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Self-organized patterns in nanoscopic metal films


In this talk, I will discuss our understanding of the self-organization that leads to spatially ordered nanostructures when nanoscopic metal films undergo multiple instances of melting and resolidification under short laser pulses. The ensuing patterns can find applications in magnetism, plasmonics and solar energy harvesting. Extensive experimental studies of the Co/SiO 2 system show that the patterning length scale increases monotonically with thickness h as ~ h 2 up to a critical thickness h c beyond which the length scale decreases with increasing h . Theoretical modeling of the laser-film thermal interactions and resulting linear (and nonlinear) thin film dynamics suggests that pattern formation in regime with h less than h c is due to a classical hydrodynamic instability (Vrij, Disc. farad. Soc. 1966) . This instability results from the competition between surface tension and attractive long range dispersion forces. On the other hand, the novel non-classical pattern formation observed for h greater than h c is a nanoscale effect. This effect results in strong thickness and time dependent film heating which produces thermocapillary effects whose magnitude and sign are h -dependent. We will also present other important findings, including the robustness of the self-organization, physical state of the nanoparticles (like crystallinity and magnetism) and complex nanoscale patterns from various experiments. This work is highly relevant towards robust nanomanufacturing strategies while also providing opportunities to further our fundamental understanding of laser-induced pattern formation in ultrathin films.