Optical microstructuring of alkali metal nanoparticle coatings on porous silica substrates induced by Bessel beam

The ability to control atomic adsorption/desorption processes by light as well as nanoparticle growth on proper substrates is nowadays recognized as a promising technique in nanotechnology. This subject is a part of a rising area of research devoted to the development of micro and nanophotonic devices, sensors, metamaterials. The experimental work in this thesis aims at obtaining ordered microstructures exploiting atomic desorption phenomena such as LIAD (Light Induced Atomic Desorption). The suitable substrates consist in nanoporous silica matrices sealed inside cells and exposed to alkaline atom vapor. A Gaussian beam generated by a laser, transformed in Bessel beam by a conical shape lens, has been used as desorption light: in such way, the nanoparticles are created on the pores and they are arranged following the Bessel beam profile, obtaining then the order microscopic structure. More efficiently a coating of alkali metal nanoparticles has been grown on the porous substrate due to LIAD induced by a Hg lamp and then the atomic desorption induced by a Bessel beam has been responsible for the redistribution of the generated nanoparticles in the micrometric concentric ring structure following the beam profile. The experiments have been performed using different alkaline atom vapors, moreover porous silica samples with different pore size and lasers with different wavelengths have been used. During the work, the experimental setup and procedure have been optimized, evaluating the best conditions for obtaining a well defined structure, in particular as regards the power emitted by the desorbing lasers and the lighting time intervals for the porous silica samples. Through adjustments to these parameters multiple, overlapping or side-by-side, illuminations, without incurring erasing phenomena, have been possible thus creating complex structures. The structures thus obtained, whether simple or complex, act as micrometric masks made out of nanoparticles, and they are persistent and reversible. These masks can be detected by illuminating them with laser light and observing the ring-shaped pattern produced by diffraction. The generated structures have been systematically analyzed verifying the effective formation of the nanoparticles by detection of their absorption spectra, observing the microstructures under a microscope and recording them with a CCD camera. By an image analysis software, the concentric ring pattern, corresponding to a 2D quasi-Bessel function, has been studied, evaluating the spacing ring distance from its Fourier transform using the FFT algorithm; in this way, the relationship between the characteristics of the Bessel beam, the wavelength of the radiation and the physical characteristics of the conical lens have been verified. Also, images of the ring-shaped pattern obtained by diffraction have also been registered, in order to estimate the contrast with the background and its persistence on the sample. The experimental results on this thesis suggest a new technique for the laser structuring of alkali metal nanoparticles in ordered patterns on micrometric scale resolution promising for many applications in all-optical photonic devices.