Improved light extraction efficiency of cerium-doped biomedical imaging scintillator by monolayers of periodic arrays of
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Improved light extraction efficiency of cerium-doped biomedical imaging scintillator by monolayers of periodic arrays of polystyrene spheres B. Liu1, 2, X. Duan3, and Y. Yi1,3* 1
University of Michigan, MI Tongji University, Shanghai, CN 3 Massachusetts Institute of Technology, Cambridge, MA 2
ABSTRACT Monolayers of arrays of periodic polystyrene (PS) spheres are designed to couple onto the surface of cerium-doped lutetium-yttrium oxyorthosilicate scintillator to improve the light extraction efficiency. The enhancement of extraction efficiency up to 38% relative to the reference case without polystyrene spheres is achieved. Combining with the simulation for the transmission as well as its dispersion relation, detailed analysis of the effect of whispering gallery modes and diffraction on the extraction mechanism are given. As a result, the optimal diameter of 414 nm is obtained based on a trade-off between the transmission loss and the diffraction enhancement. *e-mail: [email protected]
INTRODUCTION Cerium-doped lutetium-yttrium oxyorthosilicate (LYSO) with advantages of high light output (~30 000 ph/MeV), good energy resolution (~10%), and short decay time (~40 ns) has become an excellent scintillator for the applications in medical imaging and high energy physics experiments1. In spite of the high internal quantum efficiency of LYSO, the light extraction efficiency is rather low due to its high refractive index (1.83), which leads to a small critical angle (θc). The extraction efficiency from one side of the crystal-air interface is as low as 7.5% according to an approximate formula (1/4n2) 2. In order to obtain a significant increase of light extraction, photonic crystals have been widely used in inorganic semiconductor light-emitting diodes (LEDs) 3–5 and organic electroluminescence devices6,7. Although the research on the enhancement light extraction efficiency in LEDs has attracted great attention, the application of photonic structures to the field of scintillators is scarcely reported8–10. For practical purposes in scintillation detection systems, large-area patterned photonic crystals fabricated in low cost are highly desirable. Therefore, two-dimensional photonic crystals consisting of a monolayer of self-assembled hexagonal-close-packed (hcp) dielectric spheres prepared in a very economical and effective way holds a great promise11. An array of periodic wavelength-scale dielectric spheres exhibits special photonic characteristics because of both whispering gallery (WG) modes due to Mie resonance of individual spheres and Bragg diffraction arising to the periodic arrangement12. Based on the ability of controlling the redistribution of electromagnetic field in such systems, the arrays of periodic dielectric spheres obtain important applications in, such as solar cells13,14, biological and chemical sensors15, photocatalysis16, and field emission devices17. In this Letter, monolayers of periodic arrays of polystyrene (PS, n=1.59) spheres are self-assembled onto
the top surface of LYSO scintillator for the
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