Growth process of pentacene crystals obtained by physical vapor transport technique

The growth process of pentacene crystals obtained by the physical vapor transport technique is discussed on the basis of the results of optical microscopy and X-ray topography. At the initial stage of physical vapor growth, many pentacene whisker crystals were grown. After that, the morphology of the whiskers developed to dendrites. Then, the whisker crystals themselves became the main branches of the dendritic crystals formed at the initial stage, and many side branches were created from the lateral surfaces of the whisker crystals. Some of the dendritic crystals continued to develop to form plateletlike crystals with a dendritic configuration. Regarding the plateletlike crystals with a parallelogram configuration, traces of the dendrites are thought to sometimes be left inside these plateletlike crystals on the basis of the results of optical microscopy and X-ray topography. Plateletlike crystals with a parallelogram configuration are considered to sometimes grow via the formation of dendritic crystals.

Source:IOPscience

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Angular dependences of the luminescence and density of photon states in a chiral liquid crystal

Luminescence spectra of a laser dye-doped chiral liquid crystal have been studied in a wide range of angles (up to 60°) to the axis of its helical structure using a semicylindrical quartz prism, which made it possible to observe the shift and evolution of the photonic band gap in response to changes in angle. Using measured spectra and numerical simulation, we calculated the spectral distributions of the density of photon states in such a cholesteric crystal for polarised and unpolarised light, which characterise its structure as that of a chiral one-dimensional photonic crystal.

Source:IOPscience

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Depth-of-interaction measurement in a single-layer crystal array with a single-ended readout using digital silicon photomultiplier

We present the first experimental evaluation of a depth-of-interaction (DOI) positron emission tomography (PET) detector using a digital silicon photomultiplier (dSiPM). To measure DOI information from a mono-layer array of scintillation crystals with a single-ended readout, our group has previously proposed and developed a new method based on light spread using triangular reflectors. Since this method relies on measurement of the light distribution, dSiPM, which has a fully digital interface, has several merits for our DOI measurement. The DOI PET detector comprised of a dSiPM sensor (DPC-3200-22-44) coupled with a 14   ×   14 array of 2 mm  ×  2 mm  ×  20 mm unpolished LGSO crystals. All crystals were covered with triangular reflectors. To obtain a good performance of the DOI PET detector, several parameters of detector were selected as a preliminary experiment. Detector performance was evaluated with the selected parameters and the optimal experimental setup, and a DOI measurement was conducted by irradiating the crystal block at five DOI positions spaced at intervals of 4 mm. Maximum-likelihood estimation was employed for DOI positioning and the optimal DOI estimation scheme was also investigated in this study. As a result, the DOI PET detector showed clear crystal identification. The energy resolution (full-width at half-maximum (FWHM)) averaged over all depths was 10.21%  ±  0.15% at 511 keV, and time resolution averaged over all depths was 1198.61   ±   39.70 ps FWHM. The average DOI positioning accuracy for all depths was 74.22%  ±  6.77%, which equates to DOI resolution of 4.67 mm. Energy and DOI resolutions were uniform over all crystal positions except for the back parts of the array. Furthermore, additional simulation studies were conducted to verify the results of our DOI measurement method that is combined with dSiPM technology. In conclusion, our continuous DOI PET detector coupled with dSiPM is a promising PET/MRI detector with DOI-encoding capability.

single crystal.

Source:IOPscience

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Modeling Czochralski growth of oxide crystals for piezoelectric and optical applications

Numerical modeling is applied to investigate the impact of crystal and crucible rotation on the flow pattern and crystal-melt interface shape in Czochralski growth of oxide semi-transparent crystals used for piezoelectric and optical applications. Two cases are simulated in the present work: the growth of piezoelectric langatate (LGT) crystals of 3 cm in diameter in an inductive furnace, and the growth of sapphire crystals of 10 cm in diameter in a resistive configuration. The numerical results indicate that the interface shape depends essentially on the internal radiative heat exchanges in the semi-transparent crystals. Computations performed by applying crystal/crucible rotation show that the interface can be flattened during LGT growth, while flat-interface growth of large diameter sapphire crystals may not be possible.

Source:IOPscience

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Pyroelectric generation of 2D spatial soliton sets in a bulk of lithium niobate crystal

The generation of two-dimensional bright spatial soliton sets in lithium niobate sample has been experimentally demonstrated at light wavelength of 532 nm, contribution of pyroelectric effect into nonlinear optical response of the crystal, and spatial modulation of one-dimensional beam along direction normal to the crystal optical axis. Diameters of soliton beams and channel waveguides formed within the crystal bulk by these solitons are near to 20 μm at light polarization corresponding to extraordinary wave of the crystal.

Source:IOPscience

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