Clement Yuen

Random laser action in ZnO nanorod arrays embedded in ZnO epilayers

Random laser action with coherent feedback has been observed in ZnO nanorod arrays embedded in ZnO epilayers. The sample was fabricated by depositing a MgO buffer layer and followed by a layer of ZnO thin film onto a vertically well-aligned ZnO nanorod arrays grown on sapphire substrate. Under 355 nm optical excitation at room temperature, sharp lasing peaks emit at around 390 nm with a linewidth less than 0.4 nm has been observed in all directions. In addition, the dependence of the lasing threshold intensity on the excitation area is shown in good agreement with the random laser theory. Hence, it is demonstrated that random laser action can also be supported in ZnO nanorod arrays.

Zinc oxide thin-film random lasers on silicon substrate

Room-temperature ultraviolet lasing is demonstrated in mirrorless zinc oxide thin-film waveguides on (100) silicon substrate. Laser cavities, due to closed-loop optical scattering from the lateral facets of the irregular zinc oxide grains, are generated through the post-growth annealing of high-crystal-quality zinc oxide thin films obtained from the filtered cathodic vacuum arc technique. It is found that the lasing wavelength and linewidth of the zinc oxide random lasers under 355 nm optical excitation are around 390 nm and less than 0.4 nm, respectively. In addition, the lasing threshold characteristics are in good agreement with the random laser theory.

Fabrication of n-ZnO:Al∕p-SiC(4H) heterojunction light-emitting diodes by filtered cathodic vacuum arc technique

We report the low-temperature (∼150°C) fabrication of n-ZnO:Al∕p-SiC(4H) heterojunction light-emitting diodes by filtered cathodic vacuum arc technique. Diodelike rectifying current-voltage characteristics, with turn-on voltage of ∼3.8V and low reverse leakage current of <10−2μA, were measured at room temperature. In addition, ultraviolet emission with peak wavelength of ∼385nm and full width at half maximum of ∼20nm are observed at a forward biased voltage of ∼7.4V. The ultraviolet electroluminescence from the heterojunction is originated from the exciton-exciton scattering inside the n-ZnO:Al film.

Room-Temperature Ultraviolet Lasing from Zinc Oxide Microtubes

Prismatic zinc oxide microtubes have been fabricated by vapor transport. Room-temperature ultraviolet lasing action has been demonstrated in these microtube arrays. The ZnO microtubes, mainly appearing in tapped bell-mouthed shape, form natural laser cavities along the length direction. The hexagon diagonal and length of the microtube vary from 1 µm to 20 µm and 10 µm to a few hundred µm respectively. Under 355 nm optical excitation, lasing action is observed at room-temperature around 393 nm. Multi-longitudinal modes are also observed with significantly narrowed emission linewidth.

Ultraviolet amplified spontaneous emission from self-organized network of zinc oxide nanofibers

Self-organized zinc oxide (ZnO) nanofiber network with six-fold symmetry was fabricated on ZnO-buffered (0001) sapphire substrate with patterned gold catalyst by vapor-phase transport method. From the ZnO buffer layer, hexagonal ZnO nanorods with identical in-plane structure grew epitaxially along [0001] orientation to form vertical stems. The nanofiber branches grew horizontally from six side-surfaces of the vertical stem along [011¯0] and other equivalent directions. The aligned network structure constructed a waveguide array with optical gain. Ultraviolet amplified spontaneous emission was observed along the side-branching nanofibers when the aligned ZnO network was excited by a frequency-tripled Nd:YAG laser.

Ultraviolet amplified spontaneous emission from zinc oxide ridge waveguides on silicon substrate

Zinc oxide (ZnO) thin-film waveguides with ridge structures have been fabricated on n-type (100) silicon substrates. The deposition of high-crystal-quality ZnO thin films on the lattice-mismatched silicon substrate was achieved by using the filtered cathodic vacuum arc technique. A ridge structure is defined on the ZnO thin film by plasma etching. Room temperature amplified spontaneous emission with peak wavelength at 385 nm is observed under 355 nm optical excitation. The pump threshold is found to be around 0.45 MW/cm2. The maximum net optical gain of the ZnO waveguide is larger than 120 cm−1 at a pump intensity of 1.9 MW/cm2.

Low-loss and directional output ZnO thin-film ridge waveguide random lasers with MgO capped layer

Room-temperature ultraviolet lasing characteristics of ZnO thin-film ridge-waveguide random lasers with MgO capped layer fabricated on n-type (100) Si substrate are reported. It is demonstrated that highly directional emission from the facets of the random lasers can be achieved. Reduction of scattering loss inside the random cavities can also be obtained. In addition, the improvement in the efficiency of the lasing characteristics of the random lasers by optical feedback is studied.

Strain dependence of lasing mechanisms in ZnO epilayers

The lasing characteristics of highly disordered ZnO thin films deposited on SiO2∕Si substrates with and without a MgO buffer layer have been investigated. We observed that the emission spectra of the ZnO epilayers with and without a MgO buffer are associated with the radiative recombination of free-exciton (∼380nm) and electron-hole plasma (∼395nm), respectively. The difference in the lasing wavelength is due to the induced compressive (tensile) strain along the c axis of the ZnO epilayers as a result of the presence (absence) of the MgO buffer layer. It is demonstrated that the strain-induced variation of Mott density inside the ZnO epilayers is responsible for the observed lasing characteristics.

Magnetic field enriched surface enhanced resonance Raman spectroscopy for early malaria diagnosis.

Hemozoin is a by-product of malaria infection in erythrocytes, which has been explored as a biomarker for early malaria diagnosis. We report magnetic field-enriched surface-enhanced resonance Raman spectroscopy (SERRS) of β-hematin crystals, which are the equivalent of hemozoin biocrystals in spectroscopic features, by using magnetic nanoparticles with iron oxide core and silver shell (Fe(3)O(4)@Ag). The external magnetic field enriches β-hematin crystals and enhances the binding between β-hematin crystals and magnetic nanoparticles, which provides further improvement in SERRS signals. The magnetic field-enriched SERRS signal of β-hematin crystals shows approximately five orders of magnitude enhancement in the resonance Raman signal, in comparison to about three orders of magnitude improvement in the SERRS signal without the influence of magnetic field. The improvement has led to a β-hematin detection limit at a concentration of 5 nM (roughly equivalent to 30 parasites/μl at the early stages of malaria infection), which demonstrates the potential of magnetic field-enriched SERRS technique in early malaria diagnosis.

Ultraviolet lasing of ZnO whiskers prepared by catalyst-free thermal evaporation

Abstract ZnO whiskers were synthesized by vapor transportation via thermal evaporation of ZnO powders in open air. Preferred aligned hexagonal ZnO whiskers with length of 30–40 μm were obtained. Room temperature photoluminescence spectrum of the whiskers exhibits intense ultraviolet excitonic emission and weak defect related visible emission. Optical pumped lasing actions have been observed at 393 nm at room temperature when the excitation intensity exceeds 150 kW/cm 2 . Laser cavities are formed in the whiskers between the smooth interfaces of the two ends, which acted as reflectors.