Customized diffraction response of metal surfaces nanostructured by laser interference

Simposium C: Laser-material interactions for tailoring future applications. ; Lille (France), from May 2 to 6, 2016.Production of surfaces with original and personalized visual color effects is one of the major challenges of our society. It has direct applications in the technologies requiring better methods of personal identification according with the quest of the enhanced police security. Laser interference by phase mask excimer irradiation is a versatile technique for the production of 1D and 2D patterned surfaces. When applied to metal films, the laser irradiation induces the periodic dewetting of the metal from the substrate resulting to the formation of nanoparticles. These nanostructures are optically characterized by their surface plasmon resonance that depends on the size, density and metal composition. Thus, optical contrast between the regions transformed into nanoparticles and non-transformed regions, and thus the diffraction efficiency of the surface, can be tuned. In this work we report the production of bimetallic, with different atomic ratio of Ag/Au, microstructured surfaces with different motives and periodicities in the range 6.3 microns to 1.7 microns. We will present experimental data showing that these regions have different diffraction patterns according with the periodic motive and the optical response of the nanoparticles. Thus, this technique allows a personalized optical signal to be encoded with a time-efficient and single-step laser technique.Peer Reviewe

Ion-induced roughening and ripple formation on polycrystalline metallic films

We present a study of nanopattern formation on polycrystalline Ni surfaces upon low energy Ar ion bombardment. At low angles of ion incidence an isotropic, rough morphology develops on the surface while at grazing incidence a ripple structure parallel to the ion beam direction is formed (so-called perpendicular mode ripples). To explain this behavior we propose a model which is based on a combination of (a) surface roughening due to sputter yield variation between different crystalline grains and (b) anisotropic nonlinearity resulting from the oblique angle bombardment. By computer simulations we show that the combination of these two phenomena excellently reproduces the experimental behavior, in particular the dependence of the surface morphology on the ion incidence angle. Importantly, the formation of ripples at grazing incidence does not involve any linear instability, in strong contrast to the present model of the ripple formation process. © IOP Publishing and Deutsche Physikalische Gesellschaft.status: publishe

Customizing plasmonic diffraction patterns by laser interference

10 pags., 9 figs. 2 tabs. -- Open Access funded by Creative Commons Atribution Licence 3.0This work reports a versatile and efficient production of periodic structures of alloy nanoparticles (NPs) with customized diffraction patterns by using the technique of phase mask laser interference. This technique uses interfering single nanosecond laser pulses to induce the periodic dewetting of bilayer (Ag/Au) films on glass produced by pulse laser deposition. Film breaks up into alloy NPs around the regions exposed to intensity maxima and the cold regions placed in the minimum laser intensity are non-transformed. This allows fringes to be produced with a period within the range of 1.7-6.8 μm. Periodic structures of squares, diamonds, rectangles or triangles are produced by accumulating two or three laser pulses with different fringe orientations. As a film parameter, we have analyzed the pattern properties by varying the thickness of the Au layer while keeping that of Ag constant. The diameter of the NPs, their number density, percentage of the transformed region, the interface between transformed and non-transformed region or the minimum period achievable can be tuned by varying the Au concentration. In that way, isolated and big NPs, which are optically characterized by a plasmon resonance, are produced for the thinnest film, whereas a bimodal size distribution of big and small NPs, whose optical transmittance is characterized by IR absorption related with multipolar interactions between the close small NPs, are produced for the highest Au concentration. However, the periodic structure still generates visible diffractive patterns whose diffraction efficiency can increase up to a factor of 4, while their spectral trend dependences can increase or decrease as a function of the Au concentration. These optical behaviors have been explained satisfactorily by taking into account the optical contrast between the regions transformed into NPs and the non-transformed regions. Altogether, this allows the position of the diffraction orders and their relative and absolute spectral efficiency to be customized in a broad range.R. J. Pel´aez acknowledges grant no. JCI-2012-13034 from the Juan de la Cierva program. A. Ferrero, B. Bernad and J. Campos are grateful to the Comunidad de Madrid for funding the program SINFOTON-CM: S2013/MIT- 2790.Peer Reviewe

Tailoring the magnetic anisotropy, magnetization reversal, and anisotropic magnetoresistance of Ni films by ion sputtering

© 2015 American Physical Society. We studied surface morphology induced changes of magnetic anisotropy, magnetization reversal, and symmetry of the anisotropic magnetoresistance (AMR) in ion sputtered Ni films grown on MgO (001). Grazing-incidence ion sputtering generally develops anisotropic surface roughness of the Ni films, i.e., nanometer wide ripples parallel to the ion beam direction, giving rise to uniaxial magnetic anisotropy with the easy axis along the ion beam direction. The formed ripples act as domain wall nucleation and pinning sites during magnetization reversal, while two-jump domain wall motion dominates in the as-grown Ni films. More importantly, the azimuthal angular dependence of the AMR indicates a superposition of twofold symmetry and fourfold symmetry. By relying on grazing-incidence ion sputtering along specific crystallographic directions, we are able to tailor the relative weight of twofold and fourfold symmetry of AMR. We demonstrate that in contrast to the bulk case, the symmetry of the AMR becomes also sensitive to the surface morphology in thin films, which is in particular relevant for the design of magnetotransport based sensors.status: publishe

Data publication: Morphological transitions in the patterning of the crystalline Ge(001) surface induced by ion irradiation

Rohdaten Rasterkraftmikroskopi