ALF results

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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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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Announcement of defense of doctoral thesis – Javier Prada-Rodrigo

On June 8th, Javier Prada Rodrigo will present his doctoral thesis entitled “Formation and characterization of surface micro- and nanostructures in polymers and polymeric nanocomposites prepared by irradiation with pulsed nano- and femtosecond lasers” and directed by the doctors D. Pablo Moreno Pedraz and Dª. Esther Rebollar González
 

The defense act will take place at 11:30 a.m. in the Francisco de Vitoria classroom of the Escuelas Mayores Building.

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adminAnnouncement of defense of doctoral thesis – Javier Prada-Rodrigo

Announcement of defense of doctoral thesis – Miguel López Ripa

On May 26, Miguel López Ripa will present his doctoral thesis entitled “Development of ultra-stable characterization techniques for ultrashort laser beams” and directed by doctors D. Íñigo Juan Sola Larrañaga and Mr. Benjamin Alonso Fernandez.

The defense act will take place at 10:30 a.m. in the Francisco de Vitoria classroom of the Escuelas Mayores Building.

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adminAnnouncement of defense of doctoral thesis – Miguel López Ripa

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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ALF-USAL researchers participate in the annual meeting of the European Optical Society (EOSAM 2022)

From September 12 to 16, the annual meeting of the European Optical Society (EOSAM 2022) was held in Porto (Portugal). The Laser and Photonics Applications group (ALF – USAL) has participated in this conference presenting some of the most recent results of the research it is currently carrying out.

The works presented were:

  • Marina Fernández Galán, Enrique Conejero Jarque, Julio San Román. Pulse self-compression down to the sub-cycle regime in hollow capillary fibers with decreasing pressure gradients. TOM 8 Non-linear and Quantum Optics – Oral contribution (Abstract).
  • Miguel López Ripa, Iñigo J. Sola, Benjamín Alonso. Ultraestable spatiotemporal characterization of optical vortices in the visible and near infrared. TOM 13 Advances And Applications of Optics and Photonics – Oral contribution (Abstract). 
  • Rodrigo Martín Hernández, Luis Plaja, Carlos Hernández García. Fourier-limited attosecond pulse generation with magnetically pumped high-order harmonic generation. TOM 8 Non-linear and Quantum Optics – Oral contribution (Abstract)
  • Luis Plaja, Ana García Cabrera, Roberto Boyero-García, Óscar Zurrón, Julio San Román, Carlos Hernández García. Multi-beam vortex generation induced by the non-linear optical anisotropy of graphene. TOM 8 Non-linear and Quantum Optics – Poster (Abstract)
  • Victor W. Segundo Staels, Enrique Conejero Jarque, Daniel Carlson, Michaël Hemmer, Henry C. Kapteyn, Margaret M. Murnane, Julio San Román. Supercontinuum generation in the enhanced frequency chirp regime in multipass cells. TOM 8 Non-linear and Quantum Optics – Oral contribution (Abstract)
  • Carlos Hernández García. Novel ultrafast structured EUV/x-ray sources from nonlinear optics. TOM 13 Advances And Applications of Optics and Photonics – Oral contribution (Abstract). 
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adminALF-USAL researchers participate in the annual meeting of the European Optical Society (EOSAM 2022)