The generation of structured laser pulses with increasingly exotic space-temporal profiles is closely linked to the development of characterization techniques, making this interdisciplinary research line crucial in LUMES. Several researchers in the unit are international references in the development of ultrashort pulse characterization techniques. In addition to designing new characterization techniques in the visible and infrared spectra (such as the STARFISH technique), they are pioneers in the characterization of structured pulses in the extreme ultraviolet. In this regard, the article [Optica 4, 520 (2017), >80 citations] is an international reference for the generation and characterization of vectorial X-ray pulses. This work was developed in collaboration among several unit members, where numerical calculations were performed on the USAL cluster, and experiments were conducted at the Center for Pulsed Lasers (CLPU). Therefore, this is an example of collaboration between researchers and associated infrastructures, now integrated within the LUMES unit.

In addition to the generation and characterization of structured laser beams, their propagation becomes especially relevant not only for transporting pulses to potential experiments but also for modifying their properties. For over a decade, USAL has been a pioneer in the development of theoretical methods that allow understanding the propagation of ultrafast laser pulses in highly nonlinear media across different spectral regions (far-infrared, infrared, and ultraviolet). In particular, during this period, it has specialized in the study of the nonlinear propagation of light pulses in gas-filled hollow fibers, which can be structured based on the superposition of transverse modes. Noteworthy are the collaborative works with Prof. Helder Crespo at the University of Porto (Portugal), where ultrarapid single-cycle pulses have been achieved [Opt. Lett. 43, 337 (2018), >50 citations; Sci. Rep. 8, 2256 (2018), >40 citations].

The generation of high-order harmonics (HHG) is a strongly nonlinear process known for three decades. It forms the basis for attosecond pulse generation techniques and serves as the most practical solution for generating high-frequency coherent light (extending up to soft X-rays). HHG has undergone two fundamental advances in recent years: (1) the demonstration of the process’s capability to generate structured light, a field in which we were pioneers and extensively explored in research line #1; (2) the replacement of gaseous nonlinear material with nanostructured solid targets. This second aspect has led to the use of HHG not only as a new tool for generating structured light beams but also for exploring the quantum dynamics occurring in solids. The current research line corresponds to our work combining these two scenarios. At USAL, we have developed theoretical models that allowed us to describe HHG in two-dimensional materials, with special attention to graphene. Noteworthy is the work on describing the anisotropy in the nonlinear response of graphene [Optics Express, 27(5), 7776–7786 (2019), >50 citations].

The future advancement in the development of magnetic memories (MRAM) and other spintronic devices involves developing methods to control the magnetic response on the picosecond (ps) scale. This timescale is significantly shorter than that of existing MRAMs currently in the market, and the most promising control method in this regard is through ultrashort laser pulses. Researchers from the LUMES unit are pioneers in this field, having developed theoretical models that allow studying the dynamics induced by laser pulses, including contributions from the Inverse Faraday Effect and Circular Magnetic Dichroism, as well as thermal effects [V. Raposo et al. Appl. Sci. 10, 1307 (2020)]. This enables the realistic study of these processes and the correct interpretation of experimental results in various ferromagnetic [V. Raposo et al. Appl. Sci. 10, 1307 (2020)], ferrimagnetic [Phys Rev. B 105, 104432 (2022)], and synthetic antiferromagnetic materials [Adv. Sci. 1901876 (2019)]. Additionally, the thermal expansion associated with laser-induced heating in the impact zone propagates in the material as a pulse of elastic stress, which can also affect the magnetic response through magnetoelastic coupling. This effect, often overlooked, can be significant and is currently under study, as it has been observed that mechanical stress gradients can lead to the rapid movement of magnetic textures such as domain walls [M. Fattouhi et al. Phys. Rev. Appl. 18, 044023 (2022)] and skyrmions [M. Fattouhi et al. Phys. Rev. Appl. 16, 044035 (2021)].

Nonlinear optics and materials science, like most cutting-edge research areas, have not been unaffected by the development of artificial intelligence. In the LUMES environment, we have applied deep learning strategies in two main aspects. Firstly, we have implemented the use of neural networks to enhance our simulation capabilities for laser-material interaction. In a recently published article [Comp Phys Comms 291, 108823 (2023)], we have laid the foundations for integrating artificial intelligence to simulate new scenarios of structured light generation in the X-ray regime. This advancement not only accelerates the simulations we have been conducting to deepen our understanding of laser-material interaction but also allows us to explore new scenarios where traditional simulations may struggle due to high computational loads.

Secondly, we are exploring the use of neural networks for the spatiotemporal characterization of ultrashort pulses. The time required by algorithms used in ultrashort pulse characterization techniques currently limits their use in high-repetition-rate laser systems. Thanks to the implementation of neural networks, the required times can be reduced by several orders of magnitude. Therefore, we are currently investigating the possibility of characterizing structured ultrashort pulses in situ in high-repetition-rate laser systems. This research line has developed in parallel with lines #1 and #3. Although it is in its early stages, due to its interdisciplinary nature and the numerous innovative perspectives it opens up, we believe it will become a relevant research line in the near future of LUMES.

The knowledge gained from the previous research lines can be applied to the characterization of materials’ ultrafast dynamics. Therefore, an emerging line of work in LUMES involves exploring the potential of ultrafast optics in applications for the diagnosis of materials of interest. Initially, the STARFISH technique has been employed to study the behavior of integrated optical systems [Optics & Laser Technology 123, 105898 (2020)]. Expanding on the applications of this technique, it is being adapted for spatial, temporal, and polarimetric sample studies. Ultrafast pulsed rare-earth-doped optical fiber lasers, their stability, and polarization have also been studied [Optics & Laser Technology, 140, 107018 (2021)].

A second line of work aims to exploit the extraordinary potential of ultrafast optics as a tool for time-resolved spectroscopy, a highly versatile technique used in a wide variety of scientific and technological fields to investigate the dynamics of molecular and atomic systems. Its interest lies in its ability to provide information on how systems evolve over time down to the femtosecond (fs) range, with different time scales tailored to the specific characteristics of each study. For this purpose, a first time-resolved fluorescence experimental setup has been implemented using the pulsed laser beam tunable by the laboratory’s optical parametric amplifier (OPA) as the sample’s excitation light. This system can continuously modify the emission spectral region from UV to mid-IR, adapting to the studied sample. The laser beam is directed at a sample holder, and the fluorescence signal is fed into a spectrometer coupled with an intensified iCCD camera with a configurable delay relative to the recorded signal. This setup allows time-resolved fluorescence measurements with a resolution of up to 4 ns. Successful tests of the technique have been carried out. Subsequently, in collaboration with Prof. Mercedes Velázquez’s group at the Faculty of Chemical Sciences of the University of Salamanca, work is underway on the analysis of graphene quantum dot samples, studying their time-resolved spectroscopy to gain a deeper understanding of the properties of such devices. To complete the temporal range, an optical spectroscopic system with femtosecond resolution has been designed and implemented using a pump & probe scheme and proprietary development designs. Initial tests have shown that the system’s resolution is on the order of subpicoseconds. In a second phase, efforts are focused on improving the fluorescence signal collection.