Hou Kun Liang

Directional and controllable edge-emitting ZnO ultraviolet random laser diodes

Room-temperature ultraviolet random lasing action is demonstrated from a p-GaN/annealed i-ZnO:Al(3%)/n-ZnO:Al(5%) buried heterojunction diode with a 2 μm rib waveguide. Excellent electrical-to-optical conversion efficiency is achieved by strong electrical and optical confinement of a buried heterojunction rib waveguide structure. Hence, emission intensity (threshold current) can be enhanced (reduced) by ∼9 times (∼40%). Directional emission as well as controllability on the number of the random lasing modes can also be achieved.

ZnO random laser diode arrays for stable single-mode operation at high power

An array of highly disordered i-ZnO:Al(3%) random cavities, which have 1 μm width, 150 nm thickness, and 2 mm length, is sandwiched between n-ZnO:Al(5%) and p-GaN/sapphire substrate to form an array of heterojunctions. The random cavities, which are electrically isolated and optically coupled with the adjacent random cavities, are laterally separated by a 1 μm wide Al2O3 dielectric insulator. Stable single-mode operation is observed from the laser diode array under high electrical pumping (i.e., >6×threshold current) at room temperature.

Ultraviolet Electroluminescence from Randomly Assembled n-SnO2 Nanowiresp-GaN:Mg Heterojunction

Electroluminescence characteristics of a heterojunction light-emitting diode, which was fabricated by depositing a layer of randomly assembled n-SnO(2) nanowires on p-GaN:Mg/sapphire substrate via vapor transport method, were investigated at room temperature. Peak wavelength emission at around 388 nm was observed for the diode under forward bias. This is mainly related to the radiative recombination of weakly bounded excitons at the shallow-trapped states of SnO(2) nanowires, Under reverse bias, near bandedge emission from the p-GaN:Mg/sapphire leads to the observation of emission peak at around 370 nm.

Randomly packed n-SnO2 nanorods/p-SiC heterojunction light-emitting diodes

A layer of randomly packed n-SnO2 nanorods is grown by vapor transport method on the p-SiC(4H) substrate to realize heterojunction light-emitting diodes. Diodelike rectifying current-voltage characteristics, with a turn-on voltage of ∼4.5 V and reverse leakage current density of <0.25 A/m2, are obtained at room temperature. Furthermore, electroluminescent spectra with emission peaks at around 395, 434, and 497 nm are observed from the heterojunction under forward bias. This is due to the relaxation of electrons in the conduction band of SnO2 to the surface defect states and subsequent radiative recombination with holes injected from the p-SiC substrate.

Electroluminescence from n-In2O3:Sn randomly assembled nanorods/p-SiC heterojunction.

Room-temperature electroluminescence (EL) has been realized from Sn-doped In(2)O(3) (In(2)O(3):Sn) nanorods. Heterojunction light-emitting diode (LED) was formed by depositing a layer of randomly packed n-In(2)O(3):Sn nanorods onto a p-type 4H-SiC substrate. It is found that the emission intensity of the heterojunction LED under forward bias can be maximized by doping the In(2)O(3) nanorods with 3 mol. % of Sn. Furthermore, two emission peaks of the EL spectra are observed at approximately 395 and approximately 440 nm. These ultraviolet and visible peaks are attributed to the radiative recombination at Sn induced and intrinsic defect states of the In(2)O(3):Sn nanorods.

High-temperature lasing characteristics of randomly assembled ZnO nanowires with a ridge waveguide

High-temperature lasing characteristics of randomly assembled ZnO nanowires at ultraviolet wavelength are investigated. Lasing emission was observed from the randomly assembled ZnO nanowires for temperature up to 500 K. The corresponding lasing mechanism is attributed to the coherent random lasing action via the formation of closed-loop cavity modes. Furthermore, ridge waveguide lasers using the randomly assembled ZnO nanowires as the active medium were fabricated. It is found that the width of the ridge waveguides restricted the size of the closed-loop cavity modes such that the corresponding characteristic temperature can be increased by 10 K.

Edge-emitting ultraviolet n-ZnO:Al/i-ZnO/p-GaN heterojunction light-emitting diode with a rib waveguide.

An edge-emitting ultraviolet n-ZnO:Al/i-ZnO/p-GaN heterojunction light-emitting diode with a rib waveguide is fabricated by filtered cathodic vacuum arc technique at low deposition temperature (approximately 150 degrees C). Electroluminescence with emission peak at 387 nm is observed. Good correlation between electro- and photo- luminescence spectra suggests that the i-ZnO layer of the heterojunction supports radiative excitonic recombination. Furthermore, it is found that the emission intensity can be enhanced by approximately 5 times due to the presence of the rib waveguide. Only fundamental TE and TM polarizations are supported inside the rib waveguide and the intensity of TE polarization is approximately 2.2 time larger than that of TM polarization.

Planar integrated metasurfaces for highly-collimated terahertz quantum cascade lasers

We report planar integration of tapered terahertz (THz) frequency quantum cascade lasers (QCLs) with metasurface waveguides that are designed to be spoof surface plasmon (SSP) out-couplers by introducing periodically arranged SSP scatterers. The resulting surface-emitting THz beam profile is highly collimated with a divergence as narrow as ~4° × 10°, which indicates a good waveguiding property of the metasurface waveguide. In addition, the low background THz power implies a high coupling efficiency for the THz radiation from the laser cavity to the metasurface structure. Furthermore, since all the structures are in-plane, this scheme provides a promising platform where well-established surface plasmon/metasurface techniques can be employed to engineer the emitted beam of THz QCLs controllably and flexibly. More importantly, an integrated active THz photonic circuit for sensing and communication applications could be constructed by incorporating other optoelectronic devices such as Schottky diode THz mixers, and graphene modulators and photodetectors.

An Index-Guided ZnO Random Laser Array

An index-guided zinc oxide random laser array was designed and fabricated to achieve spatial overlapping of random spectral modes between the adjacent random cavities. Hence, the number of lasing spectral modes of the laser array can be limited by the phenomenon of spectral repulsion. In addition, the total spectrally coherent emission power can be enhanced by increasing the number of random cavities.

ZnO–ZnMgO Multiple Quantum-Well Ridge Waveguide Lasers

ZnO-ZnMgO multiple quantum-well (MQW) thin-film waveguides with ridge structures have been fabricated on quartz substrates. Low-temperature deposition of high-quality ZnO-ZnMgO MQW thin films was achieved by filtered cathodic vacuum arc technique. A ridge is defined on the thin film by plasma etching. Room-temperature lasing with a peak wavelength at 378 nm of 1.5-nm well width was observed under 355-nm optical excitation. Exciton-exciton scattering was attributed to the amplified spontaneous emission observed from the MQW waveguide. The net optical gain can be larger than 80 cm-1 at a pump intensity of 2 MW/cm2 .