Shahraam Afshar

Light confinement within nanoholes in nanostructured optical fibers

We report fabrication of the lead silicate microstructured fibers (MOFs) with core holes as small as 20 nm, the smallest holes fabricated within the core of an optical fiber to date. We show that light confinement and average mode intensity within such holes are strongly dependent on the hole size. Light confinement within 80 nm and 250 nm core hole within the fabricated MOFs has been experimentally characterized using Scanning Near-field Optical Microscopy (SNOM).

Fluorescence-based sensing with optical nanowires: a generalized model and experimental validation

A model for the fluorescence sensing properties of small-core high-refractive-index fibers (optical nanowires) is developed and compared quantitatively with experiment. For the first time, higher-order modes and loss factors relevant to optical nanowires are included, which allows the model to be compared effectively with experiment via the use of fluorophore filled suspended optical nanowires. Numerical results show that high-index materials are beneficial for fluorescence-based sensing. However, both numerical and experimental results show that the fluorescence signal is relatively insensitive to core size, except for low concentration sensing where nanoscale fiber cores are advantageous due to the increased evanescent field power.

Cleaving of Extremely Porous Polymer Fibers

Different cleaving techniques, based on the use of a semiconductor dicing saw, focused-ion-beam milling, and a 193-nm ultraviolet laser, have been exploited to cleave highly porous polymer fibers developed for guiding terahertz radiation. Porous fibers made up of two different polymer materials have been cleaved with the proposed methods and compared with those achieved from the conventional cleaving method. Regardless of the polymer material used for fabricating terahertz porous fibers, using an ultraviolet laser for cleaving and rotating the fiber during the process rapidly provides smooth and reproducible cleaves across the entire fiber cross section.

Bragg waveguides with low-index liquid cores.

The spectral properties of light confined to low-index media by binary layered structures is discussed. A novel phase-based model with a simple analytical form is derived for the approximation of the center of arbitrary bandgaps of binary layered structures operating at arbitrary effective indices. An analytical approximation to the sensitivity of the bandgap center to changes in the core refractive index is thus derived. Experimentally, significant shifting of the fundamental bandgap of a hollow-core Bragg fiber with a large cladding layer refractive index contrast is demonstrated by filling the core with liquids of various refractive indices. Confirmation of these results against theory is shown, including the new analytical model, highlighting the importance of considering material dispersion. The work demonstrates the broad and sensitive tunability of Bragg structures and includes discussions on refractive index sensing.

Optimization of whispering gallery resonator design for biosensing applications

Whispering gallery modes (WGMs) within microsphere cavities enable highly sensitive label-free detection of changes in the surrounding refractive index. This detection modality is of particular interest for biosensing applications. However, the majority of biosensing work utilizing WGMs to date has been conducted with resonators made from either silica or polystyrene, while other materials remain largely uninvestigated. By considering characteristics such as the quality factor and sensitivity of the resonator, the optimal WGM sensor design can be identified for various applications. This work explores the choice of resonator refractive index and size to provide design guidelines for undertaking refractive index biosensing using WGMs.

Light Enhancement Within Nanoholes in High Index Contrast Nanowires

We present systematic predictions for light enhancement in optical nanowires with nanoscale air holes in the core through numerical modeling. We show that the light intensity within such holes is strongly dependent on the hole size and refractive index of the host material and that light enhancement becomes significant only when the hole size is less than a critical value: 70 nm for silica and F2 nanowires and 50 nm for a As2S3 nanowire. High index As2S3 nanowires exhibit nearly eight times higher average mode intensity than silica glass for hole sizes of less than 10 nm. Such intensity enhancements open up new device opportunities; for example, filling nanoscale holes within silicon nanowires with silicon nanocrystals enables 30% enhancement of the nonlinear coefficient.

Self-formed cavity quantum electrodynamics in coupled dipole cylindrical-waveguide systems

An ideal optical cavity operates by confining light in all three dimensions. We show that a cylindrical waveguide can provide the longitudinal confinement required to form a two dimensional cavity, described here as a self-formed cavity, by locating a dipole, directed along the waveguide, on the interface of the waveguide. The cavity resonance modes lead to peaks in the radiation of the dipole-waveguide system that have no contribution due to the skew rays that exist in longitudinally invariant waveguides and reduce their Q-factor. Using a theoretical model, we evaluate the Q-factor and modal volume of the cavity formed by a dipole-cylindrical-waveguide system and show that such a cavity allows access to both the strong and weak coupling regimes of cavity quantum electrodynamics.

Low loss, low dispersion T-ray transmission in microwires

We present low loss, < 0.01 l/cm, and dispersion, < 10 ps/(km.nm), properties of microwires for terahertz transmission. These wires have diameters smaller than the operating wavelength, resulting in the propagation of enhanced evanescent fields.

Unified theory of whispering gallery multilayer microspheres with single dipole or active layer sources

The development of a fast and reliable whispering gallery mode (WGM) simulator capable of generating spectra that are comparable with experiment is an important step forward for designing microresonators. We present a new model for generating WGM spectra for multilayer microspheres, which allows for an arbitrary number of concentric dielectric layers, and any number of embedded dipole sources or uniform distributions of dipole sources to be modeled. The mode excitation methods model embedded nanoparticles, or fluorescent dye coatings, from which normalized power spectra with accurate representation of the mode coupling efficiencies can be derived. In each case, the emitted power is expressed conveniently as a function of wavelength, with minimal computational load. The model makes use of the transfer-matrix approach, incorporating improvements to its stability, resulting in a reliable, general set of formulae for calculating whispering gallery mode spectra. In the specific cases of the dielectric microsphere and the single-layer coated microsphere, our model simplifies to confirmed formulae in the literature.

Optimal light collection from diffuse sources: application to optical fibre-coupled luminescence dosimetry

A model is developed to evaluate the light collection of a diffuse light source located at the tip of an optical fibre. The model is confirmed experimentally and used to evaluate and compare the light collection efficiency of different fibre-coupled luminescence dosimeter probe designs. The model includes contributions from both meridional and skew rays, and considers the light collection from an optically attenuating scintillator. Hence the model enables the optimisation of different, but useful and new probe materials such as BeO ceramic. Four different dosimeter architectures are considered, including previously investigated probe designs; the butt-coupled and reflective wall, along with two novel designs. The novel designs utilise a combination of the scintillating material and transparent media to increase the light collection. Simulations indicate that the novel probes are more efficient in light collection for applications in which it is necessary to minimise the volume of the scintillating material.