Application de la théorie cinématique à l'interaction Triplet-Doublet dans les cristaux moléculaires

Suna's kinematic theory is adapted to study Triplet-Doublet interaction in molecular crystals. The applicability of this theory to crystalline anthracene where the triplet exciton motion is nearly two-dimensional permits us to describe correctly the static magnetic field effect on this interaction. In particular the resonance line shape is better reproduced with this theory than with the kinetic theory of Johnson-Merrifield. Agreement of the best fit between experiment and theory gives reasonable values of the Triplet-Doublet interaction rate (Rp ~ 4 x 10-12 cm3 s -1) and of the Triplet-Doublet pair life-time (β-1 ~ 1.4 x 10-9 s).Nous avons adapté la théorie cinématique de Suna à l'étude de l'interaction Triplet-Doublet dans les cristaux moléculaires. L'application de cette théorie à l'anthracène où le mouvement des excitons triplets est quasi bidimensionnel a permis de rendre compte convenablement de l'effet observé sur la modulation de cette interaction par un champ magnétique extérieur. En particulier la forme de la raie de résonance est mieux reproduite par cette théorie que par la théorie cinétique de Johnson-Merrifield. La recherche du meilleur accord avec l'expérience nous a permis d'atteindre la constante d'interaction Triplet-Doublet (Rp ~ 4 x 10-12 cm3 s -1) ainsi que la durée de vie de la paire corrélée Triplet-Doublet (β -1 ~ 1,4 x 10-9 s)

Finite-difference time-domain method for design and analysis of microcavity - Coupled submicron-width waveguides

In this paper the finite-difference time-domain (FDTD) method is reviewed and then used to model and predict the geometric parameters used for the design of the device. The waveguide consists of a periodic array of air gap etched into a silicon (Si) strip on a silicon dioxide (SiO2) layer. The width and the depth of the grooves of the air gap (n = 1) as well as the length of the silicon layer (n = 3.4) are investigated. Using FDTD, the optical parameters are characterized. The effect of the air gap on the field profile distribution of the whole structure is calculated and performed in the range 0.09687 μm –1.55 μm. The field profile, and the response of the microcavity against frequency are calculated from sinusoidal sources. The spectral behavior of the structure is performed and its validity is verified by calculation of the reflectance spectra

Interaction Triplet-Doublet dans les cristaux moléculaires. Cas de l'anthracène

Triplet-Doublet interaction is studied in crystalline anthracene from a static magnetic field effect on the triplet induced photodetrapping current. Experimental observations are interpreted in the frame of Johnson-Merrifield theory where Triplet-Doublet pair states are introduced. We were able to describe correctly the dependence of the effect both in function of the intensity of the applied magnetic field and of its orientation in the crystalline plane. The agreement of the best fit between experiment and theory permitted us to reach the rate constants for pair formation (10-10 cm3 s-1), separation (2.2 x 109 s-1) and reaction (3. 5 x 10 8 s-1) and to have a detailed information on the microscopic mechanism of the interaction.Nous avons étudié l'interaction Triplet-Doublet dans l'anthracène cristallin à partir de l'effet d'un champ magnétique statique sur le photocourant de dépiégeage induit par les triplets. Les observations expérimentales sont interprétées dans le cadre de la théorie de Johnson-Merrifield où on introduit l'existence d'états de paires Triplet-Doublet. On a pu ainsi rendre compte convenablement de la dépendance de l'effet aussi bien en fonction de l'intensité du champ magnétique appliqué que de son orientation dans le plan du cristal. La recherche du meilleur accord entre l'expérience et la théorie nous a permis d'atteindre les constantes de vitesse pour la formation des paires (10-10 cm3 s -1), de leur séparation (2,2 x 109 s-1) et de leur réaction (3,5 x 108 s-1) et d'avoir ainsi une information détaillée sur le mécanisme microscopique de l'interaction

Mode Converter Optical Isolator Based on Dual Negative Refraction Photonic Crystal

A new design of an optical isolator based on photonic transitions in the interbands of a honeycomb structure that generates a dual negative refraction in a photonic crystal is presented. The involved photonic transition is associated to the perturbation of the dielectric constant of the medium. The band structure is determined using the plane wave method where the transmission spectra, field profile, and mode amplitudes are obtained by applying the finite difference time domain method. Due to the time-dependent perturbation of the refractive index of the medium that constitutes the dual negative refraction, asymmetric transmission mechanism is achieved for one of the desired modes, demonstrating optical isolation. Using the dual negative refraction effect in photonic crystal structure, the optical isolation is reported for only one of the desired optical modes. It is anticipated that the proposed mode conversion mechanism can be employed for designing ultrahigh-speed optical interconnections. The proposed optical isolator model is expected to have a significant impact on designing ultrahigh-speed integrated optical platforms

Photoconductivity detected magnetic resonance (PDMR) in crystalline tetracene

A detailed experimental study of the modulation by a high power microwave of induced exciton photoconductivity in crystalline tetracene is presented. These experiments, so called PDMR (Photoconductivity Detected Magnetic Resonance) are supported by a theoretical model based on a matrix density formalism and taking into account the fission of singlet excitons, dimensionality of triplet excitons motions and singlet and triplet detrapping of charge carriers. The best agreement between the observed PDMR lines and calculated curves allows the determination of the pertinent parameters governing the different process involved in the description of the photoconductivity mechanism. Copyright Springer-Verlag Berlin/Heidelberg 2003

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 and 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