Grupo de Procesado por Láser del Instituto de Óptica “Daza de Valdés”

An Integrated Pump-Controlled Variable Coupler Fabricated by Ultrafast Laser Writing

David Benedicto, Juan C. Martín, Antonio Dias-Ponte, Javier Solis, Juan A. Vallés.
Micromachines, 14(7), 1370 (2023)
Abstract
Direct Laser Interference Patterning (DLIP) is a versatile technique that enables the fabrication of periodic micro- to nanometric scale structures over large areas in a variety of materials. The periodically modulated excitation pattern can be exploited to trigger a range of complex processes, including heating, melting, ablation, and matter reorganization.

In this work, a novel strategy is developed to combine deep-ultraviolet (UV) DLIP (λ = 193 nm, τ = 23 ns) and real-time optical reflectivity and diffraction techniques to unravel the formation dynamics of grating structures with periods down to Λ = 740 nm. Applied to crystalline Ge wafers, single-pulse topography modulation profiles with amplitudes up to 85 nm can be imprinted. Moreover, the dynamics of the melting and solidification processes, as well as the surface topography deformation, can be followed in real-time. Combined with a model, changes in topography over time with ns resolution can be obtained. The results unambiguously reveal that the formation process of the 3D structures can only be understood when taking both Marangoni convection and thermocapillary waves into account. The technique presented here has the potential to unravel the formation dynamics of a wide range of periodic structures in other materials.

Real-Time 3D Visualization of the Formation of Micrograting Structures Upon Direct Laser Interference Patterning of Ge

Miguel Alvarez-Alegria, Carlota Ruiz de Galarreta, Jan Siegel
Laser & Photonics Rev. 17, 2300145 (2023)
Abstract
Direct Laser Interference Patterning (DLIP) is a versatile technique that enables the fabrication of periodic micro- to nanometric scale structures over large areas in a variety of materials. The periodically modulated excitation pattern can be exploited to trigger a range of complex processes, including heating, melting, ablation, and matter reorganization.

In this work, a novel strategy is developed to combine deep-ultraviolet (UV) DLIP (λ = 193 nm, τ = 23 ns) and real-time optical reflectivity and diffraction techniques to unravel the formation dynamics of grating structures with periods down to Λ = 740 nm. Applied to crystalline Ge wafers, single-pulse topography modulation profiles with amplitudes up to 85 nm can be imprinted. Moreover, the dynamics of the melting and solidification processes, as well as the surface topography deformation, can be followed in real-time. Combined with a model, changes in topography over time with ns resolution can be obtained. The results unambiguously reveal that the formation process of the 3D structures can only be understood when taking both Marangoni convection and thermocapillary waves into account. The technique presented here has the potential to unravel the formation dynamics of a wide range of periodic structures in other materials.

In vitro comparative study between adhesion forces obtained on zirconia ceramic micromechanically treated with femtosecond laser (1027 nm), carbon dioxide laser (10,600 nm), and aluminum-oxide particles

Ignasi Piulachs, Luis Giner-Tarrida, Antoni España-Tost, Josep Arnabat-Dominguez & Camilo Florian
Lasers in Medical Science volume 38, Article number: 194 (2023)

High-quality, optically active and integrable EuOOH films prepared by pulsed laser deposition

A. Caño, B. Galiana, G.B. Perea, A de Andrés, A. Marical-Jiménez, J. Gonzalo, R. Serna
Applied Surface Science, Volume 640, 15 December 2023, 158236 (2023)
Abstract
Rare-earth (RE)-Oxygen-Hydrogen compounds are versatile materials whose composition and properties can be significantly varied by changing the relative O and H contents. Among them hydrides and oxyhydrides have been thoroughly investigated due to its photochromic properties. Instead, research of RE-hydroxides and oxyhydroxides (RE-OH and RE-OOH) is scarce although they show promising properties as light emitters. However, their use and integration in solid state devices have been hindered so far because their usual chemical synthesis routes yield materials in bulk or powder configurations. In this work we demonstrate a physical deposition route based on pulsed laser deposition that results in the unprecedented preparation of high-quality Eu oxyhydroxide (EuOOH) thin films. The synthetized EuOOH films show a well-defined monoclinic structure, are optically active and show a robust red emission related to the intra-f transitions of the Eu3+ ions. The excellent quality of these crystalline films has allowed us to obtain relevant properties of the monoclinic EuOOH phase not previously reported such as its refractive index and its Raman spectrum, including the identification of the characteristic phonon modes. These novel EuOOH films have been prepared both on Si and fused silica substrates, and thus are ready for potential integration in solid state optoelectronic components and devices.

Modeling optical amplification in Er/Yb-codoped integrated Bragg gratings

Ángel Sanz Felipe, Rocío Ariza, David Benedicto, Manuel Macias Montero, Juan A. Vallés, Javier Solís
Ceramics International, In Press (2023)
Abstract

Bragg gratings inscribed in active waveguides combine very efficient reflective properties with the amplifying capability of rare-earths, which may lead to large amplification and lasing performance. However, the response of these photonic structures highly depends on the grating parameters and working conditions, so modeling their behavior and dependences becomes fundamental. In this work, a numerical method has been implemented to simulate the optical power propagation along an Er/Yb-codoped integrated waveguide Bragg grating as a function of its most relevant operational parameters. The results obtained show the optimal conditions to maximize its performance as a highly amplifying reflector, but also its capability as a monolithic laser. In addition, the modeling results adequately match experimental values measured in fs-laser written structures in Er/Yb-codoped phosphate glass, supporting the accuracy of the numerical method developed and its usefulness for further optimizing these promising photonic structures.

A Route to Ultra-Fast Amplitude-Only Spatial Light Modulation using Phase-Change Materials

Joe Shields, Carlota Ruiz De Galarreta, Harry Penketh, Yat-Yin Au, Jacopo Bertolotti, C. David Wright
Advanced Optical Materials, Volume11, Issue18 Special Section: Nanoparticles with Chiroptical Responses. September 18, 2023 (2023)
Abstract
A phase-change material based, thin-film, amplitude-only spatial light modulator is presented. The modulator operates in reflection and modulates the amplitude of light incident on its surface with no effect on optical phase when the phase-change material is switched between its amorphous and crystalline states. This is achieved using a thin-film device with an embedded, switchable, GeTe phase-change layer. Test modulation patterns are written to the device using laser scans, and the amplitude and phase response measured, using optical spectroscopy and off-axis digital holography. Experimental results reveal reflected intensity to be modulated by up to 38%, with an averaged phase difference of less than ≈π/50. Since phase-change materials such as GeTe can be switched on sub-microsecond timescales, this approach maps out a route for ultra-fast amplitude spatial light modulators with widespread applications in fields such as wavefront shaping, communications, sensing, and imaging.

Temperature-Dependent Anisotropic Refractive Index in β-Ga2O3: Application in Interferometric Thermometers

Daniel Carrasco, Eva Nieto-Pinero, Manuel Alonso-Orts, Rosalía Serna, Jose M. San Juan, María L. Nó, Jani Jesenovec, John S. McCloy, Emilio Nogales, and Bianchi Méndez
Abstract
An accurate knowledge of the optical properties of β-Ga2O3 is key to developing the full potential of this oxide for photonics applications. In particular, the dependence of these properties on temperature is still being studied. Optical micro- and nanocavities are promising for a wide range of applications. They can be created within microwires and nanowires via distributed Bragg reflectors (DBR), i.e., periodic patterns of the refractive index in dielectric materials, acting as tunable mirrors. In this work, the effect of temperature on the anisotropic refractive index of β-Ga2O3 n(λ,T) was analyzed with ellipsometry in a bulk crystal, and temperature-dependent dispersion relations were obtained, with them being fitted to Sellmeier formalism in the visible range. Micro-photoluminescence (μ-PL) spectroscopy of microcavities that developed within Cr-doped β-Ga2O3 nanowires shows the characteristic thermal shift of red–infrared Fabry–Perot optical resonances when excited with different laser powers. The origin of this shift is mainly related to the variation in the temperature of the refractive index. A comparison of these two experimental results was performed by finite-difference time-domain (FDTD) simulations, considering the exact morphology of the wires and the temperature-dependent, anisotropic refractive index. The shifts caused by temperature variations observed by μ-PL are similar, though slightly larger than those obtained with FDTD when implementing the n(λ,T) obtained with ellipsometry. The thermo-optic coefficient was calculated.

Temperature behaviour of mixed-cation mixed-halide perovskite solar cells. Analysis of recombination mechanisms and ion migration

Mari Carmen López-González, Gonzalo del Pozo, Belén Arredondo, Silvia Delgado, Diego Martín-Martín, Marina García-Pardo, Beatriz Romero.
Organic Electronics 120 (2023) 106843
Abstract
In our study, we show that compositional engineering of the “A” site cation of ABX3 perovskite structure formed by a mix of organic and inorganic cations is an effective route to improve the thermal stability of perovskite solar cells (PSCs). In this work, mixed-cation mixed-halide PSCs have been fabricated and characterized with temperature, from 253 up to 333 K. The active layer based on CsRbFAMAPb(IBr)3 results in a more stable device compared to standard MAPbI3 devices. Electrical characterization reveals a decrease of the solar cell parameters with temperature. Using Impedance Spectroscopy (IS) characterization, we have estimated an activation energy for the halide ion migration of 0.63 ± 0.08 eV, an ion diffusion coefficient of 10−14 cm2 s−1, and a defect density of 7.27·1015 cm−3. To our knowledge, this is the first time that these parameters have been calculated in CsRbFAMAPb(IBr)3 based devices, resulting in improved values compared to MAPbI3 devices. The worsening of device performance for temperatures above 300 K is attributed to a decrease of the spiro-OMeTAD conductivity and the degradation of the perovskite/spiro-OMeTAD interface. It is shown that for low temperatures (from 253 to 323 K), Shockley-Red-Hall (SRH) recombination in the bulk governs, while for temperatures above 323 K the increase in surface recombination becomes dominant due to the presence of non-selective contacts.

Numerical simulations using SILVACO ATLAS corroborate the role of SRH in the perovskite active layer for low and medium temperatures, and the crucial influence of spiro-OMeTAD transport properties in the device performance parameters.

Abstract
We report the successful preparation and characterization of active silicon oxynitride thin films with controlled europium (Eu) doping by alternated pulsed laser deposition. The successful Eu doping with a nanostructured dopant distribution, as well as the stoichiometry of the oxynitride film, have been determined by ion beam analysis (RBS). The oxidation state of the incorporated Eu ions has been determined by in-depth x-ray photoelectron spectroscopy (XPS) and it is shown that the Eu ions are in the 2+ state, in contrast to the usual results found in oxide matrices for which the 3+ oxidation state prevails. The Eu-doped films show an intense broadband emission (FWHM >210 nm) associated to the optical transition 4f65d1 → 8S7/2 of the Eu2+ ions within the amorphous matrix. As expected, the intensity of the emission band increases as the Eu concentration increases, and it is remarkable that the emission shifts towards longer wavelengths. In terms of the chromatic coordinates (CIE) this implies a color tuning from a bluish to orange that enables color tunable emission, and potential white like emission by combining layers with different Eu2+ ions concentration. Therefore, the developed oxynitride films with controlled Eu2+ ions concentration achieved by PLD are promising for the development of color -tailored LED’s.

Ultrafast-laser powder bed fusion of oxygen-deficient Nb2O5 ceramics with highly improved electrical properties

B. Sotillo, R. Ariza, P. Fernández, J Solis
Materials & Design, volume 224

Abstract
In this work, Nb2O5 layers with highly improved electrical properties respect to pristine material have been produced by ultrafast-laser powder bed fusion process. The conditions required for producing uniform and compact layers of Nb2O5 from powder material have been studied and optimized. It has been established that ultrafast-laser irradiation, performed in air at room temperature, leads to the formation of dense Nb2O5 layers with the high temperature monoclinic crystal structure (H-Nb2O5) but oxygen deficient. The layers show a preferential crystal orientation with the short axis of the monoclinic structure lying in the structure plane. This preferential orientation can be controlled by the laser irradiation conditions. Anisotropic resistivity has been observed as a consequence of the induced microstructure, while the overall material resistivity is decreased by more than eight orders of magnitude due to the oxygen deficiency. These results indicate that it is feasible to use ultrafast laser processing to promote high-temperature non-stoichiometric niobium oxide phases in a few seconds and with low energy consumption. The highly improved electrical properties of the laser irradiated Nb2O5 layers are extremely interesting for different electronic and sensing applications.

Filtering and Modulation from the Infrared to the Terahertz using Phase-Change Extraordinary Optical Transmission Metasurfaces

Euan Humphreys, Jacopo Bertolotti, Carlota Ruiz de Galarreta, Noemi Casquero, Jan Siegel, C. David Wright
PSS Rapid research letters, article 2200474

Abstract
Periodic arrays of sub-wavelength-scale holes in plasmonic metal films are known to provide resonant transmission peaks via the extraordinary optical transmission (EOT) effect. Active control of the spectral position of such transmission peaks can be obtained by adding a layer of phase-change material (PCM) to the EOT device. Switching the PCM layer between its amorphous and crystalline states can shift the spectral position of the resonance, enabling potential applications in the fields of active filtering and sensing (e.g., multispectral sensing), and for signal modulation. Here, the design, fabrication, and characterization of active EOT devices are targeted at various important regions of the optical spectrum.

Optical Properties of 2D Micro- and Nanostructures of ZnO:K

Rocío Ariza, Ana Urbieta, Javier Solis, Paloma Fernández
Materials, 15(21), 7733

Abstract
ZnO nano- and microstructures doped with K were grown by the Vapor–Solid method. Wires and needles are the main morphology observed, although some structures in the form of ribbons and triangular plates were also obtained. Besides these, ball-shaped structures which grow around a central wire were also detected. Raman and cathodoluminescence investigations suggest that variations in morphology, crystalline quality and luminescence emissions are related to the different lattice positions that K occupies depending on its concentration in the structures. When the amount is low, K ions mainly incorporate as interstitials (Ki), whereas K occupies substitutional positions of Zn (KZn) when the amount of K is increased. Electron Backscattered Diffraction shows that ribbons and triangular plates are oriented in the (0001) direction, which indicates that the growth of this type of morphologies is related to distortions introduced by the Ki since this position favors the growth in the (0001) plane. In the case of the ball-shaped structures, the compositional analysis and Raman spectra show that they consist of K2SO4. Finally, the capability of the elongated structures to act as waveguides and optical resonators was investigated. Due to the size of the K ion, practically double that of the Zn, and the different positions it can adopt within the ZnO lattice (Ki or KZn), high distortions are introduced that compromise the resonators performance. Despite this, quality factor (Q) and fineness (F) show acceptable values (80 and 10 at 544 nm, respectively), although smaller than those reported for doping with smaller size alkali, such as Li.

High-quality single-crystalline epitaxial regrowth on pulsed laser melting of Ti implanted GaAs

S. Algaidya, D. Caudevilla, F. Perez-Zenteno, R. Garcia-Hernansanz, E. Garcia-Hemme, J. Olea, E. San Andres, S. Duarte-Cano, J. Siegel, J. Gonzalo, D. Pastor, A. del Prado
Materials Science in Semiconductor Processing, Volume 153, 107191 (2023)

Abstract
We present a detailed investigation on the formation of supersaturated GaAs using Ti+ implantation followed by nanosecond Pulsed Laser Melting (PLM). We have synthesized high-crystal quality supersaturated GaAs layers with concentrations of Ti above the insulator to metal transition (Mott limit). The Ti-implanted concentration depth profiles after PLM obtained by Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) show a redistribution of Ti impurities within the first hundred nanometers and superficial concentration up to 1 × 1021 cm−3. Raman spectroscopy of these Ti supersaturated, and regrown GaAs samples shows a sharp crystalline peak and tensile strain due to the Ti lattice incorporation. Scanning Transmission Electron Microscopy (STEM) and high-resolution Transmission Electron Microscopy (TEM) images show a good GaAs crystallinity after the PLM process. Energy-Dispersive X-ray Spectroscopy (EDS) reveals an enhanced Ti signal inside bubble-like structures and an appearance of interface oxide layer with all processed samples.

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