Fabrication of 50-nm period gratings on GaN in air through plasmonic near-field ablation induced by ultraviolet femtosecond laser pulses.

We demonstrate the formation of a homogeneous nanograting with 50-nm period on GaN in air, using ultraviolet femtosecond (fs) laser pulses at 266 nm in the recently developed two-step ablation technique. The experimental results have shown that the ablation technique successfully controlled the spatial mode of surface plasmon polaritons (SPP) excited on the target surface and decreased the grating period in accordance with the short wavelength of fs laser pulses. Calculation for a model target reproduces well the laser-wavelength dependent periods, being in good agreement with the observed, and supports the mechanism for nanostructuring.

Mechanism of femtosecond-laser-induced periodic nanostructure formation on crystalline silicon surface immersed in water.

Focused on silicon surface in water, superimposed multiple shots of linearly polarized 800-nm, 100-fs, 10-Hz laser pulses at lower fluence than the single-pulse ablation threshold are shown to produce two kinds of periodic nanostructures with almost constant periods of 150 nm and 400 nm. Surface plasmon polaritons excited in the surface layer illustrates well the formation of nanostructures and its dynamic properties observed. Pump and probe measurements of the ultrafast change in surface reflectivity during the interaction have demonstrated that the multiple low-fluence fs pulses are crucial to the nanostructuring through the accumulation of non-thermal bonding structure change and the subsequent nanoscale ablation.

Retrieving angular distributions of high-order harmonic generation from a single molecule.

We present a novel method to retrieve angular distributions of high-order harmonic generation from a single molecule. This technique uses an iterative procedure based only on experimental results of time and angle-dependent harmonic signals, and no actual shape of molecular orbital is assumed. The molecular axis distribution in a target gas can simultaneously be deduced in this procedure.

Measurement of molecular rotational temperature in a supersonic gas jet with high-order harmonic generation.

We apply high-order harmonic generation to sensitive measurements of the molecular rotational temperature in a thin supersonic gas beam. The method uses nonresonant pump and probe femtosecond laser pulses to generate harmonic radiation from coherently rotating molecules. The rotational temperature of molecules can be derived accurately with high spatial and temporal resolutions from the Fourier spectrum of time-dependent signals.

Dynamic properties of angle-dependent high-order harmonic generation from coherently rotating molecules.

High-order harmonic generation from coherently rotating N2 and O2 molecules has been observed for different alignment angles in a pump and probe experiment using femtosecond laser pulses. The results obtained are in excellent agreement with those calculated using a recently developed theory, which represent the characteristic properties predicted for angle-dependent harmonic generation. It is shown that polarization geometry and alignment distribution play essential roles in potential applications to probe electronic structure and dynamics of molecular systems.

Origin of periodicity in nanostructuring on thin film surfaces ablated with femtosecond laser pulses.

We report physical processes responsible for the periodic nanostructure formation in femtosecond-laser ablation of thin film surfaces. It has been found that an initial random distribution of nanoscale ablation traces is periodically structured with an increase in superimposed laser pulses or fluence on diamond-like carbon film used as the target. The results show that the formation of periodicity can be attributed to the excitation of surface plasmon polaritons to induce the periodic enhancement of local fields in the surface layer.