Yueyue Wang

Direct resonant coupling of Al surface plasmon for ultraviolet photoluminescence enhancement of ZnO microrods.

More than 170-fold ultraviolet emission enhancement was obtained from the hexagonal ZnO microrods synthesized by a simple vapor-phase transport process and decorated by Al nanoparticles (NPs). Based on the stable and transient photoluminescence (PL) spectra of the ZnO microrods sputtered with Al NPs for different times, the underlying mechanism was deduced and can be attributed to the metal surface plasmon resonance (SPR) coupling with ZnO. Interestingly, with increasing of the sputtering time, the ratio of the band gap emission to the defect-related emission was increased from 0.1 to 42.7. Our results demonstrated that ZnO microrods decorated with Al NPs shed light on developing stable and high-efficiency excitonic optoelectronic devices such as light-emitting diodes and lasers.

Single Mode ZnO Whispering-Gallery Submicron Cavity and Graphene Improved Lasing Performance

Single-mode ultraviolet (UV) laser of ZnO is still in challenge so far, although it has been paid great attention along the past decades. In this work, single-mode lasing resonance was realized in a submicron-sized ZnO rod based on serially varying the dimension of the whispering-gallery mode (WGM) cavities. The lasing performance, such as the lasing quality factor (Q) and the lasing intensity, was remarkably improved by facilely covering monolayer graphene on the ZnO submicron-rod. The mode structure evolution from multimodes to single-mode was investigated systematically based on the total internal-wall reflection of the ZnO microcavities. Graphene-induced optical field confinement and lasing emission enhancement were revealed, indicating an energy coupling between graphene SP and ZnO exciton emission. This result demonstrated the response of graphene in the UV wavelength region and extended its potential applications besides many previous reports on the multifunctional graphene/semiconductor hybrid materials and devices in advanced electronics and optoelectronics areas.

Plasmon-enhanced Electrically Light-emitting from ZnO Nanorod Arrays/p-GaN Heterostructure Devices

Effective and bright light-emitting-diodes (LEDs) have attracted broad interests in fundamental research and industrial application, especially on short wavelength LEDs. In this paper, a well aligned ZnO nanorod arrays grown on the p-GaN substrate to form a heterostructured light-emitting diode and Al nanoparticles (NPs) were decorated to improve the electroluminescence performance. More than 30-folds enhancement of the electroluminescence intensity was obtained compared with the device without Al NPs decoration. The investigation on the stable and transient photoluminescence spectraof the ZnO nanorod arrays before and after Al NPs decoration demonstrated that the metal surface plasmon resonance coupling with excitons of ZnO leads to the enhancement of the internal quantum efficiency (IQE). Our results provide aneffective approach to design novel optoelectronic devices such as light-emitting diodes and plasmonic nanolasers.

Plasmon coupled Fabry-Perot lasing enhancement in graphene/ZnO hybrid microcavity

The response of graphene surface plasmon (SP) in the ultraviolet (UV) region and the realization of short-wavelength semiconductor lasers not only are two hot research areas of great academic and practical significance, but also are two important issues lacked of good understanding. In this work, a hybrid Fabry-Perot (F-P) microcavity, comprising of monolayer graphene covered ZnO microbelt, was constructed to investigate the fundamental physics of graphene SP and the functional extension of ZnO UV lasing. Through the coupling between graphene SP modes and conventional optical microcavity modes of ZnO, improved F-P lasing performance was realized, including the lowered lasing threshold, the improved lasing quality and the remarkably enhanced lasing intensity. The underlying mechanism of the improved lasing performance was proposed based on theoretical simulation and experimental characterization. The results are helpful to design new types of optic and photoelectronic devices based on SP coupling in graphene/semiconductor hybrid structures.

Electron-hole plasma induced band gap renormalization in ZnO microlaser cavities.

We report electron-hole plasma (EHP) lasing in hexagonal ZnO microrods and thin nanobelts. Under the excitation of 325 nm line femtosecond pulsed laser, ultraviolet whispering-gallery mode (WGM) lasing was observed from hexagonal ZnO microrods. When EHP was formed at high excitation energy density, the center wavelength of the WGM lasing band presented a redshift from 387.5 nm to 397.5 nm, and the full width of half maximum (FWHM) of the WGM lasing band increased from 2.5 nm to 7 nm. Each lasing mode showed obvious blueshift and broadening. Such lasing characteristics were attributed to the band gap renormalization (BGR) due to the high carrier concentration at the EHP condition. In addition, EHP Fabry-Perot (F-P) mode lasing from thin ZnO nanobelt was also observed and discussed. According to the phenomenological BGR calculation with including the carrier density dependent screening effect, the values of the band gap of ZnO at different excitation energy densities were obtained, which agree well with the experimental results.

Electro-pumped whispering gallery mode ZnO microlaser array

By employing vapor-phase transport method, ZnO microrods are fabricated and directly assembled on p-GaN substrate to form a heterostructural microlaser array, which avoids of the relatively complicated etching process comparing previous work. Under applied forward bias, whispering gallery mode ZnO ultraviolet lasing is obtained from the as-fabricated heterostructural microlaser array. The devices electroluminescence originates from three distinct electron-hole recombination processes in the heterojunction interface, and whispering gallery mode ultraviolet lasing is obtained when the applied voltage is beyond the lasing threshold. This work may present a significant step towards future fabrication of a facile technique for micro/nanolasers.

A facile preparation route for highly conductive borate cross-linked reduced graphene oxide paper

A facile hydrothermal strategy to synthesize borate cross-linked reduced graphene oxide (B-RGO) sheets with good optical and electrical properties is proposed by using graphene oxide (GO) sheets, boric acid and sodium hydroxide as precursors in one pot without any polymer or surfactant. The resulting B-RGO sheets can be assembled into a highly conductive B-RGO paper by simple filtration. Moreover, the atomic percentage of boron (B) in the B-RGO sheet can be readily controlled, as when it reaches to a maximum of 3.33 at%, about 30.0 mA current passes through the resulting single-layer B-RGO sheet and B-RGO paper at an applied bias of 2.0 V. This current value is much higher than that of the annealed GO paper (∼1.2 mA) and very close to that of the single-layer graphene sheet (∼60.0 mA) synthesized by a chemical vapour deposition method under the same test conditions. The resulting highly conductive B-RGO sheet and B-RGO paper offer a promising transparent conductive material for electronic or optoelectronic applications.

Plasmon enhancement for Vernier coupled single-mode lasing from ZnO/Pt hybrid microcavities

It is essential to develop a single mode operation and improve the performance of lasing in order to ensure practical applicability of microlasers and nanolasers. In this paper, two hexagonal microteeth with varied nanoscaled air-gaps of a ZnO microcomb are used to construct coupled whispering-gallery cavities. This is done to achieve a stable single mode lasing based on Vernier effect without requiring any complicated or sophisticated manipulation to achieve positioning with nanoscale precision. Optical gain and the corresponding ultraviolet lasing performance were improved greatly through coupling with localized surface plasmons of Pt nanoparticles. The ZnO/Pt hybrid microcavities achieved a seven-fold enhancement of intensity of single mode lasing with higher side-mode suppression ratio and lower threshold. The mechanism that led to this enhancement has been described in detail.

Burstein-Moss Effect Behind Au Surface Plasmon Enhanced Intrinsic Emission of ZnO Microdisks

In this paper, ZnO microdisks with sputtering of Au nanoparticles were prepared to explore their plasmon/exciton coupling effect. An obvious blue shift and enhanced excitonic emission intensity were observed in the PL spectra of as-grown and Au-sputtered ZnO samples at room temperature. The investigation on the absorption spectra and temperature-dependent PL spectra has been demonstrated the Burstein-Moss effect behind the optical phenomena. These results revealed the coupling dynamics between the metal localized surface plasmon and semiconductor exciton.

Different wavelength ranges' WGM lasing from a ZnO microrod/R6G:PMMA microcavity

Dual-wavelength or multi-wavelength laser sources have potential application in many areas. The ZnO material is an important candidate for ultraviolet (UV) micro/nanolasers for integrated photonic systems. In this paper, the dual-wavelength whispering-gallery-mode (WGM) laser is fabricated by symmetrically coating Rhodamine 6G (R6G):PMMA on a ZnO microrod, and realizes the UV WGM lasing and orange WGM lasing under optical pumping condition. The performance of the UV and orange WGM lasing, including the lasing threshold, emission intensity and quality factors (Q) are discussed. The resonance mechanism of the dual-wavelength WGM lasing is analyzed in detail. The two-dimensional FDTD simulation on optical-field distribution also confirms the resonance mechanism. This work is important to the design of optically and electrically pumped dual- or multi-wavelength WGM lasers.