Hank De Bruyn

Manipulating and controlling the evanescent field within optical waveguides using high index nanolayers

We propose and demonstrate, through simulation and experiment, how the interaction of an optical field within a waveguide designed for chemical sensing and, more generally, evanescent field spectroscopy can be enhanced substantially by strategic deposition of high index surface layers. These layers draw out the optical field in the vicinity of probing and take advantage of field localisation through optical impedance matching. Localisation of the evanescent field to the inner layer in turn is accompanied by whispering gallery modes within the channels of a structured cylindrical waveguide, further enhancing sensitivity. A novel demonstration based on self-assembled layers made up of TiO2 within a structured optical fibre is demonstrated, using a simple porphyrin as the spectroscopic probe. This technique offers optimisation of the limitations imposed on practical waveguide sensors that are highly sensitive but nearly always at the expense of low loss. The principles have potential ramifications for nanophotonics more generally and these are discussed.Controlling the evanescent field within platform waveguide technologies underpins waveguide nanophotonics and is critical to optimising the interaction with integrated specialised materials or devices under test. Unfortunately, this interaction is often small since the evanescent field is a fraction of the total optical field. Here we propose and demonstrate, through simulation and experiment, how the waveguide evanescent field can be enhanced substantially by using high index interface layers, which draw out the optical field in the probe vicinity taking advantage of field localisation. This can be further enhanced by extended resonant and gallery modes within the channels of a structured cylindrical waveguide. Several orders of magnitude increased sensitivity with minimal added insertion loss is obtained using self-assembled layers of TiO2 (B) nanoparticles and porphyrin within a silica structured optical fibre. The combination of novel photonics with specialty material integration highlights the potential scope for physics, chemistry, sensing and materials research.

Characterization of electrosterically stabilized polystyrene latex; implications for radical entry kinetics

Electrosterically stabilized polystyrene latexes with a poly(acrylic acid) hydrophilic layer with either perdeuterated core or perdeuterated hydrophilic layer were prepared in situ in a styrene/acrylic acid copolymerization, in a manner similar to that commonly employed industrially. Small angle neutron scattering (SANS) measurements were made over a range of contrasts for three latexes at high and low pH. Parameters obtained by fitting to standard core/shell models were consistent with the shell being highly hydrated (about 89% at low pH and about 94% at high pH). The core was found to contain about 3% acrylic acid. Doubling the proportion of acrylic acid in the recipe increased shell thickness by about 20%, slightly reduced particle size and slightly increased the proportion of acrylic acid incorporated into the core. The maximum degree of polymerization (DOP) of the entering (and therefore grafted) species was estimated from the shell thickness to be about 44 monomer units for 0.02 M acrylic acid and 66 for 0.04 M. The observed dependence of hairy layer (shell) thickness on the initial amount of acrylic acid suggests that the critical DOP for entry (and therefore true grafting) of the electrosteric stabilizer is thermodynamically (not kinetically) controlled.

Control of Particle Morphology in Ab Initio RAFT Mediated Emulsion Polymerization

Recent advances in the use of reversible addition–fragmentation chain transfer (RAFT) polymerization in dispersed phase systems have paved the way for the fine control of the morphology of latex particles that was not possible by conventional free radical polymerization techniques. With this approach, living amphiphilic block copolymers are synthesized that self-assemble to form micelles. The hydrophilic segment is formed from a water-soluble monomer which stabilizes the latex particles as polymerization proceeds and the latex particles grow. The hydrophobic ends of the RAFT diblocks ultimately grow into the polymer that forms the body of the particles. This paper presents examples of ways in which these advances can be used to engineer latex particles with unique morphologies that exhibit specific application properties.

Self-assembly and nanotechnology within an optical fibre fFor improved evanescent field sensing

The self-assembly of TiO2 nanoparticles is used to create a high index layer within a structured optical fibre. We show both experimentally (using a novel porphyrin probe) and theoretically that this approach leads to more than order of magnitude enhanced localisation of the optical field at the layer-air interface of the hole, both through edge localisation and through novel resonance localisation as a result of a ring resonator whispering gallery modes.

Enhancing absorption and sensitivity within structured optical fibres

We propose enhancing the absorption of a species inside the channels of a structured optical fibre by depositing a high index layer within the air holes. This layer draws out the optical field within the mode increasing the overlap interaction. Simulations support the general idea and an experimental demonstration is reported using a novel approach to film formation with TiO2 nanoparticles. For the sake of demonstration, we use porphyrin with carboxylic groups that attach to the TiO2. The deposition of particles well coated with porphyrin is compared to those not fully coated prior to deposition in the holes. The latter case is found to give the best results since scattering loss is reduced when the porphyrins are not initially attached to the TiO2 particle. This is expected if film formation through intermolecular forces has occurred.rs.