Ruediger Grunwald

Femtosecond laser-induced periodic surface structures revisited: A comparative study on ZnO

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 b...

Second-harmonic efficiency of ZnO nanolayers

Pure and doped nanocrystalline ZnO films of 0.1–1.0 μm thickness were prepared by spray pyrolysis techniques. Structural properties of these thin films were investigated by electron microscopy, force microscopy, x-ray diffractometry, ellipsometry, and optical spectroscopy. Using Ti:sapphire laser pulses, the second-harmonic efficiency of the films was measured. A correlation between measured efficiency and the grain shape in the polycrystalline layers is reported. Moreover, measurements of the angular dependence of the second-harmonic intensity are indicative of the dominant role of bulk effects over surface effects for the second-harmonic generation in such films.

Formation of laser-induced periodic surface structures on fused silica upon multiple cross-polarized double-femtosecond-laser-pulse irradiation sequences

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.

Efficient laser operation with nearly diffraction-limited output from a diode-pumped heavily Nd-doped multimode fiber

Laser performance of a diode-pumped Nd-doped multimode phosphate-glass fiber is reported. The selection of the fundamental mode in a multimode fiber is demonstrated, resulting in laser emission that is close to diffraction limited and that is nearly independent of the pump power. The heavily Nd-doped fibers with core diameters of 100 microm and lengths between 13 and 32 mm deliver as much as 130 mW of cw output power at 1.053 microm. A total efficiency of 31% with respect to launched pump power has been achieved.

Growth, optical characterization, and laser operation of epitaxial Yb:KY(WO4)2∕KY(WO4)2 composites with monoclinic structure

Epitaxial monoclinic double tungstate laser crystals were grown with high crystalline quality. Based on these Yb-doped composites, laser operation was demonstrated. Continuous-wave laser emission of a Yb:KYW∕KYW crystal was achieved at 1030nm. The 25‐μm-thin Yb:KYW layer was pumped at wavelengths near 980nm by a Ti:sapphire laser. A maximum output power of 40mW was obtained at room temperature.

Efficient second harmonic generation in ZnO nanorod arrays with broadband ultrashort pulses

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.

Extended-area nanostructuring of TiO2 with femtosecond laser pulses at 400 nm using a line focus.

An efficient way to generate nanoscale laser-induced periodic surface structures (LIPSS) in rutile-type TiO(2) with frequency-converted femtosecond laser pulses at wavelengths around 400 nm is reported. Extended-area structuring on fixed and moving substrates was obtained by exploiting the line focus of a cylindrical lens. Under defined conditions with respect to pulse number, pulse energy and scanning velocity, two types of ripple-like LIPSS with high and low spatial frequencies (HSFL, LSFL) with periods in the range of 90 nm and 340 nm, respectively, were formed. In particular, lower numbers of high energetic pulses favour the generation of LSFL whereas higher numbers of lower energetic pulses enable the preferential creation of HSFL. Theoretical calculations on the basis of the Drude model support the assumption that refractive index changes by photo-excited carriers are a major mechanism responsible for LSFL. Furthermore, the appearance of random substructures as small as 30 nm superimposing low spatial frequency ripples is demonstrated and their possible origin is discussed.

Femtosecond-laser-induced quasiperiodic nanostructures on TiO2 surfaces

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 (N=10), 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 J/cm2, 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...

Multiphoton excitation of surface plasmon-polaritons and scaling of nanoripple formation in large bandgap materials

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.

Highly efficient THG in TiO 2 nanolayers for third-order pulse characterization

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.