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Controlling the electron dynamics in matter by individual oscillations of light fields has recently led to the development of attosecond metrology. One of the phenomena resulting from the nonlinear response of materials in the strong-field interaction regime is coherent emission of high energy photons. High harmonic spectra carry the fingerprints of sub-cycle electronic motion and the energy structure of the studied system. Here we show that tailoring the waveform of the driving light by using a coherent combination with its third harmonic frequency allows to control the electron tunneling time within each half-cycle of the fundamental wave with attosecond precision. We introduce an experimental scheme in which we simultaneously monitor the modulation of amplitude and emission delays of high harmonic radiation and the excited electron population generated in crystalline silicon as a function of the relative phase between the {\omega}-3{\omega} fields. The results reveal unambiguously the connection between the dynamics of electron tunneling and high harmonic generation processes in solids.

Nonlinear photoionization of dielectrics and semiconductors is widely treated in the frames of the Keldysh theory whose validity is limited to small photon energies compared to the band gap and relatively low laser intensities. The time-dependent density functional theory (TDDFT) simulations, which are free of these limitations, enable to gain insight into non-equilibrium dynamics of the electronic structure. Here we apply the TDDFT to investigate photoionization of silicon crystal by ultrashort laser pulses in a wide range of laser wavelengths and intensities and compare the results with predictions of the Keldysh theory. Photoionization rates derived from the simulations considerably exceed the data obtained with the Keldysh theory within the validity range of the latter. Possible reasons of the discrepancy are discussed and we provide fundamental data on the photoionization rates beyond the limits of the Keldysh theory. By investigating the features of the Stark shift as a function of photon energy and laser field strength, a manifestation of the transient Wannier-Stark ladder states have been revealed which become blurred with increasing laser field strength. Finally, it is shown that the TDDFT simulations can potentially provide reliable data on the electron damping time that is of high importance for large-scale modeling., Comment: Main document: 8 pages, 4 figures. Suppl. Mat.: 10 pages, 5 figures, 1 table

Thin Cr films 28-nm thick deposited on glass substrates were processed by scanning low-intensity femtosecond laser pulses with energy well below single-pulse damage threshold. Two types of laser-induced periodic surface structures (LIPSS) were produced, depending on the scanning velocity, (1) parallel to laser light polarization with periodicity somewhat smaller than laser wavelength and (2) perpendicular to polarization with spatial period much smaller than wavelength. All structures are formed as protrusions above the initial film surface and exhibit a high degree of oxidation. To explain formation of the LIPSS and their conversion from one to another type, a rigorous numerical approach for modeling surface electromagnetic waves in thin-film geometry has been developed, which takes into account the change of optical properties of material due to laser-induced oxidation and porosity. The approach addresses the multiplicity of electromagnetic modes allowed for thin films. It has been found that the low spatial frequency LIPSS parallel to laser polarization, which are formed at low scanning velocities, are well described by the Sipe theory for surfaces of low roughness. The SEW mode responsible for high spatial frequency LIPSS formation at high scanning velocities has been identified. The mechanisms of optical feedback and transformation between types of LIPSS with scanning velocity have been proposed.

The physical mechanisms of the laser-induced periodic surface structures (LIPSS) formation are studied in this paper for single-pulse irradiation regimes. The change in the LIPSS period with wavelength of incident laser radiation is investigated experimentally, using a picosecond laser system, which provides 7-ps pulses in near-IR, visible, and UV spectral ranges. The experimental results are compared with predictions made under the assumption that the surface-scattered waves are involved in the LIPSS formation. Considerable disagreement suggests that hydrodynamic mechanisms can be responsible for the observed pattern periodicity.

The possibility to excite Surface Plasmon Polaritons (SPPs) at the interface between two media depends on the optical properties of both media and geometrical aspects. Specific conditions allowing the coupling of light with a plasmon-active interface must be satisfied. Plasmonic effects are well described in noble metals where the imaginary part of the dielectric permittivity is often neglected ("perfect medium approximation"). However, some systems exist for which such approximation cannot be applied, hence requiring a refinement of the common SPP theory. In this context, several properties of SPPs such as excitation conditions, period of the electromagnetic field modulation and SPP lifetime then may strongly deviate from that of the perfect medium approximation. In this paper, calculations taking into account the imaginary part of the dielectric permittivities are presented. The model identifies analytical terms which should not be neglected in the mathematical description of SPPs on lossy materials. These calculations are applied to numerous material combinations resulting in a prediction of the corresponding SPP features. A list of plasmon-active interfaces is provided along with a quantification of the above mentioned SPP properties in the regime where the perfect medium approximation is not applicable., Comment: 9 pages, 7 figures, regular paper

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 A

The mechanisms of ripple formation on silicon surface by femtosecond laser pulses are investigated. We demonstrate the transient evolution of the density of the excited free-carriers. As a result, the experimental conditions required for the excitation of surface plasmon polaritons are revealed. The periods of the resulting structures are then investigated as a function of laser parameters, such as the angle of incidence, laser fluence, and polarization. The obtained dependencies provide a way of better control over the properties of the periodic structures induced by femtosecond laser on the surface of a semiconductor material., Comment: 11 pages, 8 figures, accepted for publication in Journal of Applied Physics

We consider the case of surface irradiation by a small number of femtosecond laser shots leading to the formation of surface ripples. To explain this effect, we propose a numerical model that accounts for the following processes: (i) interference of the laser irradiation with an electromagnetic surface wave propagating on a silicon sample; (ii) free carrier formation and laser energy absorption; (iii) energy relaxation and electron-phonon coupling. We perform numerical calculations taking into account the interference of a surface wave with laser; and present the obtained simulation results in order to explain formation mechanisms of the experimentally observed patterns.

Laser induced periodic surface structures (LIPSS) are formed by multiple irradiation of femtosecond laser on a silicon target. In this paper, we focus and discuss the surface plasmon polariton mechanism by an analysis of transient phase-matching conditions in Si on the basis of a single pulse experiment and numerical simulations. Two regimes of ripple formation mechanisms at low number of shots are identified and detailed. Correlation of numerical and experimental results is good., Comment: 4 pages, 5 figures, EMRS Spring 2011 conference, Nice