Les lasers femtosecondes à modes bloqués révèlent une structure spectrale de peigne de fréquence, couvrant plusieurs dizaines de THz. Mon travail de thèse s'est concentré sur l'étude et la mise en place d'un dispositif optique couplant le peigne laser dans une cavité optique. Le peigne et les résonances de cavités y sont délibérément désaccordés à la manière d'un Vernier, faisant apparaitre dans la transmission spectrale de la cavité un Moiré de fréquence dont la périodicité est inversement proportionnelle à ce désaccord. La première partie présente un formalisme permettant une compréhension fine de ce couplage et identifiant deux régimes en filtrages dits de «haute» résolution, où la structure de peigne est entièrement résolue, et de «basse» résolution où la résolution est donnée par le désaccord. La seconde partie décrit la réalisation expérimentale de ce couplage, détaillant la stratégie d'asservissement employée afin de stabiliser les résonances de la cavité (F=3000) par rapport au peigne laser au kHz. Enfin, ce couplage est appliqué à la spectroscopie moléculaire. Les spectres mesurés de l'air ambiant, dans des temps d'acquisition d'une seconde, exploitent l'intégralité du spectre du laser, soit 40THz (750 850nm), avec une résolution de 2GHz. La sensibilité en absorption atteint 10−9 /cm après moyenne. Cette haute sensibilité résulte d'une immunité aux bruits de conversion fréquence-Amplitude du couplage Vernier «basse» résolution et permet l'obtention d'un rapport signal sur bruit supérieur à 104. Ces performances conduisent à établir une figure de mérite de 4 × 10−11 cm−1/ √ Hz, plaçant ce résultat au troisième rang de l'état de l'art international / Femtosecond mode-Locked lasers are generators of optical frequency ‘combs’, whose distinct frequencies cover many tens or hundreds of THz. My PhD work has focused on the study and construction of a particular coupling scheme in an optical cavity, named Vernier coupling. Here, the laser comb and the cavity resonances are deliberately mismatched, as a Vernier rule. This creates Moiré pattern in the cavity spectral transmission, with a periodicity related to the inverse of the mismatch. The first part details the theory behind the coupling of laser and optical cavity modes. Two regimes are identified, called “high” resolution Vernier filtering, when the laser comb structure is probed mode by mode, and “low” resolution filtering where the linewidth of one Vernier order is given by the mismatch. The second part describes the experimental realization of this coupling scheme. It details the locking strategy used to control the resonance position of the cavity (F=3000) in regards of the laser comb (kHz scale). Finally, I present spectra recorded with this setup, focusing on molecular spectroscopy. The spectra of ambient air are recorded in acquisition times around 1 s, that cover the full bandwidth of the femtosecond laser ( 40 THz, 750-850 nm), at 2 GHz resolution. The sensitivity of the absorption measurement reaches 10−9 /cm, with averaging. This high sensitivity comes from an immunity to the frequency-To-Amplitude noise conversion of the “low” resolution Vernier coupling, leading to a signal to noise ratio better than 104. These performances give the spectrometer figure of merit of 4×10−11 cm−1/√ Hz, currently taking third place in rank international state of the art ranking

Les appariements d’ions sontomniprésents dans la nature, des océans auxaérosols, et passant par les organismes vivants.Les paires d’ions présentes dans les solutionsriches en ions y jouent un rôle crucial, notammentdans le déroulement des mécanismes réactionnelschimiques et biochimiques. En dépit de leurimportance, la caractérisation expérimentale despaires en solution reste problématique en raison dela coexistence de plusieurs types. Ainsi, le premierobjectif de ce travail est de développer uneapproche originale en phase gazeuse, pour l’étudedes paires d’ions modèles entre un groupementcarboxylate et un cation alcalin, illustrant le typed’appariement observé dans le milieu biologique.Ces premières études sont menées à l’aide d’uneapproche de spectroscopie IR et UV sélective enconformation, combinée à des calculs au niveauchimie quantique sur des modèles de formulegénérale (C6H5-(CH2)n-COO-, M+; M = Li, Na, K,Rb, Cs et n ≤ 4). L’appariement entre les ions a étéainsi caractérisé sur l’ensemble de ces systèmes, etune compétition entre les interactions cation-anionet cation-π a été observée pour les systèmes de plusgrande taille.Dans un second temps, une étude théorique estdéveloppée dans l’objectif de proposer un spectrethéorique pour chaque type de paires, et de leconfronter aux spectres expérimentaux en solutionde la littérature. L’approche repose sur le calcul dela signature vibrationnelle de paires (CH3-COO-,M+; M = Li, Na) et de l’anion libre, entouréssuccessivement de molécules d’eau explicitesdécrites au niveau chimie quantique, puis auniveau champ de force et enfin par un modèle desolvant continu.Cet apport original pour l’étude des paires d’ionsneutres ouvre la voie vers une meilleurecaractérisation de ces paires dans les solutions électrolytiques. / Ion pairs are ubiquitous in nature andwere documented in sea waters, aerosols andliving organisms, thus they play a crucial role inmany chemical and biochemical mechanisms.Although many experimental approaches aimedat identifying these ion pairs in solutions, theyfailed in characterizing their microscopicproperties, mainly due to the coexistence of manytypes of pairs in solution. This original study aimsto identify the structural properties of ion pairmodels in the gas phase that depicts theinteraction between carboxylate group and alkalication, largely present in biological media.Model systems (M+, C6H5(CH2)nCOO-, M = Li,Na, K, Rb, Cs and n ≤ 4) were the subject of thefirst studies conducted using gas phaseconformational selective IR and UV spectroscopycombined to quantum chemistry calculations.Among the identified structures, we found asecondary interaction between the cation andphenyl ring (cation-π interaction) for systems witha flexible carbon chain, which led us to study thecompetition between cation-anion and cation-πinteractions.The second study developed focused mainly oncharacterizing ion pairs (M+, CH3COO-; M = Li,Na) and free anion in solution, where the firstsolvation layer were described at the quantumlevel, followed by a solvent continuum. Thus thevibrationnal signatures proposed for theseaggregates were compared to solution spectra inlitterature, thus offering a reliable structuralassignement.This study on neutral ion pairs paves way to a betterunderstanding of ion pairing and offers a uniqueapproach to adress the structural characterization ofthese systems in solution.

This Ph.D. thesis is focused on the study of optical nonlinearities and dynamics of excited charge carriers in monocrystalline diamond, nanocrystalline diamond and silicon. The dynamics of high density carriers in bulk diamond is investigated in detail (the transition from excitons and free carriers to electron-hole liquid or plasma). We study the picosecond dynamics of electron-hole liquid condensation using several techniques of time-resolved optical spectroscopy and demonstrate its evaporation by femtosecond laser pulses. We also propose two new optical techniques for measurement of lifetime, diffusion coefficient and surface recombination velocity of excitons in diamond. The results obtained by these techniques are described theoretically using diffusion equation and compared with the results obtained by the transient grating diffraction measurement. Further we study two- and three- photon absorption and nonlinear refractive index in diamond. In nanocrystalline diamond we study the second and third harmonic generation and its physical origin. In superlattices of silicon nanocrystals in SiO2 matrix we investigate the nonlinear transient absorption dynamics and carrier diffusion.

Title: Ultrafast spectroscopy of hybrid nanosystems Author: RNDr. Pavel Galář Department: Department of Chemical Physics and Optics Supervisor: prof. RNDr. Petr Malý, DrSc. Abstract: This Ph. D. thesis is focused on physical phenomena located at the interface of hybrid nanostructure composed of polycrystalline diamond and polymer polypyrrole. The main method used in our experimental study was ultrafast laser spectroscopy that allowed us to gain new findings about electron recombination processes in polycrystalline diamond layers, polypyrrole and in their hybrid structures. The research was focused on mutual influence of both components, especially through energy and charge transfer. In the first step of our research we carried out optical characterisation of different kinds of polypyrrole and complex study of recombination processes dynamics of photoexcited charge carriers in polycrystalline diamond. The measurements were realized by the methods of time-resolved photoluminescence and transmission spectroscopy in the time scale from picoseconds to milliseconds. On the basis of the obtained results the model explaining the origin of luminescence signal related to the different kinds of electron recombination processes in non- diamond phase and on surface defects of diamond grains in polycrystalline layers was...

In this thesis experimental samples of multiple quantum wells in the InGaN/GaN structures will be compared using methods of laser spectroscopy. In particular, the optical properties of the samples will be investigated. The samples were prepared under different conditions; therefore one of the aims is to compare them. The knowledge of the influence of preparation enables utilization not only for fundamental research, but also for the construction of radiation sources or scintillation detectors. Measurements of absorption and photoluminescence will be carried out and their dynamic properties will be measured as well. There will be examined the effect of different excitation power and different excitation wavelength on the intensity of photoluminescence. From dynamic properties there will be examined the effect of different excitation wavelength on the lifetime of the absorption and how does temperature influence the lifetime of the photoluminescence. Individual quantities will be compared amongst samples and their suitability for further applications will be discussed.

Nitrogen dioxide (NO₂) is a pollutant of interest for study both because of its controlling role in the oxidant capacity of the atmosphere and the health risks it poses. Concerns about the health effects of NO₂ and its role in forming deleterious atmospheric species have made it desirable to have low-cost, sensitive ambient measurements of NO₂. A continuous-wave laser-diode laser-induced fluorescence (LIF) system for NO₂ was developed here which operates at ambient pressure, thereby eliminating the need for an expensive pumping system. The current prototype system has achieved sensitivity several orders of magnitude beyond previous efforts at ambient pressure (limit of detection of 2 ppb, 60 s averaging time). Ambient measurements of NO₂ were made in Portland, Oregon using both the standard NO₂ chemiluminescence method and the LIF instrument and showed good agreement (r² = 0.92). In addition, investigations into surface mediated chemistry involving oxides of nitrogen (namely, NO₂) have stimulated new inquiry into potential heterogeneous sources of NO₂ as well as challenged the stability of permanent sinks for NO₂. The possibility that surface mediated chemistry plays a significant role in NOy chemistry in urban air has for the past few decades received considerable attention. The AP-LIF NO₂ instrument is uniquely suited to measure surface chemistry under near ambient conditions. The so called 'renoxification' reaction of gaseous NO with surface bound HNO₃ yielding NO₂ (2HNO₃(surface) + NO--> 3NO₂ +H₂O(surface)) was suggested as a potentially important source of NO₂ which also degraded the stability of nitric acid as a sink of active oxides of nitrogen. Yet, there is disagreement in the literature as to the importance of this reaction. The disagreement stems from differing measurements of the rate for the renoxification reaction. Because there are differences in experimental setups no one research group has studied the renoxification reaction under ambient conditions, i.e., at moderate concentrations of NOy and in a static cell held at 1 atm. In this work, the production of NO₂ was measured using a novel AP-LIF. This setup made it possible to measure the rate of production of NO₂ due to the heterogeneous reaction of NO with HNO₃ under ambient conditions. Under these conditions it was found that renoxification due to gas-phase NO on surface HNO₃ is not a significant source of NO₂. However, this study did show the importance of water vapor in the renoxification of surface HNO₃.

In recent years, spin relaxation, injection, and manipulation in semiconductors have attracted considerable interest because of several potential applications in "spintronic" devices and the necessity to understand and control spin-based phenomena. In light of the growing interest in spin-related phenomena and devices, there is now renewed interest in the science and engineering of narrow gap semiconductors (NGS). NGS based heterostructures are particularly interesting for spintronic applications due to their large spin-orbit coupling, which leads to considerable zero-field spin splitting. NGS are also candidates for electronic applications, such as high-speed and low-power microprocessors; as reported recently by Intel. Furthermore, as switching rates in electronic devices are pushed to even higher frequencies, it is important to understand dynamics in semiconductors such as NGS on femtosecond time-scales. In this thesis, time-resolved studies of magnetic and non-magnetic NGS using ultrafast-laser spectroscopy techniques such as pump-probe spectroscopy and magneto-optical Kerr/Faraday effect, are reported. Our samples include: InSb-based quantum wells with different confinement potentials; InMnSb films, the newest III-V ferromagnetic semiconductors; and InAs films. The samples for these studies have been provided by the groups of Prof. Santos at the University of Oklahoma, Prof. Furdyna at the University of Notre Dame, and Prof. Guido at Virginia Tech. The objectives in this thesis have been to: a) understand charge/spin dynamics in NGS with novel confinement potentials, b) probe the effect of magnetic impurities on the spin/charge dynamics, and c) develop concepts for spin based device applications. Several specific questions and concepts have been addressed including: the effect of large spin-orbit interaction in NGS on the dynamics, how large Rashba spin splitting in these materials affect the spin coherence life time, and carrier/spin dynamics in ferromagnetic semiconductor structures. / Ph. D.

When approaching a metal surface, the electronic structure of Rydberg atoms or molecules is perturbed by the surface potential and at close enough distances resonant ionisation of the Rydberg electron into the conduction band of the surface can occur. It is possible to interfere in this process and steer the ionisation distance by making use of the polarisability of the Rydberg orbital in the presence of electric fields. The resulting ions from the surface can extracted via electric fields and subsequently detected via well established ion detection schemes. The question of how this charge-transfer process is affected by different properties of the surface (both electronic and structural) represents the main aspect of the work presented in this thesis. At first, the charge transfer of atomic hydrogen Rydberg atoms with a flat gold metal surface is investigated. While such a surface might appear homogeneous, stray fields are present in its vicinity due to local variations in the surface work function. The surface ionisation process as a function of applied electric field is therefore measured experimentally and the results are compared with classical Monte-Carlo simulations (which include stray field effects). This way the possibility to utilize Rydberg states as a probe of the magnitude of such stray fields is demonstrated. To investigate the effect the surface structure can have on the ionisation process, the interaction of Rydberg atoms with surfaces covered by nanoparticles is investigated. Surface ionisation is measured at a 5 nm nanoparticle monolayer surface and it is shown that population transfer between surface- and vacuum-oriented Rydberg states occurs. In addition, results are presented, which suggest a dependence of the ionisation process on the relative size of Rydberg orbital and nanoparticle. Furthermore, charge transfer between a Rydberg state and discrete electronic states at the surface vacuum interface are investigated by performing experiments with a Cu(100) band-gap semiconductor surface. By analysing surface ionisation as a function of collisional velocity ionisation rates can be determined and are subsequently compared with theoretical predictions. The potential of identifying resonant ionisation is thereby demonstrated. Last, a new method to produce 2s atomic hydrogen via mixing of the 2s and 2p state in an electric field is proposed and first experimental results are presented, thus demonstrating viability of the idea. The experiments presented in this thesis represent the most in depth analysis of the charge-transfer process between atomic hydrogen Rydberg states and a range of different surfaces to date. As such, they demonstrate the potential of utilizing the unique properties of Rydberg states and their applicability as surface probes. In addition, these results pave the way for further experiments involving thin films or the phenomenon of quantum reflectivity.

This thesis presents the study of a selection of gas phase chemical processes using time-resolved Fourier transform infrared (FTIR) emission spectroscopy. Such processes include molecular energy transfer, chemical reaction and photodissociation. The major focus of this thesis was the investigation of collisional energy transfer from the electronically excited states of NO and OH, with particular attention paid to the fate of the electronic energy. NO A²&Sigma;⁺(v = 0) is prepared by laser excitation, pumping the overlapped Q₁ and P₂₁ band heads of the NO A-X (0,0) transition at 226.257 nm. The quenching of this state by O₂ and CO₂ was studied. Experiments were performed to investigate what channels contribute to the quenching process, the branching ratio of these different channels and the partitioning of energy among the various products. Quenching by O₂ was found to proceed mostly through non-reactive channels. High vibrational excitation of NO X ²&Pi; was observed, with population detected in v = 22, representing 79% of the available energy. The O₂ product was found to be formed in more than one electronic state: the ground state, X ³&Sigma;^-<sub style='position: relative; left: -.3em;'>g</sub>, and a high-lying electronically excited state, such as the A ³&Sigma;⁺<sub style='position: relative; left: -.5em;'>u</sub>, A' ³&Delta;_u or c ¹&Sigma;^-<sub style='position: relative; left: -.5em;'>u</sub> states. A reactive channel producing vibrationally excited NO₂ was observed, but was found to be a minor process with an upper limit of 18% for the branching ratio. In contrast the quenching of NO A ²&Sigma;⁺(v = 0) by CO₂ was found to proceed predominately by reaction, with a branching ratio of 76 %. While emission from NO₂ was observed, it was weak, and therefore it was concluded that the main reaction products were CO, O(³P) and NO X ²&Pi;(v = 0). The nascent strong CO₂ v3 emission band from the non-reactive channel exhibited a large red-shift from its fundamental position. This indicates that the CO₂ vibrational distribution is significantly hotter than statistical. Investigations were then performed studying the quenching of NO A ²&Sigma;⁺(v = 1) by NO and CO₂, with both systems exhibiting similar characteristics to the quenching of the ground vibrational level of NO A ²&Sigma;⁺. From comparison of the emission intensity of the CO fundamental and CO₂ v3 mode following quenching of the v = 0 and 1 levels of the NO A ²&Sigma;⁺ state, it was concluded that the branching ratio for reactive quenching was larger in the latter case. Secondly, experiments were performed to measure the rate constants for the quenching of NO A ²&Sigma;⁺(v = 0) by the noble gases. The noble gases are inefficient quenchers of electronically excited NO and therefore careful experimental design was required to minimise the influence of impurities on the results. All the rate constants were found to be of the order of 10^-14 cm³ molecule^-1 s^-1. The value for Xe was 50 times smaller than reported previously in the literature. In light of this new measurement, a re-analysis of experiments, performed previously in the group, on the electronic quenching of NO A ²&Sigma;⁺(v = 0) by Xe was performed. A very hot vibrational distribution of NO X ²&Pi; was obtained. Next, the collisional quenching of OH A ²&Sigma;⁺(v = 0) by H₂ was investigated. OH radicals were generated in situ by the photolysis of HNO₃ at 193 nm, which were excited to the A ²&Sigma;⁺(v = 0) state on the overlapped Q₁(1) and P₂₁(1) rotational lines at 307.935 nm. Reactive quenching was found to be the major pathway, in agreement with the literature. Copious emission from vibrationally excited water was observed. Comparison of this emission with theoretical calculations revealed a hotter distribution than predicted. It was concluded that the energy channelled into the vibrational modes of H₂O is in excess of 60% of the available energy. Experiments performed with D₂ allowed the non-reactive channel to be studied; a cold vibrational distribution of the OH X ²&Pi; was observed. Finally the reaction between CN radicals and cyclohexane was studied. CN was generated by the photolysis of ICN at 266 nm. Prompt emission from HCN in the C-H stretching region was observed meaning the new bond was formed in a vibrationally excited state. Analysis of the emission revealed HCN was populated up to v3 = 2. Excellent agreement with the results of a theoretical study of the system was found.

This thesis reports work extending the scope of a recently developed gas sensing technique, multi-mode absorption spectroscopy (MUMAS). The ability of MUMAS to simultaneously detect multiple species from a mixture is demonstrated for the first time. The technique is subsequently extended to mid-infrared wavelengths, realising large gains in sensitivity. A solid-state, multi-mode laser has been developed to provide a high-performance comb source for use with MUMAS. This in-house constructed, diode-pumped, Er/Yb:glass laser operates on 10 longitudinal modes, separated by 18 GHz and centred close to 1565 nm. The extensive development and prototyping work leading to this final laser design is described. Multi-species detection with MUMAS is reported for the first time, thus demonstrating the ability of this technique to perform multi-gas sensing using a single laser and simple detection scheme. The previously described Er/Yb multi-mode laser was used to record MUMAS signals from a sample containing CO, C₂H₂, and N₂O. The components of the mixture were detected simultaneously by identifying multiple transitions in each of the species. Temperature- and pressure-dependent modelled spectral fits to the data were used to determine the partial pressures of each species in the mixture with an uncertainty better than +/-2%. Multi-mode radiation has been successfully generated at 3.3 μm using quasi phase matched difference frequency generation (QPM-DFG). A mid-infrared laser comb was produced by optically mixing the near-infrared, multi-mode comb produced by the previously developed Er/Yb:glass laser with the single-mode output of a Nd:YAG laser operating at 1064 nm. This multi-frequency laser source was characterised to verify performance, and subsequently used to perform proof-of-principle MUMAS measurements on the strong transitions found in this spectral region. Spectra were recorded of NH₃ and CH₄ both individually and as components of a mixture. A minimum detection level for this system was determined to be 4.3 μbar m^-1 for CH₄, a sensitivity increase of 300 over similar measurements performed in the near-IR.