Robustness and capabilities of ultrashort laser pulses characterization with amplitude swing

[EN]In this work we firstly study the influence of different parameters in the temporal characterization of ultrashort laser pulses with the recently developedamplitude swing technique. In this technique, the relative amplitude of two delayed replicas is varied while measuring their second-harmonic spectra. Herewe study the retrieval of noisy traces and the implications of having different delays or phase retardations (relative phases)between the two replicas. Then, we study the capability of the technique to characterize the pulses when the second-harmonic signal is spectrally uncalibrated or incomplete, presenting the analytical calculation of the marginal, which is used to calibrate the traces and to perform the pulse retrievals. We experimentally show the retrieval of different pulses using diverse delays and phase retardationsto perform the amplitude swing trace and demonstrate that, from an uncalibrated trace, both the pulse informationandthe response of the nonlinear process can be simultaneously retrieved.In sum, the amplitude swing technique is shown to be very robust against experimental constraints and limitations, showing a high degree of soundness.Spanish Ministerio de Economía y Competitividad (MINECO) (FIS2017-87970-R, EQC2018-004117-P), Consejería de Educación, Junta de Castilla y León (SA287P18) and FEDER Funds, Fundación General de la Universidad de Salamanca (PC_TCUE18-20_020)

Measurement of Ultrashort Vector Pulses From Polarization Gates by In-Line, Single-Channel Spectral Interferometry

[EN]The growing use of ultrashort laser pulses exhibiting time-varying polarization (vector pulses) demands simple and robust characterization techniques capable to perform measurements in a broad range of experimental and environmental conditions. Here we present in-line, single-channel setup based on spectral interferometry to characterize ultrashort vector pulses. The use of a bulk interferometer based on birefringence is key for the stability and sensitivity of the technique, thus being simple and highly robust. The technique is used to measure vector pulses corresponding to polarization gates, which are used in many applications. Those results are validated by simulations. The technique here presented has a number of potential applications in nonlinear effects (e.g. transient birefringence and nonlinear phenomena with vector pulses)

Signature of the transversal coherence length in high-order harmonic generation

It is well known that high-order harmonic generation (HHG) from extended targets differs substantially from that of isolated atoms. Phase matching during propagation plays a major role in defining the extent of the spatial regions in the target that contribute constructively to the final yield. Typically, the understanding of this process is simplified by considering phase-matching effects along the field propagation axis, defining a longitudinal coherence length. In this paper we explore the role of phase matching in the transverse direction. The definition of a transversal coherence length appears fundamental to understanding propagation of harmonics generated by focalized laser beams. We present experimental results—-supported by theory—in which transversal phase matching plays the leading role in the macroscopic HHG.We acknowledge technical support from Oscar Varela, Juan Hernández Toro, and Cruz Méndez. We acknowledge support by a Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development, under REA Grant Agreement No. 328334. We acknowledge support from Junta de Castilla y León (Consejería de Educación and Fondo Social Europeo and Projects No. SA002B08 and No. SA116U13), Spanish MINECO (Grant No. FIS2009-09522, Consolider Program SAUUL CSD2007-00013, and Ramón y Cajal Program), and Centro de Láseres Pulsados, CLP

Compact in-line temporal measurement of laser pulses with amplitude swing

[EN]A method of ultrashort laser pulse reconstruction is presented, consisting on the analysis of the nonlinear signal obtained from the interference of the pulse with a replica of itself at a given time delay while varying the relative amplitude between the pulses. The resulting spectral traces are analyzed both analytically and numerically, showing the encoding of the input pulse spectral phase. A reconstruction algorithm is discussed and applied to extract the spectral phase and, jointly to the measured spectral amplitude, reconstructing the pulse. In order to validate the technique, an experimental in-line implementation of the characterization concept is compared to the results from a stablished technique, obtaining a good agreement at different input pulse cases. In sum, a new technique is presented, showing the capability to reconstruct a broad range of temporal pulse durations while its implementation is robust and straightforward, able to be easily adapted to diverse pulse duration and central wavelength ranges.Ministerio de Economía y Competitividad (EQC2018-004117-P, FIS2017-87970-R); Consejería de Educación, Junta de Castilla y León (SA287P18) and FEDER Funds; European Regional Development Fund; Fundación General de la Universidad de Salamanca (PC_TCUE18-20_020)

Dielectric mirror optimization based on the phase-compensation method

[EN]A phase-compensation method is presented to fully optimize multilayer reflectance bandwidth, spectral phase and group delay dispersion (GDD). For a given multilayer (A), a set of different phase-compensated dielectric mirrors (B) is achieved as a function of a single parameter: the so-called reference wavelength r. With a correct selection of the parameter r, a set of different dielectric multilayers can be obtained with fixed broadband reflectance regions and smooth spectral phases (independently of the layers’ thickness distribution) so that ultrashort pulses can be entirely reflected in such dielectric mirrors with a negligible amount of absorption and distortion. Hence, with an adequate numerical analysis via our phase-compensation method, experimental designs can be easily performed to obtain ad hoc dielectric mirrors for ultrashort pulse management

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

Tailoring the spatio-temporal distribution of diffractive focused ultrashort pulses through pulse shaping

[EN]Focusing control of ultrashort pulsed beams is an important research topic, due to its impact to subsequent interaction with matter. In this work, we study the propagation near the focus of ultrashort laser pulses of ~25 fs duration under diffractive focusing. We perform the spatio-spectral and spatio-temporal measurements of their amplitude and phase, complemented by the corresponding simulations. With them, we demonstrate that pulse shaping allows modifying in a controlled way not only the spatio-temporal distribution of the light irradiance in the focal region, but also the way it propagates as well as the frequency distribution within the pulse (temporal chirp). To gain a further intuitive insight, the role of diverse added spectral phase components is analyzed, showing the symmetries that arise for each case. In particular, we compare the effects, similarities and differences of the second and third order dispersion cases

Multibeam second-harmonic generation by spatiotemporal shaping of femtosecond pulses

We present a technique for efficient generation of the second-harmonic signal at several points of a nonlinear crystal simultaneously. Multispot operation is performed by using a diffractive optical element that splits the near-infrared light of a mode-locked Ti:sapphire laser into an arbitrary array of beams that are transformed into an array of foci at the nonlinear crystal. We show that, for pulse temporal durations under 100 fs, spatiotemporal shaping of the pulse is mandatory to overcome chromatic dispersion effects that spread both in space and time the foci showing a reduced peak intensity that prevents nonlinear phenomena. We experimentally demonstrate arbitrary irradiance patterns for the second-harmonic signal consisting of more than 100 spots with a multipass amplifier delivering 28 fs, 0.8 mJ pulses at 1 kHz repetition rate.This work was supported by the Spanish Ministerio de Ciencia e Innovación (MICINN) and FEDER through the projects FIS2010-15746, FIS2009-09522, and SAUUL (CSD2007-00013) and the Fundació Caixa Castelló (P1-1B2010-26). The authors are grateful to the Serveis Centrals d’Intrumentació Científica of the Universitat Jaume I and the Centro de Láseres Pulsados, Salamanca. We are indebted to Professor Jürgen Jahns for providing us with the diffractive lenses

Optical vortex production mediated by azimuthal index of 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)

Ultrashort pulse propagation through depressed-cladding channel waveguides in YAG crystal: Spatio-temporal characterization

[EN]Inscription of optical waveguides by direct femtosecond laser irradiation has become a very versatile tool for the development of integrated photonic devices, such as waveguide lasers, frequency converters or photonic lanterns, among many others. The potential application of such devices for the control and manipulation of ultrashort pulses requires the precise knowledge of the temporal distortions that may be induced in the pulse propagation. Currently, research in this topic is scarce, and to our knowledge there is no previous experimental study on the spatio-temporal characterization at the output of waveguides inscribed inside crystals. Here, we have firstly fabricated depressed-cladding waveguides with different modal behavior in YAG crystal by direct femtosecond laser irradiation. Then, we implemented an experimental method based on the fiber coupler assisted spectral interferometry technique, that allows obtaining: (1) the temporal dispersion of a pulse at the output of an inscribed waveguide and, (2) full spatio-spectral and spatio-temporal characterization of the output of single-mode and multi-mode waveguides. Our results suggest that the main contribution to the pulse dispersion is due to the material dispersion. Moreover, we found that multimodal waveguides may induce an appreciable inhomogeneity in the temporal features of the pulses that needs to be taken into account in the design of complex devices.This work was supported by Junta de Castilla y León (Project SA046U16 and SA287P18) and MINECO (Projects FIS2017-87970-R, EQC2018-004117-P). M.M.-V. acknowledges support from Junta de Castilla y León grant (No. SA046U16) and from Generalitat Valenciana, Spain, under project CDEIGENT/2018/024