Preliminary study on the photoproduction of hydroxyl radicals in aqueous solution with Aldrich humic acid, algae and Fe(III) under high-pressure mercury lamp irradiation

Under a high-pressure mercury lamp (HPML) and using an exposure time of 4 h, the photoproduction of hydroxyl radicals ((OH)-O-.) could be induced in an aqueous solution containing humic acid (HA). Hydroxyl radicals were determined by high-performance liquid chromatography using benzene as a probe. The results showed that (OH)-O-. photoproduction increased from 1.80 to 2.74 muM by increasing the HA concentration from 10 to 40 mg L-1 at an exposure time of 4 h (pH 6.5). Hydroxyl radical photoproduction in aqueous solutions of HA containing algae was greater than that in the aqueous solutions of HA without algae. The photoproduction of (OH)-O-. in the HA solution with Fe(111) was greater than that of the solution without Fe(III) at pH ranging from 4.0 to 8.0. The photoproduction of (OH)-O-. in HA solution with algae with or without Fe(111) under a 250 W HPML was greater than that under a 125 W HPML. The photoproduction of (OH)-O-. in irradiated samples was influenced by the pH. The results showed that HPML exposure for 4 h in the 4-8 pH range led to the highest (OH)-O-. photoproduction at pH 4.0.

Effects of CO2 concentrations on the freshwater microalgae, Chlamydomonas reinhardtii, Chlorella pyrenoidosa and Scenedesmus obliquus (Chlorophyta)

In order to investigate the possible impacts of increased atmospheric CO2 levels on algal growth and photosynthesis, the influence of CO2 concentration was tested on three planktonic algae (Chlamydomonas reinhardtii, Chlorella pyrenoidosa, and Scenedesmus obliquus). Increased CO2 concentration enhanced significantly the growth rate of all three species. Specific growth rates reached maximal values at 30, 100, and 60 muM CO2 in C. reinhardtii, C pyrenoidosa, and S. obliquus, respectively. Such significant enhancement of growth rate with enriched CO2 was also confirmed at different levels of inorganic N and P, being more profound at limiting levels of N in C pyrenoidosa and P in S. obliquus. The maximal rates of net photosynthesis, photosynthetic efficiency and light-saturating point increased significantly (p<0.05) in high-CO2-grown cells. Elevation of the CO2 levels in cultures enhanced the photoinhibition of C. reinhardtii, but reduced that of C pyrenoidosa and S. obliquus when exposed to high photon flux density. The photo-inhibited cells recovered to some extent (from 71% to 99%) when placed under dim light or in darkness, with better recovery in high-CO2-grown C. pyrenoidosa and S. obliquus. Although pH and pCO(2) effects cannot be distinguished from this study, it can be concluded that increased CO2 concentrations with decreased pH could affect the growth rate and photosynthetic physiology of C. reinhardtii, C. pyrenoidosa, and S. obliquus.

Ultrafast photo-induced turning of magnetization and its relaxation dynamics in GaMnAs

We report that, by linearly polarized pumping of different wavelengths, Kerr transients appear at zero magnetic field only in the case when GaMnAs samples are initialized at 3 K by first applying a 0.8 Tesla field and then returning to zero field. We find that, instead of magnetization precession, the near-band gap excitation induces a coherent out-of-plane turning of magnetization, which shows very long relaxation dynamics with no precession. When photon energy increases, the peak value of the Kerr transient increases, but it decays rapidly to the original slow transient seen under the near-band-gap excitation.

Dynamics of photo-enhanced magneto-crystalline anisotropy in diluted ferromagnetic GaMnAs

By chopping a pump beam in conventional time-resolved Kerr rotation (TRKR) experiments and measuring the time evolution of M-shaped "major" hysteresis loops of magnetic linear dichroism (Delta MLD = MLDpump-on MLDpump-off), the differential MLD signal in the presence and the absence of the pump beam, we studied the dynamics of photo-enhanced magneto-crystalline anisotropy, and found that its very long recovering time (much longer than 13 ns) might reflect the nature of the coherent coupling between photo-excited holes and localized spins in the d shell of manganese.

Substantial photo-response of InGaN p-i-n homojunction solar cells

InGaN p-i-n homojunction structures were grown by metal-organic chemical vapor deposition, and solar cells with different p-contact schemes were fabricated. X-ray diffraction measurements demonstrated that the epitaxial layers have a high crystalline quality. Solar cells with semitransparent p-contact exhibited a fill factor (FF) of 69.4%, an open-circuit voltage (V-oc) of 2.24 V and an external quantum efficiency (EQE) of 41.0%. On the other hand, devices with grid p-contact showed the corresponding values of 57.6%, 2.36 V, 47.9% and a higher power density. These results indicate that significant photo-responses can be achieved in InGaN p-i-n solar cells.

Study on the internal reflection of the multimode-interference-type device

The internal reflection of the multimode-interference (MMI)-type device is calculated with the bidirectional beam propagation method. The calculated results indicate that the difference of the effective refractive indices between the core region and the surrounding region has a determining effect on the internal reflection of the MMI-type device. The output taper for the MMI-type combiner and splitter has a more evident effect on the internal reflection than the input taper. The internal reflection decreases with increasing the end width of the taper. For the MMI-type device with appropriate tapers, the internal reflection does not show evident degradation with the deviation of the length of the MMI region from its optimal value. (C) 2004 Society of Photo-Optical Instrumentation Engineers.

Thermo-optic variable optical attenuator with low power consumption fabricated on silicon-on-insulator by anisotropic chemical etching

A thermo-optic Mach-Zehnder (MZ) variable optical attenuator based on silicon waveguides with a large cross section was designed and fabricated on silicon-on-insulator (SOI) wafer. Multimode interferometers were used as power splitters and combiners in the MZ structure. In order to achieve a smooth interface, anisotropic chemical etching of silicon was used to fabricate the waveguides. Isolating grooves were introduced to reduce power consumption and device length. The device has a low power consumption of 210 mW and a response time of 50 mus. (C) 2004 Society of Photo-Optical Instrumentation Engineers.

Design and fabrication of compact nonblocking 4X4 optical matrix switch on silicon-on-insulator by anisotropic chemical etching

A folding nonblocking 4 X 4 optical matrix switch in simplified-tree architecture was designed and fabricated on a silicon-on-insulator wafer. To compress chip size, switch elements (SEs) were connected by total internal reflection mirrors instead of conventional S-bends. For obtaining smooth interfaces, potassium hydroxide (KOH) anisotropic chemical etching of silicon was employed. The device has a compact size of 20 X 3.2 mm(2) and a fast response of 8 +/- 1 mu s. Power consumption of 2 x 2 SE and excess loss per mirror were 145 mW and -1.1 dB, respectively. (c) 2005 Society of Photo-Optical Instrumentation Engineers.

Influence of thermal isolating grooves on the performance of the Mach-Zehnder interferometer-type thermo-optic variable optical attenuator

Two types of silicon-on-insulator thermo-optic variable optical attenuators (VOAs) based on a Mach-Zehnder interferometer and a multimode-interference coupler are fabricated, one with thermal isolating grooves to improve heating efficiency and the other without Comparison of optical and electrical properties, such as insertion losses, the maximum attenuation levels and the corresponding power consumptions, and the response times, is carried out between the two types of VOAs. The comparison results Indicate that use of thermal isolating grooves leads to better values for most characteristics and is an effective way to improve the performance of Mach-Zehnder interferometer-type thermo-optic devices. (c) 2005 Society of Photo-Optical Instrumentation Engineers.

Silicon-on-insulator thermo-optic variable attenuator module with fast response

A thermo-optic variable optical attenuator module composed of a silicon-on-insulator attenuator chip and driving circuit was designed and fabricated. The module exhibited a maximum attenuation of 21.8 dB and a response time of 10 mu s. (c) 2005 Society of Photo-Optical Instrumentation Engineers.

Effects of different modified underlayer surfaces on growth and optical properties of InGaN quantum dots

InGaN/GaN quantum dots were grown on the sapphire (0 0 0 1) substrate in a metalorganic chemical vapor deposition system. The morphologies of QDs deposited on different modified underlayer (GaN) surfaces, including naturally as grown, Ga-mediated, In-mediated, and air-passivated ones, were investigated by atomic force microscopy (AFM). Photo luminescence (PL) method is used to evaluate optical properties. It is shown that InGaN QDs can form directly on the natural GaN layer. However, both the size and distribution show obvious inhomogeneities. Such a heavy fluctuation in size leads to double peaks for QDs with short growth time, and broad peaks for QDs with long growth time in their low-temperature PL spectra. QDs grown on the Ga-mediated GaN underlayer tends to coalesce. Distinct transform takes place from 3D to 2D growth on the In-mediated ones, and thus the formation of QDs is prohibited. Those results clarify Ga and In's surfactant behavior. When the GaN underlayer is passivated in the air, and together with an additional low-temperature-grown seeding layer, however, the island growth mode is enhanced. Subsequently, grown InGaN QDs are characterized by a relatively high density and an improved Gaussian-like distribution in size. Short surface diffusion length at low growth temperature accounts for that result. It is concluded that reduced temperature favors QD's 3D growth and surface passivation can provide another promising way to obtain high-density QDs that especially suits MOCVD system. (c) 2004 Elsevier Ltd. All rights reserved.

Combined structure of ZnO vertical well-aligned nanorods and net-like structures on AIN/sapphire

ZnO vertical well-aligned nanorods were grown on A1N/sapphire by using metal-organic chemical vapor deposition. We first observed the ZnO net-like structures under the nanorods. The different strain was determined in these two layers by using double crystal X-ray diffraction, Raman spectra, which revealed that the nanorods were relaxed and the net-like structures were strained. The optical properties of two layers were measured by using the cathodoluminescence and photo luminescence and the shift of UV peaks was observed. Moreover, the growth mechanism of the ZnO nanorods and the net-like structures is discussed. (c) 2007 Elsevier B.V. All rights reserved.

Effect of electron-hole spatial correlation on spin relaxation dynamics in InAs submonolayer

Using time-resolved photoluminescence and time-resolved Kerr rotation spectroscopy, we explore the unique electron spin behavior in an InAs submonolayer sandwiched in a GaAs matrix, which shows very different spin characteristics under resonant and non-resonant excitations. While a very long spin relaxation lifetime of a few nanoseconds at low temperature is observed under non-resonant excitation, it decreases dramatically under resonant excitation. These interesting results are attributed to the difference in electron-hole interactions caused by non-geminate or geminate capture of photo-generated electron-hole pairs in the two excitation cases, and provide a direct verification of the electron-hole spatial correlation effect on electron spin relaxation. (c) 2007 Elsevier Ltd. All rights reserved.

Greatly enhanced resonant tunneling of photo-excited holes in a three-barrier resonant tunneling structure

By integrating a resonant tunneling diode with a 1.2 mu m-thick slightly doped n-type GaAs layer in a three-barrier, two-well resonant tunneling structure, the resonant tunneling of photo-excited holes exhibits a value of peak-to-valley current ratio (PVCR) as high as 36. A vast number of photo-excited holes generated in this 1.2 mu m-thick slightly doped n-type GaAs layer, and the quantization of hole levels in a 23nm-thick quantum well on the outgoing side of hole tunneling out off the resonant tunneling diode which greatly depressed the valley current of the holes, are thought to be responsible for such greatly enhanced PVCR.

Directional-coupler-based Mach-Zehnder interferometer in silicon-on-insulator technology for optical intensity modulation

Based on silicon-on-insulator (SOI) technology, a Mach-Zehnder interferometer (MZI) is fabricated, in which two directional couplers serve as power splitter and combiner. The free carrier plasma dispersion effect of Si is adopted to achieve the phase modulation and the consequent intensity modulation of optical fields. The device presents an insertion loss of 2.61 dB and an extinction ratio of 19.6 dB. The rise time and fall time are 676 ns and 552 ns, respectively. Detailed analysis and explanation of the performance behaviors are also presented. (c) 2007 Society of Photo-Optical Instrumentation Engineers.