Review

CSI Zamora-Salamanca: reconstructing vector pulses with amplitude swing

Temporally characterizing ultrashort laser pulses (on the femtosecond scale, i.e., 10-15 seconds) is akin to reconstructing a crime scene: the light pulses are so fast that we can’t catch them in the act, we can only reconstruct them from the clues they leave behind.

Typically, we work with linearly polarized scalar pulses, in which the polarization state remains constant over time (polarization refers to the trajectory described by the light in the transverse plane). To identify these pulses, we need to know their amplitude or intensity and their phase. There is another type of pulses in which polarization varies over time, known as vector pulses. These are more complex than scalar pulses, and we need to know the amplitude and phase of their two components, as well as the relative phase between them. If identifying a scalar pulse is equivalent to identifying a criminal, knowing a vector pulse would be like knowing a gang composed of two criminals, and moreover, the relationship between them.

One type of characterization techniques is based on measuring the spectrum of a nonlinear signal while the pulse undergoes some modification. In the amplitude swing technique (a-swing), developed by researchers from the ALF group, two replicas of the pulse to be measured are generated, temporally delayed from each other, and the second harmonic spectrum (frequency doubling) is measured for different relative amplitudes of these replicas. Thus, a two-dimensional trace is obtained (a map where color represents intensity), which is like a fingerprint of the pulse. In some techniques, ambiguities arise, i.e., two different pulses generate the same trace, as if two people had the same fingerprint. Through algorithms, the information of the pulse generating the trace (our clue) can be extracted.

Most techniques only allow the characterization of scalar pulses. If we want to reconstruct a vector pulse with one of these techniques, we need several traces, i.e., several fingerprints. In contrast, a single a-swing trace contains the necessary information to identify a vector pulse. Furthermore, these traces are obtained with an inline, compact, and versatile setup.

In this work, we analyze the a-swing traces analytically and numerically to study how the information of vector pulses is encoded, and we develop a strategy to extract it. This strategy is applied to simulated and experimental traces, demonstrating that a vector pulse can be reconstructed from its a-swing trace. If they don’t want to be caught, they’ll have to avoid leaving these kind of fingerprints…

More information at:
Cristian Barbero, Benjamín Alonso, and Íñigo J. Sola, “Characterization of ultrashort vector pulses from a single amplitude swing measurement,” Opt. Express 32, 10862-10873 (2024) https://doi.org/10.1364/OE.515198

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Attoscience

The shortest flashes of light we can control only last a tiny fraction of a second – mere trillionths, or attoseconds. Within this tiny timeframe, we can witness how atoms and molecules behave. Attophysics, a new area of study, has emerged from this. But how did we get here? This article tells the story of our collective effort to create shorter and shorter bursts of light, a journey that won the 2023 Nobel Prize in Physics. It’s a tale of milestones, shifts in thinking, and inspiration, giving us a new perspective on scientific progress.

More information at:
L. Plaja, “Attociencia”, Revista Española de Física 37-4, 49 (2023) 

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Triggering ultrafast magnetic dynamics using structured light

In the last decades, a growing interest has been developed around the possibility of manipulating the magnetic properties of matter at the nanoscale, with the paramount objective of obtaining high-density, ultrafast, and low-power memories. Since the ’90s, the control, and namely the demagnetization of magnetic samples using femtosecond laser pulses has been widely studied. However, the thermal effects strongly limit the demagnetization characteristical times, imposing significant restrictions to obtain the desired dynamics. 

Recently, we have studied the possibility of inducing magnetization switching using exclusively circularly polarized magnetic fields. This approach relies on developing a nonlinear magnetization dynamic induced by the circularly polarized magnetic field, avoiding the thermal imposed restrictions, and paving the way to excite ultrafast dynamics in the sub-femtosecond regime. 

Crafting a circularly polarized magnetic field is a daunting challenge, although it is nowadays feasible with the wide variety of structured beams. Specifically, thanks to the azimuthally polarized vector beams, we can obtain locally isolated magnetic fields. These intriguing beams have a ring-type intensity structure, with a zero intensity in the central area of the electric field distribution. Surprisingly, in analogy with a current coil, they present an isolated, longitudinally polarized magnetic field in the region where the electric field goes to zero. Using two non-collinear, correctly dephased, azimuthally polarized vector beams, a circularly polarized magnetic field is constructed in the crossing region, where these exotic nonlinear ultrafast dynamics take place. 

Once more, structured light demonstrates its vast versatility to study and manipulate a wide range of physical processes in a large spectrum of areas in physics

More info at:

Sánchez-Tejerina, L., Martín-Hernández, R., Yanes, R., Plaja, L., López-Díaz, L., \& Hernández-García, C. (2023). All-optical nonlinear chiral ultrafast magnetization dynamics driven by circularly polarized magnetic fields. High Power Laser Science and Engineering, 11, E82. doi: 10.1017/hpl.2023.71
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Generation of cracks in materials with ultrashort pulses: standard for fracture resistance testing

This work addresses the crack growth resistance of 3 mol% Yttria-doped Tetragonal Zirconia Polycrystalline (3YTZP) spark-plasma sintered (SPS) composites containing two types of graphene-based nanomaterials (GBN): exfoliated graphene nanoplatelets (e-GNP) and reduced graphene oxide (rGO). The crack growth resistance of the composites is assessed by means of their R-Curve behavior determined by three-point bending tests on single edge “V” _notched beams (SEVNB), in two different orientations of the samples: with the crack path perpendicular or parallel to the pressure axis during the SPS sintering. The sharp edge notches were machined by ultrashort laser pulsed ablation (UPLA). The compliance and optical-based methods for evaluating the crack length are compared on the basis of the experimental R-Curve results in composites with 2.5 vol% rGO tested in the perpendicular orientation. Moreover, the activation of reinforcement mechanisms is evaluated by both the fracture surface inspection by Scanning Electron Microscopy and a compliance analysis. It is shown that the indirect compliance method is relevant and reliable for calculating the R-Curve of 3YTZP/GBN composites. The effect of the type and content of GBN on the crack growth resistance of the composites is also discussed.

More information at:

López-Pernía, C., Muñoz-Ferreiro, C., Prada-Rodrigo, J., Moreno, P., Reveron, H., Chevalier, J., Morales-Rodríguez, A., Poyato, R., & Gallardo-López, Á. (2023). R-curve evaluation of 3YTZP/graphene composites by indirect compliance method. Journal of the European Ceramic Society, 43(8), 3486-3497. https://doi.org/10.1016/j.jeurceramsoc.2023.02.002
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Controlling light with intelligence

Thanks to a process called “high-order harmonic generation” significant progress has been made in generating ultrashort X-ray pulses over the past few years, with a duration of a few attoseconds (equivalent to dividing a second into 1,000,000,000,000,000,000 parts). This extremely short duration is comparable to the time it takes for electrons to transfer between atoms, making these pulses exceptional tools for exploring high-speed physical phenomena.

The required experimental setup and desired characteristics of the light pulses vary depending on their application. While it is possible to simulate this process to understand and predict its behavior under different circumstances, performing these calculations requires an extremely long time, even on the world’s most powerful supercomputers. Therefore, it is common to resort to approximations that provide acceptable but improvable results.

However, this can be addressed with intelligence, specifically with Artificial Intelligence (AI). A recent study conducted by the Research Group in Laser Applications and Photonics (ALF) has shown that it is possible to use artificial neural networks to accelerate these simulations and obtain nearly immediate results with a level of accuracy that had not been achieved until now.

More information at:  

José Miguel Pablos-Marín, Javier Serrano, Carlos Hernández-García, “Simulating macroscopic high-order harmonic generation driven by structured laser beams using artificial intelligence”, Computer Physics Communications, In Press – Journal Pre-proof (2023). https://doi.org/10.1016/j.cpc.2023.108823

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Fantastic Spectra and where to find them

The generation of ultra-short light pulses with a good spatial structure is the philosopher’s stone of ultrafast pulse physics. These pulses make it possible to study and modify the properties of matter at time scales unreachable by other procedures.

In recent decades, great strides have been made in the generation of high-quality ultrashort pulses among which post-compression techniques stands out. Post-compression techniques consist of widening the spectrum of a pulse during its propagation thanks to nonlinear effects and then correcting its phase to achieve the shortest possible temporal pulse. The most widely used post-compression technique today is based on the nonlinear propagation of a pulse through a hollow core fiber filled with gas. However, in the last decade, with the rise of new lasers, such as the Yb laser, other post-compression methods that do not have to deal with the restrictions presented by hollow core fibers have gained relevance. One of these new post-compression techniques is the nonlinear propagation in multipass cells.

These multipass cells are cavities formed by two spherical mirrors in which the laser beam is introduced in the cavity off-axis, in such a way that the beam is reflected multiple times forming a hyperboloid before leaving the cell. One of the advantages of these cavities is that we can introduce in them a nonlinear medium through which the beam propagates in nonlinearly during the successive round trips.

Building upon this research, we have theoretically explored a post-compression region in multipass cells that allows the generation of wide spectra with smooth profiles that prevent the pulse from presenting too much structure (pre-pulses or post-pulses) once compressed. In order to accomplish this, we have relied on a particular regime explored already in the 80s known as the enhanced frequency chirp regime, and we have adapted it to multipass cells. In this regime, nonlinear effects and dispersion go hand in hand to widen the spectrum while maintaining a smooth structure that supports a very clean temporal profile. We have optimized the parameters of this region for the case of a multipass cavity filled with argon obtaining pulses whose Fourier limit is compressed more than 10 times with respect to the duration of the initial pulse, but above all maintaining an extremely clean structure, which makes it very useful for a variety of applications.

More information at:

Staels, V. W. Segundo, E. Conejero Jarque, D. Carlson, M. Hemmer, H. C. Kapteyn, M. M. Murnane, y J. San Roman. 2023. «Numerical investigation of gas-filled multipass cells in the enhanced dispersion regime for clean spectral broadening and pulse compression». Opt. Express 31(12):18898-906. doi: 10.1364/OE.481054.
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Micro-spectrometer

Recently, researchers belonging to ALF, have been working in the development of a miniaturized spectrometer in collaboration with the European Space Agency, the Department of Physics and Swiss Nanoscience Institute (University of Basel), the Department of Chemistry and Applied Biosciences (ETH Zurich), the Swiss Federal Laboratories for Materials Science and Technology (Empa) and the Optics and Photonics Technology Laboratory (Ecole Polytechnique Fédérale de Lausanne, EPFL). The device belongs to the family of ultracompact Fourier Transform spectrometers, and it consist of a LiNbO3 chip in which a monomodal waveguide was fabricated with an optimized design to produce a light flux in the vertical direction. In the upper part of the chip a nano-detector (gold nanowire) was placed perpendicularly to the waveguide, together with a quantum dot HgTe nanolayer. The gold nanowire acts as scattering element, sensing the light confined in the waveguide. The nanolayer creates a photocurrent that can be measured. An external mirror placed at the output of the waveguide enables the creation of a standing wave that is monitorized by the nano-detector. The controlled motion of the mirror produces a spatial swept of the standing wave, thus obtaining the measurement of the confined intensity, from which the spectrum is extracted by Fourier transform.

Scheme of the device

After fabrication, it has been demonstrated the efficient operation with resolution better than 50 cm-1 in the near infrared. The active part of the device has a tiny volume as small as 100 μm×100 μm×100 μm, and it could be integrated in the new generation of ultrasmall satellites.

More information at:  

M. Grotevent et al., “Integrated photodetectors for compact Fourier-transform waveguide spectrometers” Nature Photonics 17, 59 (2023). https://doi.org/10.1038/s41566-022-01088-7

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Lasers and nanostructured polymers and compounds: influence of properties and parameters

A study of the formation of Laser Induced Periodic Surface Structures (LIPSS) using near-infrared femtosecond pulsed laser irradiation on poly(ethylene terephthalate) (PET) films deposited over gold substrates has been carried out. We report the influence of the gold substrate roughness and the PET film thickness on LIPSS formation and analyze it in terms of the features of the electric field distribution obtained by computer simulations using COMSOLTM. We obtain LIPSS with periods close to the irradiation wavelength as long as the aforementioned substrate and film parameters remain below certain threshold values, in particular for polymer thicknesses below 200 nm and substrate roughness of few nm. However, experiments show the impossibility of LIPSS formation for rough substrates as well as thick films above these threshold values. In our numerical simulations, we notice the generation of Surface Plasmon Polariton (SPP) in the film-substrate interface that gives rise to a periodical field pattern on the surface of the thin film. This periodicity is broken for a certain level of substrate roughness or film thickness. Moreover, the evolution of the period of the SPP as the substrate roughness and film thickness change for given laser parameters is qualitatively in good agreement with the experimental LIPSS period (below but close to the irradiation laser wavelength). In conclusion, the experimental findings are explained by the formation and behavior of SPP in the thin film-substrate interface. On these grounds, we propose that, for our case of study, this SPP formation and the subsequent inhomogeneous rise in temperature induced by the periodic field on the surface of the sample is the leading mechanism contributing to LIPSS formation.

More information at:  

Prada-Rodrigo, J., Rodríguez-Beltrán, R. I., Ezquerra, T. A., Moreno, P., & Rebollar, E. (2023). Influence of film thickness and substrate roughness on the formation of laser induced periodic surface structures in poly(ethylene terephthalate) films deposited over gold substrates. Optics & Laser Technology, 159, 109007. https://doi.org/10.1016/j.optlastec.2022.109007
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Use of ultrashort laser pulses as a standard for fracture resistance testing

The use of fracture mechanics for rationalizing the fracture behavior of cemented carbides is valid, as far as sharp cracks, free of residual stresses and subjected to a well-defined stress state are used for assessing fracture toughness. However, machining a very sharp notch on the surface of hardmetals for fracture toughness testing has been a critical issue during many years. Within this context, introduction of surface “through-thickness” _micronotches (SEμVNB) by means of ultrashort pulsed laser ablation (UPLA) is here proposed, implemented and analyzed as an innovative precracking-like route within flexural testing procedures for appropiated evaluation of fracture toughness of cemented carbides. UPLA parameters used for introducing the micronotch are optimized in terms of induced damage ahead of the notch tip. For comparison purposes, fracture toughness is also determined by means of flexural testing of previously cracked single-edge notch beams (SENB-Cracked) as well as specimens with V-notch tips sharpened through diamond polishing using a razor blade, and Palmqvist indentation microfracture method. The satisfactory agreement found between values measured using UPLA-micronotched and SENB-Cracked (reference) specimens allows to conclude that flexural testing of SEμVNB samples is a valid methodology for reliable determination of fracture toughness of hardmetals. This is feasible because of the extremely short time of laser-matter interaction. It yields small and somehow controlled damage in front of the notch tip as a result of shock wave propagation during ablation, which translates into effective precracking of SEμVNB specimens

More information at:  

Ortiz-Membrado, L., Liu, C., Prada-Rodrigo, J., Jiménez-Piqué, E., Lin, L. L., Moreno, P., Wang, M. S., & Llanes, L. (2022). Assessment of fracture toughness of cemented carbides by using a shallow notch produced by ultrashort pulsed laser ablation, and a comparative study with tests employing precracked specimens. International Journal of Refractory Metals and Hard Materials, 108, 105949. https://doi.org/10.1016/j.ijrmhm.2022.105949
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It was not everything perfect

 
Nowadays, the high-order harmonic generation process is an extended useful tool for the study of femtosecond dynamics. Nevertheless, there are still many doubts regarding the electron behavior inside different types of mediums.
 
Recent studies in solid targets have revealed new scenarios with extraordinary electronic dynamics compared with atoms or molecules. The process in solids can be explain through a semiclassical point of view using the electron trajectory from the excitation until the recombination with its hole in real space; the so-called perfect recollisions. However, recent studies have confirmed that part of the high-order harmonic emissions comes from trajectories where the electron and hole do not overlap in real space; the so-called imperfect recollisions.
 
In this work, we demonstrate the existence of imperfect recollisions when the medium is a single-layer graphene, and the driving laser pulse is linearly polarized. Graphene, compared to other solids, presents a singular structure band with points where the valence and conduction band are in contact. Our study has a great relevance because until this moment there were studies only with finite-gap solids and huge Berry curvature or using a driving field with elliptical polarization. We truly believe that this work takes a new step in the full understanding of the ultrafast dynamics driven by intense laser pulses in solids.
 

More information at:

Boyero-García, Roberto, Ana García-Cabrera, Oscar Zurrón-Cifuentes, Carlos Hernández-García, y Luis Plaja. «Non-classical high harmonic generation in graphene driven by linearly-polarized laser pulses». Opt. Express 30, n.o 9 (abril de 2022): 15546-55. https://doi.org/10.1364/OE.452201.
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