ALF results

Researchers from the Institute of Ion Beam Physics and Materials Research visited ALF-USAL

Researchers Rang Li and Chi Pang, from the Institute of Ion Beam Physics and Materials Research (Helmholtz-Zentrum Dresden-Rossendorf), conducted an experimentation campaign last week at the USAL Laser Laboratory.

These researchers are working on the development of new advanced materials for photonics applications such as nanomembrane microcavities.

They utilize various experimental devices based on ultrashort pulse lasers developed by the researchers of the ALF group, Carolina Romero, Ignacio López, Íñigo Sola, and Javier Rodríguez.

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adminResearchers from the Institute of Ion Beam Physics and Materials Research visited ALF-USAL

Researchers from the ALF group at USAL participated in the HILAS congress

Researchers from the ALF group at USAL, including Carlos Hernández García, Marina Fernández Galán, and Rodrigo Hernández Martín, participated in the High-Intensity Lasers and High-Field Phenomena (HILAS) congress, which took place from March 12th to March 14th in Vienna.

HILAS serves as a prominent platform for scientists and researchers to explore cutting-edge advancements and discoveries in the field of high-intensity lasers and high-field phenomena. The congress provides a forum for discussions, presentations, and collaborations among experts in various disciplines, including physics, optics, engineering, and materials science. Through keynote speeches, panel sessions, and workshops, HILAS facilitates the exchange of knowledge and fosters innovation in this rapidly evolving field.

The following works have been presented:

  • Simulating Macroscopic High-order Harmonic Generation Driven by Structured Laser Beams Using Artificial Intelligence, Carlos Hernandez-Garcia; Universidad de Salamanca, Spain.
    • Employing artificial intelligence, we integrate microscopic quantum computations based on the time dependent Schrödinger equation with macroscopic physics, to unveil hidden signatures in the ultrafast electronic dynamics of high-order harmonic generation by structured laser beams.
  • Compact Generation of Isolated Attosecond Pulses Driven by Self-compressed Subcycle Waveforms, Marina F. Galán1, Javier Serrano1, Enrique Conejero Jarque1, Rocío Borrego-Varillas2, Matteo Lucchini3, Maurizio Reduzzi3 , Mauro Nisoli3 , Christian Brahms4, John C. Travers4, Carlos Hernandez-Garcia1, Julio San Roman1; 1 Universidad de Salamanca, Spain; 2 IFN-CNR, Italy; 3 Politecnico di Milano, Italy; 4 Heriot-Watt University, United Kingdom.

We theoretically demonstrate a compact and robust scheme for the direct generation of extreme ultraviolet isolated attosecond pulses from high-order harmonics driven by self-compressed subcycle waveforms produced in a gas-filled hollow capillary fiber.

  • Generation of high-order harmonic spatiotemporal optical vortices, Rodrigo Martín Hernández1,2, Guan Gui3, Luis Plaja1,2, Henry K. Kapteyn3, Margaret M. Murnane3, Miguel A. Porras4, Chen-Ting Liao3,5, Carlos Hernandez-Garcia1,2; 1 Grupo de Investigación en Aplicaciones del Láser y Fotónica. Departamento de Física Aplicada, Universidad de Salamanca, Spain; 2 Unidad de Excelencia en Luz y Materia Estructuradas (LUMES), Universidad de Salamanca, Spain; 3 JILA and Department of Physics, University of Colorado and NIST, USA; 4 Grupo de Sistemas Complejos, ETSIME, Universidad Politécnica de Madrid, Spain; 5 Department of Physics, Indiana University, USA.

We theoretically and experimentally demonstrate the generation of high-topological charge, extreme-ultraviolet (EUV) spatiotemporal optical vortices (STOV) from high-order harmonic generation. EUV-STOVs are unique structured light tools for exploring ultrafast topological laser-matter interactions.

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adminResearchers from the ALF group at USAL participated in the HILAS congress

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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Creation of the Unit of Excellence LUMES

Last June, the Unit of Excellence in Structured Light and Matter (LUMES) was created.

The creation of the Unit of Excellence in Structured Light and Matter represents a step forward in the consolidation of the University of Salamanca as an international reference in the understanding and application of the interactions between light and matter on the ultrafast and nanometric scale. Through interdisciplinary collaboration and the training of young researchers, this unit will position itself as an international leader in the development of technologies applying structured laser light to the study of new materials. The scientific and technological advances obtained are expected to boost innovation in fields such as photonics, optoelectronics, nanotechnology and quantum optics, areas with a transversal impact on multiple disciplines of science.

The LUMES Unit of Excellence will address various cutting-edge topics in the fields of ultrafast and nonlinear optics and materials science, including the development of spatiotemporally structured ultrafast lasers in a wide spectral range (from THz to X-rays); the study of the electronic, optical and magnetic properties of 2D materials at the quantum level and their associated van der Waals heterostructures; the interaction of these materials with ultrafast structured light; the processing of materials using ultra-intense lasers; and the study of ultrafast dynamics in magnetic materials excited with structured laser pulses, among others.

The LUMES Unit of Excellence is made up of 8 guarantor researchers and a total of 32 doctoral researchers, affiliated with the Department of Applied Physics of the USAL, the Center for Pulsed Lasers, and the Department of Mechanical Engineering of the USAL. The unit will be directed by Carlos Hernández García.

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New research project: SpecX

The new research project, SpecX (Schemes for the generation of attosecond x-ray special beams using high-order harmonic generation from macroscopic targets), has started, with Julio San Román and Carlos Hernández Garcia as the principal investigators. In the SpecX project, they aim to advance in the macroscopic management of ultrarapid light beams, from the femtosecond/infrared to the attosecond/X-ray regime, with special emphasis on the study of complex topological fields. To do this, advanced simulation codes are required, given that the mechanisms for generating such short laser pulses, through nonlinear laser post-compression or high-order harmonics generation, combine microscopic and macroscopic physics, which poses a great challenge.

To achieve this goal, the following objectives have been defined:

  • Exploit high-performance computational strategies that use artificial intelligence to access these new extreme non-linear optical scenarios.
  • Design ultrarapid pulses structured in the femtosecond/infrared regime through different non-linear propagation schemes, such as the use of hollow core fibers and photonic crystals, and multipass cells.
  • Explore new schemes for the process of high-order harmonics generation in the X-ray and attosecond regime, such as crystalline solids irradiated by fields.
  • Explore the generation of high-order harmonics with post-compressed laser pulses in combined regimes.
  • Propose new experimental proposals for the generation of structured X-ray fields in the attosecond regime.

The SpecX project falls under the Call Programa Estatal para Impulsar la Investigación Científico-Técnica y su Transferencia, del Plan Estatal de Investigación Científica, Técnica y de Innovación 2021-2023. It has a duration of three years and has received funding of €127,500 from the Ministerio de Ciencia e Innovación and Agencia Estatal de Investigación, co-financed by the European Regional Development Fund (ERDF).

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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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