Surface effects on photoluminescence of single ZnO nanowires

The influence of surface effects on the temperature dependent photoluminescence (PL) spectra from individual ZnO nanowires has been studied. It is found that the surface effects of the nanowire are very important in both ultraviolet (UV) and visible emission. We propose a new luminescence mechanism based on the recombination related to oxygen vacancies to explain the temperature dependent visible emission, which is significantly influenced by the carrier depletion and band bending caused by surface effects. In addition, the observed attenuation of UV emission with increasing temperature is ascribed to the decreasing depletion region and the increasing surface states related nonradiative recombination.     

Directional coupler using gap plasmon waveguides

Here, we demonstrate that efficient nano-optical couplers can be developed using closely spaced gap plasmon waveguides in the form of two parallel nano-sized rectangular slots in a thin metal film or membrane. Using the rigorous numerical finite-difference and finite element algorithms, we investigate the physical mechanisms of coupling between two neighboring gap plasmon waveguides and determine typical coupling lengths for different structural parameters of the coupler. Special attention is focused onto the analysis of the effect of such major coupler parameters, such as thickness of the metal film/membrane, slot width, and separation between the plasmonic waveguides. Detailed physical interpretation of the obtained unusual dependencies of the coupling length on slot width and film thickness is presented based upon the energy consideration. The obtained results will be important for the optimization and experimental development of plasmonic sub-wavelength compact directional couplers and other nano-optical devices for integrated nanophotonics.     

Femtosecond waveguide writing: a new avenue to three-dimensional integrated optics

Using tightly focussed femtosecond laser pulses, waveguides can be fabricated inside various glasses and crystals. This technique has the potential to generate not only planar but three-dimensional photonic devices. In this paper we present, to the best of our knowledge, the first true three-dimensional integrated optical device, a 1×3 splitter fabricated in pure fused silica. The optical properties of this device and possibilities for the fabrication of complex high-density integrated optical elements are discussed. Received: 23 December 2002 / Accepted: 6 January 2003 / Published online: 28 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-3641/65-7680, E-mail: nolte@iap.uni-jena.de     

Chemical role of amines in the colloidal synthesis of CdSe quantum dots and their luminescence properties

The role of organic amines in the colloidal synthesis of CdSe quantum dots (QDs) has been studied. CdSe QDs were synthesized from the source solutions containing 5 vol% of amines having various alkyl chain lengths, stereochemical sizes and electron donation abilities. The role of the additional amines was evaluated on the basis of the photoluminescence (PL) properties such as PL wavelength and intensity of the obtained CdSe QDs. The observed PL spectra were explained by the fact that the amines behaved as capping ligands on the surface of the QDs in the product colloidal solution and complex ligands for cadmium in the source solutions. It was shown that the particle size was controlled by the diffusion process depending on the mass and stereochemical shape of the amines, and the luminescence intensity increased with the increasing electron donation ability and capping density of the amines.     

Electrochemical Deposition and Properties of ZnTe Nanowire Array

Pulsed electrodeposited technique is applied to fabricate ZnTe nanowire arrays with different diameters into the anodic alumina membrane in citric acid solution. The x-ray powder diffraction, scanning electron microscopy and transmission electron microscopy indicate that the high ordered, uniform and single-crystalline nanowires have been fabricated. The optical absorption spectra of the nanowire array show that the optical absorption band edge of the ZnTe nanowire array exhibit a blue shift compared with that of bulk ZnTe, and the nonlinear current-voltage characteristic is observed.     

Effective index method for planar lightwave circuits containing directional couplers

A simple and intuitive effective index method for planar lightwave circuits containing directional couplers is proposed. The optimal cladding refractive index of the effective two-dimensional planar waveguide is found according to the coupling length of the directional coupler involved. Numerical examples indicate that this proposed effective index method gives accurate results while saving significant computation time in simulation and design of planar lightwave circuits containing directional couplers.     

Analytical and numerical study on grating depth effects in grating coupled waveguide sensors

The in-coupling process for grating-coupled planar optical waveguide sensors is investigated in the case of TE waves. A simple analytical model based on the Rayleigh–Fourier–Kiselev method is applied to take into account the depth of the grating coupler, which is usually neglected in the modeling. Analytical expressions are derived both for the position and width of the in-coupling peaks to illustrate the effects of grating depth on the guided mode resonances in grating coupled waveguide sensors. Numerical computations verify the model for shallow gratings both in terms of peak shape and position and provide the limitations for the analytical formulas.     

Simple low-loss waveguide bends using ARROW effect

A new concept for reducing bend loss in dielectric planar waveguides is presented. It is based on the introduction of a set of antiresonant reflecting optical waveguides (ARROWs) on the outside of the bent core and defined in the same fabrication step as the main bend. It has been ascertained by simulation that the bending loss can be significantly reduced. The method is compatible with other bend-lossreduction strategies, as well as with different waveguiding structures, such as rib and buried waveguides and fibers.This revised version was published online in August 2005 with a corrected cover date.     

Geometry Parameter Optimization of Large Cross-Sectional S-Shaped Bent Rib Waveguides for Silicon Based Optical Switches

By using a genetic algorithm, geometry parameters of large cross-sectional S-bend rib waveguides are optimized aiming at the least total loss when the propagation loss is considered. Optimized results axe presented as an example of S-bend rib waveguides based on silicon-on-insulator （SOI） 4 × 4 optical switches. The value of 2 dB/cm is given to the propagation loss according to the experimental results. The simulation results indicate that the total loss drops from 1.0002rib down to 0.4375dB without considering a lateral offset. If the offset is adopted, the total loss reduces from 0.5463dB to 0.2365dB. In addition, the effect of the rib height ratio on the loss is analysed, and the optimal ratio is obtained to be 0.55.     

Vertically and laterally waveguide-coupled cylindrical microresonators in Si3N4 on SiO2 technology

High-finesse laterally and vertically waveguide-coupled cylindrical microresonators have been designed and realized in Si₃N₄ on SiO₂ technology using conventional optical lithography. Based on the experimental results and detailed simulations, a comparison between the lateral and vertical coupling schemes is made. Received: 16 May 2001 / Revised version: 13 August 2001 / Published online: 2 November 2001     

Binary metal and semiconductor quantum dot metamaterials with negative optical dielectric constant and compensated loss for small volume waveguides,modulators and switches

We study numerically and analytically a binary mixture of quantum dots exhibiting gain and loss. For a mixture of gain quantum dots and silver nanoparticles, we find conditions when the composite shows negative dielectric constant operation and lossless operation. The composites of this kind may be used for dense integration of photonic components as well as modulation and switching in optical interconnect systems L. Thylen is also at Dept of Microelectronics and Applied Physics, Royal Institute of Technology (KTH), 164 40 Kista, Sweden.     

Negative index materials with gain media for fast modulation

Negative index materials (NIM) enable subwavelength resolution and are promising for applications in integrated optical systems, since the mode volume is small. Most promising NIM systems essentially use noble metals (Ag, Au) with material losses much lower than in other metals, but still rather hefty, like in metal–dielectric–metal “fishnets”. Therefore, we perform extensive finite-difference time-domain modeling of NIM “fishnets” in combination with gain medium, InGaAsP multiple quantum wells in the present work. The signal recovery is weak, which is related to weak overlap between the radiation field and the gain medium. The signal modulation speed may be very large, in a picosecond range.     

Planar Er- and Yb-doped amplifiers and lasers

New results in erbium-doped planar waveguide amplifiers and lasers are presented. The waveguides are produced in silica-on-silicon technology using plasma-enhanced chemical vapor deposition. Waveguide propagation losses outside the erbium absorption band are improved to values below 0.05 dB/cm for a core–cladding index step of around 1.4%. Amplifier arrays show a net gain of more than 12 dB over the entire C-band. Received: 13 June 2001 / Published online: 23 October 2001