Fabrication and Lasing Properties of Single-Crystalline Semiconductor Microspheres with Anisotropic Crystal Structures
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Fabrication and Lasing Properties of Single-Crystalline Semiconductor Microspheres with Anisotropic Crystal Structures 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 fabricated single-crystalline microspheres of wide-gap semiconductors with anisotropic crystal structures, such as ZnO and ZnSe, by laser ablation in superfluid helium and investigated their lasing properties. Whispering gallery mode lasing at their band edges in ultraviolet region was clearly observed under the optical excitation, reflecting their high sphericity and crystal quality. INTRODUCTION Optical microcavities, which possess a high Q value and small mode volume at the same time, have attracted much attention [1]. Among a variety of microcavities, such as Fabry-Perot type, microwire, microdisk, and microtoroid, the microspheres show the highest Q values of whispering gallery modes (WGMs) in three-dimensional confinement of light [2]. As efficient lasers, polymer or glass microspheres doped with dye or impurity have been intensively studied, since they are easy to fabricate and show high sphericity [3]. On the other hand, wide-gap semiconductor microspheres are promising for efficient WGM lasing in ultraviolet (UV) region, since they work as microcavities and emitters at the same time [4]. However, it has been difficult to fabricate semiconductor microspheres, since the semiconductors tend to grow in their own shapes reflecting their crystal structures. On the other hand, recently we succeeded in the fabrication of single-crystalline microspheres of wide-gap semiconductors, such as ZnO or ZnSe with anisotropic crystal structures [5]. In addition, we observed the WGM lasing of ZnO microspheres in the whole visible region [6]. In the previous proceeding we reported timeresolved luminescence measurement of ZnO microspheres in ultraviolet region and observation of the WGMs of ZnSe microspheres in visible region [7]. Here we investigate the WGM lasing at the band edge in ZnO and ZnSe microspheres. ZnO is a well-known wide-gap semiconductor having band gap of 3.37eV and exciton binding energy of 60 meV; the material shows UV and visible luminescence due to the exciton recombination and due to oxygen vacancies, respectively, at room temperature [8], and have attracted attention as efficient light source [9]. ZnSe having band gap of 2.7 eV and exciton binding energy of 21 meV has been also studied for the potential applications such as blue lightemitting devices, though the stability is a concern [10]. Nanostructures of ZnSe, such as microwires and quantum dots, were reported [11-14]. ZnSe has two crystal structures of zincblende (cubic) and wurtzite (hexagonal) [15].
EXPERIMENT
Figure 1. TEM images of a ZnO microsphere (left) and a ZnSe microsphere (right) fabricated by laser ablation in superflu
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