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.
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 these observations possible applications to the problem of low energy electron spectroscopy on insulating samples are discussed.
Potoconductive properties and relaxation of free charges in high pure natural and CVD diamond were investigated and compared in the spectral range 1 .5-30 eV using a variety of pulsed laser sources including harmonics of radiation of 50 fs Ti:Sa laser. The resulting spectra were compared to experimental data in surface photoemission and speculated in terms of free carriers multiplication, effect of the surface layer, saturation of absorption, trapping of carriers and the following charge transfer between deep electron centers.
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.
We provide a nonperturbative theory for photoionization of transparent solids, which consistently accounts for the selection rules related to the parity of the number of absorbed photons (odd or even). We derive closed-form analytical expressions for the photoionization rate within the two-band structure model. Our model exhibits excellent agreement with measurements for the frequency dependence of the two-photon absorption and nonlinear refractive index coefficients in sapphire and silica, two highly relevant materials for industrial applications. We demonstrate the crucial role of the interference of the transition amplitudes, which in the semiclassical limit can be interpreted in terms of interfering quantum trajectories that were disregarded in Keldysh's foundational work of laser physics [Keldysh, Sov. Phys. JETP 20, 1307 (1965)], resulting in the violation of selection rules.
