscopy to structured systems, we construct an experimental apparatus that permits us to control the beam geometry used to perform four-wave mixing reflection measurements. To isolate extremely weak signals from intense background fields, we extend a conventional lock-in detection scheme to one that treats the optical fields exciting the sample on an unequal footing. To the best of our knowledge, these measurements represent a novel spectroscopic tool that has not previously been described.
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In this work the transient non-instantaneous polarization, i.e., laser-pulse injected small polarons and self-trapped excitons, is studied in the perovskite-like ferroelectric lithium niobate. The investigations span a time scale from femtoseconds to several hours. It is shown that the established small polaron picture is not able to describe transient absorption and photoluminescence of lithium niobate consistently. Several strong indications are presented demonstrating that the photoluminescence cannot be caused by geminate small polaron annihilation. Instead, the idea of radiatively decaying self-trapped excitons at the origin of the blue-green photoluminescence is revived. Excitons pinned on defect sites are proposed to lead to the already observed long-lived transient absorption in the blue spectral range in Mg- and Fe-doped crystals. Excitons pinned on iron-defects are studied in more detail. Their spectral fingerprint and absorption cross section is determined. Furthermore, it is shown that the occurrence of these pinned STEs can be tailored by chemical treatment of the samples and the experimental parameters such as the pump pulse intensity and photon energy. Based on the new experimental results and reviewing data published in literature, an atomistic picture of hopping and pinning of self-trapped excitons in lithium niobate is proposed. The question is addressed whether small polarons and self-trapped excitons in lithium niobate are coupled species in the sense that oppositely-charged polarons may merge into self-trapped excitons or STEs break into small polaron pairs. Decay kinetics of transient absorption and luminescence assigned to free small polarons and STEs indicate that this is not the case. For a more complete picture the ultrafast time scale is investigated as well. The formation times of small polarons and STEs are determined, which both lie in the range of 200 fs. No indications are found on the (sub)picosecond time scale indicating a coupling of both quasi-particle species either. In order to gain access to the formation of self-trapped excitons a custom-built femtosecond broadband fluorescence upconversion spectrometer is installed. Based on an already existing scheme, it is adapted to the inspection of weakly luminescent solid samples by changing to an all reflective geometry for luminescence collection. To avoid the necessity for an experimentally determined photometric correction of the used setup, an already established calculation method is extended considering the finite spectral bandwidth of the gate pulses. The findings presented here are important not only as fundamental research, but also regarding the technical application of lithium niobate and other similar nonlinear optical crystals. The simultaneous occurrence of both small polarons and self-trapped excitons is a rather rarely described phenomenon. Usually, the optical response of wide band gap oxide dielectrics is associated with only one of these quasi-particle species. This work may therefore be a stimulus to review the existing microscopic models for transient phenomena in other oxide dielectrics, which may help to improve their application in nonlinear optical and electro-optical devices. In this context the ultrafast transient photoluminescence spectroscopy established here for weakly luminescing solid samples may again provide valuable insight. With respect to lithium niobate, the results do not only resolve inconsistencies between the microscopic pictures described in literature, but also provide information regarding the extends to which the propagation of ultrashort laser pulses may be affected by (pinned-)STE absorption. It is shown that tailoring of the long-lived absorption center in the blue spectral range is possible, which may be used to avoid optical damage when high repetition rates are applied. It is important to emphasize that the microscopic model proposed in this work is mainly based on experimental indications. It is the task of further detailed theoretical investigations, e.g., via time-dependent density functional theory, to test whether the proposed model is justified. From an experimental perspective the important question remains whether (pinned-)STEs contribute to a photorefractive effect. In the experimentally easily accessible spectral range no absorption feature of mobile STEs is observed. As a complementary experimental technique, ultrafast holographic spectroscopy may reveal an excitonic contribution to photorefraction and provide further insight to STE transport and pinning phenomena.