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.

In-situ infrared studies performed with operating Ru-complex-sensitized wet solar cells using a total reflection technique reveal that the ruthenium complex (both tri- and mononuclear) attached to TiO2 is photoelectrochemically transformed and irreversibly consumed under conditions of insufficient regeneration by iodide or from the oxide within the nanocrystalline TiO2 pores. The sensitizer [(Ru(bpy)2(CN)2)2Ru(bpca)2]2- (bpy is 2,2‘-bipyridine, bpca is 2,2‘-bipyridine-4,4‘-dicarboxylate) decomposes into fragments; one of them was identified to be Ru(bpy)2(CN)2. For the sensitizer Ru(bpca)2(SCN)2, it is shown that a molecular fragment (absorbing at 2013 cm-1) is generated which is diffusing out of the nanostructured TiO2 layer. Due to its correlation with the photocurrent density, it is identified as a product of the oxidized sensitizer. Due to a high serial resistance introduced by the total reflection element and the resulting low fillfactor of the sensitization cell during in-situ measurements, only small photocurrents (5−10 μA cm-2) could be passed through the sensitizing interface. Since the rate of product formation should be proportional to the ratio of photocurrent density to iodide concentration, the iodide concentration was correspondingly reduced (1−10 mM) as compared to the conditions in a solar cell (10 mA cm-2, 1 M). This spectroscopic technique was developed because efforts to produce stable sensitization solar cells proved to be unsuccessful due to sealing problems. Our experiments do not seem to permit extrapolation to 107−108 electron transfer numbers for sensitizing Ru complexes, and real long-term testing is required for reevaluating long-term performance.

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.

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