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On exposure to 85 MHz sub-15 fs pulsed 800 nm Ti:sapphire laser light in the focal spot of a high-numerical aperture oil immersion objective, characteristic periodic nanostructures emerged on the surface of z-cut congruent LiNbO3 crystals. Close to the ablation threshold shallow ripples oriented parallel to the laser polarization appeared on the surface at a periodicity Λ∥ ≈ 200 nm as well as ripples running perpendicular to the laser polarization at almost the same period (Λ⊥ ≈ 190 nm). Nanostructure formation involved melting and resolidification of LiNbO3 as evidenced by scanning electron microscopy images of structural peculiarities. Micro-Raman spectroscopy demonstrated that the resolidified material crystallized in the original structural phase. Ripple formation is attributed to plasma wave interference patterns that arise in the electron–hole plasma generated by multiphoton and avalanche collisional excitation.

Due to its electro-optical, acousto-optical, ferroelectric, piezoelectric and nonlinear-optical properties lithium niobate is a material of high technological relevance. Thus, patterning of LiNbO 3 surfaces by laser light may significantly influence the performance of micro-optical devices made of this material. Here, we report on the generation of self-organized nanostructures on surfaces of unpoled LiNbO 3 crystals using tightly focused sub-15 fs pulsed Ti:Sapphire laser light (centre wavelength 800 nm, bandwidth 120 nm, repetition rate 85 MHz) at sub-nanojoule pulse energies. With the LiNbO 3 surface immersed in oil intensities close to the ablation threshold resulted in the formation of shallow ripples of 5 -25 nm in height appearing at a periodicity of approximately 220 nm. The ripples were generated by local melting and resolidification of LiNbO 3 involving minor admixture of hydrocarbons. At intensities well beyond the ablation threshold the LiNbO 3 surfaces were patterned densely with tiny cones of 100 -500 nm in height featuring diameters of a few hundred nanometers. Moreover, lines scanned inside the LiNbO 3 crystals resulted in refractive index changes along the laser traces. In contrast, with the LiNbO 3 surface in air or water, ablation was not observed even at prolonged exposure due to aberrations of the focal spot.

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Abstract. The pentadecane single droplet size of less than 20 µm and its refractive index were measured at various ambient temperatures of 100, 120, 150 and 180 °C. The experimental setup was equipped with the high speed linear CCD camera to give sharp and clearly visible MDRs (Morphology Dependent Resonances) structure from the droplet refractive index. The valve of the heated chamber was closed during the experiment until the droplet disappeared from the chamber to increase the measurement time of about 2 folds in order to obtain more experimental data of the droplet behaviors. The cooling device was mounted on the heated chamber to improve boundary condition and smooth the ambient temperature. The obtained droplet sizes were in good agreement with both the D 2 law and the Rapid Mixing Model (RMM), while the measured droplet refractive index values were closed to those calculated from the previous reported formula. This has indicated the reliability and applicability of this improved measurement technique for a better understanding of the real fuel droplet behaviors in a combustion system.