Optical Fabrication of Semiconductor Single-Crystalline Microspheres in Superfluid Helium
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Optical Fabrication of Semiconductor Single-Crystalline Microspheres in Superfluid Helium Shinya Okamoto1, Satoshi Ichikawa2, Yosuke Minowa1, and Masaaki Ashida1 1 Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyamacho, Toyonaka, Osaka 560-8531, Japan 2 Institute for NanoScience Design, Osaka University, 1-3 Machikaneyamacho, Toyonaka, Osaka 560-8531, Japan
ABSTRACT We successfully fabricated semiconductor microspheres of ZnO, ZnSe, etc., by laser ablation in superfluid helium and investigated their morphology and optical properties. Timeresolved photoluminescence spectroscopy in ultraviolet region of single ZnO microspheres shows luminescence spectra with mode structures and remarkable reduction of the luminescence decay time compared to that of polycrystals or non-spherical microparticles. This indicates strong light-matter interaction due to efficient light-confinement in the ZnO microspheres. In addition, the fabricated ZnSe microspheres also show the photoluminescence spectra with typical mode structures indicating their high sphericity.
INTRODUCTION Recently, microcavities have received a great interest, because they have high Q value and small mode volume at the same time and thus lead to low threshold lasing [1]. Various types of microcavities, such as Fabry-Perot, microwire, microdisk, microtoroid, and microsphere have been reported [2-7]. In particular, whispering gallery mode (WGM) in microsphere structures due to multiple total internal reflections at the boundary shows the highest Q value in threedimensional light confinement. Because of the easiness in their fabrication, dye-doped polymer microspheres with high sphericity have been intensively studied [8]. However, available light frequency range is usually limited in visible region when using normal polymers. On the other hand, wide-gap semiconductor microspheres are promising for efficient lasing owing to high transparency and durability even in ultraviolet region [9]. Since ZnO has direct wide band gap of 3.37 eV and large exciton binding energy of 60 meV, it shows ultraviolet luminescence due to exciton recombination even at room temperature. Also, it shows broad visible luminescence due to defects [10]. Therefore, ZnO has attracted much attention as ultraviolet and visible photonic devices [11]. Since ZnO has a hexagonal wurtzite structure, fabrication techniques of microwires have been widely studied. Actually, ultraviolet lasing in single ZnO microwires has been reported recently [12-18]. On the other hand, we successfully fabricated ZnO microspheres by laser ablation in superfluid helium and observed lasing in broad visible region due to defects in single ZnO microspheres at room temperature [19]. In this paper, we report on the strong light-matter interaction in single ZnO microspheres at room temperature in ultraviolet region. Additionally, we fabricated ZnSe microspheres by using the same method and observed the WGM structure. The results demonstrate the universality of this fabrication method of microspheres.
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