Improving the Optical Properties of Self Catalyzed GaN Microrods toward Whispering Gallery Mode Lasing

GaN microrods were grown self catalyzed by a fast and simple metal organic vapor phase epitaxy method without any processing before or after deposition. Theprismatic microrods with a regular hexagonal cross section, sharp edges, straight, and smooth sidewall facets act as a microresonator, as seen by the appearance of whispering gallery modes in the yellow defect band range. To improve their optical properties, a reduced Ga precursor flow is required during growth. However, their hexagonal microrod morphology is not maintained under these growth conditions. The approach to start growth with a high Ga precursor flow and applying a ramp to a reduced precursor flow yield in significant enhancement of the near band edge emission in the upper part of the microrods. Whispering gallery modes in superposition with the near band edge emission can now be detected by cathodoluminescence measurements. These improvements lead to stimulated emission of a single whispering gallery mode up to amp; 8764;2 MW cm2 and multimode lasing with a threshold of 2.86 MW cm2 from an as grown microrod under optical excitation at room temperatur

Emission Mössbauer spectroscopy study of fluence dependence of paramagnetic relaxation in Mn/Fe implanted ZnO

Emission Mössbauer Spectroscopy following the implantation of radioactive precursor isotope $^{57}$Mn$^{+}$ (T$_{1/2}$= 1.5 min) into ZnO single crystals at ISOLDE/CERN shows that a large fraction of $^{57}$Fe atoms produced in the $^{57}$Mn beta decay is created as paramagnetic Fe$^{3+}$ with relatively long spin-lattice relaxation times. Here we report on ZnO pre-implanted with $^{56}$Fe to fluences of 2×10$^{13}$, 5×10$^{13}$ and 8 × 10$^{13}$ ions/cm2 in order to investigate the dependence of the paramagnetic relaxation rate of Fe$^{3+}$ on fluence. The spectra are dominated by magnetic features displaying paramagnetic relaxation effects. The extracted spin-lattice relaxation rates show a slight increase with increasing ion fluence at corresponding temperatures and the area fraction of $^{3+}$ at room temperature reaches a maximum contribution of 80(3)% in the studied fluence range

Sensitivity of 57^{57}Fe emission Mössbauer spectroscopy to Ar and C induced defects in ZnO

Emission Mössbauer Spectroscopy (eMS) measurements, following low fluence (<1012 cm−2) implantation of 57Mn (t 1/2 = 1.5 min.) into ZnO single crystals pre-implanted with Ar and C ions, has been utilized to test the sensitivity of the 57Fe eMS technique to the different types of defects generated by the different ion species. The dominant feature of the Mössbauer spectrum of the Ar implanted ZnO sample was a magnetic hyperfine field distribution component, attributed to paramagnetic Fe3+, while that of the C implanted sample was a doublet attributed to substitutional Fe2+ forming a complex with the C dopant ions in the 2− state at O vacancies. Magnetization measurements on the two samples, on the other hand, yield practically identical m(H) curves. The distinctly different eMS spectra of the two samples display the sensitivity of the probe nucleus to the defects produced by the different ion species

Ultrafast Dynamics of Lasing Semiconductor Nanowires

Semiconductor nanowire lasers operate at ultrafast timescales; here we report their temporal dynamics, including laser onset time and pulse width, using a double pump approach. Wide bandgap gallium nitride GaN , zinc oxide ZnO , and cadmium sul amp; 64257;de CdS nanowires reveal laser onset times of a few picoseconds, driven by carrier thermalization within the optically excited semiconductor. Strong carrier amp; 8722;phonon coupling in ZnO leads to the fastest laser onset time of amp; 8764;1 ps in comparison to CdS and GaN exhibiting values of amp; 8764;2.5 and amp; 8764;3.5 ps, respectively. These values are constant between nanowires of di amp; 64256;erent sizes implying independence from any optical in amp; 64258;uences. However, we demonstrate that the lasing onset times vary with excitation wavelength relative to the semiconductor band gap. Meanwhile, the laser pulse widths are dependent on the optical system. While the fastest ultrashort pulses are attained using the thinnest possible nanowires, a sudden change in pulse width from amp; 8764;5 to amp; 8764;15 ps occurs at a critical nanowire diameter. We attribute this to the transition from single to multimode waveguiding, as it is accompanied by a change in laser polarization