‘‘Heterogeneous’’ versus ‘‘homogeneous’’ nucleation and growth of microcones on titanium surface under UV femtosecond-laser irradiation A. A. Ionin • S. I. Kudryashov • S. V. Makarov • A. A. Rudenko • L. V. Seleznev • D. V. Sinitsyn • E. V. Golosov • Y. R. Kolobov • A. E. Ligachev Received: 25 October 2013 / Accepted: 3 December 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract Two separate sets of microcones were observed on titanium surface under various ablative 248-nm femto- second-laser exposures near rough spallation crater edges and in crater centers, respectively. ‘‘Heterogeneous’’ nucle- ation of microcones occurred on crater edge spikes above the corresponding cumulative spallation thresholds, with their gradual growth proceeding through redeposition of the ablated matter. In contrast, quasi-periodical central micro- cones emerged through a ‘‘homogeneous’’ nucleation pro- cess owing to ablative surface instability, with average cone volume increasing exponentially versus the shot number over the broad fluence range until its stabilization, and log- arithmically versus laser fluence at the different shot num- bers above the instability threshold fluence of about 30 mJ/ cm 2 . The instability was assigned to a positive optical feedback, resulting from enhanced laser energy deposition and ablation within the inter-cone valleys, as previously suggested and demonstrated by our numerical modeling of near-surface electromagnetic intensity distribution. 1 Introduction In the last years fabrication of micro and nanostructures on titanium surfaces has become quite promising for a number of applications. One of the main advantages of femtosec- ond (fs) laser surface nano/microstructuring is the appear- ance of hierarchical (multi-scale) surface structures, which are typically represented by nanostructure-coated micro- structures [1–3]. Moreover, using a simple laser beam shaping technique, one can fabricate the surface micro- structures with different nanotopology and chemical com- position [3]. Such titanium surface structures support selective bacterial retention [4], osteointegration improve- ment [5], Leidenfrost temperature shifting [6], cell encap- sulation, and fluid sterilization. Hydrophility and biocompatibility of such multi-scale structures on titanium surfaces are subjects of intensive research in the recent years [4, 5, 7]. Another promising application of micro and nanoscale surface textures is significant increase of elec- tron temperature and hard X-ray yield from plasma pro- duced via ablation by intense (*10 16 W/cm 2 ) fs-laser pulses [8]. In spite of the two-decade research on ns-laser surface microstructuring and almost one-decade studies of fs-laser surface microstructuring [9–15], the underlying physical mechanisms are not well understood yet. In fact, several different hypothetical mechanisms of microcone formation were proposed, including surface hydrodynamical insta- bilities, redeposition of ablated material and photochemical etching [9–16]. Additionally, two different regimes of IR fs-laser surface microstructuring were qualitatively distin- guished as above-surface and below-surface growth of microcones [15]. However, no systematic studies were performed yet for quantitative analysis of mass transport during laser-induced formation of microcones as a function A. A. Ionin Á S. I. Kudryashov (&) Á S. V. Makarov (&) Á A. A. Rudenko Á L. V. Seleznev Á D. V. Sinitsyn P.N. Lebedev Physical Institute RAS, Leninskiy Prospect 53, 119991 Moscow, Russia e-mail: sikudr@sci.lebedev.ru S. V. Makarov e-mail: makser@sci.lebedev.ru E. V. Golosov Á Y. R. Kolobov Belgorod State University, Pobedy Str 85, 308015 Belgorod, Russia A. E. Ligachev A.M. Prokhorov General Physics Institute RAS, Vavilova str. 38, 119991 Moscow, Russia 123 Appl. Phys. A DOI 10.1007/s00339-013-8196-z