La thèse se positionne dans le domaine des molécules ultra-froides, c’est-à-dire des molécules qui ont des vitesses correspondant à des températures de l’ordre du µK. L’obtention de gaz dilués moléculaires à ces températures peut ouvrir la porte à des applications importantes en simulation ou en informatique quantique. La thèse s’intéresse plus particulièrement à la formation de molécules dipolaires électriques et magnétiques. Celles-ci sont présagées pour être un système idéal dans l’optique d’un simulateur quantique du système réseau-spin, permettant de décrire le magnétisme dans les solides. Nous avons choisi l’exemple de la molécule RbSr qui fait l’objet actuellement d’une expérience à Amsterdam. Nous avons donc exploré plusieurs alternatives basées sur l’emploi de laser pour la formation de molécules RbSr ultra-froides Nous avons d’abord considéré la photoassociation dont le principe est de coupler l’état de collision initial avec un état rovibrationnel d’un état électronique excité. L’étape d’émission spontanée qui suit forme des molécules dans l’état électronique fondamental. Nous avons également considéré le problème des pertes supplémentaires d’atomes lorsque le laser de photoassociation est intense et focalisé, mises en évidence dans une expérience à Bangalore. Dans la suite de la thèse, nous avons exploré des méthodes cohérentes. Nous avons montré que des molécules faiblement liées de RbSr peuvent être formées à l’aide d’un STIRAP en partant de paires d’atomes isolées et confinées dans un isolant de Mott. Nous avons ensuite étudié leur stabilisation vers le niveau le plus profond de l’état fondamental de la molécule à l’aide d’un second STIRAP. Enfin, nous avons étudié des méthodes se déroulant uniquement dans l’état électronique fondamental. La formation est induite par l’utilisation d’une impulsion à dérive de fréquence induisant un passage adiabatique ou à l’aide d’une impulsion-pi. En plus, nous avons découvert que cette méthode formation peut être reliée à une résonance de Feshbach dans la représentation habillée par les photons, que nous avons appelée Résonance de Feshbach auto-induité assistée par Laser (LASIFR en anglais). Nous montrons qu’elles sont un outil prometteur et puissant pour le contrôle des propriétés de mélange de gaz d’atomes ultra-froids, comme par exemple la longueur de diffusion. / The thesis is positioned in the ultracold domain, i.e molecules which have velocities corresponding to microkelvin temperatures. The formation of molecular diluted gas at these temperatures is promising for important applications in quantum simulation, quantum information or in precision measurements.More particularly, the thesis is focused on the formation of molecules which are polar and paramagnetic. Some recent works are predicted that these molecules could be the ideal system for creating a quantum simulator of the lattice-spin system, which can describe the magnetism in solids. We have chosen the example of RbSr molecules for whose an experience runs in Amsterdam. We explored some alternatives based on the use of lasers for the formation of ultracold RbSr molecules.First, we considered the photoassociation whose the principle is coupling the initial scattering state with a rovibrational level of an excited electronic state. The following spontaneous emission step creates molecules in the electronic ground state. We also considered the problem of atom losses observed by experiments in Bangalore, when a focused photoassociation laser is applied. In the rest of the thesis, we explored coherent methods. Firstly, we showed a STIRAP sequence could create weakly bound molecules from isolated atomic pairs confined in a Mott insulator. Lastly, we explored some of these methods where the dynamic occurs only in the electronic ground state. The formation is induced by the use of a chirped pulse or a pi-pulse. We studied the factors of the transfer. Moreover, we discovered this method is related to a new kind of Feshbach resonances in the photon dressed picture, called Laser Assisted Self-Induced Feshbach Resonance (LASIFR). We showed LASIFR present the advantages of Magnetic and Optical Feshbach Resonances. They are a promising and powerful tool for the control of properties of quantum gas mixtures, like the interspecies scattering length.

No / We show that the multidimensional Zeno effect combined with non-holonomic control allows one to efficiently protect quantum systems from decoherence by a method similar to classical random coding. The method is applicable to arbitrary error-inducing Hamiltonians and general quantum systems. The quantum encoding approaches the Hamming upper bound for large dimension increases. Applicability of the method is demonstrated with a seven-qubit toy computer.

Thesis (MSc)--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: The selective excitation of an arbitrary vibrational level of a polyatomic molecule, without passage through an intermediary electronic excited state is demonstrated. This was achieved by simulating the interaction of a shaped, femtosecond pulse with one vibrational mode of the molecule. The carrier frequency of the pulse is chosen near resonant to the ground-to- rst-excited vibrational transition of the mode, and the pulse shape is optimized via closed-loop feedback. The simulation concentrates on the rst few vibrationally excited states since the density of states is still low, thus ensuring that the inter-vibrational decoherence time is relatively long compared to the pulse length. While various molecules were investigated this study focuses onUF6 for which detailed spectroscopic data for the v3 vibrational mode is available in literature. A multilevel model was developed and can be adapted for any number of levels. The model reported here was limited to a vibrational quantum number of four. The spectroscopic data included anharmonic splitting as well as forbidden transitions. The effect of rotational levels was not included. A density matrix approach was followed because this will allow for the introduction of dephasing of the coherent excitation via thermalizing collisions with the reservoir, as well as inter-vibrational relaxation. The time evolution of the density matrix is given by the Von Neumann equations. / AFRIKAANSE OPSOMMING: Die selektiewe opwekking van 'n arbitrêre vibrasionele vlak van 'n poliatomies molekule sonder oorgang na 'n intermediëre elektroniese opgewekte toetstand word gedemonstreer. Dit was bereik deur die interaksie te simuleer van 'n gevormde, femtosekonde pulse met een vibrasionele mode van 'n molekule. Die draer frekwensie van die pulse is so gekies dat dit naby resonansie van die grond-tot-eerste-opgewekte vibrasionele oorgang van die mode is, die puls vorm word geoptimeer deur 'n geslote-lus terugvoer. Die simulasie konsentreer op die eerste paar vibrasionele opgewekte toestande, omdat die digtheid van toestande nog steeds laag is, dus verseker dit dat inter-vibrasionele de-koherensie tyd relatief lank is in vergelyking met die puls se lengte. Verskillende molekules was ondersoek vir die studie. Die fokus is op UF6 waarvoor gedetaileerde spektroskopiese data vir die v3 vibrasionele beskikbaar is in die literatuur. 'n Multivlak model was ontwikkel en kan aangepas word vir enige aantal van vlakke. Die model wat hier aangemeld is, is beperk tot die vibrasionele kwantum getal van vier. Die spektroskopiese data het anharmonies splitting so wel as nie toegelaatbare oorgange bevat. Die effek van rotasionele vlakke was nie in berekening geneem nie. 'n Digtheids matriks benadering was gevolg, omdat dit toelaat vir die dekoherensie. Die tyd evolusie van die digtheids matriks word gegee deur die Von Neumann vergelykings.

In this thesis the control of ultrafast (10⁻¹³ s) optical pulses used for metrological applications has been investigated. Two different measurement set-ups have been considered, both based around the `pump-probe' technique, where an optical pulse is divided into two parts, one to `pump' or excite a physical system of interest, the other to `probe' or measure the outcome. In both cases the measurement uses electro-optic sampling (EOS), where an electric field is measured by detecting changes in the optical probe pulse polarisation after interaction with the field. In the first study, a method for wavelength metrology in the terahertz (THz) region has been demonstrated by producing an optical pulse shaper and genetic algorithm to control pump pulses and so indirectly influence the THz spectra they generate. In the second study an OPO (optical parametric oscillator) has been developed to provide ultrafast optical pulses for the generation of < 100 fs electrical pulses for metrology using quantum interference control (QUIC). QUIC electrical signals have been demonstrated successfully by charge accumulation measurements and the QUIC electrical pulse temporally measured using EOS, though the low signal levels due to power restrictions mean the QUIC electrical pulse is unsuitable for metrology at this time. Finally, a portable optical pulse measurement device based around frequency-resolved optical gating (FROG) has been designed, built and tested. This has been shown to be capable of retrieving amplitude and phase information in both the temporal and spectral domains for optical pulses as short as 20 fs duration. The ability to characterise shaped pulses also has been demonstrated successfully, with the requirements for full automation identified.

A larga banda espectral, característica de pulsos ultracurtos de luz laser, tem possibilitado o controle coerente da interação da luz com a matéria através da manipulação das componentes espectrais da fase do pulso. Esta nova área de pesquisa tem sido responsável por avanços no entendimento e controle de fenômenos foto-induzidos, especialmente no que diz respeito a processos ópticos não lineares. Nesta tese de doutorado, estudamos o controle coerente da absorção de dois fótons (A2F) em compostos orgânicos usando pulsos de femtossegundos. O processo de A2F em derivados de perilenos foi investigado utilizando pulsos com chirp linear (máscara de fase quadrática), a partir do monitoramento da fluorescência excitada por dois fótons. A otimização da A2F desses compostos, através da formatação da fase do pulso via algorítmo genético, revelou que pulsos limitados por transformada de Fourier induzem maior A2F. Cálculos de Química Quântica, empregando o formalismo da teoria do funcional densidade, foram utilizados para caracterizar a estrutura eletrônica e determinar as transições permitidas por dois fótons nos derivados de perilenos, fundamentando nossos resultados experimentais. Além disso, estudamos também o controle coerente da A2F de compostos orgânicos aplicando uma máscara de fase senoidal ao pulso. Neste caso, demonstramos que a eficiência do controle depende da posição relativa entre o comprimento de onda central do pulso e da banda de A2F do material. Finalmente, o controle coerente da A2F foi investigado com o uso de uma máscara de fase do tipo degrau. Nossos resultados evidenciam a importância da relação entre a banda espectral do pulso e a largura de linha da A2F do material para atingir o controle da A2F. Em conclusão, os resultados obtidos neste trabalho ajudam a esclarecer aspectos do controle coerente, obtido com pulsos ultracurtos formatados, em sistemas moleculares. / The broad spectral band of ultrashort laser pulses has been used to coherently control the lightmatter interaction, by acting on the spectral phase of pulses using the so called pulse shaping methods. This new research area has been held responsible for advances in the understanding and controlling of photo-induced phenomena, especially in nonlinear optics. In this work, we studied the coherent control of two-photon absorption (2PA) processes in organic compounds, employing femtosecond pulses. We investigated the 2PA of perylene derivatives using chirped pulses (quadratic phase mask), by monitoring the two-photon excited fluorescence. Optimization of 2PA in perylene derivatives was achieved by shaping the pulse using a genetic algorithm, which revealed that Fourier transform limited pulses lead to higher 2PA. Quantum chemical calculations, using Density Functional Theory, were carried out to characterize the electronic structure and determine the allowed two-photon transitions of perylene derivatives, backing up our experimental results. Furthermore, we also studied the coherent control of 2PA in organic molecules applying a cosine-like phase mask. In such case, we demonstrated that the control efficiency depends on the detuning between the pulse central wavelength and materials 2PA band. Finally, coherent control of 2PA was explored using a step-like phase mask. Our results indicate that, in this situation, control of 2PA is only attained if a specific ratio between the pulse bandwidth and the 2PA transition bandwidth is used. In conclusion, the results obtained in this work help the understanding of coherent control in molecular systems.

Neste trabalho, investigamos o controle coerente das bandas de emissão, excitadas via absorção multifotônica, em um cristal de óxido de Zinco (ZnO) através das formatação de pulsos laser ultracurtos (790 nm, 30 fs, 80 MHz e 5 nJ). O ZnO vem se mostrado um possível candidato a dispositivos fotônicos devido a sua grande energia de ligação de éxciton (60 meV).Inicialmente, implementamos a montagem experimental do sistema de formatação de pulsos, bem como de excitação e coleta da fluorescência do ZnO. O controle coerente foi feito através de um programa baseado em um algoritmo genético (GA), também desenvolvido no transcorrer deste trabalho. Através do algoritmo genético, observamos um ganho significativo da emissão do ZnO por meio de fases espectrais impostas ao pulso laser. Monitorando o traço de autocorrelação do pulso, inferimos que este se torna mais longo após a otimização das bandas de emissão via GA. Além disso, verificamos que as funções de fase que otimizam o processo são complexas e oscilatórias. Através da análise das componentes principais (PCA), fizemos uma análise do conjunto de dados providos pelo GA, onde observamos que este método pode ser usado como um filtro para os dados, suavizando as curvas e enfatizando os aspectos mais importantes das máscaras de fase obtidas pelo controle coerente. Por fim investigamos qual a importância das máscaras suavizadas para o entendimento físico do processo. / In this work, we investigate the coherent control of the emission bands, excited via multiphoton absorption, in a zinc oxide crystal (ZnO) by pulse shaping ultrashort laser pulses (790 nm, 30 fs, 5 nJ and 80 MHz). ZnO has been preposed as a potential material for photonic devices due to its strong exciton binding energy(60 meV). Initially, we have implemented the pulse shaper experimental setup, as well as the fluorescence measurements of ZnO. The coherent control was carried out through genetic algorithm (GA) based software, also developed in the course of this work. Using the genetic algorithm, we have observed a significant increase in the ZnO emission when appropriated spectral phase masks are applied to the laser pulse. Autocorrelation measurements were used to infer the pulse duration, which get longer after optimization of the emission band via GA. Additionally, we have found that the phase masks that optimize the process are complex oscillatory functions. Through the Principal Component Analysis, we analyzed the data provided by the GA and observed that it can be used to filter the data, smoothing the curves and highlighting the most important aspects of phase masks obtained by the coherent control. Finally we investigate how important the smoothed masks are for the physical understanding of the process.

We introduce an approach to implement full coherent control on nanometer length scales. It is based on spatiotemporal modulation of the surface plasmon polariton (SPP) fields at the thick edge of a nanowedge. The SPP wavepackets propagating toward the sharp edge of this nanowedge are compressed and adiabatically concentrated at a nanofocus, forming an ultrashort pulse of local fields. The profile of the focused waveform as a function of time and one spatial dimension is completely coherently controlled. We establish the principal limits for the nanoconcentration of the terahertz (THz) radiation in metal/dielectric waveguides and determine their optimum shapes required for this nanoconcentration. We predict that the adiabatic compression of THz radiation from the initial spot size of vacuum wavelength R λ 300 μm 0 0 ≈ ≈ to the unprecedented final size of R = 100 − 250 nm can be achieved, while the THz radiation intensity is increased by a factor of 10 to 250. This THz energy nanoconcentration will not only improve the spatial resolution and increase the signal/noise ratio for THz imaging and spectroscopy, but in combination with the recently developed sources of powerful THz pulses, will allow the observation of nonlinear THz effects and a variety of nonlinear spectroscopies (such as two-dimensional spectroscopy), which are highly informative. This should find a wide spectrum of applications in science, engineering, biomedical research and environmental monitoring. We also develop a theory of the spoof plasmons propagating at the interface between a dielectric and a real conductor. The deviation from a perfect conductor is introduced through a finite skin depth. The possibilities of guiding and focusing of spoof plasmons are considered. Geometrical parameters of the structure are found which provide a good guiding of such modes. Moreover, the limit on the concentration by means of planar spoof plasmons in case of non-ideal metal is established. These properties of spoof plasmons are of great interest for THz technology.

We introduce an approach to implement full coherent control on nanometer length scales. It is based on spatiotemporal modulation of the surface plasmon polariton (SPP) fields at the thick edge of a nanowedge. The SPP wavepackets propagating toward the sharp edge of this nanowedge are compressed and adiabatically concentrated at a nanofocus, forming an ultrashort pulse of local fields. The profile of the focused waveform as a function of time and one spatial dimension is completely coherently controlled. We establish the principal limits for the nanoconcentration of the terahertz (THz) radiation in metal/dielectric waveguides and determine their optimum shapes required for this nanoconcentration. We predict that the adiabatic compression of THz radiation from the initial spot size of vacuum wavelength ~300 μm to the unprecedented final size of 100-250 nm can be achieved, while the THz radiation intensity is increased by a factor of 10 to 250. This THz energy nanoconcentration will not only improve the spatial resolution and increase the signal/noise ratio for THz imaging and spectroscopy, but in combination with the recently developed sources of powerful THz pulses, will allow the observation of nonlinear THz effects and a variety of nonlinear spectroscopies (such as two-dimensional spectroscopy), which are highly informative. This should find a wide spectrum of applications in science, engineering, biomedical research and environmental monitoring. We also develop a theory of the spoof plasmons propagating at the interface between a dielectric and a real conductor. The deviation from a perfect conductor is introduced through a finite skin depth. The possibilities of guiding and focusing of spoof plasmons are considered. Geometrical parameters of the structure are found which provide a good guiding of such modes. Moreover, the limit on the concentration by means of planar spoof plasmons in case of non-ideal metal is established. These properties of spoof plasmons are of great interest for THz technology.

A larga banda espectral, característica de pulsos ultracurtos de luz laser, tem possibilitado o controle coerente da interação da luz com a matéria através da manipulação das componentes espectrais da fase do pulso. Esta nova área de pesquisa tem sido responsável por avanços no entendimento e controle de fenômenos foto-induzidos, especialmente no que diz respeito a processos ópticos não lineares. Nesta tese de doutorado, estudamos o controle coerente da absorção de dois fótons (A2F) em compostos orgânicos usando pulsos de femtossegundos. O processo de A2F em derivados de perilenos foi investigado utilizando pulsos com chirp linear (máscara de fase quadrática), a partir do monitoramento da fluorescência excitada por dois fótons. A otimização da A2F desses compostos, através da formatação da fase do pulso via algorítmo genético, revelou que pulsos limitados por transformada de Fourier induzem maior A2F. Cálculos de Química Quântica, empregando o formalismo da teoria do funcional densidade, foram utilizados para caracterizar a estrutura eletrônica e determinar as transições permitidas por dois fótons nos derivados de perilenos, fundamentando nossos resultados experimentais. Além disso, estudamos também o controle coerente da A2F de compostos orgânicos aplicando uma máscara de fase senoidal ao pulso. Neste caso, demonstramos que a eficiência do controle depende da posição relativa entre o comprimento de onda central do pulso e da banda de A2F do material. Finalmente, o controle coerente da A2F foi investigado com o uso de uma máscara de fase do tipo degrau. Nossos resultados evidenciam a importância da relação entre a banda espectral do pulso e a largura de linha da A2F do material para atingir o controle da A2F. Em conclusão, os resultados obtidos neste trabalho ajudam a esclarecer aspectos do controle coerente, obtido com pulsos ultracurtos formatados, em sistemas moleculares. / The broad spectral band of ultrashort laser pulses has been used to coherently control the lightmatter interaction, by acting on the spectral phase of pulses using the so called pulse shaping methods. This new research area has been held responsible for advances in the understanding and controlling of photo-induced phenomena, especially in nonlinear optics. In this work, we studied the coherent control of two-photon absorption (2PA) processes in organic compounds, employing femtosecond pulses. We investigated the 2PA of perylene derivatives using chirped pulses (quadratic phase mask), by monitoring the two-photon excited fluorescence. Optimization of 2PA in perylene derivatives was achieved by shaping the pulse using a genetic algorithm, which revealed that Fourier transform limited pulses lead to higher 2PA. Quantum chemical calculations, using Density Functional Theory, were carried out to characterize the electronic structure and determine the allowed two-photon transitions of perylene derivatives, backing up our experimental results. Furthermore, we also studied the coherent control of 2PA in organic molecules applying a cosine-like phase mask. In such case, we demonstrated that the control efficiency depends on the detuning between the pulse central wavelength and materials 2PA band. Finally, coherent control of 2PA was explored using a step-like phase mask. Our results indicate that, in this situation, control of 2PA is only attained if a specific ratio between the pulse bandwidth and the 2PA transition bandwidth is used. In conclusion, the results obtained in this work help the understanding of coherent control in molecular systems.

Neste trabalho, investigamos o controle coerente das bandas de emissão, excitadas via absorção multifotônica, em um cristal de óxido de Zinco (ZnO) através das formatação de pulsos laser ultracurtos (790 nm, 30 fs, 80 MHz e 5 nJ). O ZnO vem se mostrado um possível candidato a dispositivos fotônicos devido a sua grande energia de ligação de éxciton (60 meV).Inicialmente, implementamos a montagem experimental do sistema de formatação de pulsos, bem como de excitação e coleta da fluorescência do ZnO. O controle coerente foi feito através de um programa baseado em um algoritmo genético (GA), também desenvolvido no transcorrer deste trabalho. Através do algoritmo genético, observamos um ganho significativo da emissão do ZnO por meio de fases espectrais impostas ao pulso laser. Monitorando o traço de autocorrelação do pulso, inferimos que este se torna mais longo após a otimização das bandas de emissão via GA. Além disso, verificamos que as funções de fase que otimizam o processo são complexas e oscilatórias. Através da análise das componentes principais (PCA), fizemos uma análise do conjunto de dados providos pelo GA, onde observamos que este método pode ser usado como um filtro para os dados, suavizando as curvas e enfatizando os aspectos mais importantes das máscaras de fase obtidas pelo controle coerente. Por fim investigamos qual a importância das máscaras suavizadas para o entendimento físico do processo. / In this work, we investigate the coherent control of the emission bands, excited via multiphoton absorption, in a zinc oxide crystal (ZnO) by pulse shaping ultrashort laser pulses (790 nm, 30 fs, 5 nJ and 80 MHz). ZnO has been preposed as a potential material for photonic devices due to its strong exciton binding energy(60 meV). Initially, we have implemented the pulse shaper experimental setup, as well as the fluorescence measurements of ZnO. The coherent control was carried out through genetic algorithm (GA) based software, also developed in the course of this work. Using the genetic algorithm, we have observed a significant increase in the ZnO emission when appropriated spectral phase masks are applied to the laser pulse. Autocorrelation measurements were used to infer the pulse duration, which get longer after optimization of the emission band via GA. Additionally, we have found that the phase masks that optimize the process are complex oscillatory functions. Through the Principal Component Analysis, we analyzed the data provided by the GA and observed that it can be used to filter the data, smoothing the curves and highlighting the most important aspects of phase masks obtained by the coherent control. Finally we investigate how important the smoothed masks are for the physical understanding of the process.