We report studies of multiphoton mechanisms of plasmon excitation and their influence on the femtosecond-laser induced sub-wavelength ripple generation in large-bandgap dielectric and semiconducting transparent materials. An extended Drude-Sipe formalism is applied to quantitatively estimate the real part of the dielectric function which is dependent on the carrier density. The theory is able to predict the ripple periods for selected materials in good agreement with the experimental observations. Possible limitations at very small spatial periods are also discussed.

Few-cycle high-contrast vortex beams with pulse durations around 8 fs were generated from a Ti:sapphire laser oscillator with a single diffractive–refractive component. Angular and temporal pulse properties were characterized with an advanced time-wavefront sensor. The temporal transfer indicates a fairly complete self-compensation.

Multiple quasi-Bessel beams are generated by transmission of sub-30-fs pulses from a Ti:sapphire laser through refractive thin-film microaxicon arrays. Time-integrated intensity distributions at several axial positions and for pulse durations of 26 and 12.5 fs reveal significant changes of contrast, envelope function, and spatial frequency spectrum in comparison with continuous wave data. Evidence is presented that strong space–time coupling results in a time-dependent interference zone.

High-spatial frequency, quasiperiodic structures (HSFL, Nanoripples) of 170 nm feature size were induced in rutile-type titanium dioxide surfaces by focused 150 fs Ti:sapphire laser pulses at wavelengths around 800 nm. The ripple formation is distinctly visible for numbers of pulses of $N=100–1000$ . At lower number of pulses $\left(N=10\right)$ , a significant surface roughening appears instead of ripples which is characterized by randomly meandering nanostructures. These observations confirm an essential contribution of early stage irregular material modifications to the dynamics of quasiperiodic ripple formation. The threshold fluence for ripple generation is estimated on the basis of the conventional theory of laser-induced surface structuring. The decrease in the threshold fluence from 0.34 to $0.24\text{J}/{\text{cm}}^2$ , as it was found for an increase in the number of pulses from $N=100$ to $N=1000$ , is attributed to a damage accumulation effect. Nanostructuring of spatially extended regions was enabled by utilizing a controlled sample translation at optimized energy and repetition rate of the laser pulses. A significant enhancement of the periodicity of the nanostructures was confirmed by two-dimensional Fourier transform of scanning electron microscopy data. At second harmonic wavelengths around 400 nm, the generation of nanoripples even on the sub-100 nm scale was demonstrated.

Needle beams are highly attractive for applications which take advantage from a spatial and temporal localization of photons. High intensities, high resolution and extended depth of focus lead to fundamental advances in the optical system performance. Ultrashort, fringe-free, self-reconstructing nondiffracting pulses with undistorted temporal transfer are obtained by generating truncated Bessel beams under self-apodization conditions. Nondiffracting Talbot self-imaging of needle beam arrays enables to transfer near field information to the Fraunhofer zone. With addressable arrays of needle beams, reconfigurable time-wavefront sensors are built up. Moreover, spatial light modulators and flexible axicons are used to realize structured, highly localized wavepackets, accelerating beams and nondiffracting images. GRAPHICAL ABSTRACT

Broadband frequency-doubling properties of $c$ -axis oriented zinc oxide (ZnO) nanorod arrays grown by low-temperature chemical bath method on glass substrate were studied. The maximum effective nonlinearity was found to be about 7.5 times higher than that of a type-I beta-barium borate crystal for a pump intensity of $5.5\times {10}^{10}\text{W}/{\text{cm}}^2$ . The angular dependence of second harmonic generation (SHG) was determined experimentally. The measured spectral profile of SHG was found to be in good agreement with theoretical simulations.