Effects of annealing on luminescence efficiency of large-size YAG:Ce crystal grown by temperature gradient techniques

<正>YAG:Ce crystal with a diameter of 110 mm is successfully grown by the temperature gradient technique (TGT).The effects of annealing on the luminescence efficiency of YAG:Ce crystal are investigated,and the optimal annealing temperature and atmosphere are obtained.The mechanism of variation behavior of the luminescence efficiency of YAG:Ce under different annealing conditions is discussed and some details on the luminescence associated with color centers are analyzed.     

Numerical simulations of the impact of wavefront phase distortions of pump on the beam quality of OPA

<正>A numerical model of optical parametric amplification(OPA) is introduced to investigate the impact of wavefront phase distortion of pump on the beam quality of signal.Numerical results show that the unidentical walk-off directions of the pump and the idler waves are the main factors leading to the transfer of wavefront phase distortions of the pump to the signal,and by reducing the angle between the two directions,the beam quality factor(M~2) can be greatly decreased and hence the good beam quality of the signal can be maintained.     

Subtleties in the Distribution of the Numbers of Points on Elliptic Curves Over a Finite Prime Field

Three questions concerning the distribution of the numbers ofpoints on elliptic curves over a finite prime field are considered.First, the previously published bounds for the distributionare tightened slightly. Within these bounds, there are wildfluctuations in the distribution, and some heuristics are discussed(supported by numerical evidence) which suggest that numbersof points with no large prime divisors are unusually prevalent.Finally, allowing the prime field to vary while fixing the fieldof fractions of the endomorphism ring of the curve, the orderof magnitude of the average order of the number of divisorsof the number of points is determined, subject to assumptionsabout primes in quadratic progressions. There are implications for factoring integers by Lenstra's ellipticcurve method. The heuristics suggest that (i) the subtletiesin the distribution actually favour the elliptic curve method,and (ii) this gain is transient, dying away as the factors tobe found tend to infinity.     

A necessary condition for Calderón-Zygmund singular integral operators

Calderón-Zygmund singular integral operators have been extensively studied for almost half a century. This paper provides a context for and proof of the following result: If a Calderón-Zygmund convolution singular integral operator is bounded on the Hardy space H¹ (Rⁿ), then the homogeneous of degree zero kernel is in the Hardy space H¹(S^n–1) on the sphere.     

Structural Learning: Attraction and Conformity in Task-Oriented Groups

This study extends previous research that showed how informal social sanctions can backfire when members prefer friendship over enforcement of group norms. We use a type of neural network to model the coordination of informal social control in a small group of adaptive agents confronted with a social dilemma. This model incorporates two mechanisms of social influence, informal sanctions and imitation. Both mechanisms vary with the strength of the social tie between source and target. Previous research focused on the effects of social sanctions. Here, we demonstrate a curvilinear effect of imitation on compliance with prosocial norms. Moderate doses of imitation reduce the coordination complexity of self-organized collective action and help the network achieve satisfactory levels of cooperation. High doses, however, undermine the agent-based learning required to find cooperative solutions. Increasing group size also diminishes compliance due to increased complexity, with larger groups requiring more imitation to overcome the coordination problem.     

On the contractability of the unitary group of the Hilbert space over a C⋆-algebra

If A is a C-algebra with identity, then the unitary group of L(H_A) is contractable.This research is part of the author's doctoral work under the supervision of Professor P. A. Fillmore and was supported by an NSERC Scholarship.     

Multidimensional van der Corput and sublevel set estimates

Van der Corput's lemma gives an upper bound for one-dimensional oscillatory integrals that depends only on a lower bound for some derivative of the phase, not on any upper bound of any sort. We establish generalizations to higher dimensions, in which the only hypothesis is that a partial derivative of the phase is assumed bounded below by a positive constant. Analogous upper bounds for measures of sublevel sets are also obtained. The analysis, particularly for the sublevel set estimates, has a more combinatorial flavour than in the one-dimensional case.

     

Comparison of fast field-cycling magnetic resonance imaging methods and future perspectives

ABSTRACT

Fast field-cycling (FFC) nuclear magnetic resonance relaxometry is a well-established method to determine the relaxation rates as a function of magnetic field strength. This so-called nuclear magnetic relaxation dispersion gives insight into the underlying molecular dynamics of a wide range of complex systems and has gained interest especially in the characterisation of biological tissues and diseases. The combination of FFC techniques with magnetic resonance imaging (MRI) offers a high potential for new types of image contrast more specific to pathological molecular dynamics. This article reviews the progress in FFC-MRI over the last decade and gives an overview of the hardware systems currently in operation. We discuss limitations and error correction strategies specific to FFC-MRI such as field stability and homogeneity, signal-to-noise ratio, eddy currents and acquisition time. We also report potential applications with impact in biology and medicine. Finally, we discuss the challenges and future applications in transferring the underlying molecular dynamics into novel types of image contrast by exploiting the dispersive properties of biological tissue or MRI contrast agents.     

Gastroesophageal reflux laryngitis resistant to omeprazole therapy

Gastroesophageal reflux disease (GERD) is known to cause a variety of symptoms that lead a patient to seek otolaryngologic care. New advances in the treatment of GERD have enabled otolaryngologists to eliminate most of the signs and symptoms caused by acid reflux. Omeprazole, the most recent pharmacologic advancement, has been reported to be universally successful in controlling acid release from the stomach of patients with GERD. This report describes a series of patients with GERD for whom high-dose omeprazole therapy was not successful in completely reducing gastric acid levels of GERD symptomatology.