Spot-size conversion effect on integrated photonic devices

Using numerically simulated results, we show that an efficient laser-to-optical fibre coupling is possible by incorporating a uniform spot size converter (SSC) based on diluted waveguides with low contrast index waveguide. We propose photonic devices and circuits composed of a laser coupled through a SSC to a one and more microdisks, which are coupled to an optical fibre. Using the finite-difference time-domain (FDTD) method, we simulate the propagation characteristics of light in the fundamental elements as the pulse propagates down the whole structure. Also by the use of the FDTD method, we model and predict the geometrical parameters used for the design of the different types of the proposed devices. It is shown that the coupling efficiency is less than 10% without the diluted waveguide, but the SSC couplers have over 73% power transmission. The gap width between the microdisks, coupling between the laser and the sequence of microdisks is investigated. We report the microdisk effect on the coupling efficiency between the laser-SSC structure and the optical fibre.     

FDTD study of subwavelength imaging by a photonic crystal slab

The problem of subwavelength imaging via a photonic crystal slab lens made of two-dimensional (2D) square arrays of parallel dielectric cylinders in air is studied and discussed theoretically. The finite-difference time-domain method is employed to investigate the unique features of imaging by such lens. We confirm earlier findings that a photonic crystal slab lens can provide the imaging of a point source. By analysing the transmission properties of the proposed structure, we demonstrate that inside the all-angle negative refraction, there are some favourable directions for waves to travel. We show that the surface termination of the photonic crystal is a key parameter to obtain a good quality image. The super-resolution of two sources separated by a distance less than the wavelength is also considered. It is shown that the achievable resolution is limited by the slab length.     

Application of the short and long consecutive pairs model to the triplet-doublet interaction in molecular crystals

We have adapted the model of two consecutive pairs to the study of the triplet-doublet (T-D) interaction in molecular crystals. We have applied this model to the modulation of the photoconductivity in crystalline anthracene by a static magnetic field (MFE) and a microwave field (PDMR). We were able to reproduce, for the first time, quite perfectly two types of experiments with the same set of kinetic constants.     

Dependence of hole effective mass on nitrogen concentration in W-type strained InAs(N)/GaSb/InAs(N) quantum well lasers

We have investigated the effects of nitrogen N concentration on the properties of hole subbands and effective mass in dilute-nitride type-II InAsN/GaSb laser diodes on InAs substrate with “W” design. Using a 5-bands k·p model, we obtained interesting numerical results for the heavy-hole (hh) and the light-hole (lh) subbands. The hole effective masses were found to be very sensitive to the nitrogen concentration and to the differences in the Luttinger parameters between the well and the barrier. In addition, the hole effective masses are found to be strongly affected by band-anticrossing (BAC) model.     

Modelling of photonic crystals for the design of photonic device based on SOI substrate

In this paper, we report a photonic integrated circuit (PIC), which consisting of a photonic crystal (PC) coupled by a dielectric waveguide to an optical fibre. The PC consists of a sequence of dielectric rods based on a silicon (Si) strip on a silicon dioxide (SiO₂) layer. The finite-difference time-domain (FDTD) method is reviewed and then used to model and predict the optical and the geometrical parameters used to design the fundamental elements of the PIC. The air gap width and the etching depth of the grooves are characterised. The coupling between the PC, and traditional dielectric waveguides is studied and coupling efficiency is evaluated. Diffractive losses are shown to affect strongly the performances of the proposed PIC. In addition, the effect of the air gap width on the diffractive losses and the coupling efficiency between successive neighbouring silicon sections is analysed. The field profile distribution in the structure is calculated and performed. The effects of an incorporated defect are studied, showing a high quality factor.     

Symmetrized Hamiltonian versus ‘Foreman’ Hamiltonian for semiconductor valence band: an insight

A quantitative comparison of different k·p calculations of valence-band states in quantum-confined semiconductor heterostructures is presented. The importance of using the appropriate Hamiltonian form is studied quantitatively following a numerical method which enables discrimination between existing Hamiltonians. The correct form of the Hamiltonian appears to be the Foreman Hamiltonian, which gives physically reasonable results contrary to the symmetrized Hamiltonian.     

Saturation of the ODMR effect on singlet exciton induced emission in molecular crystals: Application to crystalline tetracene

We have studied the variation of the ODMR effect according to the intensity of a microwave field. We have highlighted a saturation effect of ODMR lines. This effect has been studied as part of the kinetic theory by developing the pilot equation while utilizing for the first time the formalism of a superoperator coupled with a time dependent sinusoidal perturbation. The agreement of the best fit between the experiment and the theory gives the mixing between singlet and quintuplet states, which seems to depend on the direction of the applied magnetic field.Received: 22 September 2003, Published online: 28 May 2004PACS: 71.35.-y Excitons and related phenomena - 32.30.Dx Magnetic resonance spectra - 76.70.Hb Optically detected magnetic resonance (ODMR)     

Quenching of triplet excitions by trapped holes in crystalline pyrene

The rate of quenching of triplet excitions by trapped holes in crystalline pyrene has been measured as a function of hole concentration. The rate constant is γ=4.5×10^?11 cm³ sec^?1 within 50%, not far below that of a diffusion limited process. The quenching by free holes is not observed in the experimental conditions used.     

Design of a compact photonic crystal sensor

The development of technology in photonic crystal (PC) structures has seen rapid progress. Using PCs in biosensing area may open new venues to achieve single molecule detection, and high resolution scanning. A novel PC sensor with improved performances, in terms of size, compactness and sensitivity is presented in this paper. The sensing element consists of dielectric cylinders with varying radius introduced along <01> and <10> directions of the crystal. The results show that the peak wavelength shifts to the high frequency region when only six cylinders are filled with analytes. Also, the peaks show a larger shift compared to the structure obtained using the entire PC waveguide as sensing region. The proposed sensor shows a better sensitivity to water than other analytes, where the peak wavelength tends to shift towards the low frequency region.