Fig. 1. Synthesis of single ZnO@ZnO:Ga MW. (a) Schematic diagram of the preparation procedure of ZnO MC-decorated pre-synthesized ZnO:Ga MWs. (b) Optical photograph of the as-synthesized individual ZnO@ZnO:Ga MWs, located around the Si substrate. (c) SEM image of the pre-synthesized ZnO:Ga MWs, with perfect quadrilateral cross section displayed in the inset. (d) SEM image of the as-synthesized individual ZnO@ZnO:Ga MWs. (e)–(g) Amplified SEM image of ZnO MCs, deposited on the ZnO:Ga MWs.

Fig. 2. Optically pumped random lasing features from single ZnO MC. (a) SEM image of as-synthesized single ZnO MC. (b) Microscopic images of light emission from optically pumped single ZnO MC, with the excitation power density ranging from 9.0 to 12.0  mW/μm2. (c) Optically pumped lasing spectrum from single ZnO MC, with the excitation power density of 10  mW/μm2. (d) Optically pumped emission spectra from single ZnO MC, with the excitation power densities ranging from 5.5 to 13.0  mW/μm2. (e) Nonlinear relationship of integrated emission intensities as a function of the excitation power density, together with the spectral linewidth versus the excitation power density. The lasing threshold was extracted to be about 10.3  mW/μm2.

Fig. 3. Optically pumped lasing features from single ZnO@ZnO:Ga MW. (a) Two distinct Gaussian deconvoluted sub-bands of the PL spectrum from single ZnO@ZnO:Ga MW, with the excitation power density ∼7.0  mW/μm2. (b) Optically pumped emission spectra from single ZnO@ZnO:Ga MW, with the excitation power density ranging from 3.0 to 9.0  mW/μm2. (c) Optically pumped emission spectra as a function of pumping intensity when the excitation power energies exceed 9.0  mW/μm2. (d) The magnified lasing spectrum demonstrates that the FWHM of the lasing mode is about 0.06 nm (the excitation power density: 22.0  mW/μm2). (e) Nonlinear relationship between integrated emission intensities and the excitation power density, and FWHM versus the excitation power density. The lasing threshold was extracted to be about 9.0  mW/μm2.

Fig. 4. Working principle of optically pumped F–P mode lasing action from single ZnO@ZnO:Ga MW. (a) Comparison of normalized PL spectra from single ZnO MC and single ZnO:Ga MW. (b) Schematic illustration of optically pumped F–P mode microsized laser composed of single ZnO@ZnO:Ga MW. (c) Energy-band alignment toward the homointerface between ZnO MC and ZnO:Ga MW.

Fig. 5. Optical microscopic image of the light emitting from electrically biased single ZnO@ZnO:Ga MW-based heterojunction diode. With an increase in the injection current ranging from 5.5 to 12.0 mA, bright and blue-violet light emission can be observed, with the lighting regions located along the MW.

Fig. 6. Exciton-polariton emission characteristics from electrical-pumping single ZnO@ZnO:Ga MW-based heterojunction diode. (a) I–V characteristics curve from single ZnO@ZnO:Ga MW-based heterojunction diode. (b) Electrical-pumping emission spectra from single ZnO@ZnO:Ga MW-based heterojunction diode, with the injection current ranging from 0.005 to 12.5 mA. (c) Linear relationship between integrated emission intensity and the injection current, together with the spectral linewidth versus the injection current. (d) Energy-wavevector dispersion curve of single ZnO@ZnO:Ga MW-based heterojunction diode. (e) Normalized emission spectra of the PL emission from single ZnO@ZnO:Ga MW, PL emission from p-GaN layer, and EL emission spectrum from the heterojunction diode composed of single ZnO@ZnO:Ga MW and p-GaN layer. (f) Schematic diagram showing the energy-band alignment of the n-ZnO@ZnO:Ga/p-GaN heterojunction diode.