Fourier synthesis of 1.8-THz optical-pulse trains by phase locking of three independent semiconductor lasers

Fourier synthesis of ultrafast optical-pulse trains was demonstrated based on optical phase locking of three independent continuous-wave semiconductor lasers. Pulse repetition frequencies as high as 1.81 THz were limited by the gain bandwidth of an erbium-doped fiber amplifier. The waveforms of the pulse trains were maintained over a long period by use of an auxiliary optical phase-locked loop. The repetition frequency could be tuned continuously over a range of 100 MHz.     

Direct measurement of the parapositronium lifetime in alpha-SiO2

The mean lifetime of delocalized parapositronium in alpha-SiO2 has been determined directly for the first time using a newly developed positron lifetime spectrometer based on BaF2 scintillators and a fast digital oscilloscope. The lifetime is found to be 156+/-4 ps, which is much longer than its intrinsic lifetime of 125 ps. This indicates clearly that the primary many-body effect on positronium in alpha-SiO2 is the screening of the Coulomb interaction between the constituent particles by electrons of the medium and that the average distance between the electron and positron in positronium becomes larger than its vacuum value.     

Measurement of the chromatic dispersion of an optical fiber by use of a Sagnac interferometer employing asymmetric modulation

We describe a novel method, based on asymmetric modulation in a Sagnac interferometer, that measures the chromatic dispersion of single-mode fibers. The interferometer incorporates a phase modulator and a test fiber, so the dispersion can be determined from the interference fringe seen when a sweep rf signal is applied to the modulator. This technique provides picosecond temporal resolution without the need for any fast diagnostic equipment and is capable of accurately measuring the average dispersion of fibers several kilometers long.     

Automatic F positron source supply system for a monoenergetic positron beam

A system which supplies an intense ¹⁸F (half life 110 min) positron source produced by an AVF cyclotron through ¹⁸O(p,n)¹⁸F reaction has been constructed. Produced ¹⁸F is transferred to a low background experiment hall through a capillary. It is electro-deposited on a graphite rod and used for a source of a slow positron beam. In the meantime the next batch of target ¹⁸O water is loaded and proton irradiation proceeds. This system makes it possible to perform continuous positron beam experiments using the ¹⁸F positron source.     

Is the magic texture of Majorana neutrinos immanent in Dirac nature?

Magic textures are successful candidates of the correct texture for Majorana neutrinos. In this study, we demonstrate that several types of magic textures of Majorana neutrinos are approximately immanent in the flavor mass matrix of Dirac neutrinos. In addition, the normal mass ordering of Dirac neutrino masses is slightly preferable to inverted mass ordering in the context of magic textures.     

First experimental result with fluorescent X‐ray CT based on sheet‐beam geometry

Fluorescent X‐ray computed tomography (FXCT) enables us to reveal the cross‐sectional distribution of very low concentration of specific elements, e.g. I, Gd, or Au, in biomedical samples at a spatial resolution of several hundred micrometers, and it has been used to evaluate the states of cerebral perfusion and fatty acid metabolic function of small animals in vivo and ex vivo. However, since the current system employs the data‐acquisition scheme of the first‐generation type of CT, it requires a huge amount of time to obtain a whole set of projections. In order to overcome the problem, we propose a novel imaging geometry using a sheet incident beam. We performed a proof‐of‐concept experiment using a preliminary imaging system constructed at beam line BLNE‐5A, KEK. The efficacy of the proposed method is demonstrated by the reconstructed images of a physical phantom containing various concentrations of iodine solution and a mouse brain that is extracted after intravenous injection of ¹²⁷I‐IMP for observing the cerebral perfusion and then fixed with formalin. Copyright © 2009 John Wiley & Sons, Ltd.     

Simulation of color shift in fluorescent LED cap

Computer investigation and design of light propagation in fluorescent scattering media is considered. Suggested solution provides efficient and physically accurate model of light propagation in the media that allows simulating Stokes color shift effect and design of white LED. Based on the suggested solution the program package of simulation and design of optical devices with fluorescent scattering media was implemented and trial design of white LED was fulfilled.     

Synthesis of 2,9-dialkylated phenanthro[1,2-b:8,7-b′]dithiophenes via cross-coupling reactions and sequential Lewis acid-catalyzed regioselective cycloaromatization of epoxide

Phenanthro[1,2-b:8,7-b′]dithiophene (PDT) was prepared via Suzuki–Miyaura or Negishi cross-coupling of a 2-thienylboron or -zinc compound with 1,4-dibromobenzene, followed by Lewis acid-catalyzed regioselective cycloaromatization of the epoxide. A series of 2,9-dialkylated phenanthro[1,2-b:8,7-b′]dithiophene (PDT) derivatives could also be synthesized in good yields by Suzuki–Miyaura coupling of the brominated PDT with alkylboranes by introducing linear alkyl substituents.