Lasing characteristics of an optically-pumped single ZnO nanocrystal and nanomachining for controlling oscillation wavelength

Lasing characteristics from a single ZnO nanocrystal excited by third harmonic of a Q-switched Nd:YAG laser beam (355 nm, 5 ns) were investigated for the application to ultraviolet (UV) laser diode (LD) by using ZnO nanocrystals as building blocks. Those ZnO nanocrystals were synthesized on a silicon substrate with a catalyst of gold by a carbothermal chemical vapor deposition (CVD) method. ZnO nanowires and ZnO nanosheets were synthesized by changing the synthesis conditions and the dependence of lasing characteristics on the different forms were investigated. The emission spectra observed from a single ZnO nanowire and ZnO nanosheet showed the obvious lasing characteristics having mode structure and a threshold for lasing on the input-output characteristics. The threshold power density of a ZnO nanowire and a ZnO nanosheet was measured to be about 150 and 50 kW/cm², respectively. Then, the oscillation mechanisms were discussed on those ZnO nanocrystals, and it was concluded that each lasing mechanism was attributed to the microcavity effect due to the strong UV light confinement caused by the high refractive index of ZnO (≈2.4) for UV light. ZnO can be a superior UV laser medium and an UV nano-laser source also can be expected. However, the observed lasing spectra from both ZnO nanocrystals had mode structure, and a single longitudinal mode lasing would be required for the stabilization of the output power and the prevention of light dispersion. Therefore, we considered the possibilities of the single longitudinal mode lasing from a single ZnO nanowire using distributed Bragg reflector lasing machined by focused ion beam with Ga ions focused up to 7 nm and a single ZnO nanosheet using subwavelength machining by Fresnel diffraction for 2D photonic crystal. We also observed the laser-induced motions (LIM) of ZnO nanocrystals dispersed on a substrate in the air when they were excited by the UV laser beam at high excitation power over several MW/cm² which could be attributed to the electromotive force due to piezo effects of ZnO nanocrystals, and a simple alignment method of ZnO nanocrystals was considered by the use of the LIM and voltage-applied electrodes on a substrate.