Chuanlong Xu

Growth mechanism of tubular ZnO formed in aqueous solution

Tubular ZnO microstructural arrays were fabricated by a hydrothermal decomposition method. The dependence of the morphologies on the growth time and temperature was investigated in detail. An experiment was carried out to determine the mechanism of tubular ZnO formation. Our results showed that ZnO microtubes originated from an ageing process from ZnO microrods at a lower temperature (compared to the temperature when hydrothermal deposition of ZnO microrods was dominant) due to the preferential chemical dissolution of the metastable Zn-rich (0001) polar surfaces. A growth model was proposed based on the coexistence of hydrothermal deposition and dissolution of ZnO in the fabrication process.

Whispering gallery-mode lasing in ZnO microrods at room temperature

An individual hexagonal ZnO microrod was employed as a whispering-gallery mode (WGM) microcavity to obtain ultraviolet lasing at room temperature. The WGM lasing shows a low threshold, a high quality factor, and distinct mode structure. A typical stimulated emission from the ZnO microrod with diagonal of 6.67 μm exhibits a low lasing threshold of 255 kW/cm2. The observed discrete lasing modes match with the simulated positions very well. The spatial distribution of the lasing intensity demonstrates the lasing output direction and provides the direct evidence of the WGM resonant mechanism. We systematically investigated the lasing performance for different sized microrods.

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.

Two-photon excited whispering-gallery mode ultraviolet laser from an individual ZnO microneedle

Wurtzite structural ZnO microneedles with hexagonal cross section were fabricated by vapor-phase transport method and an individual microneedle was employed as a lasing microcavity. Under excitation of a femtosecond pulse laser with 800 nm wavelength, the ultraviolet (UV) laser emission was obtained, which presented narrow linewidth and high Q value. The UV emission, resonant mechanism, and laser mode characteristics were discussed in detail. The results demonstrated that the UV laser originated from the whispering-gallery mode induced by two-photon absorption assisted by Rabi oscillation.

Combined whispering gallery mode laser from hexagonal ZnO microcavities

The hexagonal ZnO microrods were employed as whispering gallery mode (WGM) microcavities to obtain ultraviolet laser at room temperature. For each individual ZnO microrod, the laser presented low threshold and high quality Q factor and clear WGM spectral structure. The lasing spectra from different microcavities were combined together by pumping two or more ZnO microrods simultaneously. The resonant process and lasing characteristics, such as lasing mode, threshold, and Q factor, were investigated in experiment and theory.

Exciton and electron-hole plasma lasing in ZnO dodecagonal whispering-gallery-mode microcavities at room temperature

Dodecagonal ZnO microrods were fabricated by vapor transport method. A dodecagonal ZnO microrod with diameter of 6.35 μm was employed as a whispering gallery mode microcavity, the low threshold exciton lasing was stimulated. In another smaller microrod, the electron-hole plasma lasing was observed under intense excitation as the carrier density was larger than the Mott transition density. The interference condition in the dodecagonal cavity was deduced, and the mode structures were investigated. Compared to the hexagonal microcavity with same diameter, the dodecagonal microcavity has higher lasing performance.

Velocity measurement of pneumatically conveyed solid particles using an electrostatic sensor

The paper proposes a method to measure the mean velocity of solid particles based on the spatial filtering effect of the electrostatic sensor. To determine the relationship between the spatial frequency characteristics of the sensor and solid particle velocity, a general formula is derived by analyzing quantitatively the spatial filtering characteristics of the electrostatic sensor along with the accepted assumptions. The effects of the geometric parameters of the sensor, particle velocity distribution, particle concentration distribution over the cross-section of a pneumatic pipeline, particle size, particle material type and frequency resolution on particle velocity measurement accuracy are also discussed in detail. Experiments are performed on a bench-scale gravity-fed particle flow experimental rig to test the performance of the velocity measurement system. The off-line experimental results show that the system repeatability is within ±5% over the velocity range of 2–6 m s−1 for concentrations of solid particles in the range of 0.5–6.0%.

Characteristics and Growth Mechanism of ZnO Whiskers Fabricated by Vapor Phase Transport

Microstructural ZnO whiskers have been fabricated by sintering the mixture of ZnO and graphite powders in air. Scanning electron microscopy characterization showed that the whiskers have hexagonal morphologies with prismatic tips. The measurements of X-ray diffraction, energy dispersive X-ray spectroscopy and Raman scattering spectroscopy showed that the product was composed of wurtzite phase ZnO. The photoluminescence of the micro-whsikers exhibited strong ultraviolet bandedge emission accompanied by a broad weak green band. The growth process has been interpreted by vapor-liquid-solid mechanism. The formation of the hexagonal morphologies has been attributed to the lowest energy argument.

Three-dimensional temperature field measurement of flame using a single light field camera

Compared with conventional camera, the light field camera takes the advantage of being capable of recording the direction and intensity information of each ray projected onto the CCD (charge couple device) sensor simultaneously. In this paper, a novel method is proposed for reconstructing three-dimensional (3-D) temperature field of a flame based on a single light field camera. A radiative imaging of a single light field camera is also modeled for the flame. In this model, the principal ray represents the beam projected onto the pixel of the CCD sensor. The radiation direction of the ray from the flame outside the camera is obtained according to thin lens equation based on geometrical optics. The intensities of the principal rays recorded by the pixels on the CCD sensor are mathematically modeled based on radiative transfer equation. The temperature distribution of the flame is then reconstructed by solving the mathematical model through the use of least square QR-factorization algorithm (LSQR). The numerical simulations and experiments are carried out to investigate the validity of the proposed method. The results presented in this study show that the proposed method is capable of reconstructing the 3-D temperature field of a flame.

Exciton-polariton microphotoluminescence and lasing from ZnO whispering-gallery mode microcavities

Hexagonal ZnO microrods were employed as whispering-gallery mode (WGM) optical microcavities to investigate exciton-polariton microphotoluminescence and lasing emission. Using a confocal microphotoluminescence system, the exciton-polariton emission with a large Rabi splitting of about 90 meV was observed from a ZnO microrod with the diameter of 9.38 μm. The spatial-resolved spectra demonstrated a collective nonlinear blueshift in the WGM resonance peaks along a tapered microrod and proved the anticross dispersion property of the exciton-polariton. Furthermore, the exciton-polariton WGM lasing was stimulated and blueshifted in the strong coupling region under the excitation of a 355 nm nanosecond pulsed laser.