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Thionated nucleobases are obtained by replacing oxygen with sulphur atoms in the canonical nucleobases. They absorb light efficiently in the near-ultraviolet, populating singlet states which undergo intersystem crossing to the triplet manifold on an ultrashort time scale with a high quantum yield. Nonetheless there are still important open questions about the primary mechanisms responsible for this ultrafast transition. Here we track both the electronic and the vibrational ultrafast excited-state dynamics towards the triplet state for solvated 4-thiothymidine (4TT) and 4-thiouracil (4TU) with sub-30 fs broadband transient absorption spectroscopy in the ultraviolet. A global and target analysis allows us to simultaneously resolve the contributions of the different electronically and vibrationally excited states to the whole data set. Our experimental results, combined with state-of-the-art quantum mechanics/molecular mechanics simulations and Damped Oscillation Associated Spectra (DOAS) and target analysis, support that the relaxation to the triplet state is mediated by conical intersections promoted by vibrational coherences through the population of an intermediate singlet state. In addition, the analysis of the coherent vibrational dynamics reveals that, despite sharing the same relaxation mechanism and similar chemical structures, 4TT and 4TU exhibit rather different geometrical deformations, characterized by the conservation of planarity in 4TU and its partial rupture in 4TT.

The first step to gain optical control over the ultrafast processes initiated by light in solids is a correct identification of the physical mechanisms at play. Among them, exciton formation has been identified as a crucial phenomenon which deeply affects the electro-optical properties of most semiconductors and insulators of technological interest. While recent experiments based on attosecond spectroscopy techniques have demonstrated the possibility to observe the early-stage exciton dynamics, the description of the underlying exciton properties remains non-trivial. In this work we propose a new method called extended Ptychographic Iterative engine for eXcitons (ePIX), capable of reconstructing the main physical properties which determine the evolution of the quasi-particle with no prior knowledge of the exact relaxation dynamics or the pump temporal characteristics. By demonstrating its accuracy even when the exciton dynamics is comparable to the pump pulse duration, ePIX is established as a powerful approach to widen our knowledge of solid-state physics.

Excitons determine the ultrafast response of several technologically-relevant materials. Attosecond spectroscopy provides suitable time-resolution, but the access to the exciton response remains non-trivial. We propose a novel, ptychography-based approach to solve this task.

Since the first demonstration of the generation of attosecond pulses (1 as = 10−18 s) in the extreme-ultraviolet spectral region, several measurement techniques have been introduced, at the beginning for the temporal characterization of the pulses, and immediately after for the investigation of electronic and nuclear ultrafast dynamics in atoms, molecules and solids with unprecedented temporal resolution. The attosecond spectroscopic tools established in the last two decades, together with the development of sophisticated theoretical methods for the interpretation of the experimental outcomes, allowed to unravel and investigate physical processes never observed before, such as the delay in photoemission from atoms and solids, the motion of electrons in molecules after prompt ionization which precede any notable nuclear motion, the temporal evolution of the tunneling process in dielectrics, and many others. This review focused on applications of attosecond techniques to the investigation of ultrafast processes in atoms, molecules and solids. Thanks to the introduction and ongoing developments of new spectroscopic techniques, the attosecond science is rapidly moving towards the investigation, understanding and control of coupled electron–nuclear dynamics in increasingly complex systems, with ever more accurate and complete investigation techniques. Here we will review the most common techniques presenting the latest results in atoms, molecules and solids.

The interplay between intra-band motion and inter-band transitions in photoexcited germanium is investigated by attosecond transient reflection spectroscopy. Advanced theoretical simulations interpret the sub-femtosecond optical response as charge carrier dynamics in the band structure.

Understanding the primary steps following UV photoexcitation in sulphur-substituted DNA bases (thiobases) is fundamental for developing new phototherapeutic drugs. However, the investigation of the excited-state dynamics in sub-100 fs time scales has been elusive until now due to technical challenges. Here, we track the ultrafast decay mechanisms that lead to the electron trapping in the triplet manifold for 6-thioguanine in an aqueous solution, using broadband transient absorption spectroscopy with a sub-20 fs temporal resolution. We obtain experimental evidence of the fast internal conversion from the S2(ππ*) to the S1(nπ*) states, which takes place in about 80 fs and demonstrates that the S1(nπ*) state acts as a doorway to the triplet population in 522 fs. Our results are supported by MS-CASPT2 calculations, predicting a planar S2(ππ*) pseudo-minimum in agreement with the stimulated emission signal observed in the experiment.

We present a transient absorption setup combining broadband detection over the visible-UV range with high temporal resolution (∼20 fs) which is ideally suited to trigger and detect vibrational coherences in different classes of materials. We generate and detect coherent phonons (CPs) in single-layer (1L)-MoS

[ES] Desde la primera demostración experimental en 1960, el láser ha revolucionado campos tan diversos como la Física, la Química o la Medicina. La atención se centró pronto en conseguir pulsos cada vez más cortos y energéticos. Con este propósito surgieron varias técnicas como el Q-switching o el mode locking, hasta la aparición en 1985 de la tecnología CPA (chirped pulse amplification). La potencia pico alcanzada por los sistemas CPA abrió nuevas fronteras en el procesado de materiales, la aceleración de partículas o la microcirugía, y dio lugar a nuevas disciplinas como la attociencia y la femtoquímica. Hoy en día el interés de la comunidad científica reside en implementar las aplicaciones de los láseres de femtosegundo, así como en nuevos avances en su tecnología. Muchas de estas aplicaciones requieren longitudes de onda diferentes a las proporcionadas por las fuentes láser existentes actualmente, a las que se accede a través de procesos ópticos no lineales. Aunque ha habido un gran progreso durante la última década en relación al estudio de los procesos no lineales, algunas cuestiones aún deben ser abordadas. El objetivo de esta tesis es, por un lado, la optimización de procesos no lineales con un láser CPA de Ti:zafiro (pulsos 120 fs con 1 mJ de energía a 1 kHz de repetición), a través del control del frente de ondas y, por otro, el desarrollo de herramientas de diagnóstico espacial adecuadas para este tipo de sistemas. El trabajo está organizado en cuatro bloques de conocimiento. El primero de ellos está dedicado a las bases de la detección del frente de onda y sus aplicaciones. En dicho sentido, se presenta una completa caracterización del sistema láser operativo en la Universidad de Salamanca, monitorizando para ello el comportamiento a lo largo de diferentes puntos de la cadena CPA y estudiando su estabilidad temporal. En el mismo bloque, caracterizamos la dinámica de chorros de gas utilizados en experimentos de generación de electrones y armónicos de orden alto, relacionando la densidad de gas con los cambios en la fase, lo que permite la optimización del esquema experimental. Además, la caracterización del perfil y la fase del haz también es utilizada como parámetro de entrada para códigos de simulación y propagación no lineal. Por último, se propone un esquema de postcompresión basado en filamentación con haces astigmáticos, demostrándose que es posible obtener pulsos más cortos y energéticos que en condiciones análogas con una lente esférica. El segundo bloque presenta una caracterización del proceso de generación de segundo armónico con haces aberrados, tanto a nivel teórico (a través de simulaciones) como a nivel experimental, y sus consecuencias en términos de focalización, perfil de haz, frente de ondas y chirp espacial. En la tercera sección desarrollamos un nuevo método para la medida de la fase espacial, dirigido a solventar algunos de los problemas que presentan las técnicas tradicionales. El método está basado en la generación de segundo armónico, haciendo uso de la fuerte dependencia que este proceso exhibe con el ángulo formado entre el vector de ondas (perpendicular a la superficie del frente de onda) y el eje óptico. Para ello, se desarrolla un algoritmo de reconstrucción, así como un programa de simulación que nos permite discutir la validez del método y las restricciones del mismo. Se consiguen reconstrucciones con una desviación del 3% respecto al valor dado por un sensor comercial y se demuestra también, su validez al trabajar con haces que presentan discontinuidades. Además, demostramos la capacidad de la técnica espacio-temporal STARFISH para resolver el frente de onda en función de la frecuencia. Por último, la presente tesis se centra en la generación y optimización de la generación de supercontinuo en sólidos usando óptica difractiva obteniendo novedosos resultados frente al esquema tradicional, en términos de sintonización espectral. Además se reporta la existencia de un ala anti-Stokes, consiguiendo una frecuencia de corte mayor que con una lente refractiva en las mismas condiciones, [EN] Since the first experimental demonstration in 1960, lasers have revolutionized fields as diverse as physics, chemistry or medicine. The early focus was on getting ever shorter pulses and energy. For this purpose several techniques emerged as the Q-switching or mode locking, until the appearance in 1985 of technology CPA (chirped press amplification). The peak power achieved by CPA systems opened new frontiers in materials processing, particle acceleration or microsurgery, and resulted in new disciplines such as attociencia and femtochemistry. Today the scientific community's interest lies in implementing applications of femtosecond lasers, as well as new advances in technology. Many of these applications require different wavelengths to those provided by currently existing laser sources, which are accessed through non-linear optical process. Although there has been great progress in the last decade in relation to the study of nonlinear processes, some issues still need to be addressed. The aim of this thesis is, firstly, the optimization of nonlinear processes with a laser CPA Ti: sapphire (120 fs pulses with 1 mJ energy at 1 kHz repetition rate), through control of the wavefront and secondly, the development of diagnostic tools suitable space for this type of system. The paper is organized in four blocks of knowledge. The first is devoted to the basics of wavefront sensing and its applications. In this sense, we present a complete characterization of the laser system operating at the University of Salamanca, monitoring for this behavior along different points of the chain CPA and studying their temporal stability. In the same block, characterize the dynamics of gas jets used in experiments to generate electrons and high order harmonics relating the gas density in the phase change, which enables the optimization of experimental scheme. Additionally, the characterization of the profile and the phase of the beam is also used as input to simulation codes and nonlinear propagation. Finally, we propose a scheme based postcompresión filamentation astigmatic beam, showing that it is possible to obtain energy pulses shorter than in similar conditions with a spherical lens. The second section presents a characterization of the second harmonic generation aberrated beam, both theoretically (through simulations) and experimental level, and its consequences in terms of targeting, beam profile, and wavefront spatial chirp . In the third section developed a new method for measurement of spatial phase, aimed at solving some of the problems presented by traditional techniques. The method is based on second harmonic generation, using the strong dependence of this process exhibits the angle between the wave vector (perpendicular to the wave front surface) and the optical axis. To do this, we develop a reconstruction algorithm and a simulation program that allows us to discuss the validity of the method and restrictions thereof. Reconstructions are obtained with a deviation of 3% from the value given by a commercial sensor and also demonstrates its validity when working with beams that have discontinuities. Furthermore, we demonstrate the ability of the technique spatiotemporal STARFISH to solve the wavefront based on the frequency. Finally, this thesis focuses on the generation and optimization of supercontinuum generation in solids using diffractive optical novelties results obtained compared to the traditional scheme in terms of spectral tuning. Further reported the existence of an anti-Stokes wing, achieving a cutoff frequency higher than a refractive lens in the same conditions

A compact device performing spatio-temporal amplitude and phase reconstruction of laser pulses based on spectral interferometry is presented, available for complex beams characterization as those produced by nonlinear effects or non-trivial optic systems.

A method for the analysis and control of femtosecond pulse wavefronts generated in non-linear crystals by three-wave mixing processes is presented. The possibility to improve the efficiency of these processes is discussed.

We report a novel technique for wavefront sensing of intense femtosecond lasers based on the angle dependence of second-harmonic generation. Results are compared with those obtained by a commercial sensor and supported by numerical simulations.