Laser-induced periodic surface structures (LIPSS) (ripples) with different spatial characteristics have been observed after irradiation of single-crystalline zinc oxide surfaces with multiple linearly polarized femtosecond pulses (150–200 fs, 800 nm) in air. For normal incident laser radiation, low spatial frequency LIPSS (LSFL) with a period (630–730 nm) close to the wavelength and an orientation perpendicular to the laser polarization have been found in the fluence range between ∼0.7 and ∼0.8 J/cm2 and predominantly for pulse numbers up to N=100. For lower fluences (0.5–0.7 J/cm2), a sharp transition from the LSFL features toward the formation of high spatial frequency LIPSS (HSFL) appears at any given pulse number below N=100. The HSFL are always parallel to the LSFL, exhibit spatial periods between 200 and 280 nm, and completely substitute the LSFL for pulse numbers N>100. Additionally, the influence of the angle of incidence has been studied experimentally for both LIPSS types revealing a different behavior. Experimental evidence for surface scattered second harmonic generation is presented in the regime of HSFL formation. Moreover, we will show that the HSFL structures on ZnO surfaces can be fully explained by an extension of the existing LIPSS theories if the photoexcitation of the dielectric material (affecting its transient optical properties) is considered in the frame of a simple Drude model along with the second harmonic generation at the irradiated surface. Based on our analysis, the current models of femtosecond laser-induced LIPSS are revisited and an explanation is proposed why HSFL are observed predominantly in the subpicosecond range for below band-gap excitation of dielectrics and semiconductors.

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica with multiple irradiation sequences consisting of five Ti:sapphire femtosecond (fs) laser pulse pairs (150 fs, 800 nm) is studied experimentally. A Michelson interferometer is used to generate near-equal-energy double-pulse sequences with a temporal pulse delay from −20 to +20 ps between the cross-polarized individual fs-laser pulses (∼0.2 ps resolution). The results of multiple double-pulse irradiation sequences are characterized by means of Scanning Electron and Scanning Force Microscopy. Specifically in the sub-ps delay domain striking differences in the surface morphologies can be observed, indicating the importance of the laser-induced free-electron plasma in the conduction band of the solids for the formation of LIPSS.

Abstract:We report studies of multiphoton mechanisms of plasmon excitation and their influence on the femtosecond-laser induced subwavelength 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.

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 5.5×1010 W/cm2. 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.

Third harmonic generation (THG) of femtosecond laser pulses in sputtered nanocrystalline TiO2 thin films is investigated. Using layers of graded thickness, the dependence of THG on the film parameters is studied. The maximum THG signal is observed at a thickness of 180 nm. The corresponding conversion efficiency is 26 times larger compared to THG at the air-glass interface. For a demonstration of the capabilities of such a highly nonlinear material for pulse characterization, third-order autocorrelation and interferometric frequency-resolved optical gating (IFROG) traces are recorded with unamplified nanojoule pulses directly from a broadband femtosecond laser oscillator.

We report the generation of programmable two-dimensional arrangements of ultrashort-pulsed fringe-less Bessel-like beams of extended depth of focus (referred to as needle beams) without truncating apertures. A sub-20-fs Ti:sapphire laser and a liquid-crystal-on-silicon spatial light modulator (LCoS-SLM) of high-fidelity temporal transfer in phase-only operation mode were used in the experiments. Axicon profiles with ultrasmall conical angles were approximated by adapted gray scale distributions. It was demonstrated that digitized image information encoded in amplitudephase maps of the needle beams is propagated over considerably large distances at minimal cross talk without the need for additional relay optics. This experiment represents a physical realization of Saari's proposal of spatio-temporally nondiffracting "flying images" on a few-femtosecond time scale.

The recently introduced concept of radially non-oscillating, temporally stable ultrashort-pulsed Bessel-like beams we referred to as needle beams is generalized to a particular class of highly localized wavepackets (HLWs). Spatio-temporally quasi-nondiffracting pulses propagating along extended zones are shaped from Ti:sapphire oscillator radiation with a spatial light modulator and characterized with spatially resolved second order autocorrelation. Few-cycle wavepackets tailored to resemble circular disks, rings and bars of light represent the closest approximation of linear-optical light bullets known so far. By combining multiple HLWs, complex pulsed nondiffracting patterns are obtained.

Particle acceleration and X-ray generation in different nano-structured targets irradiated by high intensity laser pulses of high contrast have been studied. It is found that maximal energy of fast particles and its directionality can be significantly enhanced, by choosing nano-structured targets. Generation and propagation of fast electrons in laser targets consisting of nano-wires are studied. Such targets exhibit a large conversion of laser energy into electron kinetic energy. An electron bunch can propagate a long distance and can be focused by bringing wires together. The results of theory and simulations were compared with the experimental data and have shown a reasonable consistency. IntroductionLaser driven nano-plasmonics deals with optical processes in plasmas at relatively low intensities and on nanoscale, i. e. on the order of or smaller than the wavelength of the laser radiation [1]. The underlying physical process is connected with a laser field enhancement due to proper nano-structuring of a target. Laser-matter interaction including nanoscale confinement of radiation and its transformation provides attractive opportunities for both, fundamental research and technological applications. Material processing uses femtosecond laser pulses exceeding the material ablation threshold to drill micro holes, to realize micro cutting or to selectively remove a particular substance. Such an extreme precision processing adds high value [2]. Due to the ultrashort pulse durations the results benefit by very clean and defined processing areas with negligible structural changes around. This is important in surgery (e. g. ophthalmology, dermatology, dentistry), for instance, when the ultrashort laser pulse irradiation enables clean surgery without damaging the side areas of the processed tissue [3].Laser nano-plasmons produced by structured surfaces have recently gained growing attention in laser plasma ion acceleration physics, as it can significantly enhance the acceleration mechanism. Today's research on laser driven particle acceleration (ions and electrons) promises future fast ion sources for different applications. Here, a laser at relativistic intensity interacts with thin solid foils, which can be well characterized and easily handled [4]. First, the laser causes an acceleration of the ionized electron distribution, which subsequently accelerates ions up to high kinetic energies. The low emittance of such ion beams is a striking feature [5]. Nevertheless, up to now the achieved acceleration efficiency is low (around a few to ten percent of the laser energy), even if the target thickness is optimized with respect to the laser parameters, thus current research is focusing on its optimization [6].