Rippled area formed by surface plasmon polaritons upon femtosecond laser double-pulse irradiation of silicon: the role of carrier generation and relaxation processes

The formation of laser-induced periodic surface structures (LIPSS, ripples) upon irradiation of silicon with multiple irradiation sequences consisting of femtosecond laser pulse pairs (pulse duration 150 fs, central wavelength 800 nm) is studied numerically using a rate equation system along with a two-temperature model accounting for one- and two-photon absorption and subsequent carrier diffusion and Auger recombination processes. The temporal delay between the individual equal-energy fs-laser pulses was varied between $0$ and $\sim 4$ ps for quantification of the transient carrier densities in the conduction band of the laser-excited silicon. The results of the numerical analysis reveal the importance of carrier generation and relaxation processes in fs-LIPSS formation on silicon and quantitatively explain the two time constants of the delay dependent decrease of the Low-Spatial-Frequency LIPSS (LSFL) area observed experimentally. The role of carrier generation, diffusion and recombination are quantified individually.Comment: 5 pages, 5 figures, Conference On Laser Ablation (COLA) 2013. The final publication is available at http://link.springer.com. Accepted for publication in Applied Physics

Rippled area formed by surface plasmon polaritons upon femtosecond laser double-pulse irradiation of silicon

International audienceThe formation of near-wavelength laser-induced periodic surface structures (LIPSS) on silicon upon irradiation with sequences of Ti:sapphire femtosecond laser pulse pairs (pulse duration 150 fs, central wavelength 800 nm) is studied theoretically. For this purpose, the nonlin-ear generation of conduction band electrons in silicon and their relaxation is numerically calculated using a two-temperature model approach including intrapulse changes of optical properties, transport, diffusion and recombina-tion effects. Following the idea that surface plasmon polaritons (SPP) can be excited when the material turns from semiconducting to metallic state, the "SPP active area" is calculated as function of fluence and double-pulse de-lay up to several picoseconds and compared to the experimentally observed rippled surface areas. Evidence is presented that multi-photon absorption explains the large increase of the rippled area for temporally overlapping pulses. For longer double-pulse delays, relevant relaxation processes are identified. The results demonstrate that femtosecond LIPSS on silicon are caused by the excitation of SPP and can be controlled by temporal pulse shaping. ©2013 Optical Society of America OCIS codes: (050.6624) Subwavelength structures; (140.3390) Laser materials processing; (160.6000) Semiconductor materials; (240.5420) Polaritons

Optimization of the Energy Deposition in Glasses with Temporally-Shaped Femtosecond Laser Pulses

International audienceBulk machining of glasses with femtosecond laser pulses enables the fabrication of embedded optical functions. Due to the nonlinear character of the laser-matter interaction, structural modifications can occur within the focal region. To reach a full control of the process, ways of controlling the deposition of the laser energy inside the material have to be unveiled. From static and time-resolved pictures of bulk-excitation of a-SiO2 and borosilicate glass, we show that particular laser temporal shapes such as picosecond sequences can better confine the energy deposition than the femtosecond sequence by reducing the propagation artifacts

Time-Resolved Observation of Energy Deposition in Fused Silica by Ultrashort Laser Pulses in Single and Cumulative Regime

International audienceWhen femtosecond laser pulses are focused in the bulk of transparent materials (glasses), deposition of energy on a restricted volume can occur owing to the non linear character of the laser matter interaction. As a consequence, the possibility to generate micrometer-sized structural modifications arises. Those local changes are often associated with a minute variation in the refractive index which, when positive, enables the fabrication of light guiding components in three dimensions through simple laser translation. Although the first corresponding experimental demonstration approaches fifteen years of age, the complete picture of the dynamics and the proc- esses leading to the local refractive index changes has still to be drawn to reach an optimal control of the laser-induced modification process. In this report, the laser-dielectric interaction is followed on an ultrashort time scale with the help of a unique time-resolved side-imaging technique allow- ing for absorption and phase contrast detection. Experimental observation of an absorptive elec- tronic cloud in the first moments of the interaction along with the launch of a pressure wave after a few ns is reported. These physical objects are shown to be reliable indicators of the success of the energy transfer to the lattice which largely depends on the pulse temporal envelope

Laser-induced modification of transparent crystals and glasses

International audienceWe analyse theoretically the processes proceeding in transparent crystals and glasses irradiated by ultrashort laser pulses in the regimes typical of different applications in optoelectronics and photonics. We consider some phenomena, which have been previously described by the authors within the developed model representations: charge of the dielectric surface due to electron photoemission resulting in a Coulomb explosion; crater shaping by using an adaptive control of the laser pulse shape; optimisation of the waveguide writing in materials strongly resistant to laser-induced compaction under ordinary irradiation conditions. The developed models and analysis of the processes based on these models include the elements of the solid-state physics, plasma physics, thermodynamics, theory of elasticity and plasticity. Some important experimental observations which require explanations and adequate description are summarised

Basics and applications of femtosecond laser interaction

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Continuum models of femtosecond laser ablation

The aim of this chapter is to provide a basic introduction to the principles that lay the foundation for established approaches that treat matter as a continuum model, in order to describe and comprehend the aspects of laser–matter interactions. The chapter starts with a consideration of the relevant processes induced in solids under laser irradiation and a description of continuum models successfully applied to quantify the laser heating and subsequent ablation processes. We intend to focus on a critical assessment of the used approaches with a clear perception of their advantages and limitations. The drift-diffusion approach of laser-induced material charging and its eventual disintegration are considered as examples. The time and length scales of its application in describing laser-induced modifications for different classes of materials are analyzed and further improvements are also discussed. In the final part of the chapter, we give a short overview of laser–solid interaction phenomena, which could be treated with continuum models, and present a number of examples too

Single-pulse ultrafast laser imprinting of axial dot arrays in bulk glasses

International audienceUltrafast laser processing of bulk transparent materials can significantly gain flexibility when the number of machining spots is increased. We present a photoinscription regime in which an array of regular dots is generated before the region of main laser focus under single-pulse exposure in fused silica and borosilicate crown glass without any external spatial phase modulation. The specific position of the dots does not rely on nonlinear propagation effects but is mainly determined by beam truncation and is explained by a Fresnel propagation formalism taking into account beam apodization and linear wavefront distortions at the air/glass interface. The photoinscription regime is employed to generate a two-dimensional array of dots in fused silica. We show that an additional phase modulation renders flexible the pattern geometry

Modeling of electron dynamics in laser-irradiated solids: progress achieved through a continuum approach and future prospects

International audienceA continuum model based on a drift-diffusion approach is applied to describe the dynamics of electronic excitation, heating and charge-carrier transport in metals and dielectrics under near-infrared femtosecond laser irradiation. The dependence of laser-induced charging of the targets on laser fluence and pulse duration is investigated. Various aspects concerning the mechanism of Coulomb Explosion (CE) are discussed. The CE threshold as a function of pulse duration is evaluated numerically for dielectric materials (sapphire and ULE glass). A special attention is paid to studies of interconnection between the electron emission yield and surface charging dynamics. It has been found that in dielectrics the photoemission yield saturates with increasing laser fluence as a result of self-regulation of the free-electron population. By contrast in metals, due to effective supply of electrons to the charging zone on the target surface, electron emission becomes unwarrantably high for short laser pulses and high fluences. However, photo- and thermionic emissions can be suppressed by the generated electric field whose amplitude is a function of pulse duration and laser fluence. The question on self-consistency of electron emission and surface charging is analyzed with outlining further studies