Guishi Wang

Hydride Vapor Phase Epitaxy Growth of Semipolar (1013)GaN on Patterned m-Plane Sapphire

We have investigated the hydride vapor-phase epitaxy growth of (1013)-oriented GaN thick films on patterned sapphire substrates (PSSs) ( 1010). From characterization by atomic force microscopy, scanning electron microscopy, double-crystal X-ray diffraction, and photoluminescence (PL), it is determined that the crystalline and optical qualities of (1013) GaN epilayers grown on the cylindrical PSS are better than those on the flat sapphire. However, two main crystalline orientations ( 1013) and (1122) dominate the GaN epilayers grown on the pyramidal PSS, demonstrating poor quality. After etching in the mixed acids, these ( 1013) GaN films are dotted with oblique pyramids, concurrently lining along the (3032) direction, indicative of a typical N-polarity characteristic. Defect-related optical transitions of the (1013) GaN epilayers are identified and detailedly discussed in virtue of the temperature-dependent PL. In particular, an anomalous blueshift-redshift transition appears with an increase in temperature for the broad blue luminescence due to the thermal activation of the shallow level.

Sensing atmospheric reactive species using light emitting diode by incoherent broadband cavity enhanced absorption spectroscopy

We overview our recent progress in the developments and applications of light emitting diode-based incoherent broadband cavity enhanced absorption spectroscopy (LED-IBBCEAS) techniques for real-time optical sensing chemically reactive atmospheric species (HONO, NO3, NO2) in intensive campaigns and in atmospheric simulation chamber. New application of optical monitoring of NO3 concentration-time profile for study of the NO3-initiated oxidation process of isoprene in a smog chamber is reported.

High-Pressure Measurements of Temperature and CO2 Concentration Using Tunable Diode Lasers at 2 μm

A sensor for simultaneous measurements of temperature and carbon dioxide (CO2) concentration at elevated pressure is developed using tunable diode lasers at 2 µm. Based on some selection rules, a CO2 line pair at 5006.140 and 5010.725 cm−1 is selected for the TDL sensor. In order to ensure the accuracy and rapidity of the sensor, a quasi-fixed-wavelength WMS is employed. Normalization of the 2f signal with the 1f signal magnitude is used to remove the need for calibration and correct for transmission variation due to beam steering, mechanical misalignments, soot, and windows fouling. Temperatures are obtained from comparison of the background-subtracted 1f-normalized WMS-2f signals ratio and a 1f-normalized WMS-2f peak values ratio model. CO2 concentration is inferred from the 1f-normalized WMS-2f peak values of the CO2 transition at 5006.140 cm−1. Measurements of temperature and CO2 concentration are carried out in static cell experiments (P = 1–10 atm, T = 500–1200 K) to validate the accuracy and ability of the sensor. The results show that accuracy of the sensor for temperature and CO2 concentration are 1.66% temperature and 3.1%, respectively. All the measurements show the potential utility of the sensor for combustion diagnose at elevated pressure.

Photoacoustic Measurement of Ethane with Near-Infrared DFB Diode Laser

A compact resonant photoacoustic sensor based on a near-infrared distributed-feedback (DFB) diode laser was developed for detection of ethane (C2H6). A DFB laser emission at 5937.25 cm−1 with a power of ∼5 mW was used as an excitation light source for generating the photoacoustic signal. Wavelength modulation and second harmonic detection scheme were employed. Modulation frequency and modulation amplitude were optimized for getting optimum performance. Performance evaluation based on the linearity response of the PAS sensor system with respect to C2H6 concentration levels was performed, and a good linear dependence of the PAS signal on the C2H6 concentration was obtained. A minimum detectable concentration of 9 ppmv was achieved for detection of C2H6 with a lock-in time constant of 10 ms.

Recent advances of laser absorption spectroscopy based technologies for sensing of atmosphere

Summary form only given. Laser absorption spectroscopy is a powerful and versatile chemical analysis tool for the identification and quantification of molecular constituents, which offers the advantages that include fast response, high sensitivity, realtime, in situ and non contact measurements of species without any sample preparation. Therefore, it was widely used in siening of atmosphere or environment monitoring. In this presentation, recent advances of laser absorption spectroscopy based technologies developed in our laboratory for sensing of atmospheric gases or aerosols, including highly sensitive laser absorptioin spectroscopy with novel compact dense-pattern multipass cell [1], photoacoustic spectroscopy, quartz enhanced photoacoustic spectroscopy [2, 3] and laser heterodyne radiometer, will be reported.

Quantitative analysis of ammonium salts in coking industrial liquid waste treatment process based on Raman spectroscopy

Quantitative analysis of ammonium salts in the process of coking industrial liquid waste treatment is successfully performed based on a compact Raman spectrometer combined with partial least square (PLS) method. Two main components (NH4SCN and (NH4)2S2O3) of the industrial mixture are investigated. During the data preprocessing, wavelet denoising and an internal standard normalization method are employed to improve the predicting ability of PLS models. Moreover, the PLS models with different characteristic bands for each component are studied to choose a best resolution. The internal and external calibration results of the validated model show a mass percentage error below 1% for both components. Finally, the repeatabilities and reproducibilities of Raman and reference titration measurements are also discussed.

Precise 13CO2/12CO2 isotopic ratio measurements for breath diagnosis with a 2-μm diode laser

A laser spectrometer based on a distributed-feedback semiconductor diode laser at 2 μm is developed to measure the changes of 13CO2/12CO2 isotope ratio in exhaled breath sample with the CO2 concentration of ~4%. It is characterized by a simplified optical layout, in which a single detector and associated electronics are used to probe CO2 spectrum. The cell has 10 cm long base length with 26.4 m optical path length in total and 330 cm3 volume. The cell pressure and temperature are controlled at 50 Torr and 28℃, respectively. The best 13δ precision of 0.06‰ was achieved by using wavelet denoising and Kalman filter.