Gaëtan Wicht
The physical-chemical properties of polymer films can be modified by embedding inorganic materials (e. g. inorganic nanoparticles) into the polymer matrix. Depending both on the material properties (e. g. nanoparticles concentration) and on the fabrication procedure, air pores can appear into such hybrid organic-inorganic matrices, thus creating nanoporous layers. At the end of the 90's, Ilford Imaging Switzerland GmbH was a pionner in the field with the development of a unique "roll-to-roll" process to coat such layers on large surfaces and on an industrial scale. First used as absorbing layers in the ink-jet printing of high quality photographs, the interest for such nanoporous films as optical films has been rapidly growing. In particular, the possibility of introducing air nanopores on demand in a polymer matrix make these layers the perfect candidates for the fabrication of optical films with a very low refractive index (n < 1.2). In optical devices based on thin film multilayers, such nanoporous materials can guarantee an excellent index matching between the air and a plastic substrate, thus acting as very good antireflection coatings. On the other hand, in a different approach, these nanoporous layers cans also yield a good index contrast between a guiding layer and its claddings, thus allowing the fabrication of good polymer waveguides. The goal of this thesis was then to demonstrate the optical potential of the industrial nanoporous layers fabricated by Ilford on flexible plastic substrates. First, both the morphology and the optical response of different nanoporous layers were thoroughly characterised with respect to the fabrication parameters. The impact of the fabrication process as well as of the nanoparticle type (silica (SiO2), bohemite (AlOOH) and titania oxyde (TiO2)), size and concentration were studied. While reflectance and transmittance measurements revealed the highly transparent nature of all these layers, ellipsometry measurements showed the large range of achievable refractive indices (1.15 < n < 2.0). With the possibility of coating several nanoporous layers one on top of the other, with independently adjusted thicknesses and indices, we designed and fabricated antireflection multilayers with graded-index, that, once coated on a plastic substrate successfully reduce its surface reflectance from 10 % to few 0.1 %. Besides the measurements, a theoretical model based on the transfer matrix method allowed us to analyse in details the response of our samples. We could thus show that the residual interface roughness is responsible for the loss of coherence of the light reflected at each interface, thus reducing the amplitude of the interference fringes in the reflectance spectrum and yielding an optical response almost constant over the entire visible spectrum. Finally, the nanoporous layers were also used as optically isolating layers for the fabrication of polymer optical waveguides. An original measurement method, based on the emission of a fluorescent dye dispersed inside the guiding layer, allowed us to clearly demonstrate the feasability of such guides. Propagation loss in the order of 1.5 dB/cm were measured, a very promising value that, after optimisation of both the material and the optical design, is likely to be further reduced in order to reach better performances than standard polymer waveguides and, eventually, to approach those of inorganic devices. Having highlighted the optical properties of our polymer nanoporous layers with the fabrication of two complementary optical devices based on polymer-nanoporous multilayers, this thesis clearly demonstrates their potential that, combined with their industrial nature, can guarantee them a very promising future, even brighter than that for similar prototype layers fabricated at a lab scale.
EPFL2010
