Abstract

Whispering-gallery-mode (WGM) resonators with high Q value and small mode volume have attracted much attention in quantum optics and/or photonics. Especially the WGM semiconductor lasers with low threshold have been studied intensively for wide-gap materials, such as GaN and ZnO, which are available in ultraviolet (UV) and/or visible region. In such hexagonal materials, which tend to grow in wire shape, however, fabrication of the microspheres that show highest Q values due to three-dimensional confinement has been impossible. Recently we successfully fabricated semiconductor microspheres with anisotropic crystal structures that tend to grow in shapes with low symmetry, such as ZnO and CdSe, whose diameters are from several nm to 3 μm by laser ablation in superfluid helium, where extremely quick quench is thought to occur. Surprisingly we discovered a number of single-crystalline spheres with high sphericity in a very wide size range [1]. Actually we found that the transmission electron microscope images of the fabricated particles show highly spherical shapes having surfaces with atomic-level smoothness without faceted structures, which means a flexible melt of the material, whose shape is determined by surface tension, is ideally cooled to grow in crystalline form, maintaining the highly spherical shape. We also found that all spots in the electron diffraction pattern taken from the single microspheres can be indexed in those of bulk crystals, which indicates that the fabricated microspheres are single-crystals.

© 2015 IEEE