Atoms, ions and molecules in intense ultrafast laser fields

The dynamics of atoms and hydrogen molecules in ultrafast intense laser pulses are studied experimentally using time of flight mass spectroscopy and fast ion beam techniques. The content of the study can be split naturally into two parts; the first dealing with the interaction of noble gas atoms and ions with laser pulses of between 40 and 50 fs in duration and around 20 mJ of energy per pulse, the second dealing with a time-resolved investigation of the nuclear dynamics of hydrogen molecules and their corresponding molecular ions with laser pulses of approximately 10 fs duration and an energy per pulse of 50 μJ. Within the first section of the study the technique of Intensity Selective Scanning with Effective Intensity Matching (ISS-EIM) has been used to observe Multi Electron Tunnelling Ionisation (METI) for the first time. These experiments were conducted with laser pulses focussed on neutral targets of argon and krypton. Also within the first section, the recombination of ionised electrons with their residual atomic cores has been observed for the first time in the atomic channel. This recombination occurs in the metastable ion population of singly charged ion beams of krypton and argon, provided by a fast ion beam apparatus. In the second section of the study, a pump-probe technique is employed using few cycle laser pulses to initiate (pump) and then image (probe) the nuclear dynamics of hydrogenic molecules and their molecular ions. In the ions, vibrational wavepackets have been studied and the phenomena of dephasing and revival observed. Additionally signatures of rotational wavepacket dynamics have been isolated in the neutral molecules. A technique to verify the laser pulse duration at the focus has also been introduced using xenon atoms and the same pump-probe technique

Photoinduced chase transitions In molecular materials

Wydział FizykiGłównym celem niniejszej pracy doktorskiej było znalezienie odpowiedzi na pytania dotyczące procesów zachodzących w materiałach molekularnych pod wpływem ultrakrótkiego impulsu laserowego. Prezentowane wyniki otrzymano za pomocą układów laserowych typu pompa-sonda, modyfikowanych na potrzeby prowadzonych badań wraz z pierwszymi próbami zastosowania nowej techniki wykorzystującej tylko jedna parę impulsów (ang.: single shot technique). W badanych kryształach typu spin-crossover, w których impuls świetlny zmienia stan układu z nie-magnetycznego do magnetycznego po raz pierwszy śledzono odpowiedz układu w czasach opóźnień od 100 femtosekund do milisekundy od chwili wzbudzenia. Wnioski płynące z tych badań pozwalają sądzić, że droga całego procesu jest złożona z kilku etapów związanych z oddziaływaniami najpierw w mikro, a następnie w makroskali. Drugą grupą badanych związków były kryształy molekularne, w których zachodzi przeniesienie ładunku, prowadzące do przejścia układu z fazy izolującej do metalicznej. W otrzymanych czasowo rozdzielczych widmach reflektancji obserwowano kilka fononów optycznych, a badania w zależności od temperatury i z wykorzystaniem impulsów o różnych energiach pozwalają stwierdzić, że stan fotoindukowany z fazy izolującej różni się od stanu metalicznego indukowanego termicznie. W pracy przedstawiono także wyniki badań testowych cienkiej warstwy złota z użyciem techniki pomiarowej wykorzystującej tylko jedną parę impulsów. Układy tego typu z powodzeniem mogą być wykorzystane do badań przejść nieodwracalnych.The main purpose of this Ph. D. thesis is to study the photo-induced transformations by a laser pulse in molecular materials. The results have been obtained thanks the use of pump-probe optical techniques. This required innovative experimental developments, including first attempts with a single shot technique. In the spincrosover family of molecular materials, in which light may induce the switching from a non magnetic to a magnetic state, for the first time we followed the transformation dynamics over ten decades in time scale, from 100 femtoseconds to a millisecond. It reveals that the process follows a complex pathway from molecular to material scale through a sequence of steps. A charge transfer organic compound, which exhibits an insulatior-to-metal phase transition, has also been investigated. A dynamics implying several coherent optical phonon modes has been clearly observed. The behaviour as a function of laser pulse intensity and temperature shows that the state photo-induced from the insulating phase differs from the metallic phase at thermal equilibrium. The newly developed single shot set-up proved able of recording changes upon an irreversible transformation, for instance inside a hysteresis regime. This set has been tested by observing photo-induced damages of a thin gold layer

Advanced radiating systems based on leaky wave and nondiffracting waves

In recent years, microwave, millimeter-wave, and THz applications such as medical and security imaging, wireless power transfer, and near-field focusing, just to mention but a few, have gained much attention in the area of ICT due to their potentially high social impact. On one hand, the need of highly-directive THz sensors with tunable radiating features in the far-field region has recently boosted the research activity in the design of flexible, low-cost and low-profile devices. On the other hand, it is of paramount importance to focus energy in the near-field region, and thus the generation of limited-diffraction waves in the microwave and millimeter-wave regime is a topic of recent increasing interest. In this context, leaky-wave theory is an elegant and extremely useful formalism which allows for describing in a common fashion guiding and radiating phenomena in both the near field and the far field, spanning frequencies from microwaves to optics passing through THz. In this PhD thesis we aim to exploit the intrinsic versatility of the leakywave approach to design advanced radiating systems for controlling the far-field radiating features at THz frequencies and for focusing electromagnetic radiation in the near field at millimeter waves. Specifically, the use of relatively new materials such as graphene and liquid crystals has been considered for the design of leaky-wave based radiators, achieving very promising results in terms of reconfigurability, efficiency, and radiating capabilities. In this context, an original theoretical analysis has provided new general formulas for the evaluation of the radiating features (e.g., half-power beamwidth, sidelobe level, etc.) of leaky-wave antennas. Indeed, the current formulations are based on several simplifying hypotheses which do not allow for an accurate evaluation of the beamwidth in different situations. In addition to the intriguing reconfigurable capabilities offered by leaky waves in far-field applications, interesting focusing capabilities can be obtained in the near field. In particular, it is shown that leaky waves can profitably be used to generate limited-diffraction Bessel beams by means of narrow-band radiators in the microwave range. Also, the use of higher-order leaky-wave modes allows for achieving almost the same performance in the millimeter-wave range, where previous designs were subjected to severe fabrication issues. Even more interestingly, the limited-diffractive character of Bessel beams can also be used to generate limited-diffraction pulses as superpositions of monochromatic Bessel beams over a considerable fractional bandwidth. In this context, a novel theoretical framework has been developed to understand the practical limitations to efficiently generate limited-diffraction, limited-dispersion pulses, such as X-waves, in the microwave/millimeter-wave range. As a result of this investigation, a class of wideband radiators has been thoroughly analyzed, showing promising capabilities for the generation of both zeroth-order and higher-order Xwaves. The latter may pave the way for the first localized transmission of orbital angular momentum in the microwave range