Technological progress in commercializing ultrafast lasers and detectors has allowed realization of cooperative processes on an ultrashort time scale, which demand a re-evaluation of the conventional cooperative phenomena with a new insight. Ultrafast cooperative phenomena in coherently prepared media and various applications of superfluorescence and coherent Raman scattering are studied in this dissertation. In particular, a simple theoretical testimony on analogy between a cooperative emission and coherent Raman scattering is presented by offering an opportunity to perform parallel research on these two processes from a unified point of view. On one hand, the superfluorescent pulse with a time duration of a few tens of picoseconds (ps) from alkali metal vapor is observed for the first time, even though cooperative phenomena in atomic vapor have been extensively studied for more than five decades. A dense rubidium vapor pumped by ultrashort (100 femtosecond, fs) pulses allows a realization of the ultrafast superfluorescence while a time-resolved study of superfluorescence is accomplished by using a streak camera with 2 ps time resolution. Experimental research on quantum nature of cooperative emissions has been “frozen” over the years (three decades) possibly because of the technical difficulties. Quantum fluctuations of superfluorescence development are explored experimentally by taking advantage of the ultra fast streak camera. Presumable applications of the superfluorescent pulse in e.g., a remote sensing, and an ultraviolet upconversion of the input infrared laser pulse are presented. The quantum interference due to different excitation pathways is revealed by the temporal coherent control technique while observing interferometric signals from alkali metal vapors. On the other hand, a new spectroscopic technique based on ultrafast coherent Raman scattering is developed. The key advantage of the presented technique is to suppress the non-resonant background noise which usually obscures possible applications of the other conventional coherent Raman techniques in practice. A reduction of the background noise is achieved by shaping and delaying the third pulse which probes the coherence of the medium (i.e., an enhancement of specific vibrations of the target molecules in unison) firstly prepared by two broadband pulses. We demonstrate a robustness and superiority of signal-to-noise ratio of the developed technique by identifying as few as 10000 bacterial spores at a single laser shot level. Finally, several comparative studies between cooperative and uncooperative processes are presented. A picosecond cooperative phenomenon in a three-photon resonant medium induced by a single as well as two-color ultrashort pulses is investigated. A time-resolved study shows that a picosecond cooperative effect is crucial in the well-established fields of resonant-enhanced multiphoton ionizations and harmonic generations. We also present a quantitative analysis for spontaneous versus broadband coherent Raman scattering on pyridine molecules. The spontaneous Raman signal is enhanced by 5 orders as a result of cooperative phenomena.

Submitted by Isaac Francisco de Souza Dias (isaac.souzadias@ufpe.br) on 2016-03-02T19:08:33Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Tese José Ferraz.pdf: 2743054 bytes, checksum: 9154337dac4a587934aab71697b77a90 (MD5) / Made available in DSpace on 2016-03-02T19:08:33Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Tese José Ferraz.pdf: 2743054 bytes, checksum: 9154337dac4a587934aab71697b77a90 (MD5) Previous issue date: 2008-05-29 / CNPQ / Neste trabalho, utilizamos as técnicas de controle temporal coerente e de manipulação da fase óptica do campo óptico para investigar e controlar os diferentes caminhos quânticos induzidos em transições de dipolo elétrico envolvendo a absorção de dois fótons no átomo de rubídio. As transições foram excitadas por pares de pulsos de separação temporal variável e analisamos a resposta do meio atômico em função dessa separação. Três situações experimentais são analisadas envolvendo transições com diferentes características. No primeiro experimento, utilizamos luz incoerente de um laser de corante com duração temporal de nanosegundos, mas com tempo de coerência de picosegundos, para excitar a transição de dois fótons, envolvendo níveis altamente excitados, níveis de Rydberg. A resposta do sistema é analisada através de um processo de mistura de quatro ondas, resolvida no tempo, e a seleção dos diferentes caminhos quânticos envolvidos no processo é feita a partir do controle da polarização dos campos do laser incidente. Interferências na freqüência central do laser, interferências "ópticas", e no dobro dessa freqüência, interferências quânticas, são observadas. Nos outros dois estudos, a transição de dois fótons é excitada por pulsos com duração temporal da ordemde 100 fentosegundos. No primeiro caso, investigamos uma transição de dois fótons pura, entre os níveis 5S e 7S do rubídio, onde uma fase externa, dependente da freqüência, é adicionada em um dos pulsos, enquanto o outro tem seu atraso temporal controlado. A fluorescência detectada é uma medida direta da população do estado excitado. Novamente, um sinal interferométrico é observado, cujo controle coerente é efetuado por uma combinação da fase externa e do atraso temporal. O último experimento envolve uma transição seqüencial, cuja ressonância de um fóton leva a efeitos de propagação observados no sinal de interesse. Outro aspecto importante é que a taxa de repetição do laser era maior que as taxas de relaxação dos níveis envolvidos, de forma que efeitos de acumulação na população e na coerência também estão presentes. A resposta do sistema é analisada através do processo de mistura paramétrica de quatro ondas, resolvido no tempo. O sinal interferométrico, com controle de polarização e da freqüência de detecção, permite uma demonstração clara da origem quântica nas interferências "ópticas". / In this work, we use the techniques of temporal coherent control and phase manipulation of the optical field to investigate and control different quantum pathways in the two-photon absorption in rubidium atoms. Three experimental situations are studied involving transitions with different characteristics. In all of them, the medium response is analyzed as a function of the temporal delay between the pulse pairs responsible for the two-photon transition. In the first experiment, we use incoherent light from a dye laser with pulse duration in the nanosecond scale, but with coherence time in the picosecond scale, to excite the two-photon transition, which involves highly excited levels, Rydberg states. The system response is analyzed through a time-resolved four-wave mixing process, and the quantum pathway selection is realized by polarization control of the incident laser fileds. Interferences in the central laser frequency - "optical" interferences - and at twice this frequency - quantum interferences - are observed. In the subsequent two studies, the two-photon transition is excited with 100 fs pulses. In the first case, we investigate a pure two-photon transition, the 5S - 7S rubidium transition, where an external frequency depedent phase is added in one pulse, while the temporal delay of a second pulse is controlled. We observe an interferometric signal, in which coherent control is achieved with external phase and temporal delay combination. The last experiment involves a sequential transition, which has a one-photon resonance that leads to propagation effects observed in the signal. The laser repetition rate is greater than the atomic system relaxation rates, leading to accumulation effects in the population and coherence, which leads to important effects. A time-resolved parametric four-wave mixing is used to investigate the system response. The interferometric signal, with polarization and detection frequency control, allows us to clearly demonstrate the quantum origin of the "optical" interferences.