Influence of the spatial confinement on the self-focusing of ultrashort pulses in hollow-core fibers

The collapse of a laser beam propagating inside a hollow-core fiber is investigated by numerically solving different nonlinear propagation models. We have identified that the fiber confinement favors the spatial collapse, especially in case of pulses with the input peak power close to the critical value. We have also observed that when using pulses in the femtosecond range, the temporal dynamics plays an important role, activating the spatial collapse even for pulses with input peak powers below the critical value. The complex self-focusing dynamics observed in the region below the critical power depends on the temporal evolution of the pulse and, also, on the interaction between the different spatial modes of the hollow-core fiber.The authors thank support from Spanish Ministerio de Economía y Competitividad (FIS2016-75652-P) and from Junta de Castilla y León (SA287P18). A.C. is funded by the FPI grant program of Spanish Ministerio de Economía y Competitividad and European Social Fund (BES-2017-080280)

Scalable sub-cycle pulse generation by soliton self-compression in hollow capillary fibers with a decreasing pressure gradient

Advances in the generation of the shortest optical laser pulses down to the sub-cycle regime promise to break new ground in ultrafast science. In this work, we theoretically demonstrate the potential scaling capabilities of soliton self-compression in hollow capillary fibers with a decreasing pressure gradient to generate near-infrared sub-cycle pulses in very different dispersion and nonlinearity landscapes. Independently of input pulse, gas and fiber choices, we present a simple and general route to find the optimal self-compression parameters which result in high-quality pulses. The use of a decreasing pressure gradient naturally favors the self-compression process, resulting in shorter and cleaner sub-cycle pulses, and an improvement in the robustness of the setup when compared to the traditional constant pressure approach

Optimization of pulse self-compression in hollow capillary fibers using decreasing pressure gradients

The improvement of techniques for the generation of near-infrared (NIR) few-cycle pulses is paving the way for new scenarios in time-resolved spectroscopy and the generation of ultrashort extreme-ultraviolet pulses through high-harmonic generation. In this work, we numerically study how to optimize the self-compression of NIR pulses using decreasing pressure gradients in hollow capillary fibers (HCFs). We identify a moderate nonlinear regime in which sub-cycle pulses are obtained with very good temporal quality from an input 30 fs pulse centered at a 800 nm wavelength and coupled as the fundamental mode of an argon-filled HCF fully evacuated at the output end. Surprisingly, we observe that there is a relatively broad region of parameters for which the optimum self-compression takes place, defined by a simple relation between the input pulse energy and the initial gas pressure.This work was supported by grant PID2019-106910GB-I00, funded by the Spanish Ministry of Science and Innovation, MCIN/AEI/ 10.13039/501100011033

Ultrashort visible energetic pulses generated by nonlinear propagation of necklace beams in capillaries

The generation of ultrashort visible energetic pulses is investigated numerically by the nonlinear propagation of infrared necklace beams in capillaries. We have developed a (3+1)D model that solves the nonlinear propagation equation, including the complete spatio-temporal dynamics and the azimuthal dependence of these structured beams. Due to their singular nonlinear propagation, the spectrum broadening inside the capillary extends to the visible region in a controlled way, despite the high nonlinearity, avoiding self-focusing. The results indicate that the features of these necklace beams enable the formation of visible pulses with pulse duration below 10 fs and energies of 50 μJ by soliton self-compression dynamics for different gas pressures inside the capillary.Junta de Castilla y León (SA287P18); Ministerio de Economía y Competitividad (BES-2017-080280, FIS2016-75652-P); Ministerio de Ciencia e Innovación (PID2019-106910GB-I00)

Quantum-path signatures in attosecond helical beams driven by optical vortices

High-order harmonic generation (HHG) driven by beams carrying orbital angular momentum has been recently demonstrated as a unique process to generate spatio-temporal coherent extreme ultraviolet (XUV)/x-ray radiation with attosecond helical structure.Weexplore the details of the mapping of the driving vortex to its harmonic spectrum. In particular we show that the geometry of the harmonic vortices is complex, arising from the superposition of the contribution from the short and long quantum paths responsible of HHG. Transversal phase-matching and quantum path interferences provide an explanation of the dramatic changes in theXUVvortex structure generated at different relative positions of the target respect to the laser beam focus. Finally, we show how to take advantage of transversal phase-matching to select helical attosecond beams generated from short or long quantum paths, exhibiting positive or negative temporal chirp respectively.CH-G acknowledges fruitful discussions with Mette B Gaarde and support from the Marie Curie International Outgoing Fellowship within the EUSeventh Framework Programme for Research and Technological Development (2007–2013), under REA grant Agreement No 328334 CH-G, JSR and LP acknowledge support from Junta de Castilla y León (Project SA116U13) and MINECO (FIS2013-44174-P). AP acknowledges financial support of the US Department of Energy, Basic Energy Sciences, Office of Science, under contract No DE-AC02- 06CH11357

Trains of attosecond pulses structured with time-ordered polarization states

[EN]Ultrafast laser pulses generated at the attosecond timescale represent a unique tool to explore the fastest dynamics in matter. An accurate control of their properties, such as polarization, is fundamental to shape three-dimensional laser-driven dynamics. We introduce a technique to generate attosecond pulse trains whose polarization state varies from pulse to pulse. This is accomplished by driving high-harmonic generation with two time-delayed bichromatic counter-rotating fields with proper orbital angular momentum (OAM) content. Our simulations show that the evolution of the polarization state along the train can be controlled via OAM, pulse duration, and time delay of the driving fields. We, thus, introduce an additional control into structured attosecond pulses that provides an alternative route to explore ultrafast dynamics with potential applications in chiral and magnetic materials.Junta de Castilla y León (SA287P18); FEDER funds; Ministerio de Ciencia, Innovación y Universidades (FIS2016-75652-P, RYC-2017-22745, PID2019-106910GBI00); Ministerio de Educación, Cultura y Deporte (FPU16/02591); EuropeanResearch Council (ERC) (851201); Barcelona Supercomputing Center (FI-2019-1-0013)

Preparación de prácticas de laboratorio de las asignaturas "óptica cuántica" y "óptica avanzada" impartidas en los másteres en "física y tecnología de los láseres" y "física"

Memoria ID-0131. Ayudas de la Universidad de Salamanca para la innovación docente, curso 2013-2014

Temporal and spectral structure of the infrared pulse during the high order harmonic generation

We present, for the first time, the complete pulse characterization of the infrared pulse after generating harmonics. A systematic study of the high harmonic generation process, and the generating infrared pulse characterization, has been done by changing the focus-gas-jet relative position. We have concluded, supported by nonlinear propagation simulations, that there is a correlation between the spectral and temporal nonlinear evolution of the infrared generating field and the structures shown in the harmonic signal. We have identified two different pressure regimes: the low pressure regime, characterized by the effects produced by the plasma generated by the infrared pulse, and the high pressure regime where the plasma and the Kerr effect generated by the infrared field are both present. These observations highlight the important role played by the nonlinear propagation of the generating field in the high harmonic generation context.Spanish Ministerio de Ciencia e Innovación (MICINN) through the Consolider Program SAUUL ( CSD2007-00013) and Research Project FIS2009-09522, from the Junta de Castilla y León (Project No. SA116U13) and from Centro de Láseres Pulsados. B. A. acknowledges Fundaçao para a Ciencia e a Tecnologia (FCT) through grant No. SFRH/BPD/88424/2012. W. H. and I. J. S. also acknowledge support from the Spanish Ministerio de Ciencia e Innovación through the Formación de Personal Investigador and Ramón y Cajal grant programs respectively

Elaboración de modelos de informe para la asignatura de Laboratorio de Óptica

Memoria ID-0195. Ayudas de la Universidad de Salamanca para la innovación docente, curso 2014-2015

Spatiotemporal-dressed optical solitons in hollow-core capillaries

[EN]The nonlinear propagation of a laser beam in a hollow-core capillary is studied by solving the spatiotemporal nonlinear propagation equation. Although we assume to initially couple the light into only one high spatial mode of the capillary, we have identified that the beam can propagate as a new type of multi-mode solitonic structure, the spatiotemporal-dressed soliton, which consists of a mixture of spatial modes in which one has most of the energy while the rest of them, with small contributions, module (dress) the propagation of the main spatial mode. As a consequence of such behavior, we observe a clean self-compression process, obtaining a pulse in the single-cycle limit, accompanied by a giant blue dispersive wave and a new type of multi-mode dispersive wave that appears in the mid-IR region