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

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

Advantages of phase retrieval for fast x-ray tomographic microscopy

In near-field imaging with partially coherent x-rays, the phase shifting properties of the sample are encoded in the diffraction fringes that appear as an additional intensity modulation in the x-ray projection images. These Fresnel fringes are often regarded as purely an enhancement of the visibility at the interfaces. We show that retrieving the phase information contained in these patterns significantly advances the developments in fast micro-tomography. Improving temporal resolution without intensifying radiation damage implies a shortening of the exposure time rather than increasing the photon flux on the sample. Phase retrieval, to a large extent, compensates the consequent photon count moderation in the images, by fully exploiting the stronger refraction effect as compared with absorption. Two single-distance phase retrieval methods are evaluated for the case of an in situ 3 Hz micro-tomography of a rapidly evolving liquid foam, and an in vivo 6 Hz micro-tomography of a blowfly. A new dual-detector setup is introduced for simultaneous acquisition of two near-field diffraction patterns. Our goal is to couple high temporal, spatial and density resolution in a single imaging system in a dose-efficient manner, opening further options for dynamic four-dimensional studies. © 2013 IOP Publishing Ltd

Design and verification of diffractive optical elements for speckle generation of 3-D range sensors

The optical projection using speckles is one of the structured light methods that have been applied to three-dimensional (3-D) range sensors. This paper investigates the design and fabrication of diffractive optical elements (DOEs) for generating the light field with uniformly distributed speckles. Based on the principles of computer generated holograms, the iterative Fourier transform algorithm was adopted for the DOE design. It was used to calculate the phase map for diffracting the incident laser beam into a goal pattern with distributed speckles. Four patterns were designed in the study. Their phase maps were first examined by a spatial light modulator and then fabricated on glass substrates by microfabrication processes. Finally, the diffraction characteristics of the fabricated devices were verified. The experimental results show that the proposed methods are applicable to the DOE design of 3-D range sensors. Furthermore, any expected diffraction area and speckle density could be possibly achieved according to the relations presented in the paper