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The advent of ultrafast optics and its several applications has implied the development of a wide range of pulse time domain diagnostics (autocorrelation, FROG, SPIDER, spectral interferometry, d-scan, etc.). Nowadays, the growing application of ultrafast pulses at out-of-lab environments (e.g., industrial) with noise and instability sources is a challenge, requiring a simple and robust measurement technique.

Special light beams exhibiting a structured field and carrying orbital angular momentum (OAM) appear as interesting tools in different fields such as particle manipulation, telecommunications or in several areas of light-matter interaction, including materials processing and nonlinear optics, among others. A very interesting possibility of these special light beams is the coupling of spatially varying linear polarization fields ( i.e. radial or azimuthal) with spin and orbital angular momenta. Taking advantage of this interaction, we propose an in-line method to obtain collinear optical vortices exhibiting different topological charges [1] . The proposed method can be applied to focused or collimated beams, being continuous wave or pulsed.

Over the past decades, the use of ultrafast laser beams at a highly varied range of applications (e.g., biophotonics, material science, telecommunications, physics, chemistry, etc.) has evolved, demanding more complex light structures not only in time (i.e., ultrafast pulses), but also spatiotemporally and in polarization (e.g., vector beams, time evolving polarization pulses). Therefore, the characterization of such complex structured beams has become a hot topic nowadays.

We present a new concept for temporal pulse measurement, the amplitude swing, based on the amplitude variation of two delayed pulse replicas. The in-line compact set-up is versatile for pulses with different durations and bandwidths.

We perform a complete measurement of ultrashort pulsed beams where the polarization evolves spatially and temporally. We apply our technique to characterize ultrafast shaped vector beams which combine temporal polarization gating and radial polarization.

[EN]The use of structured ultrashort pulses with coupled spatiotemporal properties is emerging as a key tool for ultrafast manipulation. Ultrafast vector beams are opening exciting opportunities in different fields such as microscopy, time-resolved imaging, nonlinear optics, particle acceleration or attosecond science. Here, we implement a technique for the full characterization of structured time-dependent polarization light waveforms with spatiotemporal resolution, using a compact twofold spectral interferometer, based on in-line bulk interferometry and fibre-optic coupler assisted interferometry. We measure structured infrared femtosecond vector beams, including radially polarized beams and complex-shaped beams exhibiting both temporal and spatial evolving polarization. Our measurements confirm that light waveforms with polarization evolving at the micrometer and femtosecond scales can be achieved through the use of structured waveplates and polarization gates. This new scale of measurement achieved will open the way to predict, check and optimize applications of structured vector beams at the femtosecond and micrometer scales., We acknowledge funding from Junta de Castilla y León (SA287P18) and FEDER Funds, and from Spanish Ministerio de Economía y Competitividad (FIS2016-75652-P, FIS2017-87970-R, EQC2018-004117-P). C.H.-G. acknowledges support Ministerio de Ciencia, Innovación y Universidades for a Ramón y Cajal contract (RYC-2017-22745), cofunded by the European Social Fund. B.A. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Individual Fellowship grant agreement No. 798264. PGK acknowledges support of ERC project ENIGMA. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 851201).

We perform a complete measurement of ultrashort pulsed beams where the polarization evolves spatially and temporally. We apply our technique to characterize ultrafast shaped vector beams combining temporal polarization gating and radial polarization.

[EN]Special light beams are becoming more and more interesting due to their applications in particle manipulation, micromachining, telecommunications or light matter-interaction. Both spin and orbital angular momenta of light are exploited often in combination with spatially varying linear polarization profiles (e.g. radial or azimuthal distributions). In this work we study the interaction between those polarization profiles and the spin-orbit angular momenta, finding the relation involved in the mode coupling. We find that this manipulation can be used for in-line production of collinear optical vortices with different topological charges, which can be filtered or combined with controlled linear polarization. The results are valid for continuous wave and ultrashort pulses, as well as for collimated and focused beams. We theoretically demonstrate the proposal, which is further confirmed with numerical simulations and experimental measurements with ultrashort laser pulses., This work was partially funded by Junta de Castilla y León (SA287P18) and FEDER Funds; Spanish Ministerio de Economía y Competitividad (MINECO) (FIS2017-87970-R, EQC2018-004117-P); European Research Council (ENIGMA).

[EN]The chirped pulse amplification (CPA) technique applied to Titanium Sapphire lasers has made intense near-infrared (NIR) ultrashort pulses in the 20−100 fs range widely available for scientific, biomedical and industrial applications. Special efforts have been devoted to generate even shorter pulses, in the few- and single-cycle regime, due to a number of interesting applications. In particular, such pulses have paved the way for attosecond physics and metrology, via the extreme ultraviolet (XUV) attosecond pulse trains and isolated attosecond pulses that can be obtained by high-harmonic generation (HHG). The use of few-cycle optical pulses with durations close to or shorter than 10 fs in the near-infrared, visible and near-ultraviolet spectral regions has been extended in recent years to a wide range of spectroscopic techniques such as impulsive vibrational spectroscopy, time-resolved stimulated Raman spectroscopy, and ultrafast pump-probe absorption spectroscopy. Few-cycle optical pulses have also become an interesting tool for transient absorption microscopy, near-field imaging techniques and for generating ultrashort terahertz radiation. While it is possible to obtain sub-10 fs pulses from CPA or from optical parametric amplification systems, the former is not easy to accomplish, and the latter is not commonplace. Therefore, post-compression techniques are usually employed for the generation of intense few- and even single-cycle pulses in the near- and mid-infrared spectral regions. In order to post-compress ultrafast pulses down to the few-cycle regime, two steps are usually needed. First, nonlinear processes broaden the pulse spectrum, thus decreasing the Fourier-limited pulse duration. In a second step, the spectral phase resulting from the previous stage is compensated, typically using chirped mirrors, gratings, prisms, or other dispersive systems, resulting in a temporally compressed pulse. This scheme was first proposed in the context of optical fibers in the 1980s, and enabled achieving 6 fs pulses when compensating simultaneously the outcoming group delay dispersion (GDD) and third-order dispersion (TOD).

Intense few- and single-cycle pulses are powerful tools in different fields of science Today, third- and higher-order terms in the remnant spectral phase of the pulses remain a major obstacle for obtaining high-quality few- and single-cycle pulses from in-line post-compression setups. In this Letter, we show how input pulse shaping can successfully be applied to standard post-compression setups to minimize the occurrence of high-order phase components during nonlinear propagation and to directly obtain pulses with durations down to 3 fs. Furthermore, by combining this pulse shaping of the input pulse with new-generation broadband chirped mirrors and material addition for remnant third-order phase correction, pulses down to 2.2 fs duration have been measured., Ministerio de Economía y Competitividad (MINECO) (FIS2013-44174-P, FIS2015-71933-REDT, FIS2016-75652-P, FIS2017-87970-R); Consejería de Educación, Junta de Castilla y León (SA046U16, SA116U13); Fundação para a Ciência e a Tecnologia (FCT) (M-ERANET2/0002/2016); European Regional Development Fund (ERDF) (NORTE-01-0145-FEDER-022096).

In this work we study the impact of chromatic focusing of few-cycle laser pulses on high-order-harmonic generation (HHG) through analysis of the emitted extreme ultraviolet (XUV) radiation. Chromatic focusing is usually avoided in the few-cycle regime, as the pulse spatiotemporal structure may be highly distorted by the spatiotemporal aberrations. Here, however, we demonstrate it as an additional control parameter to modify the generated XUV radiation. We present experiments where few-cycle pulses are focused by a singlet lens in a Kr gas jet. The chromatic distribution of focal lengths allows us to tune HHG spectra by changing the relative singlet-target distance. Interestingly, we also show that the degree of chromatic aberration needed for this control does not degrade substantially the harmonic conversion efficiency, still allowing for the generation of supercontinua with the chirped-pulse scheme, demonstrated previously for achromatic focusing. We back up our experiments with theoretical simulations reproducing the experimental HHG results depending on diverse parameters (input pulse spectral phase, pulse duration, and focus position) and proving that, under the considered parameters, the attosecond pulse train remains very similar to the achromatic case, even showing cases of isolated attosecond pulse generation for near-single-cycle driving pulses., We acknowledge support from Consejería de Educación,Junta de Castilla y Le n (Projects No. SA116U13 and No. SA046U16), Spanish MINECO (Grants No. FIS2009-09522, No. FIS2013-44174 P, No. FIS2015-71933-REDT, and No. FIS2016-75652-P). This work was partly supported by Fundacio para a Ciencia e Tecnología, Portugal, co-funded by COMPETE and FEDER, via Grants No. PTDC/FIS/122511/2010 and No. UID/NAN/50024/2013. F.S. acknowledges support from Grant No. SFRH/BD/69913/2010 and B.A. acknowledges support from Post-Doctoral Fellowship Grant No. SFRH/BPD/88424/2012; H.C. acknowledges support from Sabbatical Leave Grant No. SFRH/BSAB/105974/2015. C.H.-G. acknowledges support from a Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development, under REA Grant Agreement No. 328334.

Intense few-to single-cycle pulses are a key tool for attosecond science and ultrafast spectroscopy, and are also enabling a new generation of compact laser-plasma accelerators [1], Achieving such short pulse durations is a challenging task as it requires not only the generation of very broad (potentially over octave-spanning) spectra but also the compensation of the spectral phase over such bandwidths, in order to get pulses as close as possible to their Fourier-transform limit. Presently, the most used scheme to obtain such pulses is gas-filled hollow-core fiber (HCF) post-compression of amplified multi-cycle pulses, where chirped mirrors are normally employed for dispersion compensation. In general, researchers optimize their HCF compressors by empirically adjusting several key parameters, such as gas type and pressure, input pulse characteristics and coupling conditions, with the final result usually involving a delicate compromise between output efficiency, amount of spectral broadening and achievable degree of compression (pulse duration and quality) for their particular system and chimed mirror set.

We present observations of the emission of XUV continua in the 20-37 eV region by high harmonic generation (HHG) with $4$-$7\ \mathrm{fs}$ pulses focused onto a Kr gas jet. The underlying mechanism relies on coherent control of the relative delays and phases between individually generated attosecond pulse, achievable by adjusting the chirp of the driving pulses and the interaction geometry. Under adequate negative chirp and phase matching conditions, the resulting interpulse interference yields a continuum XUV spectrum, which is due to both microscopic and macroscopic (propagation) contributions. This technique opens the route for modifying the phase of individual attosecond pulses and for the coherent synthesis of XUV continua from multicycle driving laser pulses without the need of an isolated attosecond burst., 14 pages, 5 figures. Submitted to Physical Review A

We present a laboratory experiment for advanced undergraduate or graduate laser-related classes to study the performance of a neodymium laser. In the experiment, the student has to build the neodymium laser using an open cavity. After that, the cavity losses are modulated with an optical chopper located inside, so the $\mathcal {Q}$-switching regime is achieved. Also a nonlinear crystal can be inserted in the cavity in order to have second harmonic generation. Finally, the relation between the transverse modes and the temporal emission in the $\mathcal {Q}$-switching regime can be observed., We acknowledge support from Spanish Ministerio de Ciencia e Innovación through the Consolider Program SAUUL (CSD2007-00013), the ‘Programa de Implantación y Seguimiento de Acciones de Mejora de los Másteres Universitarios’ and the ‘Becas de Colaboración en Másteres Oficiales’ program of theUniversidad de Salamanca and the research project FIS2009-09522, from Junta de Castilla y Le on through the Program for Groups of Excellence (GR27) and from the EC’s Seventh Framework Programme (LASERLABEUROPE, grant agreement no 228334). We also acknowledge Luis Plaja for his valuable comments and support. WH and IJS acknowledge the support from the Spanish Ministerio de Ciencia e Innovaci on through the ‘Formación de Personal Investigador’ and ‘Ramon y Cajal’ grant programs, respectively.

We demonstrate a novel, single-shot ultrafast diagnostic, based on the dispersion-scan (d-scan) technique. In this implementation, rather than scanning wedges to vary the dispersion as in standard d-scan, the pulse to be measured experiences a spatially varying amount of dispersion in a Littrow prism. The resulting beam is then imaged into a second-harmonic generation crystal and an imaging spectrometer is used to measure the two-dimensional trace, which is analyzed using the d-scan retrieval algorithm. We compare the single-shot implementation with the standard d-scan for the measurement of sub-3.5-fs pulses from a hollow core fiber pulse compressor. We show that the retrieval algorithm used to extract amplitude and phase of the pulse provides comparable results, proving the validity of the new single-shot implementation down to near single-cycle durations., 6 pages, 4 figures

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.

We make a review of the high harmonic generation (HHG) process, its characteristics and properties, presenting our main results in the field. The HHG is a very interesting source on the deep ultraviolet region with high potential in applications such as time resolved measurements and high spatial resolution imaging

We present evidence for self-stabilization of the relative spectral phase of high-order harmonic emission against intensity variations of the driving field. Our results demonstrate that, near the laser focus, phase matching of the harmonic field from a macroscopic target can compensate for the intensity dependence of the intrinsic phase of the harmonics emitted by a single radiator. As a consequence, we show experimentally and theoretically the insensitivity of the harmonic spectra produced at the laser focus against variations of the carrier-envelope phase (CEP) of a sub-two-cycle driving field. In addition, the associated attosecond pulse trains exhibit phase locking against CEP changes of the few-cycle driver., The authors acknowledge support from MINECO ( FIS2013-44174-P), from Junta de Castilla y León (Project No. SA116U13) and from Centro de Láseres Pulsados. C.H.-G. acknowledges support from the Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007–2013), under REA grant Agreement No. 328334. W.H. acknowledges ESF support through SILMI Research Networking Programme. B. A. acknowledges Fundação para a Ciência 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. This work was partly supported by grant PTDC/FIS/122511/2010 from Fundação para a Ciência e Tecnologia, Portugal, co-funded by COMPETE and FEDER.

Parallel to the rising of a number of scientific and industrial applications of ultrashort laser pulses, their precise temporal characterization has become a major issue. Recently, the new pulse measurement technique of dispersion-scan (d-scan) was introduced, which is based on measuring the spectrum of a nonlinear signal as a function of varying amounts of known dispersion added to the pulse [1]. A reconstruction algorithm then enables extracting the amplitude and/or the spectral phase of the pulse from the measured two-dimensional (nonlinear spectrum vs. dispersion) trace. The first implementation of the technique used chirped mirrors and a pair of wedges that introduce varying amounts of dispersion depending on how much wedge is inserted or removed into the beam path in a scanning sequence. This implementation is very well suited for very short pulses (i.e., with broad bandwidths and high sensitivity to dispersion) being capable of measuring even single-cycle pulses [2-4]. On the other hand, longer pulses (i.e., with narrower spectra) present lower sensitivity to dispersion and the wedge setup is not enough to perform a complete d-scan, so a more complex (and expensive) calibrated dispersion scanning module is needed, e.g., a prism/grating/grism compressor or an acousto-optical programmable dispersive filter (AOPDF).

We demonstrate that supercontinuum generation with a diffractive lens in a sapphire plate gives rise to new spectral phenomenology. Applicability of these pulses is supported by a spatial and temporal study.