Finite-Difference Time-Domain Study of Guided Modes in Nano-plasmonic Waveguides

A conformal dispersive finite-difference time-domain (FDTD) method is developed for the study of one-dimensional (1-D) plasmonic waveguides formed by an array of periodic infinite-long silver cylinders at optical frequencies. The curved surfaces of circular and elliptical inclusions are modelled in orthogonal FDTD grid using effective permittivities (EPs) and the material frequency dispersion is taken into account using an auxiliary differential equation (ADE) method. The proposed FDTD method does not introduce numerical instability but it requires a fourth-order discretisation procedure. To the authors' knowledge, it is the first time that the modelling of curved structures using a conformal scheme is combined with the dispersive FDTD method. The dispersion diagrams obtained using EPs and staircase approximations are compared with those from the frequency domain embedding method. It is shown that the dispersion diagram can be modified by adding additional elements or changing geometry of inclusions. Numerical simulations of plasmonic waveguides formed by seven elements show that row(s) of silver nanoscale cylinders can guide the propagation of light due to the coupling of surface plasmons.Comment: 6 pages, 10 figures, accepted for publication, IEEE Trans. Antennas Propaga

Photonic crystals for confining, guiding, and emitting light

We show that by using the photonic crystals, we can confine, guide, and emit light efficiently. By precise control over the geometry and three-dimensional design, it is possible to obtain high quality optical devices with extremely small dimensions. Here we describe examples of high-Q optical nanocavities, photonic crystal waveguides, and surface plasmon enhanced light-emitting diode (LEDs)

Optimized magneto-optical isolator designs inspired by seedlayer-free terbium iron garnets with opposite chirality

Simulations demonstrate that undoped yttrium iron garnet (YIG) seedlayers cause reduced Faraday rotation in silicon-on-insulator (SOI) waveguides with Ce-doped YIG claddings. Undoped seedlayers are required for the crystallization of the magneto-optical Ce:YIG claddings, but they diminish the interaction of the Ce:YIG with the guided modes. Therefore new magneto-optical garnets, terbium iron garnet (TIG) and bismuth-doped TIG (Bi:TIG), are introduced that can be integrated directly on Si and quartz substrates without seedlayers. The Faraday rotations of TIG and Bi:TIG films at 1550nm were measured to be +500 and -500°/cm, respectively. Simulations show that these new garnets have the potential to significantly mitigate the negative impact of the seedlayers under Ce:YIG claddings. The successful growth of TIG and Bi:TIG on low-index fused quartz inspired novel garnet-core waveguide isolator designs, simulated using finite difference time domain (FDTD) methods. These designs use alternating segments of positive and negative Faraday rotation for push-pull quasi phase matching in order to overcome birefringence in waveguides with rectangular cross-sections

The ultra-wideband pulse

Since the birth of mode-locking the temporal duration of optical pulses has radically diminished. In parallel to this, bandwidths have grown so large that almost entire frequency octaves are present in today’s few-cycle pulses. This thesis investigates the character of ultra-wideband pulses in nonlinear environments. Because of the growth in optical bandwidths, traditional definitions and propagation models break down, requiring newer more accurate numerical techniques. A novel approach capturing the uni-directionality of pulses is presented in the form of Gvariables by combining the electric and magnetic field descriptions. These G-variables have the advantage of both an accurate spectral representation and a reduced computational overhead, making them significantly more efficient than existing direct Maxwell solvers. Such approaches are particularly important where large propagation distances and/or transverse dimensions are concerned. Pseudo-spectral techniques play a key role in the success of these wideband models enabling sub-cycle dynamics to be studied. One such phenomenon is Carrier Wave Shocking (CWS), where the optical carrier undergoes self-steepening in the presence of third-order nonlinearity. This process is carefully studied, focussing on the effect of dispersion and the feasibility of its physical realisation. The process is then generalised to arbitrary nonlinear order, where the quadratic form finds potential applications in High Harmonic Generation (HHG). Shock detection schemes are also developed, and agree with analytical solutions in the dispersionless regime. To fully characterise few-cycle pulses, the absolute Carrier Envelope Phase (CEP) must be known. A novel 0 − f self-referencing scheme relying on wideband interference is investigated. By applying robust frequency domain definitions a proposal is made to convert this scheme into one that determines absolute CEP. The scheme maps the level of spectral interference to absolute CEP using numerical simulations

FDTD modelling of electromagnetic transformation based devices

PhDDuring this PhD study, several finite-difference time-domain (FDTD) methods were developed to numerically investigate coordinate transformation based metamaterial devices. A novel radially-dependent dispersive FDTD algorithm was proposed and applied to simulate electromagnetic cloaking structures. The proposed method can ac- curately model both lossless and lossy cloaks with ideal or reduced parameters. It was demonstrated that perfect “invisibility” from electromagnetic cloaks is only available for lossless metamaterials and within an extremely narrow frequency band. With a few modifications the method is able to simulate general media, such as concentrators and rotation coatings, which are produced by means of coordinate transformations techniques. The limitations of all these devices were thoroughly studied and explo- red. Finally, more useful cloaking structures were proposed, which can operate over a broad frequency spectrum. Several ways to control and manipulate the loss in the electromagnetic cloak ba- sed on transformation electromagnetics were examined. It was found that, by utili- sing inherent electric and magnetic losses of metamaterials, as well as additional lossy materials, perfect wave absorption can be achieved. These new devices demonstrate super-absorptivity over a moderate wideband range, suitable both for microwave and optical applications. Furthermore, a parallel three-dimensional dispersive FDTD method was introdu- ced to model a plasmonic nanolens. The device has its potential in subwavelength imaging at optical frequencies. The finiteness of such a nano-device and its impact on the system dynamic behaviour was numerically exploited. Lastly, a parallel FDTD method was also used to model another interesting coordinate transformation based device, an optical black hole, which can be characterised as an omnidirectional broad- band absorber