Thibault J.-Y. Derrien

Possible surface plasmon polariton excitation under femtosecond laser irradiation of silicon

The mechanisms of ripple formation on silicon surface by femtosecond laser pulses are investigated. We demonstrate the transient evolution of the density of the excited free-carriers. As a result, the experimental conditions required for the excitation of surface plasmon polaritons are revealed. The periods of the resulting structures are then investigated as a function of laser parameters, such as the angle of incidence, laser fluence, and polarization. The obtained dependencies provide a way of better control over the properties of the periodic structures induced by femtosecond laser on the surface of a semiconductor material.

Plasmonic formation mechanism of periodic 100-nm-structures upon femtosecond laser irradiation of silicon in water

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of silicon by multiple (N = 100) linearly polarized Ti:sapphire femtosecond laser pulses (duration τ = 30 fs, center wavelength λ0 ∼ 790 nm) is studied experimentally in air and water environment. The LIPSS surface morphologies are characterized by scanning electron microscopy and their spatial periods are quantified by two-dimensional Fourier analyses. It is demonstrated that the irradiation environment significantly influences the periodicity of the LIPSS. In air, so-called low-spatial frequency LIPSS (LSFL) were found with periods somewhat smaller than the laser wavelength (ΛLSFL ∼ 0.7 × λ0) and an orientation perpendicular to the laser polarization. In contrast, for laser processing in water a reduced ablation threshold and LIPSS with approximately five times smaller periods ΛLIPSS ∼ 0.15 × λ0 were observed in the same direction as in air. The results are discussed within the frame of recent LIPSS theories and compleme...

Formation of femtosecond laser induced surface structures on silicon: Insights from numerical modeling and single pulse experiments

Abstract Laser induced periodic surface structures (LIPSS) are formed by multiple irradiation of femtosecond laser on a silicon target. In this paper, we focus and discuss the surface plasmon polariton mechanism by an analysis of transient phase-matching conditions in Si on the basis of a single pulse experiment and numerical simulations. Two regimes of ripple formation mechanisms at low number of shots are identified and detailed. Correlation of numerical and experimental results is good.

Study on laser-induced periodic structures and photovoltaic application

We have irradiated silicon with a series of femtosecond laser pulses to improve light absorption at the silicon surface. The laser treated surface namely black silicon shows excellent optical properties on mono and multicrystalline silicon wafers with a reflectivity reduction down to 3%, without crystal orientation dependence. After the laser process, the front side of samples has been boron‐implanted by Plasma Immersion Ion Implantation to create the 3D p+ junction. Improved electrical performances have also been demonstrated with a 57% increase in the photocurrent, compared to non‐texturized surface.

Wavelength dependence of picosecond laser-induced periodic surface structures on copper

Abstract The physical mechanisms of the laser-induced periodic surface structures (LIPSS) formation are studied in this paper for single-pulse irradiation regimes. The change in the LIPSS period with wavelength of incident laser radiation is investigated experimentally, using a picosecond laser system, which provides 7-ps pulses in near-IR, visible, and UV spectral ranges. The experimental results are compared with predictions made under the assumption that the surface-scattered waves are involved in the LIPSS formation. Considerable disagreement suggests that hydrodynamic mechanisms can be responsible for the observed pattern periodicity.

Femtosecond Laser Interactions with Semiconductor and Dielectric Materials

Electronic excitation-relaxation processes induced by ultra-short laser pulses are studied numerically for semiconductors and dielectric materials (Si, quartz). A detailed kinetic approach is used in the calculations accounting for electron-photon-phonon, electron-phonon and electron-electron scatterings. In addition, both laser field ionization ranging from multi-photon to tunneling one, and electron impact (avalanche) ionization processes are included in the model. Based on the performed calculations we study the relaxation time as a function of laser parameters. It is shown that this time depends on the density of the created free carriers, which in turn is a nonlinear function of laser intensity. In addition, a simple damage criterion is proposed based on the mean electron energy density rather than on critical free electron density. This criterion gives a reasonable agreement with the available experimental data practically without adjustable parameters. Furthermore, the performed modeling provides energy absorbed in the target, conditions for damage of dielectric materials, as well as conditions for surface plasmon excitation and for periodic surface structure formation on the surface of semiconductor materials.

Insights into Laser-Materials Interaction Through Modeling on Atomic and Macroscopic Scales

Computer simulations and theoretical analysis of laser-materials interactions are playing an increasingly important role in the advancement of modern laser technologies and broadening the range of laser applications. In this chapter, we first provide an overview of the current understanding of the laser coupling and transient variation of optical properties in metals, semiconductors and dielectrics, with the focus on the practical implications on the energy deposition and distribution in the irradiated targets. The continuum-level modeling of the dynamic evolution of laser-induced stresses, nonequilibrium phase transformations, and material redistribution within the laser spot are then discussed, and the need for the physical insights into the mechanisms and kinetics of highly nonequilibrium processes triggered by the laser excitation is highlighted. The physical insights can be provided by atomistic modeling, and several examples are discussed where large-scale molecular dynamics simulations are used for investigation of the mechanisms of the generation of crystal defects (vacancies, interstitials, dislocations, and twin boundaries) and the material redistribution responsible for the formation of laser-induced periodic surface structures in the single-pulse ablative regime. The need for the integrated computational approach fully accounting for the strong coupling between processes occurring at different time- and length-scales is highlighted.

Modeling of silicon in femtosecond laser-induced modification regimes: accounting for ambipolar diffusion

During the last decades, femtosecond laser irradiation of materials has led to the emergence of various applications based on functionalization of surfaces at the nano- and microscale. Via inducing a periodic modification on material surfaces (band gap modification, nanostructure formation, crystallization or amorphization), optical and mechanical properties can be tailored, thus turning femtosecond laser to a key technology for development of nanophotonics, bionanoengineering, and nanomechanics. Although modification of semiconductor surfaces with femtosecond laser pulses has been studied for more than two decades, the dynamics of coupling of intense laser light with excited matter remains incompletely understood. In particular, swift formation of a transient overdense electron-hole plasma dynamically modifies optical properties in the material surface layer and induces large gradients of hot charge carriers, resulting in ultrafast charge-transport phenomena. In this work, the dynamics of ultrafast laser excitation of a semiconductor material is studied theoretically on the example of silicon. A special attention is paid to the electron-hole pair dynamics, taking into account ambipolar diffusion effects. The results are compared with previously developed simulation models, and a discussion of the role of charge-carrier dynamics in localization of material modification is provided.

Nanofabrication with pulsed lasers

The employment of pulsed lasers offers a novel unique tool for nanofabrication [1]. When focused on the surface of a solid target, pulsed laser radiation causes a variety of phenomena, including heating, melting, and finally ablation of the target, and such processes can lead to an efficient material nanostructuring. First, the laser-assisted removal of material from the irradiated spot can result in a spontaneous formation of variety of periodic nanoarchitectures within this spot. Second, laser ablation of material from a solid target leads to the production of nanoclusters, which can then be either deposited on a substrate to form a nanostructured film or released into a liquid environment to form a colloidal nanoparticle solution. Our on-going projects on laser nanofabrication include the following activities: 1. Laser-assisted self-structuring to form nanoscale features on the surface. In this case, we consider spontaneously formed architectures on the surface under laser-matter interactions. In the first method, fs laser ablation in residual gas leads to a formation of micro-scale spikes on Si surface, which condition a drastic increase of the absorption of the treated surface (“black silicon”) that is important for photovoltaic applications [2]. In the second method, we create hot, highly absorbing laser plasma by a phenomenon of laser-induced gas breakdown and use it to treat surfaces and thus form unique “photon crystal-like” structures, which are of importance for optoelectronics applications [3,4]. 2. Pulsed Laser ablation in liquids. In this method, fs laser radiation is used to ablate a solid target in liquid ambience (aqueous solutions of biopolymers etc) to form colloidal nanoparticles [5–7]. Nanomaterials synthesized by this method exhibit unique proper-ties, which can not be reproduced by conventional chemical routes, including small size (down to 1 nm) and size dispersion, unique surface chemistry, and the absence of toxic contaminants on nanoparticle surface. Such properties give a promise for successful “in vivo” applications of nanoparticles. In particular, we showed that Si nanoparticles pre-pared by laser ablation are fluorescent and capable of exhibiting singlet oxygen under photoexcitation, making them excellent candidates for PDT of cancer [8]. 3. Near-field nanoparticle-assisted nanostructuring of surfaces: fabrication of patterned nanoarrays. In these methods, laser ablation through glass nanoparticles dispersed on the surface is used in combination with photolithography to form nanoplasmonics arrays for biosensing applications [9].