Secondary electron yield reduction by femtosecond pulse laser-induced periodic surface structuring

The electron-cloud phenomenon is one cause of beam instabilities in high intensity positive particle accelerators. Among the proposed techniques to mitigate or control this detrimental effect, micro-/nano-geometrical modifications of vacuum chamber surfaces are promising to reduce the number of emitted secondary electrons. Femtosecond laser surface structuring readily allows the fabrication of Laser Induced Periodic Surface Structures (LIPSS) and is utilized in several fields, but has not yet been tested for secondary electron emission reduction. In this study, such treatment is carried out on copper samples using linearly and circularly polarized femtosecond laser pulses. The influence of the formed surface textures on the secondary electron yield (SEY) is studied. We investigate the morphological properties as well as the chemical composition by means of SEM, AFM, Raman and XPS analyses. Surface modification with linearly polarized light is more effective than using circularly polarized light, leading to a significant SEY reduction. Even though the SEY maximum is only reduced to a value of ~1.7 compared to standard laser-induced surface roughening approaches, the femtosecond-LIPSS process enables to limit material ablation as well as the production of undesired dust, and drastically reduces the number of redeposited nanoparticles at the surface, which are detrimental for applications in particle accelerators. Moreover, conditioning tests reveal that LIPSS processed Cu can reach SEY values below unity at electron irradiation doses above 10−3 C/mm2

Deterministic Realization of Quasicrystal Surface Relief Gratings on Thin Azopolymer Films

Spatially structured UV–visible light fields generate topographic modulations on the surface of films of azobenzene-containing polymers. The geometry of the surface reliefs depends on the spatiotemporal distribution of light over the sample. If multistep sequential irradiations are used, even illumination configurations as simple as the interference of two linearly polarized beams produce complex surface textures. This is the case of the quasicrystal geometries, obtainable as the superposition of multiple sinusoidal surface relief gratings oriented in different directions over the surface. The quantitative relief design would require a comprehensive theoretical description of the light-induced relief formation mechanism, which is still elusive for azopolymers. Here, despite limiting the description at a phenomenological level, the deterministic design of the quasicrystal surface reliefs obtained in sequential light exposures of an azopolymer film is demonstrated. The model provides an excellent agreement between simulated and experimental relief textures, predicting also the dependence of relief structural features on illumination parameters as the number of exposure steps and the beam interference angle. The deterministic texture design allows the controlled tailoring of surface functionalities related to the relief geometry like light diffraction properties of the samples, which are here exploited to manipulate, split, and trap diffracted light

Holographic Laser Scanning Microscopy

We realize a new optical microscopy technique (Holographic Laser Scanning Microscopy, H-LSM) based on holographic laser scanning illumination of the sample. In each scanning step, a multispot illumination pattern is generated by phase modulating a laser beam via Computer Generated Holography (CGH). A CCD acquires an image of the light signal backscattered from the sample in each scanning step, and the elaboration software (Henriques R, et al. Nat Methods 7:339-340, 2010) reconstructs the image of the sample by localizing the centroid of each recorded spot (Bobroff N, Rev Sci Instrum 57:1152-1157, 1986). The elaboration software operates a spatial filtering on the recorded spots, accepting and processing only those satisfying the minimum SNR and the maximum FWHM controls. The centroid of the valid signals is displayed in the elaborated image (H-LSM image) as a single bright pixel. We implement two completely different methods in illumination schemes. A first illumination condition is realized by focusing an array of light spots onto the sample plane. This intensity pattern is obtained in objective focal plane imposing the calculated phase map (kinoform) on the laser wavefront via a Spatial Light Modulator (SLM). The array of light spots can be made scanning the sample by imposing a grating-like function to the original kinoform (Leach J, et al. Appl Opt 45:897-903, 2006). The second illumination scheme is realized by dynamically changing the random Speckle pattern originating from the random mutual interference of the laser beam wavefronts. Such Speckle pattern is easily generated by imposing a random phase profile on the laser beam using the CGH apparatus, while the mapping of the sample is realized by changing the phase profiles in time. We tested the H-LSM method on some samples presenting structures whose lateral dimensions are below the resolution limit of the optical setup. The H-LSM image of a region of a sample illuminated with an array of light spots, realizing the ordered illumination scheme, is shown in Fig. 64.1a. For comparison, in Fig. 64.1c the image obtained summing up the CCD acquired stack of the scanning procedure, is also shown. Fig. 64.1b,c,d display respectively the H-LSM and the summed image of the sample obtained with the random illumination scheme

Photoinduced Surface Reshaping from Azopolymer Micropillars with Programmable Anisotropy

Tailoring surface-driven functionalities requires versatile techniques for surface micropatterning. Here surfaces of a photosensitive azobenzene-containing polymer, prepatterned with an array of discrete cylindrical micropillars, are reconfigured through a light-driven polarization-sensitive surface deformation

Morphometric analysis on benthic foraminifera through Atomic Force Microscopy

We developed a new non-destructive procedure for quantitatively describing morphological features in benthic foraminiferal tests. The proposed approach is based on nanometric three-dimensional analysis of test outlines under an Atomic Force Microscope (AFM). The AFM provided us a useful tool to analyse some new morphometric features that cannot be investigated with other methods. Specifically, pore density, pore diameter, pore depth, pore surface area, porosity and roughness can be easily estimated in Ammonia tepida tests. In respect to all the far-field microscopy optical methods the AFM characterization allows a high spatial resolution analysis (≈nanometres) in the definition of the pore characteristics especially regarding their vertical extent (i.e. 3D). The proposed approach, tailored for Ammonia tepida, could be used to further explore the potential of pores as an environmental proxy

Light-Induced Structuration of Azopolymer Films for Reconfigurable Diffractive Optical Elements

Reconfigurable diffractive optical elements, having on-demand practical uses, are realized in single-step all-optical structuration processes of a photosensitive re-shapable polymer film

Large-Scale Multiplexed Azopolymer Gratings with Engineered Diffraction Behavior

The diffraction of polychromatic light from periodic superficial structures is often responsible for the structural colors observed in Nature. Similarly, engineered microtextures fabricated on metallic or dielectric surfaces can be used to design diffracted optical patterns with desired shapes and colors. To this aim, advanced diffraction gratings with exceptional design and functionality are continuously proposed, and new fabrication methods follow to stay abreast with the improving design capabilities. Multiplexed surface reliefs, acting as complex gratings with tunable diffraction behavior, can be readily produced on films of azobenzene containing materials by exposing the surface to controlled sequences of holographic interference patterns. This work fully investigates, both theoretically and experimentally, the use of light-induced surface relief on azopolymers for the realization of large-scale multiplexed gratings with optimized diffraction performances. The reconfigurable diffraction gratings able to diffract polychromatic light in the same direction with controllable relative color intensities by tuning exposure parameters in a switchable two-beam interference setup are designed and fabricated. The results can be generalized to more complex diffractive devices, usable in emerging display application areas

Programmable surface anisotropy from polarization-driven azopolymer reconfiguration

he ability to accurately realize complex textures is of great relevance for tailoring surface-driven functionalities as wettability, adhesion and light diffraction. The fabrication of superficial micro-Textures, in a simple and cost-effective way, is high desiderable in this framework. A versatile technique for surface micropatterning is based on reconfiguration of photosensitive azobenzene-containing polymers, in which a macroscopic light-induced motion of polymer chains, fueled by the photo-isomerizing azobenzene molecules, allows the controlled optical reshaping of prestructured superficial micro-Textures. Here, azopolymer surfaces, prepatterned with an array of discrete cylindrical micropillars, are reconfigured through a polarization-driven large-scale surface deformation until achieving superficial gratings with programmable amplitude, orientation and periodicity. The high degree of structural surface anisotropy, the possibility to program the directionality of such anisotropy from the reconfiguration of basic pristine surfaces, and the simplicity of the optical setup, make the proposed structuration method attractive for versatile and cost-effective surface patterning