Stimulated Emission In Zinc Oxide/Graphene Hybrid Microcavity

Zinc oxide (ZnO) is regarded as an important candidate for short-wavelength photo-electronic materials and devices due to its wide direct band gap (3.37 eV) and high exciton binding energy (60 meV) at room temperature. Recently, researches on the ultraviolet (UV) lasing of ZnO has revealed the high quality factor and low lasing threshold of the whispering gallery mode (WGM) lasing of ZnO and has attracted intense interest among scientists from many fields. Though ZnO microrod always has hexagonal cross-section, which facilitate the occurrence of the WGM lasing, the big optical loss is inevitable with decreasing of the diameter of the microrod. So it is significant to reduce the optical loss, as well as improve the lasing performance of an optical cavity. Many researches have indicated that metal surface plasmon (SP) can significantly enhance the optical performance of the semiconductor materials. However, the metal SP is inconveniently tunable in fixed devices and generally has large Ohmic loss, which largely hinders the flexible design and development of novel functional photonic materials and devices with high performance. Graphene, a new star in the material sciences, has been considered as a plasmonic material alternative to noble metals based on its unique conductivity and optical properties. More importantly, plasmon in graphene can be tuned by chemical doping and gating potentials, thus has more wide potential applications than that of the metal SP. However, the researches of grephene SP are mainly concentrated on the terahertz (THz) region, its response in UV region is lacked in both physical understanding and experimental realization. Few reports about the PL enhancement from the graphene-coated ZnO have the question of the unclear mechanism and lacking of the direct evidence of the coupling between graphene SP and the excitation of ZnO.To clarify the problems mentioned above, we fabricated ZnO microcavity (microrod, microbelt and sub-micron sized microrod) through a vapor phase transport method, and high quality monolayer graphene used in our experiment was fabricated by a chemical vapor deposition method, then the hybrid microcavity was fabricated by covering monolayer graphene on the ZnO microcavity. The main research works are carried out as follows:1. A graphene/ZnO microrod hybrid microcavity was fabricated by covering a piece of monolayer graphene on the ZnO microrod. The optical field confinement and photoluminescence (PL) enhancement induced by graphene SP was investigated based on theoretical simulation and experimental characterization. Time resolved photoluminescence (TRPL) analysis was employed to analyze the coupling dynamics between graphene SP and ZnO interband emission. TRPL experiments revealed the accelerated dynamics for the coupling process and indicated more effective recombination process in the graphene coated microcavities. As a functional application, the improved WGM lasing performance was realized. It demonstrated stronger lasing intensity, lower threshold and higher lasing quality factor as the bare ZnO microrod was coated with graphene. Our research give direct evidence on the response of graphene SP in the UV region.2. The Fabry-Perot (F-P) resonant mechanism was investigated based on a ZnO microbelt with rectangular cross-section. Under the excitation of 325 nm UV laser, the microbelt emits bright blue-violet light from the lateral sides of the microbelt, giving a direct evidence of the F-P lasing resonant in the microbelt. The F-P lasing performance was significantly improved due to the confinement effect of graphene SP through reducing the optical loss at the ZnO/air interfaces.3. Single-mode lasing of ZnO was realized in a submicron-sized ZnO rod based on the systematic investigation of the WGM lasing of the microrod with different diameters. The lasing performance, such as the quality factor and the lasing intensity, was remarkably improved by facilely covering a monolayer graphene on the ZnO submicro-rod through utilizing the optical field confinement and reducing of the big optical loss from the cavity. The single-mode lasing may have many potential applications, such as optoelectronic integration, microspectroscopy photometers, and ultrasensitive chemical/biological sensors.