Estructuras superficiales auto-organizadas generadas por láser

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid,Facultad de Ciencias, Departamento de Física Aplicada. Fecha de lectura: 29-11-2019En este trabajo se aborda la formación controlada de estructuras periódicas superficiales inducidas por láser, LIPSS (Laser Induced Periodic Surface Structures), producidas mediante pulsos láser de femtosegundos. Primero, se estudia la formación de micro y nanoestructuras amorfas en silicio cristalino. A continuación, se fabrican franjas amorfocristalinas auto-organizadas en superficie (ac-LIPSS) desarrollando control sobre el periodo, el espesor y las dimensiones de dichas franjas. En especial, se descubre cómo seleccionar solo uno de los dos periodos de franjas que se forman al irradiar con ángulo barriendo el haz sobre la muestra en uno u otro sentido. Se indaga asimismo sobre la formación de LIPSS ablativos en metales. En concreto en cobre. Se logra fabricar un régimen en el que se producen dos periodos distintos que coexisten separados espacialmente. Además, dado que el modelo plasmónico tiende a sobrestimar los periodos producidos en irradiaciones oblicuas, se introduce el parámetro de la rugosidad de superficie dando forma a un modelo plasmónico y de rugosidad. Los resultados teóricos se ajustan mucho mejor a las franjas así obtenidas. Por otro lado, en lámina delgada de cobre, se verifica de forma experimental la importancia del número de barridos en la formación de LIPSS, además del fenómeno de franjas viajeras: franjas que aparentemente se desplazan en la superficie de la muestra durante la irradiación. Se discute su papel en la degradación de los LIPSS formados con un único barrido. Se continúa el trabajo para el entendimiento de la formación de LIPSS cambiando a acero. Se descubre el efecto pluma, que permite seleccionar de forma fina el periodo de las franjas invirtiendo el sentido del barrido del haz. Adicionalmente, se adapta el modelo de rugosidad-plasmónico a materiales absorbentes (como es el caso del acero), haciendo uso de una constante dieléctrica efectiva. Por último, se estudia la dinámica de reflectividad en acero mediante técnicas de microscopía con resolución de femtosegundos y de medidas de reflectividad en tiempo real.The present work addresses the controlled formation of Laser Induced Periodic Surface Structures (LIPSS) using femtosecond laser pulses. It started with the formation of amorphous micro- and nanostructures on crystalline silicon. Afterwards, ripples of amorphous-crystalline LIPSS (ac-LIPSS) were fabricated. The study is extended in order to control the period, thickness and lateral dimensions of the fringes. In particular, it was discovered how to select just one of the two periods of ripples that are formed when performing oblique irradiation. The technique consists in sweeping the beam on the sample rather in one or the other direction due to front- or backscatter of the incident laser beam. In a second stage, we study ablative LIPSS in copper. We identified a processing regime which produces two different periods coexisting with spatial separation. In addition, we introduced a roughnessplasmonic model given that the simple plasmonic model tends to overestimate the periods that are fabricated when irradiations are made with an oblique angle. This new model takes into account the influence of the surface’s roughness on the surface plasmon polariton (SPP) formation. The theoretical results fit adequately the ripples thus obtained. Additionally, in thin layers of copper we verified the importance of the number of overscans for the LIPSS formation, along with the traveling fringes phenomenon: ripples that are apparently moving across the surface during the irradiation. We have discussed its role in the degradation of LIPSS that are formed with a single scan. The final part is dedicated to the formation of LIPSS in steel. The quill writing effect or non-reciprocal writing was discovered when processing the surfaces at oblique angle while scanning the sample. This phenomenon allows us to select with great finesse the ripples period just by inverting the scan direction. The underlying mechanisms are related to the asymmetric roughness formation at angled incidence. Consequently, different values of roughness at different positions generate different wavevector of the surface plasmon polariton in function of the scan direction. In addition, we have adapted the roughness-plasmonic model to absorbent materials such as steel, using an effective dielectric constant. Finally, we have performed femtosecond pump-probe imaging and real time reflectivity measurements to study the reflectivity dynamics when the sample is irradiated by fs pulses

Influence of climate change and pesticide use practices on the ecological risks of pesticides in a protected Mediterranean wetland: A Bayesian network approach

Pollution by agricultural pesticides is one of the most important pressures affecting Mediterranean coastal wetlands. Pesticide risks are expected to be influenced by climate change, which will result in an increase of temperatures and a decrease in annual precipitation. On the other hand, pesticide dosages are expected to change given the increase in pest resistance and the implementation of environmental policies like the European ´Farm-to-Fork` strategy, which aims for a 50 % reduction in pesticide usage by 2030. The influence of climate change and pesticide use practices on the ecological risks of pesticides needs to be evaluated making use of realistic environmental scenarios. This study investigates how different climate change and pesticide use practices affect the ecological risks of pesticides in the Albufera Natural Park (Valencia, Spain), a protected Mediterranean coastal wetland. We performed a probabilistic risk assessment for nine pesticides applied in rice production using three climatic scenarios (for the years 2008, 2050 and 2100), three pesticide dosage regimes (the recommended dose, and 50 % increase and 50 % decrease), and their combinations. The scenarios were used to simulate pesticide exposure concentrations in the water column of the rice paddies using the RICEWQ model. Pesticide effects were characterized using acute and chronic Species Sensitivity Distributions built with toxicity data for aquatic organisms. Risk quotients were calculated as probability distributions making use of Bayesian networks. Our results show that future climate projections will influence exposure concentrations for some of the studied pesticides, yielding higher dissipation and lower exposure in scenarios dominated by an increase of temperatures, and higher exposure peaks in scenarios where heavy precipitation events occur right after pesticide application. Our case study shows that pesticides such as azoxystrobin, difenoconazole and MCPA are posing unacceptable ecological risks for aquatic organisms, and that the implementation of the ´Farm-to-Fork` strategy is crucial to reduce them.publishedVersio

Ultrafast Moving-Spot Microscopy: Birth and Growth of Laser-Induced Periodic Surface Structures

7 págs.; 5 figs.Laser-induced periodic surface structures (LIPSS) are a universal phenomenon observed in all classes of solid materials, giving rise to a variety of self-assembled subwavelength structures with different symmetries. These promising features have opened new opportunities for laser structuring of materials in a wide range of applications, including plasmonics, nanophotonics, nanoelectronics, sensing, and even mechanics. However, there is an ongoing debate about the formation mechanism of LIPSS, and the current picture stems mainly from the combined effort of theoretical modeling and experimental studies of the final structures produced. Here we demonstrate femtosecond-resolved imaging of the formation process of such structures produced by ultrashort laser pulses in silicon. The particular type of LIPSS studied are well-aligned amorphous-crystalline fringes generated in dynamic processing conditions, whose period can be tuned and which can be extended over large areas. Using a moving-spot, multiple-pulse irradiation approach we are able to spatially and temporally resolve the birth and growth of individual fringes. We demonstrate that the formation process is initiated by free electron generation leading to nonthermal melting, liquid phase overheating, and rapid solidification into the amorphous phase. © 2016 American Chemical SocietyThis work has been supported by the LiNaBioFluid project (H2020-FETOPEN-2014-2015RIA, grant 665337) of the European Commission as well as by the Spanish Ministry of Economy and Competiveness through research grant TEC2014-52642-C2-1-R and an FPU fellowship for M.G.-L from the Spanish Ministry of EducationPeer Reviewe

Ultrafast Electron Dynamics and Optical Interference Tomography of Laser Excited Steel

Artículo con 1 tabla y 5 figurasFemtosecond laser machining of materials has transformed into a mature high-precision technology, giving access to a wide range of applications. Yet, for a complete exploitation of its full potential, a detailed knowledge of the complex laser–matter interaction processes is required. For the case of the application-relevant material steel, a complete study of the electron dynamics and material transformation upon single laser pulse irradiation (800 nm, 120 fs) is reported. Detailed analysis of the obtained time- and fluence-dependent reflectivity map reveals that ultrafast electron heating during the pulse is directly followed by energy transfer to the lattice within a few picoseconds, reaching fluence-dependent peak temperatures from below melting threshold to above the boiling point. Moreover, an innovative approach to obtain the ultrafast-evolving 3D structure of a femtosecond laser excited material is reported, unraveling the dynamics of complex processes as melting, ablation, and solidification. Combined with modeling, the evolving optical properties, multilayer structure, and expansion velocities can be precisely determined. The information obtained from this study will contribute to further increase the achievable precision and wealth of structures that can be produced by designing efficient pulsed energy deposition schemes for selective re-excitation of the material.This work was partly funded by the Spanish Research Agency (MCIU/AEI/Spain) through project ULS_PSB (PID2020-112770RB-C21) and the European Regional Development Fund (ERDF)

Laser-induced periodic surface structures on germanium by femtosecond irradiations

USTS 2019 Meeting, Madrid, November 6th to 8th, 2019Laser induced ripples that are also known as Laser Induced Periodic Surface Structures (LIPSS) have gained considerable attention by researchers and industry due to their surface functionalization applications. Surface modification with LIPSS takes place in metals, semiconductors or dielectrics [1]. Their morphology depends strongly on many parameters, both on material properties and irradiation conditions (pulse fluence, wavelength, light polarization, pulse repetition rate, and processing speed). Two types of LIPSS are distinguished by their spatial periodicity, namely high-spatial frequency LIPSS (HSFL) having a periodicity significantly smaller than the laser wavelength and low-spatial frequency LIPSS (LSFL) having a periodicity close to the laser wavelength. In this work, we have investigated the LIPSS formation on germanium by using a Ti:Sapphire (800nm, 120 fs) laser. The results show different scenarios depending on the irradiation conditions. One of these scenarios is presented in Figure 1, showing images of LIPSS formed by the same fluence but different number of pulses. For a low number of pulses, LIPSS of type HSFL were formed parallel to polarization, whereas for larger number of pulses they were of type LSFL and perpendicular to polarization. This evolution of spatial periodicity by increasing the number of pulses is fundamentally different regarding the behaviour in silicon [2], which will be discussed for comparison. On the other hand, since the spatial intensity distribution of the laser beam is Gaussian, the central disk of the irradiated spot has been exposed to the peak fluence. Suprisingly, this central disk did not present ablation but LIPSS appeared on its surroundings, as can be seen in the images. This suggests that a phase change from the crystalline phase into the amorphous one could be the origin of these LIPSS. References 1 Bonse, J., Höhm, S., Kirner, S.V., Rosenfeld, A. & Krüger, J. IEEE J. Sel. Top. Quantum Electron 2017, 23, 9000615 2 Fuentes-Edfuf, Y., Garcia-Lechuga, M., Puerto, D., Florian, C., Garcia-Leis, A., Sanchez-Cortes, S., Solís, J., Siegel, J. Scientific Reports 2017, 7, 459

Generation, control and erasure of dual LIPSS in germanium with fs and ns laser pulses

15 pags., 6 figs.,Laser-induced periodic surface structures (LIPSS) can readily be fabricated in virtually all types of materials and benefit from an efficient parallel patterning strategy that exploits self-organization. The wide range of different LIPSS types with different spatial scales and symmetries is continuously growing, addressing numerous of applications. Here, we report on the formation of two fundamentally different types of LIPSS on germanium upon exposure to femtosecond laser pulses (λ=800 nm, 130 fs), featuring different periods and orthogonal orientations. On the one hand, the well-known low-spatial frequency LIPSS (LSFL) with a period ≈λ and perpendicular orientation to the laser polarization are formed, which can be extended homogeneously in 2D by sample scanning. Additionally, extremely smooth ripples with a period ≈λ /2 and parallel orientation were generated at lower pulse numbers. We show that this new kind of ripples, named HSFL-∥, can be superimposed onto LSFL by increasing the pulse number, forming complex dual LIPSS with nanohill-like morphology. While exposure to multiple nanosecond laser pulses is found to trigger also the formation of LSFL, HSFL-∥ cannot be formed under these conditions, which points out the role of ultrafast excitation in the formation of the latter. By performing time-resolved reflectivity measurements, we are able to resolve the melting and solidification dynamics, revealing melting of a very shallow surface layer (< 20 nm) and melt durations of a few ns for both pulse durations pulses at the fluences employed for LIPSS formation. Finally, we demonstrate erasure of both types of LIPSS by exposure to single nanosecond pulses at high fluences, which paves the way for erasable multi-level data storage.Laser-induced periodic surface structures (LIPSS) can readily be fabricated in virtually all types of materials and benefit from an efficient parallel patterning strategy that exploits self-organization. The wide range of different LIPSS types with different spatial scales and symmetries is continuously growing, addressing numerous of applications. Here, we report on the formation of two fundamentally different types of LIPSS on germanium upon exposure to femtosecond laser pulses (λ=800 nm, 130 fs), featuring different periods and orthogonal orientations. On the one hand, the well-known low-spatial frequency LIPSS (LSFL) with a period ≈λ and perpendicular orientation to the laser polarization are formed, which can be extended homogeneously in 2D by sample scanning. Additionally, extremely smooth ripples with a period ≈λ /2 and parallel orientation were generated at lower pulse numbers. We show that this new kind of ripples, named HSFL-∥, can be superimposed onto LSFL by increasing the pulse number, forming complex dual LIPSS with nanohill-like morphology. While exposure to multiple nanosecond laser pulses is found to trigger also the formation of LSFL, HSFL-∥ cannot be formed under these conditions, which points out the role of ultrafast excitation in the formation of the latter. By performing time-resolved reflectivity measurements, we are able to resolve the melting and solidification dynamics, revealing melting of a very shallow surface layer (< 20 nm) and melt durations of a few ns for both pulse durations pulses at the fluences employed for LIPSS formation. Finally, we demonstrate erasure of both types of LIPSS by exposure to single nanosecond pulses at high fluences, which paves the way for erasable multi-level data storage.The authors are grateful to S. Sanchez-Cortes at Instituto de Estructura de la Materia of the CSIC for performing micro-Raman measurements. JSi and JSo acknowledge financial support through the national research grant UDiSON (TEC2017-82464-R) from the Spanish Research Agency(AEI, Ministry of Research and Innovation) and the European Regional Development Fund (ERDF), as well as the Consejo Superior de Investigaciones Científicas for the intramurales project (201850E057). The authors acknowledge a pre-doctoral fellowship from the MICINN for NCa and a pre-doctoral grant for RZa from the regional government of Madrid and financed by the European Social Fund (ESF) and the national “Iniciativa de Empleo Juvenil” (YEI). The authors acknowledge EU-H2020 grant agreement No 654360 (NFFA-Europe) and the corresponding access to SEM and AFM facilities at FORTH within NFFA project ID 507.Peer reviewe

Fabrication of Novel Biomimetic Structures on Steel Via Femtosecond Laser Over-Scans

Conference on Lasers and Electro-Optics OSA Technical Digest (online) (Optical Society of America, 2017), paper STh4J.3. -- San Jose, California United States 14–19 May 2017We present different biomimetic structures on steel fabricated with a high repetition rate femtosecond laser. We show that their wetting properties can be defined by the irradiation conditions and the overall number of over-scans.Peer Reviewe

Influence of Surface Roughness on the Formation of Laser Induced Periodic Surface Structures (LIPSS) in Copper

Cancún, Quintana Roo, México, September 23-27, 2019Introduction/Purpose The generally accepted mechanism leading to LIPSS formation relates to the interference of incident laser light with a wave propagating at the surface, leading to a periodic intensity modulation that is imprinted into the material [1,2]. Amongst the irradiation parameters, the angle of incidence (tetha) of the laser beam directly influences the period of the so-called Low Spatial Frequency (LSF) LIPSS (parallel ripples perpendicular to the laser polarization). Here, we report a strong deviation of the angle dependence of the LIPSS period on Cu from the predictions of existing models based on Surface Plasmon Polariton (SPP) propagation, and correlate it to the strong dependence of the SPP wave-vector on surface roughness [3,4]. Methods LIPSS in Cu where formed upon irradiation with 120 fs laser pulses at 800 nm at different angles of incidence and effective number of pulses per unite surface. Results Fig. 1(a) shows a SEM image of LIPSS in Cu formed upon irradiation at a fixed angle of incidence (theta= 52o). The periodic structures are aligned perpendicularly to the laser polarization. Notably, two different ripple periods are observed in different regions of the laser-written track. Fig. 1(b) shows in direct comparison of modelling results taking as an input the specific surface roughness of the exposed regions measured with atomic force microscopy. In order to explain such differences, we introduce a model incorporating the strong dependence of the SPP¿s wave-vector on surface roughness parameters [3,4]. Fig. 1(c) shows the periods of the two types of ripples for different angles of incidence, combining the experimental data with the results of our model. Conclusions The model fits the experimental data very well for all angles, much better than the simple plasmonic model not taking into account the surface roughness, which highlights the potential of our model for the general understanding of LIPSS in metals

Surface Plasmon Polaritons on Rough Metal Surfaces: Role in the Formation of Laser-Induced Periodic Surface Structures

The formation of self-organized laser-induced periodic surface structures (LIPSS) in metals, semiconductors, and dielectrics upon pulsed laser irradiation is a well-known phenomenon, receiving increased attention because of their huge technological potential. For the case of metals, a major role in this process is played by surface plasmon polaritons (SPPs) propagating at the interface of the metal with the medium of incidence. Yet, simple and advanced models based on SPP propagation sometimes fail to explain experimental results, even of basic features such as the LIPSS period. We experimentally demonstrate, for the particular case of LIPSS on Cu, that significant deviations of the structure period from the predictions of the simple plasmonic model are observed, which are very pronounced for elevated angles of laser incidence. In order to explain this deviation, we introduce a model based on the propagation of SPPs on a rough surface that takes into account the influence of the specific roughness properties on the SPP wave vector. Good agreement of the modeling results with the experimental data is observed, which highlights the potential of this model for the general understanding of LIPSS in other metals