Our aim is to better understand and observe the many events occurring during the interaction of an intense and ultrashort pulse with a dielectric material. It is a challenging task, due to the competition between many different elementary physical mechanism, all occurring at sub-picosecond or femtosecond time scale: electron-phonon interaction, elastic and inelastic electron-electron scattering - including impact ionization, formation of transient or permanent defect states, exciton self-trapping, exciton-exciton interaction, etc. The direct observation of these processes is beyond the capacity of traditional time resolved femtosecond experiment due to a lack of temporal resolution. To encompass this intrinsic difficulty, we have carried out experiments with a double exciting pulse scheme. Under appropriate conditions, we can control independently the two key parameters: plasma density and temperature. Then, using time resolved interferometry as a probe, we could directly observe for the first time an electronic avalanche induced by a laser pulse in a solid, namely crystalline SiO2 [1]. A complete modeling, using multiple rate equation and taking into account the laser propagation, allows to fully describe the experimental results. In materials where exciton self-trapping does not occur, no evidence of electronic avalanche is found, and the lifetime of excited carrier is much longer [2]. Our investigations on a large set of different materials (NaCl, KBr, CaF2, etc) provide a strong indication that a link exists between two apparently opposite relaxation mechanisms, exciton self-trapping and laser induced impact ionization and avalanche.

The relaxation of excited carriers in α–Al2O3 is complex, depending for instance on the type of ionizing radiation. Using femtosecond time-resolved absorption spectroscopy, we can induce a controllable excitation density on a wide range, and follow the relaxation dynamics from 30 fs to 7 ns. We show that the excited carrier decay is non-exponential: it is dependent on the pump intensity, i.e. on the initial carrier concentration. We describe the relaxation as a two-steps process, involving the trapping of initially free electron–hole pairs, followed by recombination. A numerical model taking into account the initial electronic excitation by multiphoton absorption and the subsequent relaxation allows to quantitatively reproduce the amplitude of the measured absorption and its temporal evolution.

Solids irradiated by energetic photons can be eroded in two modes, depending on the radiation intensity. High average, low-peak power sources, e.g., synchrotron radiation and high-order harmonics, induce desorption of the material at a low etch rate. In contrast, high-peak-power radiation from extreme ultraviolet and x-ray lasers usually causes a massive removal of the material even by a single shot. In this contribution, an effective material erosion is reported in PMMA exposed to multiple accumulated pulses generated by the free-electron x-ray-laser Linac Coherent Light Source (LCLS, tuned at a photon energy of 830 eV in this study, operated in Menlo Park at Stanford, CA, USA) at a fluence below the single-pulse ablation threshold. The effect is caused by polymer-chain scissions initiated by single photons carrying enough energy to break the C-C bounds. High efficiency of the erosion is supposed to occur due to a correlation of the single-photon effects. The subthreshold damage exhibits a nonlinear dose dependence resulting from a competition between chain scissions and cross-linking processes. The cross-linking is proven by Raman spectroscopy of the irradiated polymer. Two theoretical models of the x-ray free-electron-laser-induced erosion are suggested, which provide an excellent agreement with the experimental results.

When an intense ultrashort light pulse – in the visible domain- interacts with a wide band gap dielectric, a plasma can be generated by non-linear photoexcitation of carrier from the valence band to the initially empty conduction band. These carriers can be further excited in the conduction band, leading to an increase of their energy distribution, and thus of the amount of energy transferred to the material. If this deposited energy exceeds some critical threshold, permanent modification like damage or ablation may take place. The two key parameters determining the energy deposition are the density and the temperature of the plasma. In this presentation, we wish to demonstrate that a sequence of double pulse can be used to better control these two parameters, and thus to optimize energy deposition and facilitate for instance ablation of insulators and semi-conductors in the VUV domain. First, in the visible domain, using the second harmonic and the fundamental of a Ti-Sa laser, we show that time resolved double pump- and probe experiment allows to directly observe the sequence of events carrier excitation/ carrier heating, provided the parameters (energy, duration, delay) are appropriately chosen. Then, the ablation threshold (due to the first pulse) is dramatically reduced by the presence of the second pulse, while the characteristic of ablation are still determined by the first pulse [1]. Finally, new information regarding the excitation mechanisms, in particular impact ionization and avalanche are obtained [2]. In the second part, we show that this double pulse technique can be extended in the VUV domain. Using high order harmonics of a Ti-Sa laser (harmonic 25, ie. wavelength of 32 nm), whose intensity of far too low to damage any material, we could observe direct ablation of a dielectric, namely quartz, a-SiO2, when the VUV pulse if followed by an IR pulse, whereas no effect is observed with each of these pulses. [1] Guizard, S.; Klimentov S.; Mouskeftaras A. ; Fedorov N., Geoffroy G.; Vilmart G., Applied Surface Science 336, p. 206, 2015. [2] A Mouskeftaras, S Guizard, N Fedorov, S Klimentov Applied Physics A 110 (3), 709-715, 2013.

Due to lattice disorder, glassy semiconductors have energy levels in their bandgaps (gap states) that can be populated by sub-gap illumination or due to recombination of photo-excited carriers. The non-linear optical (NLO) response induced by high-intensity femtosecond (fs) laser pulses develops together with the gap states excitation. These effects in non-crystalline semiconductors have not yet been well studied. In this work, we use two realizations of the pump-probe method [1,2] to study the NLO response of chalcogenide glassy semiconductors of the systems As 40 S x Se 1-x and As 40 Se x Te 1-x with fs resolution in time upon illumination by fs laser pulses with the peak wavelengths λ p of 0.79 and 1.57 μm.

We report the first direct observation of laser induced electronic avalanche in a wide bandgap dielectric, namely silicon dioxide (SiO2). A double pulse excitation scheme allows to independently control the plasma density and temperature. Under appropriate conditions, the sequence laser heating-impact ionization can be separated from other photoionization mechanisms, and lead to an obvious increase of the excited carrier density observable by time resolved interferometric measurement of the dielectric function. A model taking into account the non-linear photoexcitation and the dynamics of excited carrier submitted to the heating laser field is used to describe the evolution of the dielectric function and extract quantitative information about the different mechanism efficiency. The interaction of intense light pulses with transparent materials is a domain of research with a long history which is however more active than ever, and this revival concerns both applied science and fundamental research. It has been shown in the last decade that femtosecond lasers can be used to drill, ablate, cut, or more importantly, to permanently modify – in 3 dimensions the optical properties of glasses, giving rise to numerous applications in photonics, data storage, microfluidics, etc [1]. As an example, the paper relating the discovery of selfstructuring at nanometer scale in the bulk of silica irradiated by femtosecond laser pulses [2] has been cited more than 1000 times. This is because the nano-structuring allows to inscribe and modulate birefringent properties in glasses, hence opening a huge field of application, like 5D data storage [3]. Very recently, another attractive domain has emerged, linked to the possibility to modulate, by using extremely short few cycleslaser pulses, the optical or insulating properties of dielectric materials [4,5]. Because the period of optical fields is on the order of a femtosecond, the current switching and its control by an optical field may pave a way to petahertz optoelectronic devices [6, 7]. Last but not least, we will mention another active field of research, which is the generation of high order harmonics in transparent solids – for a review see ref [8] while very recently, laser amplification could be observed in an excited Sapphire sample [9]. The shared issue of all this domain is a better knowledge of electronic excitation and relaxation processes in the solid during and immediately after the exciting pulse. It is a challenging task, due to the competition between many different elementary physical mechanism, all occurring at sub-picosecond or femtosecond time scale: electron phonon interaction, elastic and inelastic electron-electron scattering including impact ionization, formation of transient or permanent defect states, exciton self-trapping, exciton-exciton interaction, etc. The direct observation of these processes is beyond the capacity of traditional time resolved femtosecond experiment due to a lack of temporal resolution. In this work, we tackle the problem with an alternative technique, using a pump-probe scheme involving a double exciting pulse configuration. An appropriate choice of the characteristics of these two pulses allows to play with the two main parameters of the excited state: plasma density and plasma temperature. The evolution of the solid’s dielectric constant is probed during and after the two pulses by time resolved Fourier transform interferometry. The experimental results and numerical simulations support evidence for the first direct observation of laser induced avalanche in a wide band gap dielectric. The schematic of experimental is drawn on figure 1. The exit of a chirped pulse amplified TiSa laser is split in two parts, which are sent into two separate compressors. One beam, optimally compressed to provide the shortest possible pulses is again split to generate the probe pulse and, after frequency doubling, the first pump pulse. The duration of the second pump pulse, at 800 nm, is expanded for two purposes: reduce the photoexcitation probability, and fit the lifetime of free carrier in SiO2, which are known to form self-trapped excitons with a trapping rate of 150 fs [10,11]. The probing part of the setup is built for measuring the variation of the optical constant of the solid during and after the exciting – pump –pulse. The probe beam is going through the sample at varying delay. Then it is split in two parts in a Michelson interferometer, to get two identical probe beams. The two mirrors of this Michelson interferometer are slightly shifted, and their distance to beam splitter is also slightly different. Finally, the image of the entrance surface of the sample is formed with a single lens at the entrance of a spectrometer. This imaging scheme is set such that what is passing through the entrance slit of the monochromator corresponds to an unperturbed part of the beam for the first pulse (reference), and the perturbed part of the beam (which has crossed the excited region of the sample) for the second pulse (signal). These two beams do interfere at the exit of the monochromator. This interference pattern is recorded with a CCD camera and analyzed with a Fourier transform algorithm, allowing to extract the phase shift and the change of fringe contrast, thus giving access to the modification of the refractive index change induced by the pump pulses. A first, double pump interferometry measurement is displayed on Figure 2. The time evolution of the phase shift (and absorption) induced by a single pulse at 400 nm (blue curve), at 800 nm (red curve) and when both pulses are applied (black curve). When the sample is excited by the second harmonic only, three parts can be distinguished: first a positive phase shift due to Kerr effect, then a negative phase shift, due to excited carrier, is visible. The population of free carriers decays rapidly, and the signature of the self-trapping process leads to a stable positive phase shift, the third and last part of this curve. The red curve is obtained with the pulse at 800 nm alone. Again, a large positive peak due to Kerr effect is observed. This Kerr signal allows us to check the time position and width of this second pulse. Its intensity is too low to induce a significant excitation density, and no negative phase shift is observed under these conditions. Finally, the black curve is obtained when both pulses are impinging the sample. Similarly, we observe the sequence of positive phase shift, in that case with a double peak due to the two exciting pulses, followed by a short lived negative phase shift which becomes finally positive while excited carriers are self-trapping. The most remarkable feature is an obvious increase of the signal after the Kerr effect when the two pulses are present. The increase is about a factor three for the final positive phase shift, and about a factor 2 for the negative phase shift. The difference between these two factors is essentially due to the Kerr effect which partially overlaps and hides the initial negative phase shift. We claim that this increase of measured phase shift induced by the sequence of two pump pulses is a first direct observation of an increase of excited carrier density due to laser induced avalanche. Indeed, under our experimental conditions, the only mechanism that can lead to such an increase of the excited carrier density is the following set of events: free carriers excitation by the first pulse heating of these carrier by the second pulse impact ionization: collision between highly excited electrons in the CB and electrons from the top of the valence band, giving two “low energy” carriers in the conduction band. This process can be repeated during the duration of the second pulse, hence the observed increase for the population in the conduction band that we can clearly identify in our double pulse data. finally, all excited carriers coming from photoexcitation by the first pulse or from impact ionization, will form self-trapped excitons. To test this hypothesis, we have increased the delay between the two pump pulses, up to 400 fs. The same set of tree temporal pump and double pump scans is reported on Figure 2 (bottom part). The increase of the signal, which can only be seen in the final positive phase shift, is strongly reduced. This behavior is interpreted as follows: with this larger delay between the two pump pulses, a larger fraction of carriers has already been trapped before the second heating pump impinges the sample. These trapped carriers cannot absorb photons from the second pump pulse, and thus do not contribute to the heating-impact ionization or avalanche mechanism. In order to get more quantitative information from these experimental data, we have performed numerical simulations based on the multiple rate equation model first introduced by B. Rethfeld [12]. The various populations are described in the following fashion: the valence band is described as a single energy level with an electron density ρvb and the conduction band, as a series of energy levels of electron density ρi for level i. The amount of energy separating each of these levels is set to be the one of a photon from the IR pump. Finally, a level is added in the forbidden band to model the population of STE. The set of equations governing the population of the conduction band levels is displayed below: ∂ρ1 ∂t = ρvb(σ6IIR 6 + σ3IUV 3 + σcrossIUV 2 IIR 2 ) − (σheat,IR IIR hνIR + σheat,UV IUV hνUV ) ρ1 + 2?̃? ρvb ρvb,i ρk − ρ1 τste + ρsteσsteIIR nste ∂ρ2 ∂t = σheat,IR IIR hνIR (ρ1 − ρ2) − σheat,UV IUV hνUV ρ2 − ρ2 τste

An analysis of the results of measurements by using the pump - probe method with a femtosecond resolution in time and computer simulation of the charge carrier kinetics have revealed two types of a nonlinear optical response in samples of chalcogenide glasses belonging to the As - S - Se system, irradiated by 50-fs laser pulses with a wavelength of 0.79 mu m. The difference in the nonlinear dynamics is due to the difference in the photoexcitation character, because laser radiation can be absorbed either through bound states in the band gap or without their participation, depending on the ratio of the pump photon energy to the bandgap energy.

In this Letter, a novel method to evaluate nonlinear refractive index using time-resolved digital holographic microscopy is introduced. To demonstrate the viability of the method, cross-correlative nonlinear refractive index values for sapphire are measured experimentally: 2.75·10-20  m2/W at 1030 nm and 4.10·10-20    m2/W 515 nm wavelengths. The obtained results for sapphire are compared to those available in literature obtained by other methods.

Understanding the dynamics of excited carriers in wide band gap materials is a requirement to describe a broad range of physical mechanisms such as scintillator response, radiation induced damage of crystals, or laser-induced breakdown in optical materials and coatings. The difficulty arises from the competition between all the different relaxation channels: electron-phonon collisions, impact ionization, exciton and transient or permanent defects formation. Ultrashort laser pulses are ideal tool to investigate transparent materials since they allow to induce a large excitation density, and provide a temporal resolution high enough to track in real time the carrier relaxation. Two results concerning material which are extremely important for numerous application, namely silica (SiO2) and sapphire (Al2O3), and using different techniques, will be presented. First, in Al2O3, we have measured in a broad temporal range – from 30fs to 8 ns - the absorption induced by photo-excited carriers using time revolved absorption spectroscopy. By changing the intensity of the pump pulse, and thus the initial excitation density, we could measure the induced absorption on more than two orders of magnitude and demonstrate that the carrier relaxation dynamics exhibit a complex decay, and strongly depends on the initial density of excited carriers. We have developed a two steps model based on rate equations and taking into account the laser damping, which allows to fully reproduce the decay and the amplitude of the measured absorption. We demonstrate that in sapphire the electrons are mobile and can recombine with any hole. With this experiment and our modelling we can explain for instance the complex decay of luminescence observed when sapphire is irradiated with heavy ions or VUV photons. In SiO2, an important problem related to optical breakdown is the impact ionization which can lead to avalanche: electron excited by an intense laser can gain high kinetic energy in the conduction band and collide with valence electron (impact ionization) thus multiplying the excited carrier density. By using a sequence of double pump pulse we could control independently the two key parameters: plasma density and temperature. Under appropriate conditions, using time resolved interferometry as a probe, we could directly observe for the first time an electronic avalanche induced by a laser pulse. Again a complete modeling, using multiple rate equation and taking into account the laser propagation,; allow to completely describe the experimental results.

Magnitudes of the non-linear coefficients of absorption and refraction have been evaluated near the bandgap wavelengths of chalcogenide glasses of the system As-S-Se by using the interferometric pump-probe method and are compared with literature data. Photo-excited plasma dynamics and long-time scale variation of the dielectric constant have been studied by comparison with the results of numerical modelling of the behaviour of the glasses when heated by the ultra-short laser pulses.

We provide a nonperturbative theory for photoionization of transparent solids. By applying a particular steepest-descent method, we derive analytical expressions for the photoionization rate within the two-band structure model, which consistently account for the $selection$ $rules$ related to the parity of the number of absorbed photons ($odd$ or $even$). We demonstrate the crucial role of the interference of the transition amplitudes (saddle-points), which in the semi-classical limit, can be interpreted in terms of interfering quantum trajectories. Keldysh's foundational work of laser physics [Sov. Phys. JETP 20, 1307 (1965)] disregarded this interference, resulting in the violation of $selection$ $rules$. We provide an improved Keldysh photoionization theory and show its excellent agreement with measurements for the frequency dependence of the two-photon absorption and nonlinear refractive index coefficients in dielectrics.

Simultaneous time-and-space resolved reflectivity and interferometric measurements over a temporal span of 300 ps have been performed in fused silica and sapphire samples excited with 800 nm, 120 fs laser pulses at energies slightly and well above the ablation threshold. The experimental results have been simulated in the frame of a multiple-rate equation model including light propagation. The comparison of the temporal evolution of the reflectivity and the interferometric measurements at 400 nm clearly shows that the two techniques interrogate different material volumes during the course of the process. While the former is sensitive to the evolution of the plasma density in a very thin ablating layer at the surface, the second yields an averaged plasma density over a larger volume. It is shown that self-trapped excitons do not appreciably contribute to carrier relaxation in fused silica at fluences above the ablation threshold, most likely due to Coulomb screening effects at large excited carrier densities. For both materials, at fluences well above the ablation threshold, the maximum measured plasma reflectivity shows a saturation behavior consistent with a scattering rate proportional to the plasma density in this fluence regime. Moreover, for both materials and for pulse energies above the ablation threshold and delays in the few tens of picoseconds range, a simultaneous ``low reflectivity'' and ``low transmission'' behavior is observed. Although this behavior has been identified in the past as a signature of femtosecond laser-induced ablation, its origin is alternatively discussed in terms of the optical properties of a material undergoing strong isochoric heating, before having time to substantially expand or exchange energy with the surrounding media.

Two time-resolved experimental methods have been used for characterization of the non-linear optical response of chalcogenide glasses of the system As-S-Se-Te in IR and THz ranges upon excitation by femtosecond laser pulses at 800 nm wavelength. Photoinduced conductivity and refractivity were studied by using a rate equation model.

We investigate the mechanisms involved in the modification of dielectric materials by ultrashort laser pulses. We show that the use of a double pulse (fundamental and second harmonic of a Ti–Sa laser) excitation scheme allows getting new insight in the fundamental processes that occur during the interaction. We first measure the optical breakdown (OB) threshold map (intensity of first pulse versus intensity of second pulse) in various materials (Al2O3, MgO, α-SiO2). Using a simple model that includes multiphoton excitation followed by carrier heating in the conduction band, and assuming that OB occurs when a critical amount of energy is deposited in the material, we can satisfactorily reproduce this evolution of optical breakdown thresholds. The results demonstrate the dominant role of carrier heating in the energy transfer from the laser pulse to the solid. This important phenomenon is also highlighted by the kinetic energy distribution of photoelectrons observed in a photoemission experiment performed under similar conditions of double pulse excitation. Furthermore, we show, in the case of α-SiO2, that the formation of self-trapped exciton is in competition with the heating mechanism and thus play an important role especially when the pulse duration exceeds a few 100 fs. Finally, also in quartz or silica, we observe that the initial electronic excitation plays a key role in the formation of surface ripples and that their characteristics are determined by the first pulse, even at intensities well below OB threshold. The consequence of all these experimental results in the domain of UV or VUV induce damage will be discussed. In particular we demonstrate the possibility to dramatically increase the ablation efficiency by VUV light by using such double pulse scheme.

Laser processing and machining of dielectrics is a growing field, involving increasingly complex laser temporal and spatial pulse shaping. Predicting and modeling the optimum pulse characteristic for a given application requires a detailed knowledge of all the elementary events involved during the interaction. To understand and observe these physical mechanisms in detail, we carry out different time resolved experiments: spectral interferometry, holography, reflectivity change and photoelectron spectroscopy. Thus, we can measure in real time the excitation density achieved in the solid and the following relaxation of excited carriers. Since many processes (non-linear excitation, impact ionization, modification of pulse shape and propagation) arise during the pump laser pulse itself, usual pump-probe experiments are not capable to distinguish and directly observe them. To encompass this difficulty, we used a flexible double pump scheme [1], allowing modulating the excitation density and controlling the carrier heating steps.

Traditional methods of nonlinear refraction index (n2) measurements assume it as a constant measure. Keeping in mind its possible variation in time we explore an alternative approach of time-resolved digital holography for such measurements.

Nonlinear interaction of electronic excitations has been shown to play a remarkable role in luminescent material performance. Recently, it has been intensely studied in semiconductors and wide-gap scintillators. The nonlinear interaction is a cause of the nonproportionality of scintillator response to the energy of the absorbed ionizing radiation and can limit its energy resolution, which results in crucial deterioration of the scintillator functionality. The type and degree of nonproportionality are material specific and depend little on impurity content or sample treatment. Theoretical models have been developed, which explain the effect of scintillator response nonproportionality by the interplay between the density and mobility of charge carriers created in a track of ionizing radiation [1-3]. Energy losses in relaxation processes and the final distribution of excited luminescence centres in a material are determined by a number of processes including thermalisation of hot carriers, their recombination, interaction, trapping at defects and impurities and exciton formation. In the present contribution, we demonstrate how the mutual interaction of small-radius Frenkel excitons created at high densities by ionizing radiation or intense laser pulses can reveal itself in the luminescence characteristics of a luminescent material. It is shown that the related phenomena observed in various materials under X or γ rays are similar to those detected under UV-XUV excitation, which allows their theoretical description proceeding from the same principles and their experimental study in substantially better controlled experimental conditions of excitation by ultrashort laser pulses with well-defined timing and spatial parameters. A remarkable effect of saturation of excitonic absorption was discovered under excitation by ultrashort UV pulses from the energy region of phonon-assisted electronic transitions. The changes in the luminescence decay kinetics and various saturation effects in excitonic and impurity luminescence induced by high-density or high-energy excitation are demonstrated and theoretically modelled for several wide-gap materials. The experimental results of this contribution originate mainly from the study of time-resolved luminescence in tungstate crystals of different structures, wolframites CdWO4 and ZnWO4, and homologous scheelites CaWO4, SrWO4, BaWO4, by using femtosecond UV-XUV sources such as free-electron lasers, tuneable femtosecond lasers, high-order harmonic generation systems. It is shown that the emission decay and saturation can be described within the frame of Förster’s dipole-dipole interaction model on the basis of theoretical statements developed in [1]. The importance of studying exciton-exciton interactions is demonstrated by revealing a direct link between the Förster interaction radius and nonproportionality of the material light response. A good correlation between the radius of cation, radius of exciton and radius of Förster interaction is shown for the sequence of homologous scheelites CaWO4, SrWO4, BaWO4. References: A. N. Vasil’ev, IEEE Trans. Nucl. Sci. 55, 1054 (2008). G. Bizarri, W. W. Moses, J. Singh et al., J. Appl. Phys. 105, 044507 (2009). X. Lu, Q. Li, G. A. Bizarri et al., Phys. Rev. B 92, 115207 (2015).

An interferometric pump-probe method with the femtosecond resolution in time is used to study the third-order non-linear optical response and photosensitivity of a variety of chalcogenide glasses of the system As-S-Se.

We investigate the mechanisms involved in the modification of dielectric materials by ultrashort laser pulses. We show that the use of a double pulse (fundamental and second harmonic of a Ti–Sa laser) excitation allows getting new insight in the fundamental processes that occur during the interaction. We first measure the optical breakdown (OB) threshold map (intensity of first pulse versus intensity of second pulse) in various materials (Al2O3, MgO, α-SiO2). Using a simple model that includes multiphoton excitation followed by carrier heating in the conduction band, and assuming that OB occurs when a critical amount of energy is deposited in the material, we can satisfactorily reproduce this evolution of optical breakdown thresholds. The results demonstrate the dominant role of carrier heating in the energy transfer from the laser pulse to the solid. This important phenomenon is also highlighted by the kinetic energy distribution of photoelectrons observed in a photoemission experiment performed under similar conditions of double pulse excitation. Finally we show, in the case of α-SiO2, that the initial electronic excitation plays a key role in the formation of surface ripples and that their characteristics are determined by the first pulse, even at intensities well below OB threshold.