The effect of surface roughness was studied on the formation of femtosecond laser-induced ripple structures for 4H-SiC. Of the two types of ripple structures, fine and coarse, the threshold fluence for the formation of the fine ripple was remarkably reduced on the surface roughened by intentional rubbing, while the threshold for the formation of the coarse ripple was not affected. The result suggests that the initial surface roughness plays a significant role only for the formation of fine ripples. A brief explanation will be given on the qualitative basis for the specific effect of surface morphology to the fine ripple formation.

We report on temperature dependence of excitonic photoluminescence ͑PL͒ from ZnO/͑Mg, Zn͒O multiple quantum wells ͑MQWs͒. Two kinds of MQWs having different barrier heights grown by laser molecular-beam epitaxy showed significantly different temperature dependences of PL spectra; in ZnO/Mg 0.27 Zn 0.73 O MQWs, the PL peak energy at 50-200 K was a monotonically increasing function of temperature, which was opposite to that ascribed by band gap shrinkage. Moreover, spectra taken at 95-200 K encompassed two peaks, both of which originated from recombination of localized excitons. The temperature-induced shift ͑redshift-blueshift-peak duplication-redshift͒ at 5-300 K is caused by a change in the exciton dynamics with increasing temperature due to inhomogeneity and the exciton localization effect. On the other hand, the corresponding dependence in ZnO/Mg 0.12 Zn 0.88 O MQWs ͑lower barrier height͒ was similar to that in bulk II-VI semiconductors.

We observed the formation of subwavelength periodic strained layers associated with nanovoids in the cross section of femtosecond laser-irradiated lines written inside 4H-SiC single crystals. Both conventional and high-voltage transmission electron microscopies were carried out for microstructural analyses. The cross section of the irradiated lines consists of four to six groups of fine periodic structures. Each group is composed of strained layers with a typical spacing of 150 or 300 nm. The layers extend along the irradiated lines, aligned parallel to the electric field of the laser light. Tiny voids approximately 20 nm in diameter are found in the layers.

This paper describes the formation and subsequent behavior of silver nanoparticles (NPs) observed by optical spectra, atomic force microscopy, and transmission electron microscopy in primary alcohols by nanosecond pulsed-laser irradiation (1064 and 532 nm, typically at 1 J/(cm 2 pulse)) of silver flakes. Effects of the carbon chain length of the solvents and the irradiation atmosphere of the solutions on the Ag NP formation were investigated. The effect of alcohol chain length in aerated solutions can be described as follows: (1) in shortchain alcohols such as methanol and ethanol, the NPs are extremely unstable and easily settled down to form precipitates by centrifugation treatment; (2) very stable NPs are formed with an appreciably smaller particle size distribution in alcohols with chain lengths from C-3 to C-5 than in alcohols with longer chain length than C-5; (3) the yield of NPs is dependent on the alcohol chain length. On the other hand, the yield of NPs is greater in Ar-and N 2 -saturated solutions than in aerated solutions. Additionally, the yield is similar regardless of the chain length, with smaller size distributions than those in air-equilibrated solutions. Oxygen molecules dissolved in the solvents are responsible for these observations. The oxygen effect consists of two parts: (1) the scavenging of electrons generated by the plasma formation and thermionic emission due to extremely high temperature under the ablation condition; (2) the formation of an oxide layer on the surface of particles that hampers further growth processes to form NPs. Furthermore, we observed the formation of string segments in evacuated ethanol due to coagulation and coalescence of bare metal particles, giving rise to the splitting of the plasmon band. Thus we demonstrated that the systematic change in the solvent and irradiation atmosphere can control the particle size and size distribution. The present findings may add a new aspect to better manipulate NP fabrication based upon the laser ablation method.

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