Ultrafast laser surface structuring with Gaussian and Optical Vortex beam

The progress in the physics of interaction of electromagnetic radiation with matter and related fundamental processes largely benefits from the contribution of the laser sources and many areas of investigation just commenced after the breakthrough given by the realization of the first pulsed laser in 1960 by T. H. Maiman. Contactless processing and high localization of energy possibly achieved with laser processing increased its acceptance in diverse fields. In general, reduced pulse duration offered by ultrashort laser pulses is utilized for direct monitoring of temporal dynamics of several fast atomic processes. On the other hand, the high intensity provided by such pulses gives a new twist in the field of laser ablation of materials. Study related to fundamental aspects of laser interaction and ablative processes opened numerous investigations and many possible applications. Because of the ultrashort duration, fs laser holds the capacity to achieve ablation with considerably less heat effected zone which in turns provides maximum spatial resolution in term of material processing. Controlled ablation processes with fs laser offer the possibility to generate periodic surface structures with a periodicity much less than the laser wavelength. This further proves the fs laser as a precise and versatile tool for micro- and nano-fabrication processes with great accuracy. Since many surface properties of solids (optical, wetting, mechanical etc.) are closely associated to their morphology, such phenomenon can have great impact on a variety of applications. Further applications include generation of microfluidic channels for biological uses and optical integrated circuits for quantum applications. This thesis describes surface processing of crystalline silicon (100 orientation) with fs laser pulses with Gaussian as well as with some modified beam generated by a special optical device called q-plate. The experimental details and results are discussed in five chapters, which include a brief introduction about laser ablation and ultrafast laser processing. The first chapter gives a brief introduction about laser-matter interaction, underlying mechanisms of material ablation with ultrashort laser pulses along with a short overview of characterization methods used in laser induced plasma and material removal analyses. The material removal by the laser irradiation results in the generation of specific surface structures, generally called laser induced periodic surface structures (LIPSS). This chapter discusses also some basic aspects in the formation of LIPSS generated with Gaussian and Optical Vortex (OV) beams reported earlier. Finally, a short section illustrates the experiments on laser surface structuring using Gaussian and OV beams presented in the next chapters of this thesis. The second chapter deals with experimental methods used in fs surfaces structuring experiments during this thesis. In particular, the laser sources and the method used to generate OV beams using q-plate are discussed. Moreover, it includes a brief description of sample material, optical setup and instrumentations used for surface structuring with different kind of fs laser beams and surface characterization. Chapter 3 illustrates experimental results on the various surface structures generated on crystalline silicon (100) by irradiation with fs laser pulses having a Gaussian spatial intensity profile. In particular, characteristic effects related to the number of pulses and energy are described. Moreover, bending and bifurcation phenomena characterizing quasi-periodic structures are presented. These aspects are, then, rationalized in the frame of a model of surface structure formation based on surface scattered wave theory. Finally, results of some experiments on the effect of the ambient gas pressure on the surface structures are also illustrated. The fourth chapter is dedicated to surface structuring with fs OV beam with orbital angular momentum (OAM) m=1 generated by a q-plate (q=+1/2). OV beams with azimuthal, radial, spiral and linear state of polarization (SoP) in the transverse plane are employed in the process, which enables not only to generate complex surface patterns but also to characterize the OV beam itself in the focal point. Interaction with more complex OV as the one generated with additional λ/4 wave plates gives interesting patterns of ripples and grooves. Results showing the laser structuring with higher OAM beams, such as m=2 and 5, are also included in order to demonstrate the ability of laser induced structures in following the much more complex beam polarization pattern. The fifth chapter addresses an experimental investigation on direct femtosecond laser surface structuring with a higher class of vector beams generated by electrically tuning the optical retardation of a q-plate with q=+1/2. This allows generating a family of ultrashort laser beams with a continuous spatial evolution of polarization and fluence distribution in the focal plane by controlled coherent superposition of a uniformly polarized Gaussian beam with an OV beam. The use of these generalized vector beams in laser structuring can considerably improve the possibility of achieving a number of asymmetric surface patterns. Finally, a brief summary of the experimental findings is reported in the last chapter also shortly addressing the scope of surface structuring and the possible applications of process and resulting periodic surface structures formed

Femtosecond laser surface structuring of silicon with Gaussian and optical vortex beams

We report an experimental analysis of femtosecond laser induced surface structuring of silicon by exploiting both Gaussian and Optical Vortex beams. In particular, we show how different surface patterns, consisting of quasi-periodic ripples and grooves, can be obtained by using different states of polarization offered by optical vortex beams. Both for Gaussian and optical vortex beams, an increase of the number of laser pulses, N, or beam energy, E-0, leads to a progressive predominance of the grooves coverage, with ripples confined in specific regions of the irradiated area at lower fluence. The average period of ripples and grooves shows a different dependence as a function of both E-0 and N, underlying important differences in mechanisms leading to the formation of ripples and grooves. In particular, our experimental characterization allows identifying a preliminary stage of grooves generation with rudimental surface structures, preferentially directed parallel to the laser polarization. This supports the idea that one possible mechanism of grooves formation lies in the progressive aggregation of clusters of nanopartides densely decorating the ripples. Our experimental findings provide important indications on the basic understanding of the processes involved in laser surface structuring with ultrashort pulses that can guide the design of the surface patterns. (C) 2016 Elsevier B.V. All rights reserved

Surface Structuring with Polarization-Singular Femtosecond Laser Beams Generated by a q-plate

In the last few years femtosecond optical vortex beams with di erent spatial distributions of the state of polarization (e.g. azimuthal, radial, spiral, etc.) have been used to generate complex, regular surface patterns on di erent materials. Here we present an experimental investigation on direct femtosecond laser surface structuring based on a larger class of vector beams generated by means of a q-plate with topological charge q = +1/2. In fact, voltage tuning of q-plate optical retardation allows generating a family of ultrashort laser beams with a continuous spatial evolution of polarization and uence distribution in the focal plane. These beams can be thought of as a controlled coherent superposition of a Gaussian beam with uniform polarization and a vortex beam with a radial or azimuthal state of polarization. The use of this family of ultrashort laser beams in surface structuring leads to a further extension of the achievable surface patterns. The comparison of theoretical predictions of the vector beam characteristics at the focal plane and the generated surface patterns is used to rationalize the dependence of the surface structures on the local state of the laser beam, thus o ering an e ective way to either design unconventional surface structures or diagnose complex ultrashort laser beams

Vector vortex beams generated by q-plates as a versatile route to direct fs laser surface structuring

We report an experimental investigation on direct laser surface structuring with femtosecond vector vortex beams generated by means of q-plates with topological charges q = 1, 3/2, 2, 5/2. Structured light beams with spatially variant state of polarization and intensity are generated and applied to multi-pulse irradiation of a solid crystalline silicon target. The creation of a variety of surface structures, like laser induced periodic surface structures, multi-spot arrays and shaped ablation craters, is demonstrated by direct laser surface structuring with vector vortex beams at different values of q. The features of the surface structures are compared with the vector vortex beam characteristics at the focal plane, evidencing their relationship with the polarization and intensity profile of the laser beams. Our experimental findings show that vector vortex beams produced by q-plates can offer a valuable and versatile route to imprint unconventional surface structures on a solid target through a mask-free ablative process and step scan processing

Surface structures with unconventional patterns and shapes generated by femtosecond structured light fields

We present an investigation on ultrashort laser surface structuring with structured light fields generated by various q-plates. In particular, q-plates with topological charges q = 1, 3/2, 2, 5/2 are used to generate femtosecond (fs) vector vortex beams, and form complex periodic surface structures through multi-pulse ablation of a solid crystalline silicon target. We show how optical retardation tuning of the q-plate offers a feasible way to vary the fluence transverse distribution of the beam, thus allowing the production of structures with peculiar shapes, which depend on the value of q. The features of the generated surface structures are compared with the vector vortex beam characteristics at the focal plane, by rationalizing their relationship with the local state of the laser light. Our experimental findings demonstrate how irradiation with fs complex light beams can offer a valuable route to design unconventional surface structures

Simple method for the characterization of intense Laguerre-Gauss vector vortex beams

We report on a method for the characterization of intense, structured optical fields through the analysis of the size and surface structures formed inside the annular ablation crater created on the target surface. In particular, we apply the technique to laser ablation of crystalline silicon induced by femtosecond vector vortex beams. We show that a rapid direct estimate of the beam waist parameter is obtained through a measure of the crater radii. The variation of the internal and external radii of the annular crater as a function of the laser pulse energy, at fixed number of pulses, provides another way to evaluate the beam spot size through numerical fitting of the obtained experimental data points. A reliable estimate of the spot size is of paramount importance to investigate pulsed laser-induced effects on the target material. Our experimental findings offer a facile way to characterize focused, high intensity complex optical vector beams which are more and more applied in laser-matter interaction experiments

Generation of Supra-Wavelength Grooves in Femtosecond Laser Surface Structuring of Silicon

Extensive research work has been carried out on the generation and application of laser-induced periodic surface structures (LIPSS). LIPSS with a sub-wavelength period generated by femtosecond laser irradiation, generally indicated as ripples, have been extensively investigated. Instead, the other ordered surface structures characterized by a supra-wavelength period, indicated as grooves, have been much less studied. Grooves typically form at larger irradiance levels or for higher number of laser pulses. Here, we report a comprehensive overview of recent investigations on the supra-wavelength grooves formed on crystalline silicon irradiated by femtosecond laser pulses. The authors’ recent experimental work is mainly addressed giving an explicit picture of the grooves generation process, namely illustrating the influence of the various experimental parameters, including, e.g., polarization, wavelength, fluence and repetition rate of the laser beam as well as number of laser pulses hitting the surface of the material. The effect of irradiation of a static or moving target and of the environmental conditions (e.g., vacuum or air ambient) will also be discussed. Finally, possible mechanisms envisaged to explain grooves formation and still open issues are briefly discussed

Generation of Supra-Wavelength Grooves in Femtosecond Laser Surface Structuring of Silicon

Extensive research work has been carried out on the generation and application of laser-induced periodic surface structures (LIPSS). LIPSS with a sub-wavelength period generated by femtosecond laser irradiation, generally indicated as ripples, have been extensively investigated. Instead, the other ordered surface structures characterized by a supra-wavelength period, indicated as grooves, have been much less studied. Grooves typically form at larger irradiance levels or for higher number of laser pulses. Here, we report a comprehensive overview of recent investigations on the supra-wavelength grooves formed on crystalline silicon irradiated by femtosecond laser pulses. The authors' recent experimental work is mainly addressed giving an explicit picture of the grooves generation process, namely illustrating the influence of the various experimental parameters, including, e.g., polarization, wavelength, fluence and repetition rate of the laser beam as well as number of laser pulses hitting the surface of the material. The effect of irradiation of a static or moving target and of the environmental conditions (e.g., vacuum or air ambient) will also be discussed. Finally, possible mechanisms envisaged to explain grooves formation and still open issues are briefly discussed

Direct femtosecond laser surface structuring with complex light beams generated by q-plates

Direct femtosecond (fs) laser surface structuring became a versatile way to generate surface structures on solid targets demonstrating a high degree of flexibility and controllability in creating different types of structures for many applications. This approach demonstrated an alteration in various properties of the surface, such as optical properties, wetting response, etc. This paper focuses on direct fs laser surface structuring using complex light beams with spatially variant distribution of the polarization and fluence, with emphasis on the results obtained by the authors by exploiting q-plate beam converters. Although striking scientific findings were achieved so far, direct fs laser processing with complex light fields is still a novel research field, and new exciting findings are likely to appear on its horizon

Femtosecond laser surface structuring of silicon in dynamic irradiation conditions

The process of surface structuring of silicon in dynamic irradiation conditions is investigated by analyzing the features of the surface structures generated on a target moving at different scanning velocities under the irradiation by a 1030 nm femtosecond laser beam. Scanning electron microscopy analysis of the shallow linear craters produced in different experimental conditions allows characterizing the dependence of threshold fluence for the formation of surface structures (ripples and grooves) as well as their spatial periods on the effective number of laser pulses. Moreover, the effect of the accumulated laser fluence dose has been also addressed. Our findings evidence clear trends of the morphological features of the surface structure on the effective number of laser pulses as well as the existence of interesting differences in their characteristics when processing occurs at the same value of the accumulated laser fluence dose achieved by appropriate selection of laser pulse energy and scanning speed