M. H. Chan

OPTICAL ABSORPTION OF FREE-STANDING POROUS SILICON FILMS

The optical absorptions of anodically etched p+ and n+ porous silicon (PS) films were investigated by photothermal deflection spectroscopy. Si–H stretching overtones and combination bands of Si–F and Si–H were observed. The defect model in hydrogenated amorphous silicon was used to explain the Urbach edge and the subgap absorptions of PS. The dangling bond defect densities in PS were estimated.

Fourier transform ultraviolet-visible spectrometer based on a beam-folding technique

A beam-folding technique in optical interferometry, where the number of beam folds used can be very large, is reported. This technique can be used as a low-cost position-tracking method in a Fourier transform spectrometer (FTS) to cover the broad spectral range from UV to IR. The main advantage gained is the simple position-tracking algorithm used in sampling the interferogram. We have developed a UV-visible FTS, whose wavelength coverage is limited only by the optical elements (350 nm(-1) microm with off-the-shelf components). Preliminary results show that it can achieve a resolution of approximately 4 cm(-1) even with a ball-bearing translation stage.

Acousto-Optic Differential Optical Absorption Spectroscopy for Atmospheric Measurement of Nitrogen Dioxide in Hong Kong

Measurement of the atmospheric concentration of nitrogen dioxide (NO2) pollutant was demonstrated by differential optical absorption spectroscopy (DOAS) using a visible acousto-optic tunable filter. In a traditional spectral scanning DOAS system for atmospheric concentration monitoring, a highly stable light source is required. When the light intensity fluctuates during scanning, the concentration retrieval will be inaccurate. In order to reduce the error due to intensity fluctuations, a modified DOAS system has been developed by introducing a broadband light intensity monitoring channel. Using the measured intensity of the broadband channel as the intensity of the light source, the spectrum can be de-biased and the residual intensity variation will primarily result from atmospheric extinction. In addition, by employing the lock-in detection technique, the background light interference is also removed in the modified DOAS system. The atmospheric NO2 concentration measurement was performed at the campus of City University of Hong Kong, and the results were compared with the concentration reported from a nearby monitoring station in Sham Shui Po, operated by the Hong Kong Environmental Protection Department.

Photothermal deflection spectroscopy of polymer thin films

Photothermal Deflection Spectroscopy (PDS) is known to be one of the most sensitive techniques for measuring the absorption of weakly absorbing materials. We have applied PDS for measuring the optical absorption of a few polymer thin-film samples over the wavelength region from 0.4–2.0 μm. The results are useful for optical evaluation of these polymers.

Low-Grade Waste Vibration Energy Recovery from Hong Kong Franchised Buses

Vibration characterization in moving franchised buses has been performed. Considering the frequency spectrum of vertical acceleration, two distinct vibration peaks at 2.2 and 14 Hz were observed. The root-mean-square acceleration peak values were 5.59×10−6 and 1.95×10−6 g2/Hz, where g is the acceleration due to gravity. Based on the experimental results of vibration frequency characteristics, a conventional piezoelectric cantilever energy harvester with natural frequency tuned at 14 Hz was constructed. This paper presents the application of piezoelectric energy harvester to obtain the low-grade waste vibration energy from Hong Kong franchised buses. Ten different bus routes were selected for the energy harvesting. On average, statistical data showed that for buses traveling from one terminus to another, by using the cantilever piezoelectric energy harvesting technique, the feasible power generated from bus vibration was around 90 μW.

Airborne solar spectroscopic measurements of nitrogen dioxide column density beneath the boundary layer

Airborne atmospheric measurement of nitrogen dioxide (NO2) column density was performed using the solar spectroscopic method. The measurement was carried out at different altitudes, from 460 to 700 meters above sea level during a flight in Hong Kong (22.2°N, 114.1°E), the Peoples Republic of China (PRC). In the territory, the boundary layer height in a sunny day is about 1000 meters. Below the boundary layer, most of the NO2 exists. The airborne solar spectroscopic measurement gives the NO2 vertical profile below the boundary layer. In particular, the solar spectroscopic measurement requires a solar tracking system to collect the direct sunlight. However, in an acceptable small percentage of error in tracking the sun position, it is possible to collect the direct sunlight manually. In this paper, to reduce the complexity of the experimental setup, the sunlight is collected by a portable miniature CCD spectrometer. In the retrieval of NO2 column density, the airborne solar spectrum is normalized to a reference solar spectrum, which is taken at a high altitude (11,230 meters) during another flight in Xinjiang (42.208°N, 83.949°E) province, PRC. The column density retrieval is achieved from the normalized solar spectrum using the differential optical absorption spectroscopy. A ground-based off-axis control experiment is also performed to estimate the error in the slant column density from the airborne measurement.

Data inversion of eye-safe micropulse Mie lidar for internal boundary layer studies

In studies of the internal boundary layer, it is important to make the lidar both eye-safe and capable of measuring near range extinction to high accuracy. The inversion of data is much more challenging due to the weaker signal from an eye-safe lidar. The method of inversion employed in this paper is the Fernalds method. Because the digitization system is capable of obtaining returns in the time interval of a few seconds, formal statistical analysis and error propagation are introduced in the inversion. Different weighting schemes are used during the averaging of the inversion and the results from scattered cloud data as well as pure aerosol profiles will be discussed. Results from several sites and under different meteorological conditions will be reported.

Monitoring of environmental nitrogen dioxide concentration using visible acousto-optic differential optical absorption spectroscopy

Atmospheric concentration measurement of nitrogen dioxide (NO2) pollutant was demonstrated by a new type of differential optical absorption spectroscopy (DOAS) using a spectral scanning device of a visible acousto-optic tunable filter. The measurement requires a stable artificial light source such as a xenon lamp, and the light beam is directed into the environment where the concentration of NO2 is to be monitored. The retrieval of NO2 concentration is then achieved by analyzing the residual light using the DOAS signal processing. In this paper, we present results obtained from this new DOAS system during continuous measurement of atmospheric NO2 concentration in the campus of the City University of Hong Kong. Another DOAS system, using a miniature CCD grating spectrometer, was established as a control experiment. The CCD spectrometer acts as a traditional DOAS system for performance evaluation. Instead of using the NO2 absorption cross section for concentration retrieval, differential absorption area provides an alternative quantity for concentration retrieval. The monitoring results from both of the DOAS systems are compared with the pollutant concentrations reported in a nearby pollutant monitoring station, operated by the Hong Kong Environmental Protection Department.

STUDY OF POROUS SILICON SURFACE PROPERTIES

Porous silicon is a material with a coral-like structure which has a fractal surface. To study these aspects of porous silicon and its relationship with the luminescence property, we have used atomic force microscopy (AFM). Samples were prepared using either pure HF or HF diluted with ethanol. From the results of AFM, distinct structural difference was observed from samples prepared by these two etchants. If we relate the structures to their respective photoluminescence spectra, it appears that finer structure produced shorter wavelength peak photoluminescence. However, the columns of the samples were too large for one to attribute the luminescence to quantum confinement only. Hence, an alternative model may be required to explain the luminescence mechanism. We have also observed that the composition of the etchant can also affect the evolution of the fractal dimension with respect to etching time. Probing of the surfcace electron states was performed using photothermal deflection spectroscopy (PDS). In order to ensure that only porous silicon layer was probed, free-standing films of various porosity were produced for the PDS measurement. The probe energy range was from 0.56 eV to 2.5 eV so that both the bulk states and the surface states were probed. The results showed that there is a clear blueshift of the energy band gap with respect to porosity, and the absorption coefficient decreases with porosity increase at a fixed photon energy. Overtones of hydrides and fluorides of silicon were also observed.

Optical Absorptions and EL2-Like Defects in Low Temperature Grown Molecular-Beam-Epitaxial GaAs

The optical absorptions and the defect densities of GaAs grown by low temperature molecular-beam-epitaxy at growth temperatures between 200-580 °C were evaluated by photothermal deflection spectroscopy. The shapes of the absorption spectra exhibit EL2-like characteristics. Defect densities were found to be in the range of 10 18 -10 19 cm −3 . The PDS phase spectra were shown to be useful to differentiate the absorptions of the epilayer from those of the bulk.