Review

Generating isolated attosecond pulses in semi-infinite gas cells

In this paper, published in collaboration with researchers from the Politecnico di Milano, ETH Zürich, and Lund University, we explore an innovative approach to generate isolated attosecond pulses using a semi-infinite gas cell. Traditionally, experiments generating these extremely short pulses employ gas jets or short gas cells at high pressures, ensuring the necessary phase matching for efficient high-order harmonic generation. However, longer, lower-pressure media have not been used due to the difficulties in achieving good phase matching over such a long distance, often leading to less effective results.

Our work challenges this conventional view by demonstrating that it is possible to efficiently generate isolated attosecond pulses in an extended medium configuration, such as a semi-infinite gas cell (SIGC) filled with a noble gas at low pressure. Our results show that the incident infrared field, during its nonlinear propagation through the SIGC, creates a plasma channel in the final part that plays a crucial role in self-regulating its spatiotemporal structure and facilitating the phase matching conditions necessary to produce these isolated attosecond pulses.

Experiments, conducted by our collaborators at the Politecnico di Milano, have managed to characterize, for the first time and clearly in an extended medium, isolated pulses with a duration of 180 attoseconds and a continuous spectrum in the 20-45 eV energy range, making them very useful for ultrafast spectroscopy experiments.

Supporting the experiments, we present detailed simulations developed by our team that provide a better understanding of the mechanisms involved. These simulations, which combine the atomic-level quantum dynamics of the harmonic generation process with the nonlinear propagation of the incident laser pulse, confirm the experimental findings. One of the most important aspects we observe is that the gradual increase in gas pressure in the cell creates a temporal window of phase matching during which the emission of attosecond pulses is especially efficient. As the pressure increases, this window is reduced, allowing the generation of a single attosecond pulse in half a cycle of the incident laser.

In summary, our work demonstrates that long media configurations, such as the semi-infinite gas cell, are not only viable for generating isolated attosecond pulses but, under the right conditions, can overcome some of the traditional limitations of short media, opening up new possibilities for experiments requiring extremely high temporal resolution. This advance could have applications in fields such as ultrafast spectroscopy and studies of electronic dynamics in complex materials.

More information in:

Vismarra, M. F. Galán, D. Mocci, L. Colaizzi, V. W. Segundo, R. Boyero-García, J. Serrano, E. C. Jarque, M. Pini, L. Mai, Y. Wu, H. J. Wörner, E. Appi, C. L. Arnold, M. Reduzzi, M. Lucchini, J. San Roman, M. Nisoli, C. Hernández-García, and R. Borrego-Varillas, “Isolated attosecond pulse generation in a semi-infinite gas cell driven by time-gated phase matching,” Light Sci. Appl. 13, 197 (2024). https://doi.org/10.1038/s41377-024-01564-5

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Reverse engineering of ultrashort laser pulses

In this work, we explore a novel theoretical approach to enhance our understanding of ultrashort laser pulse compression in gas-filled hollow-core fibers. These pulses are essential in ultrafast science, where they are used to study atomic and molecular dynamics on extremely short timescales. However, compressing these pulses to very short durations without the appearance of relevant secondary structures (pre-pulses and/or post-pulses) is a significant technical challenge. To address this challenge, we apply a method called “reverse nonlinear propagation,” which allows us to predict the ideal shape of the input pulse to achieve an optimal compressed pulse at the output. The key to this approach is that, instead of directly designing the input pulse, we simulate what an ideal pulse would look like at the output and reverse its propagation in the fiber to determine the characteristics the initial pulse should have.

The process of pulse compression typically involves using a hollow capillary filled with gas in which a laser pulse is coupled to broaden its spectrum during propagation, mainly due to self-phase modulation, a nonlinear effect that generates new frequencies very efficiently. Then, the phase of the new spectrum obtained at the output of the capillary is adjusted in an external compressor, composed of dispersive elements, to shorten the temporal duration of the pulse. The problem is that compressed pulses often exhibit undesired secondary structures, such as additional peaks that distort the pulse shape. Our method allows us to design an input pulse that minimizes or eliminates these secondary structures.

One of the most interesting findings is that the ideal pulse predicted by the reverse propagation technique has a characteristic profile: its spectrum always presents small modulations around the main peak. Our simulations demonstrate that these initial spectral modulations allow compensating for the nonlinear effects that occur inside the capillary during the pulse propagation to produce the clean pulse at the output.

The reverse propagation method is not new, but its application in this context presents particular challenges due to the high energy losses of hollow-core fibers and the symmetries of the equation that describes the nonlinear propagation of ultrashort pulses. Despite these complications, we demonstrate that it is possible to numerically reverse the pulse propagation and accurately predict the characteristics of the input pulse required to obtain optimal compression.

Furthermore, our study highlights the high sensitivity of the compression process to small changes in the phase and amplitude profile of the input pulse. Even slight variations in the initial phase or amplitude can lead to significantly different results at the output, underlining the importance of controlling both aspects in the design of experiments.

In summary, this work proposes a new theoretical tool that can guide the design of ultrashort pulse compression experiments in laboratories. While some of our results still need to be experimentally validated, we believe that this method opens the door to generating clean and ultrashort pulses that could improve applications in ultrafast spectroscopy, strong-field physics, and other areas of ultrafast science.

More information in:

F. Galán, E. C. Jarque, and J. San Roman, “Reverse design of the ideal pulse for hollow capillary fiber post-compression schemes,” Phys. Rev. Res. 6(2), 023111 (2024). https://doi.org/10.1103/PhysRevResearch.6.023111

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ALF-USAL drives innovations in biomedical implants with MELTIO’s 3D technology.

The research group ALF-USAL from the University of Salamanca is participating in the ATILA project, which focuses on developing new applications for biomedical implants. This project, led by AIDIMME and also involving the FIHGUV foundation, uses MELTIO’s metal 3D printing technology.

ALF-USAL is responsible for initial studies on the parameters needed to create models simulating the additive manufacturing process. These studies are crucial for improving implant biocompatibility and customization.

The project faces challenges in material precision and adaptability but has made significant progress in creating personalized, biocompatible implants.

For more details, visit the COPE press release.

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Isolated and intense polarization-controlled optical magnetic fields

Usually, when talking about laser-matter interaction, only the electric field associated with such electromagnetic radiation is taken into account. One of the reasons for this is that the excitations induced by the magnetic field are orders of magnitude smaller than those driven by the electric field. However, the interest in coherently probing magnetic systems on specific time and space scales, outside the scope of traditional magnetic field sources such as electromagnets, demonstrates the need to develop new schemes for the design and control of the electromagnetic field that forms light. This is possible thanks to the large structured light zoo, being able to manipulate different degrees of freedom such as intensity, phase or polarization state. Although there are several studies that address the separation of the magnetic field from the associated electric field in a light beam, in most cases it is necessary the interaction with matter to induce electrical currents for the creation of a sufficiently intense and isolated longitudinal linearly polarized magnetic field.

With our theoretical study we go one step further in this scenario, looking for a magnetic field whose polarization state can be controlled, ranging from linear to circular through elliptical. When such optical magnetic field with cylindrical symmetry along the beam propagation axis is introduced into Maxwell’s equations that govern classical electromagnetism, the result is an extremely complex associated electric field distribution. This consists of an optical vortex (a beam in which the phase or wavefront forms a helix as it propagates; this is known as the orbital angular momentum of light) with a single polarization component along the propagation axis. This challenging solution is beyond the current laser technology, so other more realistic schemes need to be adopted.

In our work we propose the coherent superposition of several dephased structured beams, in a way that only by their optical manipulation one can have direct control over the polarization state of the resulting isolated magnetic field in a given region of space. On one hand, we use azimuthally polarized vector beams as drivers to exploit their magnetic longitudinal component linearly polarized along the axis where the electric field is zero due to the polarization singularity. By tightly-focusing them with a large numerical aperture optical system outside the paraxial regime, this component can be confined and intensified starting from relatively low intensity lasers. By combining two or four of these focused beams in a cross geometry with the respective focus at the same point and applying the corresponding phase shifts, it is possible to achieve an intense magnetic field, isolated from the electric field and with circular polarization laying in the plane in which the driving beams are arranged, in a sub-wavelength region.

Our results obtained from a a feasible experimental setup point of view open the doors to new perspectives in such wide applications as optical and magnetic spectroscopy, force microscopy or ultrafast magnetization dynamics. In particular, the inspection of magnetic interactions with intense lasers in the ultrafast regime with phenomena such as the nonlinear dynamics of magnetization in ferromagnetic samples, the study of chiral materials or applications in the potential improvement of spatial resolution in the optical interaction with magnetic systems are particularly attractive.

More info at:

Sergio Martín-Domene, Luis Sánchez-Tejerina, Rodrigo Martín-Hernández, Carlos Hernández-García; Generation of intense, polarization-controlled magnetic fields with non-paraxial structured laser beams. Appl. Phys. Lett. 20 May 2024; 124 (21): 211101.

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2×1 in ultrashort laser pulses

In the last decades, ultrashort laser pulses have revolutionized our way of studying the microscopic world through the interaction of coherent light with matter. The generation and manipulation of these ephemeral electromagnetic fields allows us to access the fastest atomic phenomena in nature, occurring on the femto to attosecond (10-15-10-18 s) time scale. The rapid advancement of laser technology has made it possible, in recent years, to synthesize infrared pulses with sub-cycle durations, in which the most intense structure of the electric field of light barely has time to complete an oscillation at its central frequency. These pulses provide a unique tool for exploring electron motion in atoms and molecules, but their generation is still limited to extremely expensive and complex setups.

Recently, we demonstrated that these sub-cycle pulses can be obtained much more simply in standard systems based on the propagation of light through gas-filled hollow capillary fibers with a decreasing pressure gradient. This proposal is based on a surprising phenomenon of nonlinear optics, known as soliton self-compression, where an intense laser pulse can, by itself, simultaneously broaden and organize its frequency spectrum, reducing its duration almost to the limit. By following some scaling rules to design the fiber and input pulse parameters, this technique allows for the generation of high quality sub-cycle infrared pulses.

Not content with reaching durations of just one femtosecond, in our latest work, conducted in collaboration with researchers from Politecnico di Milano and Heriot-Watt University, we have explored the application of these sub-cycle fields to generate even shorter laser pulses in the attosecond regime. To do so, we have exploited the phenomenon of high-order harmonic generation, which arises from the interaction of an intense infrared pulse with the atoms of a gas. When the interaction is performed with a conventional laser, this process works as a production chain of attosecond pulses in the extreme ultraviolet, giving rise to a series of light flashes that occur at regular time intervals. However, if the interaction is driven by one of our previous sub-cycle pulses, the harmonic generation process is naturally confined to a single event, resulting in the direct emission of an isolated attosecond pulse. These solitary ultraviolet pulses are a highly sought-after tool in ultrafast science applications where very precise control and high temporal resolution are needed.

Thus, our study opens the door to a new generation of compact fiber-based systems in which, starting from a standard infrared laser pulse, we combine for the first time its extreme self-compression down to the sub-cycle regime and its direct application to generate extreme-ultraviolet isolated attosecond pulses.

More information in:

  1. F. Galán, J. Serrano, E. C. Jarque, R. Borrego-Varillas, M. Lucchini, M. Reduzzi, M. Nisoli, C. Brahms, J. C. Travers, C. Hernández-García, and J. San Roman, “Robust isolated attosecond pulse generation with self-compressed sub-cycle drivers from hollow capillary fibers,” ACS Photonics 11(4), 1673-1683 (2024).

https://doi.org/10.1021/acsphotonics.3c01897

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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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