Intrachain and interchain triplet-triplet exciton annihilation in a quasi-one-dimensional crystal: 1,4-dibromonaphthalene (DBN)

The modulation of delayed fluorescence in 1,4-dibromonaphthalene by magnetic fields has been studied at room temperature. The analysis of high field resonance positions shows that, besides annihilation of triplet excitons in the same one-dimensional stack, annihiliation also occurs between triplets localized on different, neighbouring stacks, in particular those corresponding to the two molecules of the asymmetric pair.     

Mutual annihilation of triplet excitons in a nearly one-dimensional system: 1,4-dibromonaphthalene

The analysis of the variation with incident flux of the time dependence of the delayed fluorescence in conjunction with the determination of the absolute ground state-first excited triplet absorption coefficients at room temperature, yields the value of γ_tot = (5.5 ± 2.0) × 10^?12 cm³ s^?1, for the total triplet-triplet annihilation rate constant in 1,4-dibromonaphthalene crystals. The one-dimensional mutual annihilation rate constant for the triplet exciton motion restricted to linear chains along the crystal c axis is γ¹_tot = (1.0 ± 0.4) × 10³ cm s^?1. The results are discussed in terms of recent theories of mutual annihilation of triplets in one-dimensional systems.     

Effect of the oligothiophene chain length on the performance of organic photovoltaic cells

We investigate photovoltaic cells based on α-oligothiophene molecules with different chain lengths nT (n=4,6,8) deposited on a sandwich structure between two electrodes: ITO and Al. From the analysis of the absorption and photocurrent spectrum, we have shown that the structure of the oligothiophene affects considerably the absorbing character of the evaporated film, which is increasingly significant from 4T towards 8T, and a significant photocurrent in agreement with strong absorption in the region Al/nT. Moreover, a theoretical model of Ghosh is used to display the photocurrent origin, proved to be sufficient after adjustment with a random term. Then we deduce that the crystalline order of the film improves the transport of charge and the photocurrent. The results are confirmed by electrical characterization under illumination that shows effectively an influence of the expansion of the molecular chains on the increase in the energy efficiencies and on the performances of organic photovoltaic cells.     

Analysis of ultra-small photonic large scale integrated circuits

A detailed study of a platform of ultra-small photonic large-scale integrated circuits was conducted. Bandgap structure calculations of silicon-on-insulator (SOI) based photonic crystals have been investigated. The photonic crystal consists of dielectric cylinders in air. Using the band structure calculations we obtained design parameters for the proposed structures. The coupling between the photonic crystal and a waveguide fabricated from SOI system has been analysed. It is shown that the optical coupling is improved by interfacing different types of spot-size converters (SSCs) between the SOI waveguide and the photonic crystal. Also, the possibility and limitations of silicon doped germanium and SOI photonic crystals to analyse the light guiding in the third dimension is discussed.     

Detailed investigation of self-imaging in multimode photonic crystal waveguides for applications in power and polarization beam splitters

Properties of the self-imaging effect based on multimode interference (MMI) in multimode photonic crystal waveguides (PCWs) are investigated and analyzed in detail. By combining photonic band gap (PBG) and total internal reflection (TIR) effects in PCWs, self-imaging phenomena are achieved for both TE and TM polarizations. To observe the images reproduced by this self-imaging phenomenon, finite-difference time-domain (FDTD) simulations are performed on a multi-mode PCW. From these numerical simulations the reproduced images are identified and their positions along the propagation axis are described. We also, present the design and simulation of novel polarization-insensitive power splitter and polarization splitter. The simulation results show that the optimized devices have excellent transmission efficiencies as well as wide operating frequency bandwidths and small structure size. So, the proposed devices are promising and may play an important role in high-density photonic integrated circuits in the future.     

Design of photonic crystal superlens with improved image resolution

In this paper, we theoretically investigate the spatial resolution of a triangular two-dimensional photonic crystal superlens. We prove that this resolution can be improved by optimizing the air hole radius as well as the air hole shape. First, it is found that by decreasing the radius of the air holes, the spatial resolution is improved significantly. Next, we demonstrate that using elliptic air holes instead of circular air holes in a triangular two-dimensional photonic crystal can leads to good-quality images and focusing, with effective refractive index n_eff=−1 and enhanced image resolution. It is also shown that for the case of two sources, the resolution of such a photonic crystal lens can be made indeed better than the radiation wavelength.     

Design of a novel sensor based on tapered photonic crystal with negative refraction structures

We report a novel design of an ultracompact sensor consisting of two-dimension photonic crystal with negative refraction coupled to a taper ended by multicavity structure. It is shown that this system exhibits promising properties in the area of sensing. The presence of nanocavities enables the detection of small amount of analytes. By controlling their radii, a large quality factor, Q is reached. It is demonstrated that the quality factor is a key parameter in determining the sensitivity to the analytes.     

A novel design of a photonic crystal sensor with improved sensitivity

Biosensors, based on photonic crystal (PC), are emergent subject. The use of PCs in this area brought solutions to both miniaturization and integration challenges that have been facing research groups for long time. We are only recently, by engineering such defects, able to propagate light in complex structures containing molecules of different sizes and shapes. We propose a novel structure containing defects with various sizes. The PC is formed by a dielectric cylinders with permittivity 8.9 (alumina Al₂O₃) and a radius r = 0.2a (a is the square lattice constant), arranged in a square lattice. We use the finite difference time domain to investigate the sensitivity of the proposed sensor to water. The defect based sensing element is introduced in two directions 〈0 1〉 and 〈1 0〉. These simulations show a better sensitivity to water than other analytes. It appears in the transmission curves where the peak shifts to high frequency when the refractive index is changed.     

Study by 1H NMR of the concentration of monomer-units on interface polymer grafted on silica using magic angle spinning and relaxation techniques

The segmental motion of poly(ethylene oxide) grafted on silica has been investigated by 1H and 13C magnetic resonance. The local monomer units concentration on an interface is evaluated. Two independent techniques, namely magnetic relaxation and magic angle spinning, are used and give a good agreement for the physical parameters describing the monomer units concentration on the surface. This behavior has been studied as a function of molecular weight of the chains, and each time also as a function of temperature.