Nonparaxial description of reflection and transmission at the interface between an isotropic medium and a uniaxial crystal

Angular spectra of reflected and transmitted fields, induced by an arbitrary electromagnetic beam passing through the planar interface between a homogeneous medium and a uniaxially anisotropic medium, are derived and related to the incident medium. By using these formulas, we obtain the expressions for paraxial and slightly nonparaxial fields. The reflected paraxial field is related to the incident one by means of Fresnel relations; the transmitted paraxial field is the superposition of an ordinary and an extraordinary beam, multiplied by the Fresnel coefficient. We find that the nonparaxial corrections, owing to the medium discontinuity, are larger than their free-propagation counterparts and that they are very simply related to the paraxial solutions of the incident beam. The case of two homogeneous media with different refractive indices is also discussed. The general expressions obtained are applied to the case of a nonparaxial Gaussian beam.     

Alternative Translation Techniques for Propositional and First-Order Modal Logics

We describe and analyze techniques, other than the standard relational/functional methods, for translating validity problems of modal logics into first-order languages. For propositional modal logics we summarize the -as-Pow method, a complete and automatic translation into a weak set theory, and then describe an alternative method, which we call algebraic, that achieves the same full generality of -as-Pow but is simpler and computationally more attractive. We also discuss the relationships between the two methods, showing that -as-Pow generalizes to the first-order case. For first-order modal logics, we describe two extensions, of different degrees of generality, of -as-Pow to logics of rigid designators and constant domains.     

Micro-SPE method for sample introduction in capillary HPLC/MS

A new solid-phase extraction on-line device for micro-HPLC is presented. This device optimizes the injection of very dilute samples into a packed capillary column. It consists of two capillary, reversed-phase, HPLC columns of different length that can be linked together as a single chromatographic column. The first segment, only 2 cm long is connected to the HPLC injector. When disconnected from the longer column, several milliliters of an aqueous sample can be passed through at a high flow rate for fast trapping. On the basis of the retention mechanism, all suitable compounds are focused on the short column head in a sharp band. As soon as the chromatographic column is recomposed, the trapped analytes are eluted and separated at the optimal flow rate and gradient conditions. Due to the high preconcentration factor, trace-level analysis can be performed successfully. Different classes of analytes of various polarities and molecular weights can be determined, depending on the stationary phase and on the detector used. Some pesticides belonging to different classes were chosen to evaluate the performance of the device using an electron ionization mass spectrometer as HPLC detector. A fungicide in an irrigation canal water was determined at a concentration level of 4.5 microg x L(-1).     

Computing using signals: from cells to P systems

In cell biology a fundamental topic is the study of how biological signals are managed by cells. Signals can arise from inside the cell or from the external environment and the correct answer to certain signals is essential for bacteria to survive in a certain environment. Starting from these biological motivations we consider a model of P systems where the computation is controlled by signals which move across the regions. In particular, we consider signals-based P systems where the symbol-objects cannot be moved and the evolution rules can be activated/inactivated using a finite number of signals (signal-promoters) moved across the membranes; differently from standard P systems using promoters, in our case signal-promoters cannot be created during the computation. After discussing the biological motivations we show how this model becomes universal when it uses one catalyst and a bounded number of signal-promoters. Also results concerning signals-based P systems using non cooperative rules together with several open problems are presented.     

Magnetic resonance imaging and67ga scan versus computed tomography in the staging and in the monitoring of mediastinal malignant lymphoma: a prospective pilot study

Purpose: To assess the potential value of magnetic resonance imaging (MRI) combined with⁶⁷Ga single-photon emission computed tomography (SPECT) versus computed tomography (CT) in the staging and in the monitoring of mediastinal malignant lymphoma. Materials and methods: Twenty-three patients, referred to our institute for the evaluation of lymphoma, underwent CT,⁶⁷Ga scan, and MRI between April 1993 and February 1996 at sequential intervals. The tests studied (MRI,^67Ga, and CT) were performed according to the following schedule: 1) before start of therapy; 2) after four courses of chemotherapy; and 3) 2, 6, 12, and 18 months after the end of treatment. Results: All patients studied at the time of diagnosis had abnormal gallium accumulation in the mediastinum as well as pathologic CT and pathologic signal intensity at MRI. Six months after the end of treatment full consistency was found between the results of MRI and SPECT, whereas during treatment and 2 months after the end of therapy MRI and⁶⁷Ga scan were not in agreement in nine patients. In the 23 patients in follow-up, in CT there were nine false-positive and three false-negative findings; in SPECT three false negatives; in MRI one false positive and one false negative. Conclusion: MRI can give morphologic information similar to CT, even superior due to multiplanarity and with major precision in the distinction between fibrosis and active disease. MRI is thus an alternative to CT. The association with SPECT allows a great diagnostic accuracy in the positive and negative predictive value.     

Wavelength-flattened directional couplers: a geometrical approach

A new approach to design a wavelength-insensitive optical power splitter is presented. First, a coupled-mode theory is cast in operatorial form. This allows us to solve the equivalent of coupled differential equations as simple limits. The operators are then represented on a generalized Poincaré sphere, and the resulting graphical tool is applied to different structures, giving a clear interpretation of previous results in literature as well as hints on how to find improved solutions.     

Rapid detection of Paenibacillus larvae from honey and hive samples with a novel nested PCR protocol

The bacterial pathogen Paenibacillus larvae is the causative agent of American foulbrood disease in honeybees (Apis mellifera). A touchdown nested PCR protocol was developed to detect the presence of P. larvae spores directly in honey and hive samples. This approach allows early discovery of the bacteria even at concentrations below pathogenic levels, opening the door to new prophylactic approaches against American foulbrood and real-time epidemiological studies.