Theoretical study and development of photonic devices and sensors

Abstract

In this thesis emerging photonic platforms based on either plasmonic structures or sensing schemes using functional polymeric materials are studied both theoretically and experimentally. The first studied structure is metal-coated optical fiber tips with integrated plasmonic slot nanoresonators (PSNRs). The guiding and modal properties of metal-coated optical fiber tips with embedded PSNRs are investigated through Finite Element Method (FEM) simulations towards the identification of their optimization parameters. It was found that the placement of a PSNR at the cut-off radius of a metal-coated fiber tip, where the group velocity tends to zero, leads to considerable intensity enhancement of the field confined beyond the diffraction limit. Maximum intensity enhancement of optimally placed PSNRs at different radii shows a linear dependence between excitation wavelength and radius, making it feasible to engineer the proper radius for a specific wavelength for maximum enhancement. The second studied plasmonic platform are metal tips which can offer high field intensity at the tip apex and high confinement on the nanoscale. The proposed platform based on hybrid composite glass metal microwires can offer robustness, ease of light coupling as well as continuous re-excitation of the plasmon modes due to repeated total internal reflection at the glass/air interface which can dramatically reduce the high losses induced by the metal core. An optimized fabrication process of high-quality all-fiber plasmonic tips by tapering such hybrid metal core/dielectric cladding microfibers is proposed and demonstrated experimentally. For this purpose the Plateau-Rayleigh instability in such hybrid fibers is theoretically investigated by inducing surface tension perturbations and by comparing them to the Tomotika instability theory. The continuous-core breakup time is calculated via FEM microfluidic simulations for different temperatures. The theoretical results are in close agreement with the experimental observations providing insight into the engineering of fibers, towards the development of plasmonic tips. Plasmonic tips were successfully demonstrated in a highly controllable manner, and their performance was related to simulation results predicting high field enhancement factors up to 105. Optical fibers coated with novel polymers are also studied towards biosensing applications. The sensing mechanism is based on the variations of the measured power at the distal end of the fiber due to the interaction between the fiber’s evanescent field and the changes induced to the polymeric film by the adsorption of the under study molecules. Two novel amphiphilic block copolymers, the cationic PMMA117-b-PDMAEMA16 and the cationic vinyl-sulfone functionalized PMMA117-b-P(DMAEMA17-VSTEMA2), having both hydrophobic poly(methyl methacrylate) (PMMA) and hydrophilic poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) blocks have been designed and synthesized for efficient protein detection in photonic-based sensing. The presence of the cationic PDMAEMA block and the vinyl-sulfone double bonds led to reversible electrostatic binding of negatively charged proteins like bovine serum albumin (BSA) and non-reversible chemical binding by thiol-ene reactions with cysteine in proteins, respectively. The sensing properties of these materials were assessed and confirmed by ATR-FTIR analysis and by the characterization of fabricated sensing heads on silica optical fibers functionalized with suitably deposited overlayers. The sensing assessment revealed the requirements for deposited overlayer characteristics towards proteins' detection sensitivity and selectivity enhancement. The fabrication of cost-effective, polymer-based electrospun fluorescent fibrous grids and their evaluation as candidates for sensing is also reported. The formation of 3D grids can provide large interaction area with gas analytes and thus overcome quenching limitations induced by polymeric films, for more efficient sensing. Two different polymer-based electrospun fibers having fluorescent moieties were fabricated. The first was fabricated by a well-defined, methacrylic homopolymer functionalized with anthracene moieties as fluorescent elements that has been blended with a commercially available poly(methyl methacrylate) for the production of fluorescent electrospun polymer fibers. These materials have been evaluated for ammonia sensing based on the fluorescence quenching of the anthracene fluorophores in the presence of ammonia vapors, exhibiting fast response at concentration up to 10000 ppm. For the fabrication of the second fluorescent electrospun fiber system, ferrous core-shell nanoparticles consisting of a magnetic γ-Fe2O3 multi-nanoparticle core and an outer silica shell have been synthesized and covalently functionalized with Rhodamine B (RhB) fluorescent molecules (γ-Fe2O3/SiO2/RhB NPs). The resulting γ-Fe2O3/SiO2/RhB NPs were integrated with a renewable and naturally-abundant cellulose derivative (i.e. cellulose acetate, CA) that was processed in the form of electrospun fibers to yield multifunctional fluorescent fibrous nanocomposites. The encapsulation of the nanoparticles within the fibers and the covalent anchoring of the RhB fluorophore onto the nanoparticle surfaces prevented the fluorophore’s leakage from the fibrous mat, enabling thus stable fluorescence-based operation of the developed materials. These materials were further evaluated as dual fluorescent sensors (i.e. ammonia gas and pH sensors), demonstrating consistent response for very high ammonia concentrations (up to 12000 ppm) and fast and linear response in both alkaline and acidic environments. The superparamagnetic nature of embedded nanoparticles provides means of electrospun fibers morphology control by magnetic field-assisted processes and additional means of electromagnetic-based manipulation making possible their use in a wide range of sensing applications.