Single Shot Generation of High‐Aspect‐Ratio Nano‐Rods from Sapphire by Ultrafast First Order Bessel Beam

International audienceAbstract Engineering the polarization and spatial phase of ultrafast laser pulses represents a compelling strategy for enhancing control over laser–matter interaction and enabling rapid and innovative nano‐fabrication processes. Here, the single‐shot, ultrafast laser fabrication of high‐aspect‐ratio, vertically standing nano‐pillars with a diameter of ≈800 nm and height up to 15 µm on the surface of sapphire, is reported. To achieve this, the distinctive properties of diffraction‐free, first‐order Bessel beams endowed with either radial or azimuthal polarization distributions, are harnessed under tight focusing conditions. The highly intense laser–matter interaction in this configuration generates a tubular‐shaped, high‐pressure field beneath the material surface, leading to the rapid expulsion of material across the surface. Three distinct regimes for the pillar generation are identified in addition to a mechanism based on the Rayleigh‐Plateau theory that explains the distinct morphological regimes observed. The findings not only shed light on the underlying physical mechanisms of intense excitation of transparent dielectrics but also offer exciting prospects for the rapid fabrication of positive nano‐structures and material compression across various fields of application

The giant monazite crystals from Manangotry (Madagascar)

International audienceThe Manangotry area in the South-East of Madagascar is famous for its giant monazite crystals. The studied occurrence is an outcrop near the Manangotry pass, were giant, cm-sized crystals have been found. The crystals are embedded in biotitite levels, also containing cm-sized xenomorphic ilmenite, and are associated with a 1 m thick layer of apatite. Besides giant crystals, monazite is also present as abundant small (mm-sized) grains. The surrounding rocks are leucogranites and charnockites. The monazite giant crystals are chemically homogeneous and thorium-rich, whereas small crystals tend to be heterogeneous. Both biotite and apatite are F-rich. Temperature estimates from biotite and apatite yielded 750 to 800 °C. Despite their high radioactivity, the giant crystals preserve a high degree of crystallinity. Electron microprobe U-Th-Pb dating produced ages of 537 ± 14, 534 ± 10 and 530 ± 10 Ma, considered to be the age of the main monazite crystallization episode but some minor domains in small grains yielded 482 ± 12 Ma. The enclosing biotites yielded 453 ± 6 Ma Ar-Ar ages. More than 98 % of radiogenic helium have been retained in the studied monazite samples, and kinetic experiments indicates the intrinsic diffusion parameters for high-temperature to be Ea = 53 kcal/mol and D0 = 3085 cm2/s. The paragenesis (monazite-apatite-ilmenite-biotite) is different from any other occurrences of larges monazite elsewhere. Based on the current data and literature, the preferred genetic model for these giant monazites is crystallization from a high-temperature fluorine-rich fluid, possibly originating from surrounding charnockites