The role of asymmetric excitation in self-organized nanostructure formation upon femtosecond laser ablation AIP Conf. Proc. 1464, 428 (2012) . In addition, the percentage of transformed to non-transformed region normalized to the pattern period follows similar fluence dependence regardless the period and thus becomes an excellent control parameter. This dependence is fitted within a thermal model that allows for predicting the in-depth profile of the pattern. The model assumes that transformation occurs whenever the laser-induced temperature increase reaches the melting temperature of nanoPS that has been found to be 0.7 of that of crystalline silicon for a porosity of around 79%. The role of thermal gradients across the pattern is discussed in the light of the experimental results and the calculated temperature profiles, and shows that the contribution of lateral thermal flow to melting is not significant for pattern periods !6.3 lm.

Discontinuous metal films are converted into either almost round, isolated, and randomly distributed nanoparticles (NPs) or fringed patterns of alternate non transformed film and NPs by exposure to single pulses (20 ns pulse duration and 193 nm wavelength) of homogeneous or modulated laser beam intensity. The dynamics of NPs and pattern formation is studied by measuring in real time the transmission and reflectivity of the sample upon homogeneous beam exposure and the intensity of the diffraction orders 0 and 1 in transmission configuration upon modulated beam exposure. The results show that laser irradiation induces melting of the metal either completely or at regions around intensity maxima sites for homogeneous and modulated beam exposure, respectively, within 10 ns. The aggregation and/or coalescence of the initially irregular metal nanostructures is triggered upon melting and continues after solidification (estimated to occur at 80 ns) for more than 1 ls. The present results demonstrate that real time transmission rather than reflectivity measurements is a valuable and easy-to-use tool for following the dynamics of NPs and pattern formation. They provide insights on the heat-driven processes occurring both in liquid and solid phases and allow controlling in-situ the process through the fluence. They also evidence that there is negligible lateral heat release in discontinuous films upon laser irradiation. V C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4908251]Metal nanoparticles (NPs) either randomly distributed or periodically organised have a high potential for several applications including camouflage, sensors, photovoltaic or optical devices, and tissue engineering scaffolds. 1,2 Most of the applications relate to their unique optical response that is dominated by the surface plasmon resonance (SPR). 3 Heating of metal thin films is a method used since very long to produce them because metal films on dielectric substrates are thermodynamically unstable and tend to dewet for reducing their surface to volume ratio and thus the free energy of the system. [4][5][6] Laser techniques offer a faster means not only for producing NPs on a surface by a heat driven process but also for producing periodic structures, thus representing an alternative route to the costly lithographic methods such as electron beam writing, 3 particularly for those applications in which large-scale and low-cost manufacturing is essential. Irradiation with homogeneous or Gaussian converts metal films into nanoislands, beads or NPs and even self-organised structures with well-defined length scales. 7-11 Irradiation with modulated beams such as those produced by beam interference produces structures with controlled motives and periods in large areas (>mm 2 ). 12-16 These earlier works aim either to the understanding of the length scales of the dewetting and/or mass flow mechanisms or the fabrication of nanostructures. There is only a very recent work studying the dewetting process in real time upon Gaussian ns laser pulses 11 by ...

Patterns are fabricated on 290 nm thick nanostructured porous silicon layers by phase-mask laser interference using single pulses of an excimer laser (193 nm, 20 ns pulse duration). The dynamics of pattern formation is studied by measuring in real time the intensity of the diffraction orders 0 and 1 at 633 nm. The results show that a transient pattern is formed upon melting at intensity maxima sites within a time <30 ns leading to a permanent pattern in a time <100 ns upon solidification at these sites. This fast process is compared to the longer one (>1 μs) upon melting induced by homogeneous beam exposure and related to the different scenario for releasing the heat from hot regions. The diffraction efficiency of the pattern is finally controlled by a combination of laser fluence and initial thickness of the nanostructured porous silicon layer and the present results open perspectives on heat release management upon laser exposure as well as have potential for alternative routes for switching applications.