FABRICATION AND CHARACTERIZATION OF NANOCOMPOSITE-BASED ELASTOMERIC OPTICAL DEVICES
Creators: name: GHISLERI, CRISTIAN
Publisher: name: Universit� degli Studi di Milano
Year: 2014

description: Adaptive and tunable optics, consisting in the correction of perturbed light wavefronts by means of deformable optical devices, was first introduced in the 50?s by Babcock [1] and developed in the 70?s by Buffington and Hardy independentely. Nowadays it represents a well established technology mainly exploited in astronomy in order to reduce the effects of atmospheric perturbations limiting the attainable resolution of large astrophysical telescopes. Adaptive and tunable optics is exploited in ophthalmology, microscopy and photonics as well for the improvement of imaging performances. However exploitation of adaptive and tunable optics is nowadays limited to high-level or prototype-level instrumentation because of the high complexity and fabrication costs of the deformable optical devices, responsible for the correction of aberrations in light wavefronts. The fabrication of functional, simpler and cheaper deformable optical devices is fundamental in order to apply adaptive and tunable optics in more commonly available instrumentation. 

Such deformable optical devices typically consist in deformable mirrors or gratings, able to change their optical properties (shape, focal length or pitch) dynamically, according to the perturbations of the wavefront or the wanted outgoing optical features. The technologies on which such deformable optical devices rely (for example segmented mirrors, reflective thin membrane, MEMS tunable gratings) suffer of several drawbacks in terms of high weight and complexity or low deformability and tunability of the optical properties. New technologies are required, aiming to overcome these issues. 

The fabrication of non planar diffraction gratings, required in a number of optical mounts for the correction of spherical or higher order aberrations affecting the diffracted beam or to add focusing to diffractive capabilities, is of concern as well for the necessary high fabrication costs. The possibility to easily fabricate low-cost arbitrarily shaped reflective
diffraction gratings would be a breakthrough for example in monochromoators or spectral imaging techniques. 

Elastomeric optics represents a promising technology for the fabrication of optical devices on highly deformable and conformable elastomeric substrates. Reflective elastomeric optics, obtained by metallization of elastomeric substrates, is particularly interesting because of the possibility to limit the dimensions of elastomeric device based optical instruments respect to devices working in transmission. However the metallization of elastomeric substrates by classic coating techniques is problematic because of the low adhesion and resilience of the reflective metal layer deposited on the elastomer surface.

Metal-elastomer nanocomposites may represent an alternative and effective approach for the fabrication of reflective elastomeric optical devices since the reflective metal is embedded in the elastomeric matrix and does not consist in a continuous rigid metal layer, solving adhesion and resilience problems of coating techniques. However currently available nanocomposites synthesis techniques (chemical or metal ion implantation approaches) do not guarantee the achievement of reflectivity and surface smoothness required for deformable optical devices. Driven by the motivations described above, the present thesis is devoted to the fabrication, characterization and exploitation of metal-elastomer nanocomposite based deformable optical components (mirrors and diffraction gratings) obtained by means of Supersonic Cluster Beam Implantation (SCBI). SCBI allows implanting electrically neutral metal nanoparticles (silver in this work) with low kinetic energy in elastomeric substrates like Polydimethylsiloxane (PDMS). The optical and morphological characterization of the Ag/PDMS nanocomposite will demonstrate that the issues encountered for the fabrication of reflective elastomeric optical components by currently available nanocomposite synthesis techniques can be overcome by using SCBI.

In particular optical properties of reflective elastomeric optical components synthesized by SCBI are affected by Surface Plasmon Resonance (SPR) characterizing silver particles of nanometric size implanted in the elastomeric matrix. A characterization of SPR in light of the theoretical model describing the optical behavior of metal nanoparticles embedded in a dielectric matrix and upon applied strain is necessary for a better understanding and control of the optical properties of the devices during the fabrication process.