Category: Review

Review

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.

29 July, 2022 admin ALF results, Review No comments

High fashion jewelry in non-linear optics

Sterling silver jewelry, rose gold with exclusive designs, rings, earrings, bracelets… made out of light? Like a high fashion jewelry workshop, the Laser and Photonics Applications research group of the University of Salamanca (ALF-USAL) focuses its efforts on designing light jewels through non-linear optics. Jewels, not only for their beauty in the form of high-frequency lasers, but also for their usefulness in observing and controlling unknown processes in nature. In fact, the design of coherent and high-frequency laser light beams (towards the X-rays) has become a unique tool to access processes that take place in very small sizes (nanometers) and in very short times (trillionths of seconds).

The theoretical-experimental collaboration between ALF-USAL and the group of Profs. Murnane and Kapteyn at the University of Colorado at Boulder (USA) have been developing fine jewelry for the last few years. After generating high-frequency rings with properties never seen before (see references [1,2]), the researchers have gone a step further, entering the world of necklaces. The design of an infrared laser in the form of a necklace allows to control the frequency and divergence (or spatial size) of the X-rays that are produced after the non-linear process of high-order harmonic generation. In this work, published in the journal Science Advances, the researchers also demonstrate that the spectral content of these X-ray lasers can be calibrated by means of the number of beads in the necklace. One more step in the high fashion jewelry with lasers that tries to help in the understanding of the fastest processes of nature.

More information in:

“Necklace-structured high harmonic generation for low-divergence, soft X-ray harmonic combs with tunable line spacing”, Laura Rego, Nathan J. Brooks, Quynh L. D. Nguyen, Julio San Román, Iona Binnie, Luis Plaja, Henry C. Kapteyn, Margaret M. Murnane, Carlos Hernández-García, Science Advances 8, eabj7380 (2022).

References::

[1] “Generation of extreme-ultraviolet beams with time-varying orbital angular momentum”, Laura Rego, Kevin M Dorney, Nathan J Brooks, Quynh Nguyen, Chen-Ting Liao, Julio San Román, David E Couch, Allison Liu, Emilio Pisanty, Maciej Lewenstein, Luis Plaja, Henry C Kapteyn, Margaret M Murnane, Carlos Hernández-García, Science 364, eaaw9486 (2019).

[2] “Controlling the polarization and vortex charge of attosecond high-harmonic beams via simultaneous spin-orbit momentum conservation”, Kevin M. Dorney, Laura Rego, Nathan J. Brooks, Julio San Román, Chen-Ting Liao, Jennifer L. Ellis, Dmitriy Zusin, Christian Gentry, Quynh L. Nguyen, Justin. M. Shaw, Antonio Picón, Luis Plaja, Henry C. Kapteyn, Margaret M. Murnane, Carlos Hernández-García, Nature Photonics 13, 123–130 (2019).

25 April, 2022 admin ALF results, Review No comments

Compressing light pulses in pressure gradients

In spite of having an extremely short life, of just a few quadrillionths of a second, ultrashort femtosecond laser pulses have become an indispensable tool in many areas of science and technology, as they allow to probe the most fundamental properties of matter on ultrafast time scales.

Generating these such short pulses of light in a controlled manner and with a good quality is not an easy task, and in the last years various strategies have been proposed. The main idea is to generate a very wide light spectrum, made up of many frequencies, by means of nonlinear processes starting from a narrower one, and then correcting its phase to synchronize all the frequencies, giving rise to an ultrashort pulse. A widely used method to achieve this large spectral broadening is to propagate an initial light pulse through a hollow cylindrical fiber filled with a gas. In this case, one of the parameters that most influences the propagation is the pressure of the filling gas, which allows for a continuous tuning of the dispersion and the intensity of the nonlinear effects experienced by the pulse. In particular, if the fiber and gas parameters are carefully chosen, the incident pulse can broaden its spectrum while correcting its phase due to the interaction between the linear and nonlinear processes. In this way, the pulse reduces its duration on its own, in a process known as soliton self-compression.

Usually, these experiments are carried out keeping the gas at constant pressure, homogeneously filling the fiber. However, in one of our latest studies we have shown that applying a decreasing pressure gradient, causing the gas concentration to gradually decrease during propagation, can improve the quality of the self-compressed pulses and reduce their duration even more than constant pressure.

You can look up for all the details of this work at:

F. Galán, E. C. Jarque, and J. San Roman, Optimization of pulse self-compression in hollow capillary fibers using decreasing pressure gradients, Optics Express 30(5), 6755–6767 (2022). https://doi.org/10.1364/OE.451264

Download the full paper at Gredos @Universidad de Salamanca: http://hdl.handle.net/10366/148576

21 February, 2022 admin ALF results, Review No comments

Tailoring complex structures in high-frequency light

The prestigious journal Optica has just published a new article demonstrating the generation of high-frequency light with multiple vibration directions and a spiral phase structure. The research is the result of an international theoretical-experimental collaboration between the Laser and Photonics Applications Group of the University of Salamanca, the University of Paris-Saclay and the Colorado School of Mines. This work was developed within the European project ERC ATTOSTRUCTURA.

One of the great advantages of laser light is that we can shape its spatial properties, with the aim of exploring new scenarios in light-matter interaction and optimizing some applications such as imaging techniques or optical communications.

In this work we organize the distribution of the phase (the instantaneous oscillation position) in the form of a helix, which is the characteristic of optical vortices or “tornadoes of light”. In addition, we configure different polarizations (oscillation directions) in a single laser beam. Forms of light that combine both properties are called vector-vortex beams.

In the high-frequency regime, it is more challenging to structure laser light, since most conventional devices are not efficient for ultraviolet radiation, X-rays or gamma rays. However, we can circumvent this problem thanks to the generation of high-order harmonics. This nonlinear optics process, in which a high-intensity visible or infrared laser interacts with the atoms of a gas, allows us to up-convert the properties into the extreme ultraviolet or X-rays.

Our work demonstrates that we can generate vector-vortex beams in the extreme ultraviolet thanks to the physical conservation laws in high-order harmonic generation. Our theoretical proposal of a new conserved quantity, the Pancharatnam topological charge, in high harmonic generation has been confirmed experimentally in the laboratory of Paris-Saclay.

More information at:

Heras, A. de las, Pandey, A. K., Román, J. S., Serrano, J., Baynard, E., Dovillaire, G., Pittman, M., Durfee, C. G., Plaja, L., Kazamias, S., Guilbaud, O., & Hernández-García, C. (2022). Extreme-ultraviolet vector-vortex beams from high harmonic generation. Optica, 9(1), 71-79. https://doi.org/10.1364/OPTICA.442304

Download the full paper at Gredos @Universidad de Salamanca: http://hdl.handle.net/10366/146004

17 January, 2022 admin Attostructura_En, Review No comments

The Lord of the Rings: the Graphene Match

As in the books of JRR Tolkien, the Research Group of Laser Applications and Photonics (ALF-USAL) continues their adventure in the search of the Ring.

After finding the Phase-Matched Ring in Argon based High-Harmonic Generation [1], now they’ve gone a step beyond to find the Phase-Matched Ring in graphene. It turns out that when a graphene layer is illuminated by an intense laser beam, ultrafast electronic dynamics result in the emission of higher frequency radiation.

This emission can be understood as a result of an excitation of electrons from the valence to the conduction band, a subsequent acceleration in such conduction band, and a final recombination with the holes that were left in the valence band. Such process, known as high-order harmonic generation, is very sensitive to the intensity of the incoming laser field, and thus to the spatial profile of the beam.

As a consequence, the radiation emitted from different parts of the graphene layer can interfere, both destructively or constructively. It turns out that an adequate matching between the emissions from different parts of the graphene layer is required to obtain bright high-frequency radiation.

In their latest work, the ALF-USAL group has demonstrated that when a graphene layer is illuminated by an intense Gaussian beam, the high-order harmonic emission is dominated by an annular region: the Phase-Matched Ring. This finding gives light into the macroscopic physics of high harmonic generation in graphene, offering a tool to engineer the process, and to increase its efficiency.

After finding the Phase-Matched Ring in Argon and in Graphene, the researchers of ALF-USAL group continue their adventures to rule intense light-matter interactions.

More information in:

“Transverse phase matching of high-order harmonic generation in single-layer graphene”, Roberto Boyero-García, Óscar Zurrón-Cifuentes, Luis Plaja, and Carlos Hernández-García, Optics Express 29, 2488-2500 (2021).

References:

[1] “Carrier-envelope-phase insensitivity in high-order harmonic generation driven by few-cycle laser pulses”, C. Hernández-García, W. Holgado, L. Plaja, B. Alonso, F. Silva, M. Miranda, H. Crespo, and I. J. Sola, Optics Express 23, 21497 (2015)

Download in Gredos@Universidad de Salamanca: http://hdl.handle.net/10366/146004

22 November, 2021 admin Attostructura_En, Review No comments

Optical nanotechnology using electrons.

In 1924, Louis-Victor Pierre Raymond, pair of France and Duke of Broglie, astonished his contemporaries in proposing that electrons behave as waves, when moving along spatial dimensions of a few millionths of a milimeters. These distances, dubbed nanometers, remained technologically unaccessible until some decades ago. Roughly speaking, if the electrons appear as waves under these circumstances, we should expect their behaviour close to that of light. One of the most obvious manifestations of the wave nature of light is diffraction, the modification of its structure when passing through small slits. In 1836, the british physicist Henry Fox Talbot observed an extraordinary remarkable diffraction phenomenon: when light passes through a mask composed by a regular disposition of slits, it forms an image of the slits at some particular distances from the mask. This image if formed spontaneously, without he use of any focalizing element as lenses or mirrors. Our study predicts theoretically that this very same effect arises in the electron waves, if the electrons are detached from a periodic crystal due to the interaction with an electromagnetic field. The Talbot imaging effect would allow to control the shape of the electron wave and, therefore, some of the properties of their emission of light. Almost magically, the spatial structure of the Talbot image is codified in the temporal structure of the light emission. All this, of course, at nanometric scales!. More info at: [1]

A. García-Cabrera, C. Hernández-García, and L. Plaja, Ultrafast Sub-Nanometer Matter-Wave Temporal Talbot Effect, New Journal of Physics 23, 093011 (2021).

Download in Gredos Universidad de Salamanca http://hdl.handle.net/10366/147197

19 November, 2021 admin Review No comments