Dynamics of Femtosecond Laser Interactions with Dielectrics

review article written in common (LBNL+CEA)Femtosecond laser pulses appear as an emerging and promising tool for processing wide band-gap dielectric materials for a variety of applications. This article aims to provide an overview of recent progress in understanding the fundamental physics of femtosecond laser interactions with dielectrics that may have the potential for innovative materials applications. The focus of the overview is the dynamics of femtosecond laser-excited carriers and the propagation of femtosecond laser pulses inside dielectric materials

Photoconductivity and Photoemission of Diamond Under Femtosecond Vuv Irradiation

In order to gain some insight on the electronic relaxation mechanisms occuring in diamond under high intensity laser excitation and/or VUV excitation, we studied experimentally the pulsed conductivity induced by femtosecond VUV pulses, as well as the energy spectra of the photoelectrons released by the same irradiation. The source of irradiation consists in highly coherent VUV pulses obtained through high order harmonic generation of a high intensity femtosecond pulse at a 1.55 eV photon energy (titanium-doped sapphire laser). Harmonics H9 to H17 have been used for photoconductivity (PC) and harmonics H13 to H27 for photoemission experiments (PES). As the photon energy is increased, it is expected that the high energy photoelectrons will generate secondary e-h pairs, thus increasing the excitation density and consequently the PC signal. This is not what we observe : the PC signal first increases for H9 to H13, but then saturates and even decreases. Production of low energy secondary e-h pairs should also be observed in the PES spectrum. In fact we observe very few low energy electrons in the PES spectrum obtained with H13 and H15, despite the sufficient energy of the generated free carriers. At the other end (H21 and above), a very intense low energy secondary electron peak is observed. As a help to interprete such data, we realized the first ab initio calculations of the electronic lifetime of quasiparticles, in the GW approximation in a number of dielectrics including diamond. We find that the results are quite close to a simple "Fermi-liquid" estimation using the electronic density of diamond. We propose that a quite efficient mechanism could be the excitation of plasmons by high energy electrons, followed by the relaxation of plasmons into individual e-h pairs

Mechanisms of femtosecond laser ablation of dielectrics revealed by double pump-probe experiment

International audienceWe study experimentally the electronic excitation mechanisms involved in the breakdown and ablation of wide band gap dielectrics. A femtosecond pump-probe interferometry technique, with 100 fs temporal resolution, allows measuring the modification of refractive index induced by ultra-short intense laser pulses. To get more information in the complex process of excitation and relaxation mechanisms involved during and after the interaction, we use a sequence of two excitation pulses: a first short pulse at 400 nm excites a controlled density of carriers, and a second one at 800 nm with variable pulse duration, from 50 fs to 10 ps, reaches an excited solid. In Al2O3, we show that the total density of carriers never exceeds the sum of the densities excited by the two pulses sent independently. This means that the second pulse deposits further energy in the material by heating the previously excited carriers, and that no electronic multiplication occurs. On the other hand, in SiO2, it is possible, under specific conditions, to observe an increase of carrier density due to impact ionization. All these results demonstrate that the avalanche process, which is often invoked in the laser breakdown literature, does not play a dominant role in optical breakdown induced by short pulses

Ultrafast Optical Measurements of Defect Creation in Laser Irradiated SiO2_{2}

Optical methods using sub-picosecond laser pulses allow to study the kinetic of defect creation in SiO$_{2}$, caused by an intense electronic excitation. A first intense “pump” pulse is used to create a high density (up to 10$^{19}$ cm$^{-3}$) of e-h pair. A second, weaker pulse is then used to probe the state of the material after an adjustable delay, with a time resolution of the order of 10$^{-13}$ s. A first investigation using photoelectron spectroscopy shows that the electrons can reach kinetic energies in the conduction band in large excess of the photon energy, through three-body electron-photon-phonon transitions (a sequential absorption process). “Transient Frequential Interferometry” is used to measure the instantaneous refractive index, i.e. the free carrier density (conduction electrons), and to confirm the existence of the absorption by conduction electrons. Transient absorption can be used to monitor the appearance of point defects following the trapping of the free carriers. We show that, contrary to what is observed in other oxides (Al$_{2}$O$_{3}$ and MgO), the trapping process is extremely fast (150 fs), and occurs at all temperatures in the triplet state of the Self Trapped Exciton (STE). A permanent absorption is shown to appear at room temperature only, resulting from the thermal conversion of STE into colored centers. Finally, we study from a theoretical point of view the transport of conduction electrons with help of two different methods: Monte-Carlo simulations, which allow to introduce in a convenient way the effect of the laser field, and solving the time-evolution of the density matrix equations, a more exact treatment in principle required in SiO$_{2}$ because of the strong electron-phonon coupling, but which does not yet allow to include the effect of a strong laser field

Time resolved study of laser induced coloured centres in SiO2

We report the investigation of the onset of an absorption band in the UV in crystalline quartz (alpha-SiO2) induced by an intense femtosecond laser pulse. Using a conventional pump– probe technique, we have measured the absorptions at 219 nm (5.66 eV) and at 240 nm (5.16 eV) as functions of time, at two different temperatures (10 and 300 K). The rise time of the absorption is measured to be 150 (50) fs. It is independent of probe wavelength and sample temperature. The absorption coefficients are similar at both probe wavelengths and the values at room temperature are about twice these at 10 K. We attribute the onset of the absorption to the ultra-fast formation of self-trapped excitons (STEs). The STEs recombine radiatively at a low temperature. At room temperature, we observe a cumulative effect. This demonstrates that, at 300 K, some of the STEs are converted to permanent colour centres, which we tentatively identify as neutral oxygen vacancies

Probing ultrashort-pulse laser excitation of sapphire: From the initial carrier creation to material ablation

International audienceUltrashort-pulse laser excitation of dielectrics has been investigated over a large span of intensities. Experimentally, single-shot studies on single-crystal sapphire samples combine time-resolved spectral interferometry with time-resolved reflectivity and ablation-rate measurements. The complete development of the excitation from the first creation of conduction-band electrons at low intensities to the formation of a highly excited plasma and associated material fragmentation is observed experimentally and explained by a single theoretical model, which combines material excitation in a multiple-rate equation description with light propagation. Copyright (C) EPLA, 201

Interaction of intense femtosecond laser pulses with KDP and DKDP crystals in the short wavelength regime

We investigate the electronic photo-excitation and relaxation mechanisms involved in the optical breakdown of potassium dihydrogen phosphate crystal (KH2PO4) and its deuterated form. The dynamics and spectroscopic properties of electron–hole pair formation are investigated using time-resolved measurement of the dielectric function, and luminescence spectroscopy. The non-common mechanical and electronic characteristics of these dielectric materials are revealed by the particular structure of ablation craters and also by the complex dynamics observed in the relaxation of excited carriers. This relaxation occurs in two steps, and varies with the initial carrier density and thus with the laser intensity. We show that the defect states play a key role in the excitation pathways, and also determine the relaxation stage. The latter also depends upon the initial amount of energy of the electron–hole pair after photo-excitation. A model based on kinetic equations describing the evolution of the different level populations allows us to successfully interpret and reproduce the experimental data

Evaluation of a new On-Stream Supercritical Fluid Deposition process for sol–gel preparation of silica-based membranes on tubular supports

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Heating of Conduction Band Electrons by Intense Femtosecond Laser Pulses

We present photoelectron spectra for \chem{CsI} excited by intense femtosecond \chem{Ti}-Sapphire laser pulses. A high-energy plateau is detected in the spectra, at excitation intensities above 0.5\un{TW/cm^2}. This plateau extends up to 24\un{eV} at 3\un{TW/cm^2} and 90% of the emitted electrons have energy higher than twice the laser photon energy. Such intensive electron heating in solids cannot be explained in terms of phonon-assisted transitions. A model of direct interbranch transitions in the conduction band is used for the simulation of the heating process

Femtosecond laser-induced damage: lessons learned from time-resolved measurements

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