Nanoparticles of: Synthesis and superparamagnetic properties

By using oil in water micelles, cobalt ferrite particles having an average diameter around 3 nm were synthetised. These nanoparticles are characterized by the presence of cation vacancies and no Fe(II) is observed, as it has been described in literature previously. Chemical interfacial treatment allows to coat the particles with citrate derivatives. The magnetic properties of uncoated and coated particles strongly diluted in a polymer substrate are compared by magnetization measurements and ⁵⁷Fe M?ssbauer spectroscopy. The anisotropy constant is shown to be independent of coating, whereas the magnetization is found to be larger in the uncoated particles. Received 3 February 1998     

Power factors of late rare earth-doped Ca3Co2O6 oxides

Polycrystalline samples of (Ca_1−xR_x)₃Co₂O₆ with R = Gd, Tb, Dy and Ho at x=0-0.1 were synthesized and the effects of rare earth substitution on their thermoelectric properties were investigated. In the high-temperature region, the rare earth substitution resulted in an increase in the Seebeck coefficients (S), and the S values increased with decreasing ionic radius of rare earth elements in the order Gd³⁺>Tb³⁺>Dy³⁺>Ho³⁺. In contrast, the influence of rare earth substitution on the electrical resistivity was small. The high-temperature power factor was thereby improved by the late rare earth substitutions, particularly those with Ho³⁺ for Ca²⁺. For the Ho-doped samples (x≤0.05), the power factor was significantly improved by increasing Ho concentration.     

Chirped fiber Bragg gratings with arbitrary group delay responses using stepped-stretching fiber method

We present the design and development of piece-wise stepped-chirp fiber Bragg gratings (FBGs) with arbitrary group delay responses using a uniform phase mask in a pre-stretched fiber. The method involves writing a series of sub-gratings on a pre-stretched fiber whose length is varied during the UV exposure process. Two motorized stages are employed to control the length of the pre-stretched fiber in order to adjust the grating pitch at each writing step to achieve piece-wise stepped chirping of the FBG. To demonstrate the effectiveness of the method to produce chirped FBGs with arbitrary group delay responses, a linear-chirp FBG and a quadratic-chirp FBG were fabricated, and their measured performances agree well with analytical predictions. PACS 42.81.-i; 42.79.Dj; 42.79.Ci     

Polymer-based waveguides with low propagation loss and polarization-dependent loss

Polymeric optical waveguides based on a new synthesized bisphenol A–aldehyde polymer (PA-1 resin) have been fabricated using photolithography and reactive ion etching technique. The polymer is novel with relatively high refractive index, low birefringence and absorption at the optical communication wavelengths. The single-mode channel waveguides exhibit a propagation loss of 0.41±0.05 dB/cm at a wavelength of and 0.5±0.05 dB/cm at for both the TE and TM polarizations. The polarization-dependent loss of the waveguides is 0.1±0.05 dB/cm at these wavelengths.     

Performance of optical chaotic communication systems using multimode vertical cavity surface emitting lasers

Influence of side modes on the performance of optical chaotic communication systems is studied theoretically. Three coding techniques, namely chaotic modulation, chaotic masking and chaotic shift keying, are also considered in the investigation. It can be shown that communication system using chaotic shift keying has better immunity to side mode. On the other hand, frequency division multiplexing using multimode vertical cavity surface emitting laser is proposed to double the transmission capacity of the communication system.     

Engaging Copper(III) Corrole as an Electron Acceptor: Photoinduced Charge Separation in Zinc Porphyrin–Copper Corrole Donor–Acceptor Conjugates

An electron‐deficient copper(III) corrole was utilized for the construction of donor–acceptor conjugates with zinc(II) porphyrin (ZnP) as a singlet excited state electron donor, and the occurrence of photoinduced charge separation was demonstrated by using transient pump–probe spectroscopic techniques. In these conjugates, the number of copper corrole units was varied from 1 to 2 or 4 units while maintaining a single ZnP entity to observe the effect of corrole multiplicity in facilitating the charge‐separation process. The conjugates and control compounds were electrochemically and spectroelectrochemically characterized. Computational studies revealed ground state geometries of the compounds and the electron‐deficient nature of the copper(III) corrole. An energy level diagram was established to predict the photochemical events by using optical, emission, electrochemical, and computational data. The occurrence of charge separation from singlet excited zinc porphyrin and charge recombination to yield directly the ground state species were evident from the diagram. Femtosecond transient absorption spectroscopy studies provided spectral evidence of charge separation in the form of the zinc porphyrin radical cation and copper(II) corrole species as products. Rates of charge separation in the conjugates were found to be of the order of 10¹⁰ s^?1 and increased with increasing multiplicity of copper(III) corrole entities. The present study demonstrates the importance of copper(III) corrole as an electron acceptor in building model photosynthetic systems.     

Cs oxide aggregation in SIMS craters in organic samples for optoelectronic application

This paper presents evidence for Cesium (Cs) oxide aggregation in secondary ion mass spectrometry (SIMS) craters in organic samples used for optoelectronic devices, after exposure to air. The morphology of craters after Cs⁺ bombardment at low impact energy was investigated under vacuum and after exposure to air by different techniques, including surface roughness characterization by Atomic Force Microscopy (AFM), surface chemical mapping and quantification by SIMS, Auger Electron Spectroscopy (AES) and X-ray Photon Spectroscopy (XPS). Altogether, two different organic molecules used in optoelectronic devices, Tris(8‐hydroxyquinolinato) aluminum (Alq₃) and Copper Phthalocyanine (CuPc), different layer thicknesses and deposition methods were studied as a function of depth and as a function of duration to air exposure. The results show that, after exposure to air, Cs containing particles appear at the crater bottom under certain conditions. These results are useful to understand the behavior of implanted Cs in view of avoiding artifacts when Cs-implanted samples need to be transferred by air to other instruments for subsequent analyses (e.g. the analysis technique mentioned above).