S. Guizard

Time-resolved study of laser-induced colour centres in

We report the investigation of the onset of an absorption band in the UV in crystalline quartz () 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 . 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.

Contrasted Behaviour of an Electron Gas in MgO, Al2O3 and SiO2

We report on the observation, by use of a sub-picosecond time-resolved phase-sensitive technique, of an electron gas pumped by an intense femtosecond laser pulse into the conduction band of three different wide-band-gap oxides. The free-carriers lifetime is measured to be two orders of magnitude longer in MgO and in Al2O3 than in SiO2.

Femtosecond multiphoton generation of the self‐trapped exciton in α‐SiO2

Nonlinear optical excitation of crystalline quartz with intense femtosecond UV pulses yields the 2.8 eV recombination luminescence of the self‐trapped exciton. The relation between the excitation and emission intensities reveals two‐ and three‐photon kinetics for photon energies of 4.4 and 4.0 eV at excitation densities below 1018 cm−3. These power laws are not sizably influenced by transient linear absorption, self‐focusing, and filamentation.

Desorption mechanisms in PMMA irradiated by high order harmonics

The recent development of intense sources in the XUV range (10-100 nm), such as X-ray laser, Free Electron Laser and High order Harmonics (HoH), allows the study of high flux processes and ultra-fast dynamics in various domains. At the SLIC facility of CEA-Saclay, we have built a gas-harmonic beamline to investigate the interaction of intense XUV pulse with solids. High Harmonics of an IR laser (Ti:Sa at 800 nm, 35 fs, 13 mJ/pulse, 1 kHz) are generated in a rare gas cell (Xe). The useful XUV range (40-60 nm) is selected with metallic filters. The harmonic beam is focused with a parabolic mirror to a 10 μm focal spot on sample, leading to a fluence per shot of up to 1 mJ/cm2 (within a typical 10 fs pulse duration). Studies aimed at understanding the damaging mechanisms caused by XUV irradiation on surface of various samples by systematically varying of fluence and exposure time. For PMMA irradiated in the desorption regime (fluence/shot ≤ 0.2 mJ/cm2), the surface presents craters whose profile depends on the dose (Grey [Gy] = 1 J/kg). The crater evolution proceeds from the competition between two main degradation processes, that is chain scission and cross linking. Namely, at low dose (≤ 1 GGy) polymer chain scission is followed by the blow up of the volatile, molecular fragments, forming the crater. At high dose (> 10 GGy) the broken chain-ends, in the near-surface layer of the remaining material, recombine by cross-linking, opposing desorption by surface hardening. In a recent experiment at LCLS FEL facility, PMMA was irradiated at high fluence; the cross-linking signature was identified from Raman spectroscopy. A kinetic model could be adapted for interpreting these original and very promising results.

Temperature effects in the multiphoton photoemission of laser irradiated α‐SiO2

The effects of the sample temperature on laser induced electron emission of α‐SiO2 is discussed. The sample was irradiated using the third harmonic of a Nd:YAG laser, with pulse durations of 30 ps, at intensities in the 109 W cm−2 range, leading to an electron emission due to multiphoton absorption. We measured both the total photocurrent and the photoelectron energy spectrum for temperatures ranging between room temperature and 250 °C. We observed a strong increase of the photocurrent, which is associated with the disappearance of the charging effect due to the holes left by the emission. We interpret this as a result of a thermally induced, trapping/detrapping, electron conductivity. This interpretation is based on the fact that hole conductivity is too small to account for our observations and that intrinsic electron conductivity does not show the correct temperature behavior. The increase of the photocurrent is, however, mainly due to an increase of the defect creation yield with the temperature. From...

Non-Linear Wave Propagation in α-Quartz

We present a theoretical investigation of the excitation density induced in the conduction band of α-quartz by an intense laser field in the TW/cm2 regime using the Maxwell time-independent propagation equation. Competition between two-photon absorption (TPA), self-focusing process (SFP), free-carriers defocusing process (FCDFP), Drude losses, and its consequences on the excitation density are analysed in detail.

Ultrafast breakdown of dielectrics: new insight from double pump-probe experiments (Conference Presentation)

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.

Time-Resolved Studies of Free Carriers in Insulators

Electronic excitation in insulators is known to originate numerous defects in these materials. Today, ultrashort laser pulses allow the observation of the different stages of electron relaxation and can bring new informations about these processes and especially about defect formation time. In the present set of experiments, interferometry in the frequency domain1 has been applied to three different wide band gap materials. The principle of the experiment and the laser description have been given in details elsewhere2, 3. Let us just recall that the sample is probed by two twin laser pulses separated by a fixed time delay, one impinging the solid before and the second after the pump pulse. A phase shift in the interference pattern appears in the frequency spectrum of the sequence of the two probe pulses. This phase shift, induced by the high intensity pump pulse, is proportional to the modification of the refractive index, and its temporal evolution is obtained by changing the delay between the pump pulse and the two probe pulses. The absorption of the second probe pulse can also be deduced from the variation of the fringe contrast. The laser pulses duration and wavelength are respectively 70 fs and 310 nm (4 eV photons) for the pump beam, 60 fs and 560 nm (2.2 eV) for the probe beam.

Picosecond and subpicosecond laser heating of electrons in the conduction band of SiO 2

Photoelectron spectroscopy on an α-quartz surface with intense subpicosecond laser pulses reveals strong differences between the spectra obtained with two different wavelengths. For intensities above 100 GW/cm2, a high-energy tail spreads out in the photoemission spectra at 620 nm, but no effect is observed at 310 nm. Multiple photon absorption by free electrons in the conduction band can explain the dependence of the spectra on the laser wavelength and intensity, and Monte Carlo simulations including such mechanisms are in good agreement with the experimental results.

Multiphoton generation of STE in α-SiO2 using subpicosecond UV laser pulses

Abstract Multiphoton STE generation in α-SiO 2 was investigated in the high intensity regime (from 0.1 to a few TW cm −2 ) using 250 fs UV pulses, for photon energies of 4.4 and 4 eV respectively, by monitoring the 2.8 eV luminescence of the STE. The corresponding two- and three-photon dynamics are observed except in the 4.4 eV case, for intensities in excess of 0.5 TW cm −2 . A theoretical study of the laser propagation, using a numerical solution of the Maxwell equation with non-linear terms (representing the effects of two-photon absorption and intensity induced self-focusing) leads us to the conclusions that propagation effects cannot account for this observation, except for the fact that it generates very high electron-hole pair densities (between 10 19 and 10 20 cm −3 ), so that interaction between neighbouring pairs could play a decisive role in the STE generation.