Sistema de blogs Diarium
Universidad de Salamanca
Benjamín Alonso Fernández
Área de Óptica. Departamento de Física Aplicada


List of Publications:


Retrieving Ultrashort Pulses with Time-Varying Polarization Using Amplitude Swing [Preprint PDF]
Submitted (2023).
DOI: 10.2139/ssrn.4443101
C. Barbero, B. Alonso, Í. J. Sola.
 vector swing retrieved 4 Abstract: Ultrashort vector pulses are those whose polarization is time (and frequency) dependent. Its raising interest in many fields demands simple, robust, and versatile techniques to characterize the polarization state of these complex pulses. Amplitude swing is a recently demonstrated technique to measure scalar pulses (polarization state constant in time), with a compact in-line set-up, easily adaptable for a wide range of pulse durations. Here, we analyze the capability of amplitude swing to characterize complex vector pulses without any additional changes in the set-up, simulating, recording, and retrieving experimental traces. We show that a single trace encodes all the information needed to characterize vector pulses, including the relative phase between both polarization projections. To extract this information, we developed a reconstruction strategy, based on the Levenberg-Marquardt algorithm. In sum, an ultrashort vector pulse can be fully characterized from its amplitude swing trace.


Generalizing amplitude swing modulation for versatile ultrashort pulse measurement [Open PDF]
Opt. Express 31(21), 34428-34442 (2023).
DOI: 10.1364/OE.500271
M. López-Ripa, Í. J. Sola, B. Alonso.
 GenerAswingThumbnail01 Abstract: In this work we broaden the amplitude modulation concept applied to the temporal characterization of ultrashort laser pulses with the amplitude swing technique. We theoretically study the effect of diverse types of relative amplitude and phase modulations. This variation of the replicas can be implemented by means of rotating zero-order waveplates to manipulate the delayed pulse replicas produced in a following multi-order waveplate, which can be more practical under certain conditions. We numerically simulate and study different scenarios under different modulations and for different noise levels and pulses. The proposed schemes are validated and compared through the experimental application to compressed and chirped pulses, confirming the applicability of the work. The simplicity, robustness and versatility of this ultrashort pulse measurement benefits the applications of ultrafast optics.


Amplitude swing ultrashort pulse characterization across visible to near-infrared [Open PDF]
Opt. Laser Technol. 164, 109492 (2023).
DOI: 10.1016/j.optlastec.2023.109492
M. López-Ripa, Í. J. Sola, B. Alonso.
 topas vis nir a-swing Abstract: There is a growing interest in obtaining robust ultrashort temporal characterization techniques able to operate in different spectral ranges with different pulse bandwidths. Here we demonstrate that the recently introduced amplitude swing technique can measure ultrashort laser pulses in different spectral regions over more than one octave from the visible to near infrared spectral ranges, without significant modification in the technique. Moreover, we implement a new strategy for the amplitude swing retrievals using a differential evolution algorithm.


Roadmap on spatiotemporal light fields [Open PDF]
Journal of Optics 25, 093001 (2023).DOI: 10.1088/2040-8986/ace4dc
DOI: 10.48550/arXiv.2210.11273
Y. Shen, Q. Zhan, L. G. Wright, D. N. Christodoulides, F. W. Wise, A. E. Willner, K. Zou, Z. Zhao, M. A. Porras, A. Chong, C. Wan, K. Y. Bliokh, C.-T. Liao, C. Hernández-García, M. Murnane, M. Yessenov, A. F. Abouraddy, L. J. Wong, M. Go, S. Kumar, C. Guo, S. Fan, N. Papasimakis, N. I. Zheludev, L. Chen, W. Zhu, A. Agrawal, M. Mounaix, N. K. Fontaine, J. Carpenter, S. W. Jolly, C. Dorrer, B. Alonso, I. Lopez-Quintas, M. López-Ripa, Í. J. Sola, J. Huang, H. Zhang, Z. Ruan, A. H. Dorrah, F. Capasso, and A. Forbes.
 Imagen1 Abstract: Spatiotemporal sculpturing of light pulse with ultimately sophisticated structures represents a major goal of the everlasting pursue of ultra-fast information transmission and processing as well as ultra-intense energy concentration and extraction. It also holds the key to unlock new extraordinary fundamental physical effects. Traditionally, spatiotemporal light pulses are always treated as spatiotemporally separable wave packet as solution of the Maxwell’s equations. In the past decade, however, more generalized forms of spatiotemporally nonseparable solution started to emerge with growing importance for their striking physical effects. This roadmap intends to highlight the recent advances in the creation and control of increasingly complex spatiotemporally sculptured pulses, from spatiotemporally separable to complex nonseparable states, with diverse geometric and topological structures, presenting a bird’s eye viewpoint on the zoology of spatiotemporal light fields and the outlook of future trends and open challenges.


Bulk lateral shearing interferometry for spatiotemporal study of time-varying ultrashort optical vortices [Open PDF]
Photonics Research 10, 922-931 (2022).
DOI: 10.1364/PRJ.448339
M. López-Ripa, Í. J. Sola, B. Alonso.
  Abstract: Mode-locked erbium-doped fibre lasers are ultrashort pulsed sources widely studied due to their versatility and multiple applications in the near infrared range. Here we present the experimental study of the emission of a passive mode-locked erbium-doped fibre laser with an amplification stage outside the cavity by means of Frequency Resolved Optical Gating (FROG) and spectral interferometry. Due to shot-to-shot instabilities, the FROG traces can be understood as the combination of two different traces, corresponding to the coherent artifact and the average pulse characteristics. We have modified a Principal Components Generalized Projections Algorithm, in order to make it able to retrieve efficiently both the coherent artifact and the average pulse. In addition, we study the temporal dependence of the polarization, showing that the pulses present time-dependent polarization with a stable spectral relative phase between the horizontal and vertical projections. Up to our knowledge, this is the first experimental study that shows the FROG measurements of unstable pulse trains associated with the coherent artifact and analyses the time-dependent polarization in ultrafast fibre lasers.


Coherent artifact and time-dependent polarization in amplified ultrafast erbium-doped fibre lasers [Open PDF]
Opt. Laser Technol. 140, 107018 (2021).
DOI: 10.1016/j.optlastec.2021.107018
M. López-Ripa, B. Alonso, S. Jarabo, F. J. Salgado-Remacha, J. C. Aguado, Í. J. Sola.
 cafrog_pic1 Abstract: Mode-locked erbium-doped fibre lasers are ultrashort pulsed sources widely studied due to their versatility and multiple applications in the near infrared range. Here we present the experimental study of the emission of a passive mode-locked erbium-doped fibre laser with an amplification stage outside the cavity by means of Frequency Resolved Optical Gating (FROG) and spectral interferometry. Due to shot-to-shot instabilities, the FROG traces can be understood as the combination of two different traces, corresponding to the coherent artifact and the average pulse characteristics. We have modified a Principal Components Generalized Projections Algorithm, in order to make it able to retrieve efficiently both the coherent artifact and the average pulse. In addition, we study the temporal dependence of the polarization, showing that the pulses present time-dependent polarization with a stable spectral relative phase between the horizontal and vertical projections. Up to our knowledge, this is the first experimental study that shows the FROG measurements of unstable pulse trains associated with the coherent artifact and analyses the time-dependent polarization in ultrafast fibre lasers.


Robustness and capabilities of ultrashort laser pulses characterization with amplitude swing [Open PDF]
Scientific Reports 10, 18364 (2020).
DOI: 10.1038/s41598-020-75220-4
Íñigo J. Sola, Benjamín Alonso.
 swing_delay_diarium Abstract: In this work we firstly study the influence of different parameters in the temporal characterization of ultrashort laser pulses with the recently developed amplitude swing technique. In this technique, the relative amplitude of two delayed replicas is varied while measuring their second-harmonic spectra. Here we 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 relative phases to perform the amplitude swing trace and demonstrate that, from an uncalibrated trace, both the pulse information and the 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.


Compact in-line temporal measurement of laser pulses with amplitude swing [Open PDF]
Optics Express 28(10) 15625-15640 (2020).
DOI: 10.1364/OE.386321
Benjamín Alonso, Warein Holgado, Íñigo J. Sola.
MATLAB Handle Graphics Abstract: 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.


Complete spatiotemporal and polarization characterization of ultrafast vector beams [Open PDF]
Communications Physics 3, 151 (2020). DOI: 10.1038/s42005-020-00419-w B. Alonso, I. López-Quintás, W. Holgado, R. Drevinskas, P. G. Kazansky, C. Hernández-García, and Í. J. Sola.
 xtpol Abstract: The use of structured ultrashort pulses with coupled spatiotemporal properties is emerging as a key tool for ultrafast manipulation. In particular, the ultrafast vector beams are opening exciting opportunities in different fields such as microscopy, time-resolved imaging, nonlinear optics, particle acceleration or attosecond science. We propose and demonstrate a technique for the full characterization of structured time-dependent polarization light waveforms with spatiotemporal resolution. We have implemented a compact twofold spectral interferometer, based on in-line bulk interferometry and fibre-optic coupler assisted interferometry. We have experimentally measured 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 ultrafast —femtosecond— and micrometer scales.


Optical vortex production mediated by azimuthal index of radial polarization [LINK] [Free PDF]
Journal of Optics 22, 095402 (2020).
DOI: 10.1088/2040-8986/aba813
I. López-Quintás, W. Holgado, R. Drevinskas, P. G. Kazansky, Í. J. Sola, and B. Alonso.
 R11_vortex_diarium Abstract: 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.


Characterizing ultrashort laser pulses with second harmonic dispersion scans [Open PDF]
J. Opt. Soc. Am. B 38, 1546-1555 (2021).
DOI: 10.1364/JOSAB.412535
I. Sytcevich, C. Guo, S. Mikaelsson, J. Vogelsang, A.-L. Viotti, B. Alonso, R. Romero, P. T. Guerreiro, I. J. Sola, A. L’Huillier, H. Crespo, M. Miranda, C. L. Arnold.
 dscan_trace_snapshot Abstract: The dispersion scan (d-scan) technique has emerged as a simple-to-implement characterization method for ultrashort laser pulses. D-scan traces are intuitive to interpret and retrieval algorithms that are both fast and robust have been developed to obtain the spectral phase and the temporal pulse profile. Here, we give a review of the d-scan technique based on second harmonic generation. We describe and compare recent implementations for the characterization of few- and multi-cycle pulses as well as two different approaches for recording d-scan traces in single-shot, thus showing the versatility of the technique.


Single-shot d-scan technique for ultrashort laser pulse characterization using transverse second-harmonic generation in random nonlinear crystals [Link; Open PDF]
Optics Letters 45(14), 3925-3928 (2020)
DOI: 10.1364/OL.397033
F. J. Salgado-Remacha, B. Alonso, H. Crespo, C. Cojocaru, J. Trull, R. Romero, M. López-Ripa, P. T. Guerreiro, F. Silva, M. Miranda, A. L’Huillier, C. L. Arnold, Í. J. Sola.
 ssdscan Abstract: We demonstrate a novel dispersion-scan (d-scan) scheme for single-shot temporal characterization of ultrashort laser pulses. The novelty of this method relies on the use of a highly dispersive crystal featuring antiparallel nonlinear domains with a random distribution and size. This crystal, capable of generating a transverse second-harmonic signal, acts simultaneously as the dispersive element and the nonlinear medium of the d-scan device. The resulting in-line architecture makes the technique very simple and robust, allowing the acquisition of single-shot d-scan traces in real time. In addition, the technique can be further simplified by avoiding the need of dispersion pre-compensation. The retrieved pulses are in very good agreement with independent FROG measurements. We also apply the new single-shot d-scan to a terawatt-class laser equipped with a programmable pulse shaper, obtaining an excellent agreement between the applied and the d-scan retrieved dispersions.


Detection and elimination of pulse train instabilities in broadband fibre lasers using dispersion scan [Open PDF]
Scientific Reports 10, 7242 (2020).
DOI: 10.1038/s41598-020-64109-x
B. Alonso, S. Torres-Peiró, R. Romero, P. T. Guerreiro, A. Almagro-Ruiz, H. Muñoz-Marco, P. Pérez-Millán, and H. Crespo.
 inst-dscan Abstract: We use self-calibrating dispersion scan to experimentally detect and quantify the presence of pulse train instabilities in ultrashort laser pulse trains. We numerically test our approach against two different types of pulse instability, namely second-order phase fluctuations and random phase instability, where the introduction of an adequate metric enables univocally quantifying the amount of instability. The approach is experimentally demonstrated with a supercontinuum fibre laser, where we observe and identify pulse train instabilities due to nonlinear propagation effects under anomalous dispersion conditions in the photonic crystal fibre used for spectral broadening. By replacing the latter with an all-normal dispersion fibre, we effectively correct the pulse train instability and increase the bandwidth of the generated coherent spectrum. This is further confirmed by temporal compression and measurement of the output pulses down to 15 fs using dispersion scan.


Ultrashort pulse propagation through depressed-cladding channel waveguides in YAG crystal: spatio-temporal characterization [Link] [Free PDF]
Opt. Laser Technol. 123, 105898 (2020). DOI: 10.1016/j.optlastec.2019.105898 M. Morales-Vidal, Í. J. Sola, G. R. Castillo, J. R. Vázquez de Aldana, and B. Alonso.
 hex_wg_thumbnail Abstract: 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.


Measurement of ultrashort vector pulses from polarization gates by in-line, single-channel spectral interferometry [LinkOpen PDF]
IEEE J. Sel. Top. Quantum Electron. 25(4), 8900307, 1-7, (2019). B. Alonso and I.J. Sola.
 jstqe_pol_image Abstract: 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).


Tailoring the spatio-temporal distribution of diffractive focused ultrashort pulses through pulse shaping [Open PDF]
Optics Express 26, 10762-10772 (2018). B. Alonso, J. Pérez-Vizcaíno, G. Mínguez-Vega, and I.J. Sola.
 thumbnail_OE_chirpdoes01 Abstract: 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.


Self-calibrating d-scan: measuring ultrashort laser pulses on-target using an arbitrary pulse compressor [Open PDF]
Scientific Reports 8, 3264 (2018). B. Alonso, I.J. Sola, and H. Crespo.
 self_imag Abstract: In most applications of ultrashort pulse lasers, temporal compressors are used to achieve a desired pulse duration in a target or sample, and precise temporal characterization is important. The dispersion-scan (d-scan) pulse characterization technique usually involves using glass wedges to impart variable, well-defined amounts of dispersion to the pulses, while measuring the spectrum of a nonlinear signal produced by those pulses. This works very well for broadband few-cycle pulses, but longer, narrower bandwidth pulses are much more difficult to measure this way. Here we demonstrate the concept of self-calibrating d-scan, which extends the applicability of the d-scan technique to pulses of arbitrary duration, enabling their complete measurement without prior knowledge of the introduced dispersion. In particular, we show that the pulse compressors already employed in chirped pulse amplification (CPA) systems can be used to simultaneously compress and measure the temporal profile of the output pulses on-target in a simple way, without the need of additional diagnostics or calibrations, while at the same time calibrating the often-unknown dispersion of the compressor itself. We demonstrate the technique through simulations and experiments under known conditions. Finally, we apply it to the measurement and compression of 27.5 fs pulses from a CPA laser.


Strategies for achieving intense single-cycle pulses with in-line post-compression setups [Link]
Optics Letters 43, 337-340 (2018). F. Silva, B. Alonso, W. Holgado, R. Romero, J. San Román, E. Conejero Jarque, H. Koop, V. Pervak, H. Crespo, and I.J. Sola.
 single_imag Abstract: 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.


Universal route to optimal few- to single-cycle pulse generation in hollow-core fiber compressors [Open PDF]
Scientific Reports 8, 2256 (2018). E. Conejero Jarque, J. San Román, F. Silva, R. Romero, W. Holgado, M.A. González-Galicia, B. Alonso, I.J. Sola, and H. Crespo.
univ_imag Abstract: Gas-filled hollow-core fiber (HCF) pulse post-compressors generating few- to single-cycle pulses are a key enabling tool for attosecond science and ultrafast spectroscopy. Achieving optimum performance in this regime can be extremely challenging due to the ultra-broad bandwidth of the pulses and the need of an adequate temporal diagnostic. These difficulties have hindered the full exploitation of HCF post-compressors, namely the generation of stable and high-quality near-Fourier-transform-limited pulses. Here we show that, independently of conditions such as the type of gas or the laser system used, there is a universal route to obtain the shortest stable output pulse down to the single-cycle regime. Numerical simulations and experimental measurements performed with the dispersion-scan technique reveal that, in quite general conditions, post-compressed pulses exhibit a residual third-order dispersion intrinsic to optimum nonlinear propagation within the fiber, in agreement with measurements independently performed in several laboratories around the world. The understanding of this effect and its adequate correction, e.g. using simple transparent optical media, enables achieving high-quality post-compressed pulses with only minor changes in existing setups. These optimized sources have impact in many fields of science and technology and should enable new and exciting applications in the few- to single-cycle pulse regime.


Tunable high-harmonic generation by chromatic focusing of few-cycle laser pulses [Link; Open PDF]
Physical Review A 95, 063823 (2017) W. Holgado, C. Hernández-García, B. Alonso, M. Miranda, F. Silva, Ó. Varela, J. Hernández-Toro, L. Plaja, H. Crespo, and I. J. Sola.
 holgado pra17 thumbnail Abstract: 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 spatio-temporal 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 to 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, 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.


Continuous spectra in high-harmonic generation driven by multicycle laser pulses [Link; Open PDF]
Physical Review A 93, 013816 (2016) W. Holgado, C. Hernández-García, B. Alonso, M. Miranda, F. Silva, L. Plaja, H. Crespo, and I. J. Sola.
 holgado pra16 thumbnail Abstract: We present observations of the emission of XUV continua in the 20–37-eV region by high-harmonic generation with 4–7-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 pulses, 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.


Carrier-envelope-phase insensitivity in high-order harmonic generation driven by few-cycle laser pulses [Free PDF]
Opt. Express 23, 21497-21508 (2015) C. Hernández-García, W. Holgado, L. Plaja, B. Alonso, F. Silva, M. Miranda, H. Crespo, and I. J. Sola.
 getThumbnail.cfm Abstract: 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.


Temporal and spectral structure of the infrared pulse during the high order harmonic generation [Free PDF]
Opt. Express 22, 10191-10201 (2014) W. Holgado, B. Alonso, J. San Román, and I. J. Sola.
 Imagen2 Abstract: 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.


Simultaneous compression, characterization and phase stabilization of GW-level 1.4 cycle VIS-NIR femtosecond pulses using a single dispersion-scan setup [Free PDF]
Opt. Express 22, 10181-10191 (2014) Francisco Silva, Miguel Miranda, Benjamín Alonso, Jens Rauschenberger, Vladimir Pervak, and Helder Crespo.
 getThumbnail.cfm Abstract: We have temporally characterized, dispersion compensated and carrier-envelope phase stabilized 1.4-cycle pulses (3.2 fs) with 160 µJ of energy at 722 nm using a minimal and convenient dispersion-scan setup. The setup is all inline, does not require interferometric beamsplitting, and uses components available in most laser laboratories. Broadband minimization of third-order dispersion using propagation in water enabled reducing the compressed pulse duration from 3.8 to 3.2 fs with the same set of chirped mirrors. Carrier-envelope phase stabilization of the octave-spanning pulses was also performed by the dispersion-scan setup. This unprecedentedly simple and reliable approach provides reproducible CEP-stabilized pulses in the single-cycle regime for applications such as CEP-sensitive spectroscopy and isolated attosecond pulse generation.


Characterization of sub-two-cycle pulses from a hollow-core fiber compressor in the spatiotemporal and spatiospectral domains [Link]
Appl. Phys. B 112, 105-114 (2013) Benjamín Alonso, Miguel Miranda, Francisco Silva, Vladimir Pervak, Jens Rauschenberger, Julio San Román, Íñigo J. Sola, and Helder Crespo.
 HCF starfish 3D color Abstract: We have post-compressed 25 fs (Fourier limit) amplified pulses in an argon-filled hollow-core fiber. The output pulses were compressed using a pair of wedges and chirped mirrors down to 4.5 fs (Fourier limit of 4.1 fs), which corresponds to less than two optical cycles. We then performed the characterization of the pulses by combining the d-scan and the STARFISH techniques. The temporal (and spectral) measurement of the pulses is done with d-scan, which is used as the reference to extend the characterization to the spatiotemporal (and spatiospectral) amplitude and phase of the pulses by means of STARFISH. The post-compressed pulses at the output of the hollow-fiber had an energy of 150 μJ. The analysis of the pulses revealed larger spectral broadening and blue-shift, and shorter duration at the center of the beam. For the first time, we demonstrate the complete characterization of intense ultra-broadband pulses in the sub-two-cycle regime, which provides an improved insight into the properties (space–time and space–frequency) of the pulses and is highly relevant for their applications.


Spatiotemporal characterization of few-cycle laser pulses [PDF] (Erratum)
Opt. Express 20, 17880-17893 (2012)
Erratum: Opt. Express 21, 5582-5584 (2013)
Benjamín Alonso, Miguel Miranda, Íñigo J. Sola, and Helder Crespo.
 thumbnail01_onaxis Abstract: In this paper we apply a broadband fiber optic coupler interferometer to the measurement of few-cycle laser pulses. Sub-8-fs pulses delivered by an ultrafast oscillator were characterized spatiotemporally using STARFISH, which is based on spatially resolved spectral interferometry. The reference pulse was measured with the d-scan technique. The pulses were focused by an off-axis parabolic mirror and were characterized at different transverse planes along the focusing region. The evolution of the retrieved pulses is analyzed, exhibiting small variations in the temporal (and spectral) amplitude and phase during propagation. Finally, the peak irradiance evolution is estimated from the integration of the spatiotemporal intensity.


Characterization of broadband few-cycle laser pulses with the d-scan technique [PDF]
Opt. Express 20, 18732-18743 (2012) Miguel Miranda, Cord L. Arnold, Thomas Fordell, Francisco Silva, Benjamín Alonso, Rosa Weigand, Anne L’Huillier, and Helder Crespo.
 dscan_OE2_thumbnail Abstract: We present an analysis and demonstration of few-cycle ultrashort laser pulse characterization using second-harmonic dispersion scans and numerical phase retrieval algorithms. The sensitivity and robustness of this technique with respect to noise, measurement bandwidth and complexity of the measured pulses is discussed through numerical examples and experimental results. Using this technique, we successfully demonstrate the characterization of few-cycle pulses with complex and structured spectra generated from a broadband ultrafast laser oscillator and a high-energy hollow fiber compressor.


Frequency resolved wavefront retrieval and dynamics of diffractive focused ultrashort pulses [PDF]
J. Opt. Soc. Am. B 29, 1993-2000 (2012) Benjamín Alonso, Rocío Borrego-Varillas, Omel Mendoza-Yero, Íñigo J. Sola, Julio San Román, Gladys Mínguez-Vega, and Luis Roso.
 image035 Abstract: In this work we demonstrate the ability of the spatiotemporal characterization technique STARFISH to retrieve the wavelength dependent wavefront of focused ultrashort laser pulses. The high resolution achievable with this technique allows measuring the wavefront at the focal spot. In particular, the method is applied to study the effects of focusing with a kinoform diffractive lens. The evolution from converging to diverging wavefronts as the pulse propagates along the focal region is analyzed for each wavelength. The spatiotemporal intensity and spatially-resolved spectrum structure of the pulses, as well as their profiles on-axis, are also presented. Numerical simulations of the propagation of such pulses confirm the experimental results.


Synthesis of fractal light pulses by quasi-direct space-to-time pulse shaping [PDF]
Opt. Lett. 37, 1145-1147 (2012) Omel Mendoza-Yero, Benjamín Alonso, Gladys Mínguez-Vega, Íñigo Juan Sola, Jesús Lancis, and Juan A. Monsoriu.
 image022 Abstract: We demonstrated a simple diffractive method to map the self-similar structure shown in squared radial coordinate of any set of circularly symmetric fractal plates into self-similar light pulses in the corresponding temporal domain. The space-to-time mapping of the plates was carried out by means of a kinoform diffractive lens under femtosecond illumination. The spatiotemporal characteristics of the fractal pulses obtained in this way were measured by means of a spectral interferometry technique assisted by a fiber optics coupler (STARFISH). Our proposal allows synthesizing suited sequences of focused fractal femtosecond pulses potentially useful for several current applications, such as femtosecond material processing, atomic, and molecular control of chemical processes or generation of nonlinear effects.


Multibeam second-harmonic generation by spatiotemporal shaping of femtosecond pulses [PDF]
Opt. Lett. 37, 957-959 (2012) Raúl Martínez-Cuenca, Omel Mendoza-Yero, Benjamín Alonso, Íñigo Juan Sola, Gladys Mínguez-Vega, and Jesús Lancis.
 image024 Abstract: 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.


Compensation of second-order dispersion in femtosecond pulses after filamentation using volume holographic transmission gratings recorded in dichromated gelatin [PDF]
Appl. Phys. B 136, 135-141 (2012) A. Villamarín, I. J. Sola, M. V. Collados, J. Atencia, O. Varela, B. Alonso, C. Méndez, J. San Román, I. Arias, L. Roso, and M. Quintanilla.
 image026 Abstract: We have designed and developed a pulse compressor with volume transmission holographic gratings to be implemented in post-compression experiments based on filamentation in gases. Pulse compression down to 13 fs has been demonstrated. The gratings have been recorded in commercial PFG-04 dichromated gelatin emulsions with a recording wavelength of 532 nm, attaining sufficient index modulation to achieve high efficiency when they are illuminated by an 800-nm laser.


Enhancement of filamentation post-compression by astigmatic focusing [PDF]
Opt. Lett. 36, 3867-3869 (2011) Benjamín Alonso, Rocío Borrego-Varillas, Íñigo J. Sola, Óscar Varela, Ayalid Villamarín, M. Victoria Collados, Julio San Román, Juan M. Bueno, and Luis Roso.
 image020 Abstract: The energy scaling up of pulse postcompression is still an open issue. In this work we analyze the use of astigmatic focusing to improve the output pulses in a filamentation based postcompression setup. Unlike spherical conditions, astigmatic focusing enhances the output energy and the spectral broadening of the filament. This is due to the increase of critical power, allowing a considerable improvement of the postcompression energy and stability in a simple way. We demonstrated compression from FWHM 100 fs, 10 nm, 3mJ input pulses to 13 fs, 142 nm, near 1mJ pulses.


Spatiotemporal evolution of light during propagation in filamentation regime [PDF]
J. Opt. Soc. Am. B 28, 1807-1816 (2011) Benjamín Alonso, Íñigo J. Sola, Julio San Román, Óscar Varela, and Luis Roso.
 image018 Abstract: The full description of the evolution of light during its nonlinear propagation represents a valuable help to the complete understanding of important nonlinear phenomena such as light filamentation. In this paper we present a comparison between theoretical and experimental results of the spatiotemporal structure of a light filament at different propagation distances. In order to obtain the experimental spatiotemporal structure, we have used a technique based on spatially resolved spectral interferometry called STARFISH, for spatiotemporal amplitude-and-phase reconstruction by Fourier transform of interference spectra of high-complex beams. We have been able to observe important nonlinear pulse dynamics during the nonlinear propagation, including pulse splitting and the subsequent competition among the pulses that result from the splitting, obtaining a full insight into the general nonlinear behavior.


El láser, la luz de nuestro tiempo (outreach book) [PDF]
Globalia Ediciones Anthema, 2010199 p.: il. col. y n., gráf.; 24 cm
ISBN 9788492997107
B. Alonso, R. Borrego Varillas, C. Hernández García, J.A. Pérez-Hernández y C. Romero (editors).B. Alonso, I. Arias, R. Borrego-Varillas, A. Chacón, E. Conejero, A. García, C. Hernández-García, C. Méndez, J.-A. Pérez-Hernández, L. Plaja, C. Prieto, J. Ramos, J. R. Vázquez de Aldana, C. Romero, L. Roso, C. Ruiz, J. San Román, J. Santamaría, Í.-J. Sola, R. Torres (authors).
 laser_book_1 Resumen: Desde su invención en 1960, el láser ha pasado de ser una curiosidad científica sin aparente utilidad a convertirse en un objeto cotidiano en nuestra vida. Por el camino, ha revolucionado las comunicaciones, la industria, la biomedicina y otras muchas ramas de la ciencia y de la tecnología. Este libro aborda, de forma accesible pero rigurosa, los aspectos más relevantes del láser y de sus aplicaciones. Ahora que gracias a él se pueden lograr en el laboratorio densidades de energía comparables a las que se encuentran en las estrellas, o medir los fenómenos más breves que podamos imaginarnos, está más que justificado definir a l láser como la luz de nuestro tiempo.


Self-compression controlled by the chirp of the input pulse [PDF]
Opt. Lett. 35, 3649-3651 (2010) O. Varela, B. Alonso, I. J. Sola, J. San Román, A. Zaïr, C. Méndez, and L. Roso.
 image016 Abstract: Self-compressed (SC) pulses have been achieved through the filamentation process in air without any additional dispersion compensation, using the input pulse chirp as the control parameter. For any studied input pulse energy (3–5 mJ), we have found two opposite sign input group-delay dispersion values for which SC pulses can be achieved systematically. In addition, we have observed that the energy coupled into the inner core of the filament is always of the order of 20% of the total input pulse energy, which opens the way to a scalable technique to obtain intense short pulses directly from the filamentation process.


Femtosecond multi-filamentation control by mixture of gases: towards synthesised nonlinearity [PDF]
Opt. Express 18, 15467-15474 (2010) Benjamín Alonso, Amelle Zaïr, Julio San Román, Oscar Varela, and Luis Roso.
 image012 Abstract: We have investigated femtosecond multi-filamentation process in a mixture of gases controlling the concentration of atoms versus molecules in the gas cell. The experimental results show that this control could provide a new freedom degree to deterministic spatial distribution control of the multiple filaments. Our simulation indicates surprisingly that only difference of the gases nonlinearity (referred to as “synthesized nonlinearity”) is sufficient to be responsible for this control. This study opens the way to provide few-cycle pulses spatial distributed source for spatially encoded measurements and experiments.
Energy scaling-up of stable single filament [PDF]
Appl. Phys. B 101, 15-22 (2010) B. Alonso, O. Varela, I.J. Sola, J. San Román, A. Zaïr, C. Méndez and L. Roso.
 image014 Abstract: The influence of the input pulse chirp and energy on the filamentation process is studied. Output beam profiles, spectra and energies were systematically measured by changing the pulse chirp for different input energies. A map of the different energy-chirp regions was compiled. It shows that high-energy stable single filament can be obtained by using chirped input pulses, allowing the scaling-up of the energy throughput in the filament. Moreover, under high-energy regimes nonlinear effects induced by chirped pulses could produce pulse post-compression.
Spatio-temporal characterization of ultrashort pulses diffracted by circularly symmetric hard-edge apertures: theory and experiment [PDF]
Opt. Express 18, 20900-20911 (2010) Omel Mendoza-Yero, Benjamín Alonso, Oscar Varela, Gladys Mínguez-Vega, Íñigo Juan Sola, Jesús Lancis, Vicent Climent, and Luis Roso.
 image010 Abstract: We carry out a complete spatio-temporal characterization of the electric field of an ultrashort laser pulse after passing through a diffractive optical element composed of several binary amplitude concentric rings. Analytical expressions for the total diffraction field in the time and spectral domain are provided, using the Rayleigh-Sommerfeld formulation of the diffraction. These expressions are experimentally validated. The spatio-temporal amplitude and phase structure of the pulse are measured at different planes beyond the diffractive optical element using spatially-resolved spectral interferometry assisted by an optical fiber coupler (STARFISH). Our results allow corroborating theoretical predictions on the presence of multiple pulses or complex spectral distributions due to the diffraction-induced effects by the hard-edge ring apertures.


Spatiotemporal amplitude-and-phase reconstruction by Fourier-transform of interference spectra of high-complex-beams [PDF]
J. Opt. Soc. Am. B 27, 933-940 (2010) Benjamín Alonso, Iñigo J. Sola, Óscar Varela, Juan Hernandez-Toro, Cruz Mendez, Julio San Román, Amelle Zaïr, and Luis Roso.
 image008 Abstract: We propose what we believe to be a novel method to reconstruct the spatiotemporal amplitude and phase of the electric field of ultrashort laser pulses using spatially resolved spectral interferometry. This method is based on a fiber-optic coupler interferometer that has certain advantages in comparison with standard interferometer systems, such as being alignment-free and selection of the reference beam at a single point. Our technique, which we refer to as the SpatioTemporal Amplitude-and-phase Reconstruction by Fourier-transform of Interference Spectra of High-complex-beams, offers compactness and simplicity. We report its application to the experimental characterization of chirped pulses and to spatiotemporal reconstructions of a convergent beam as well as plane-plane and spherical-plane waves interferences, which we check with our simulations.


Spatio-temporal characterization of laser pulses by spatially resolved spectral interferometry [PDF]
Opt. Pura Apl. 43, 1-7 (2010) Benjamín Alonso, Iñigo J. Sola, Óscar Varela, Cruz Mendez, Isabel Arias, Julio San Román, Amelle Zaïr, and Luis Roso.
 image006 Abstract: We have implemented a Mach-Zehnder interferometer scheme to characterize the spatio-temporal amplitude and phase of ultrashort laser pulses. We perform spatially resolved spectral interferometry using a homogeneous plane beam as reference that is achieved after spatially filtering a replica of the test beam. We have tested our system and reconstruction algorithm with simulations and experiments. We study the limits of spectral interferometry in our system and, as an example, we reconstruct a linearly tilted beam. This method is expected to be used to characterize pulses with more complex spatio-temporal structure.


Above-millijoule super-continuum generation using polarisation dependent filamentation in atoms and molecules [PDF]
Opt. Express 17, 3630-3639 (2009) Óscar Varela, Amelle Zaïr, Julio San Román, Benjamín Alonso, Iñigo Juan Sola, Camilo Prieto and Luis Roso.
 image004 Abstract: We demonstrate for the first time that input polarisation control inducing one single filamentation is a very robust technique to accurately control the filamentation dynamics enhancing throughput energy of the supercontinuum generation up to 1.2 millijoule. Reaching the above-millijoule regime opens the way to post-compression of multi-terawatt laser pulses.


Simulating beam-shape effects in non-collinear second harmonic generation [PDF]
Opt. Pura Apl. 42, 71-81 (2009) Benjamín Alonso, Javier R. Vázquez de Aldana, and Luis Roso.
 image002 Abstract: We present a simplified model for the simulation of Second Harmonic Generation (SHG) with two fundamental beams that propagate in different directions: non-collinear SHG. In spite of its simplicity (diffraction is not included and the slowly varying envelope approximation in space is assumed) it can be used in many realistic situations. It has been implemented on Mathematica and is freely available to the reader. Beam-shape effects, conversion efficiencies, input intensity, phase-matching and other parameters can be studied with our code in a virtual experiment. We have applied the model to SHG of 1064 nm radiation in KDP crystal as an example.



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