Volume 5,Issue 6,2009 Table of Contents
2009, 5(6):401-404.
Abstract:
A novel method to design Y-branch waveguides is proposed by using total internal reflection, and the optical properties are analyzed. The simulated results show that the optical loss values of two Y-branch waveguides with the branching angles of 12° and 30° are only 0.310 dB and 0.645 dB, respectively. Compared with conventional Y-branch waveguides, the optical performance of the proposed ones is well improved, which has many advantages such as wide angle, low loss, simple structure and easy for fabrication.
A novel method to design Y-branch waveguides is proposed by using total internal reflection, and the optical properties are analyzed. The simulated results show that the optical loss values of two Y-branch waveguides with the branching angles of 12° and 30° are only 0.310 dB and 0.645 dB, respectively. Compared with conventional Y-branch waveguides, the optical performance of the proposed ones is well improved, which has many advantages such as wide angle, low loss, simple structure and easy for fabrication.
2009, 5(6):405-408.
Abstract:
To enhance the light extraction efficiency of traditional light-emitting diodes (LEDs) by reducing the total internal reflection, an omni-directional reflector (ODR) and photonic crystal are adopted in the paper. The structures of photonic crystal and the ODR are designed by diffraction theory and finite difference time domain (FDTD) method. The photonic crystal is employed in the p-GaN layer and the ODR composed of TiO2/SiO2 is designed between the active region and substrate. The simulation results indicate that the light extraction of LEDs can be enhanced by 11.6 times, and the external quantum efficiency of LEDs will be effectively improved.
To enhance the light extraction efficiency of traditional light-emitting diodes (LEDs) by reducing the total internal reflection, an omni-directional reflector (ODR) and photonic crystal are adopted in the paper. The structures of photonic crystal and the ODR are designed by diffraction theory and finite difference time domain (FDTD) method. The photonic crystal is employed in the p-GaN layer and the ODR composed of TiO2/SiO2 is designed between the active region and substrate. The simulation results indicate that the light extraction of LEDs can be enhanced by 11.6 times, and the external quantum efficiency of LEDs will be effectively improved.
2009, 5(6):409-412.
Abstract:
Pentacene-based organic field effect transistors (OFETs) are fabricated using poly(methyl methacrylate) (PMMA) and polyimide (PI) as gate dielectrics, respectively. The fabricated OFETs exhibit reasonable device characteristics. The field-effect mobility, threshold voltage, and on/off current radio are determined to be 3.214 × 10−2 cm2/Vs, −28 V, and 1 ×103 respectively for OFETs with PMMA as gate dielectrics, and 7.306×10−3cm2/Vs, −21 V, and 2 ×102 for OFETs with PI. Furthermore, the dielectric properties of gate insulator layer are tested and the dipole effect at the semiconductor/dielectrics interface is also analyzed by a model of energy level diagram.
Pentacene-based organic field effect transistors (OFETs) are fabricated using poly(methyl methacrylate) (PMMA) and polyimide (PI) as gate dielectrics, respectively. The fabricated OFETs exhibit reasonable device characteristics. The field-effect mobility, threshold voltage, and on/off current radio are determined to be 3.214 × 10−2 cm2/Vs, −28 V, and 1 ×103 respectively for OFETs with PMMA as gate dielectrics, and 7.306×10−3cm2/Vs, −21 V, and 2 ×102 for OFETs with PI. Furthermore, the dielectric properties of gate insulator layer are tested and the dipole effect at the semiconductor/dielectrics interface is also analyzed by a model of energy level diagram.
2009, 5(6):422-426.
Abstract:
The model of monocrystalline silicon solar cells is established, and the effects of wafer parameters, such as the p-Si (100) substrate thickness, the defect density, and the doping concentration, on the electronic properties of monocrystalline silicon solar cells are analyzed. The results indicate that the solar cells with an Al back-surface-field will have good electronic properties when the wafers meet the following three conditions: (i) the defect density is less than 1.0×1011 cm−3; (ii) the doping concentration is from 5×1015 cm−3 to 1×1017 cm−3, i.e. the bulk resistivity is from 0.5 Ω·cm to 10 Ω·cm; (iii) the cells substrate thickness is in the range of 100 μm to 200 μm.
The model of monocrystalline silicon solar cells is established, and the effects of wafer parameters, such as the p-Si (100) substrate thickness, the defect density, and the doping concentration, on the electronic properties of monocrystalline silicon solar cells are analyzed. The results indicate that the solar cells with an Al back-surface-field will have good electronic properties when the wafers meet the following three conditions: (i) the defect density is less than 1.0×1011 cm−3; (ii) the doping concentration is from 5×1015 cm−3 to 1×1017 cm−3, i.e. the bulk resistivity is from 0.5 Ω·cm to 10 Ω·cm; (iii) the cells substrate thickness is in the range of 100 μm to 200 μm.
2009, 5(6):427-429.
Abstract:
The design of an ultra-compact SOI-based 1×2 3 dB multimode interference (MMI) coupler is proposed. The symmetric multimode interference structure and deeply-etched parabolically tapered multimode waveguide are used in the device. The length of the device is reduced by about 87% compared with general multimode interference straight couplers. Based on effective index method (EIM) and two dimensional beam propagation method (2D-BPM), the structure parameters are optimized and the fabrication tolerances are analyzed. The results indicate that the performance of the coupler is deeply affected by the coupler’s width, etching depth and photolithography alignment error in perpendicular direction.
The design of an ultra-compact SOI-based 1×2 3 dB multimode interference (MMI) coupler is proposed. The symmetric multimode interference structure and deeply-etched parabolically tapered multimode waveguide are used in the device. The length of the device is reduced by about 87% compared with general multimode interference straight couplers. Based on effective index method (EIM) and two dimensional beam propagation method (2D-BPM), the structure parameters are optimized and the fabrication tolerances are analyzed. The results indicate that the performance of the coupler is deeply affected by the coupler’s width, etching depth and photolithography alignment error in perpendicular direction.
2009, 5(6):430-433.
Abstract:
The frequency-dependent optical and dielectric properties of annealed ZnO nanoparticles in the range of 0.1 to 0.9 THz are studied by using terahertz time-domain spectroscopy (THz-TDS). The refractive index, power absorption and complex dielectric constants are obtained and the experimental results are well fit with a simple effective medium theory in conjunction with a pseudo-harmonic model. This study reveals that annealed ZnO nanoparticles exhibit the similar phonon response characteristics to the single ZnO crystal and other ZnO nanostructures, such as tetrapods and nanowires.
The frequency-dependent optical and dielectric properties of annealed ZnO nanoparticles in the range of 0.1 to 0.9 THz are studied by using terahertz time-domain spectroscopy (THz-TDS). The refractive index, power absorption and complex dielectric constants are obtained and the experimental results are well fit with a simple effective medium theory in conjunction with a pseudo-harmonic model. This study reveals that annealed ZnO nanoparticles exhibit the similar phonon response characteristics to the single ZnO crystal and other ZnO nanostructures, such as tetrapods and nanowires.
2009, 5(6):434-436.
Abstract:
The optical lattice transformation from 2 dimensions (2D) to 3 dimensions (3D) by holographic interference is studied. The 2D optical lattices can be formed by 3 noncoplanar beams, and in this paper by adding another three beams, a 3D interference lattice structure is obtained. The added beams and the original ones are mirror symmetry (by using a reflective mirror). The simulation results show that the lattices become ellipsoids from 2D columns, which are periodic in the z direction. The calculations and numerical simulations provide a useful theoretical guide for fabricating 3D photonic crystals (PCs).
The optical lattice transformation from 2 dimensions (2D) to 3 dimensions (3D) by holographic interference is studied. The 2D optical lattices can be formed by 3 noncoplanar beams, and in this paper by adding another three beams, a 3D interference lattice structure is obtained. The added beams and the original ones are mirror symmetry (by using a reflective mirror). The simulation results show that the lattices become ellipsoids from 2D columns, which are periodic in the z direction. The calculations and numerical simulations provide a useful theoretical guide for fabricating 3D photonic crystals (PCs).
2009, 5(6):437-440.
Abstract:
An all-optical quantization based on Raman self-frequency shift (RSFS) in a photonic crystal fiber (PCF) and spectral compression in a dispersion-increasing fiber (DIF) is analyzed, and the evolution of femtosecond pulse in fibers is described by numerically solving the generalized nonlinear Schr?dinge equation (GNLSE). Gaussian pulse with the width of 300 fs and center wavelength of 1550 nm is injected into 15 m-long PCF and 100 m-long DIF. The simulation results show that the center wavelength increases linearly with the input peak power which changes from 110 W to 165 W. The RSFS of 65.3 nm and maximal spectral compression ratio of 3.38 can be obtained. The resolution of the quantization is improved from 2.4 bits to 4 bits by using the spectral compression in the DIF.
An all-optical quantization based on Raman self-frequency shift (RSFS) in a photonic crystal fiber (PCF) and spectral compression in a dispersion-increasing fiber (DIF) is analyzed, and the evolution of femtosecond pulse in fibers is described by numerically solving the generalized nonlinear Schr?dinge equation (GNLSE). Gaussian pulse with the width of 300 fs and center wavelength of 1550 nm is injected into 15 m-long PCF and 100 m-long DIF. The simulation results show that the center wavelength increases linearly with the input peak power which changes from 110 W to 165 W. The RSFS of 65.3 nm and maximal spectral compression ratio of 3.38 can be obtained. The resolution of the quantization is improved from 2.4 bits to 4 bits by using the spectral compression in the DIF.
2009, 5(6):441-444.
Abstract:
Aiming at implementing the digital pulse interval modulation (DPIM) for optical wireless communications (OWC), a dualmapping technique is presented. The scheme of DPIM train based upon the dual-mapping technique is given. Its slot error rate is derived for the avalanche photonic diode (APD) receiver model, and is compared with that of classical DPIM. Simulation results show that the dual-mapping DPIM (D-DPIM), which has a fixed slot length, only has marginally inferior error performance, but can solve waiting slots or buffer overflowing in comparison with DPIM. Hence, it is suitable for the optical wireless communication systems.
Aiming at implementing the digital pulse interval modulation (DPIM) for optical wireless communications (OWC), a dualmapping technique is presented. The scheme of DPIM train based upon the dual-mapping technique is given. Its slot error rate is derived for the avalanche photonic diode (APD) receiver model, and is compared with that of classical DPIM. Simulation results show that the dual-mapping DPIM (D-DPIM), which has a fixed slot length, only has marginally inferior error performance, but can solve waiting slots or buffer overflowing in comparison with DPIM. Hence, it is suitable for the optical wireless communication systems.
2009, 5(6):445-449.
Abstract:
The dynamic image information of typical gas-liquid two-phase flow patterns in vertical upward pipe is captured by a highspeed dynamic camera. The texture spectrum descriptor is used to describe the texture characteristics of the processed images whose content is represented in the form of texture spectrum histogram, and four time-varying characteristic parameter indexes which represent image texture structure of different flow patterns are extracted. The study results show that the amplitude fluctuation of texture characteristic parameter indexes of bubble flow is lowest and shows very random complex dynamic behavior; the amplitude fluctuation of slug flow is higher and shows intermittent motion behavior between gas slug and liquid slug, and the amplitude fluctuation of churn flow is the highest and shows better periodicity; the amplitude fluctuation of bubble-slug flow is from low to high and oscillating frequence is higher than that of slug flow, and includes the features of both slug flow and bubble flow; the slug-churn flow loses the periodicity of slug flow and churn flow, and the amplitude fluctuation is high. The results indicate that the image texture characteristic parameter indexes of different flow pattern can reflect the flow characteristics of gas-liquid two-phase flow, which provides a new approach to understand the temporal and spatial evolution of flow pattern dynamics.
The dynamic image information of typical gas-liquid two-phase flow patterns in vertical upward pipe is captured by a highspeed dynamic camera. The texture spectrum descriptor is used to describe the texture characteristics of the processed images whose content is represented in the form of texture spectrum histogram, and four time-varying characteristic parameter indexes which represent image texture structure of different flow patterns are extracted. The study results show that the amplitude fluctuation of texture characteristic parameter indexes of bubble flow is lowest and shows very random complex dynamic behavior; the amplitude fluctuation of slug flow is higher and shows intermittent motion behavior between gas slug and liquid slug, and the amplitude fluctuation of churn flow is the highest and shows better periodicity; the amplitude fluctuation of bubble-slug flow is from low to high and oscillating frequence is higher than that of slug flow, and includes the features of both slug flow and bubble flow; the slug-churn flow loses the periodicity of slug flow and churn flow, and the amplitude fluctuation is high. The results indicate that the image texture characteristic parameter indexes of different flow pattern can reflect the flow characteristics of gas-liquid two-phase flow, which provides a new approach to understand the temporal and spatial evolution of flow pattern dynamics.
2009, 5(6):454-458.
Abstract:
Taking the feature that the energy of the image would gather and spread on four components (LL 2, LH 2, HL 2 and HH 2) in the sub-image after first-order GHM multi-wavelet-transform. And by using the color control ability of lαβ color space in color transfer theory (CTT), an information hiding algorithm based on GHM-CTT is proposed. In this way, the robust parameters are embedded in the LL 2, the hidden information is set in LH 2 and HL 2 with RAID4, and fragile sign is set in HH 2. The consistence between the embedded data bits’ order and the embedded code of the sub-image is improved by using the chaotic mapping and the genetic algorithm. Experimental results indicate that the GHM-CTT can increase the imperceptibility by 15.72% averagely and robustness by 18.89% at least.
Taking the feature that the energy of the image would gather and spread on four components (LL 2, LH 2, HL 2 and HH 2) in the sub-image after first-order GHM multi-wavelet-transform. And by using the color control ability of lαβ color space in color transfer theory (CTT), an information hiding algorithm based on GHM-CTT is proposed. In this way, the robust parameters are embedded in the LL 2, the hidden information is set in LH 2 and HL 2 with RAID4, and fragile sign is set in HH 2. The consistence between the embedded data bits’ order and the embedded code of the sub-image is improved by using the chaotic mapping and the genetic algorithm. Experimental results indicate that the GHM-CTT can increase the imperceptibility by 15.72% averagely and robustness by 18.89% at least.
2009, 5(6):464-467.
Abstract:
To accurately estimate the motion parameters, especially for rotation, scale and transition, a new invariant feature descriptor is proposed based on simplified polar sampling. The Gaussian weighed regions in original version are replaced by the uniform weighted regions. Fast calculation of gradient-orientation histogram by means of integral map is adopted to improve the efficiency. Then a new method is proposed based on linear fitting of motion path and it can estimate the current frame position by local fitting parameters. The frame rate of stabilized output can reach 30 f/s, which gains notable improvement compared with the general invariant feature methods such as SIFT and SURF.
To accurately estimate the motion parameters, especially for rotation, scale and transition, a new invariant feature descriptor is proposed based on simplified polar sampling. The Gaussian weighed regions in original version are replaced by the uniform weighted regions. Fast calculation of gradient-orientation histogram by means of integral map is adopted to improve the efficiency. Then a new method is proposed based on linear fitting of motion path and it can estimate the current frame position by local fitting parameters. The frame rate of stabilized output can reach 30 f/s, which gains notable improvement compared with the general invariant feature methods such as SIFT and SURF.
2009, 5(6):468-471.
Abstract:
To improve the detecting effects of moving objects, an adaptive background reconstruction algorithm based on inertial filtering is proposed in this paper. According to different properties of the moving foreground and ever-changing background, the current frame is added to the background with a specific weight value. So the background can not only keep steady, but also be reconstructed at a specific speed. Experimental results show that the algorithm can reconstruct the background quickly and effectively whenever the background changes slowly or suddenly, or the background is mixed with moving foreground, and it can improve the veracity and robustness of objects detection effectively.
To improve the detecting effects of moving objects, an adaptive background reconstruction algorithm based on inertial filtering is proposed in this paper. According to different properties of the moving foreground and ever-changing background, the current frame is added to the background with a specific weight value. So the background can not only keep steady, but also be reconstructed at a specific speed. Experimental results show that the algorithm can reconstruct the background quickly and effectively whenever the background changes slowly or suddenly, or the background is mixed with moving foreground, and it can improve the veracity and robustness of objects detection effectively.
2009, 5(6):472-475.
Abstract:
Based on the nonlinear medium transfer theory, optical bistability of nonlinear microcavity is investigated at oblique incidence. It is found that the critical frequency of incident light inducing bistability phenomenon is related to the polarization and incident angle. The critical frequency increases with the incident angle. Increasing the incident angle can make bistability happen easier for focusing Kerr medium, but will hinder bistability for defocusing Kerr medium. The bistability switch threshold of TE mode is significantly lower than that of TM mode at a large incident angle.
Based on the nonlinear medium transfer theory, optical bistability of nonlinear microcavity is investigated at oblique incidence. It is found that the critical frequency of incident light inducing bistability phenomenon is related to the polarization and incident angle. The critical frequency increases with the incident angle. Increasing the incident angle can make bistability happen easier for focusing Kerr medium, but will hinder bistability for defocusing Kerr medium. The bistability switch threshold of TE mode is significantly lower than that of TM mode at a large incident angle.
2009, 5(6):476-480.
Abstract:
In order to improve the precision of profile measurement based on ultra-precise thin light beam scanning, an assessment method that compares different numerical integration algorithms in frequency-domain is put forward. The compared numerical integration methods are regarded as recursive digital filters. Through comparing their functions of frequency response in frequency-domain, the delivering role of noise with different frequencies can be analyzed directly and clearly in the process of integrating measured slope data. Analyzing results show that the method of cubic spline is better than trapezoidal, Simpson and 3/8 Simpson rules.
In order to improve the precision of profile measurement based on ultra-precise thin light beam scanning, an assessment method that compares different numerical integration algorithms in frequency-domain is put forward. The compared numerical integration methods are regarded as recursive digital filters. Through comparing their functions of frequency response in frequency-domain, the delivering role of noise with different frequencies can be analyzed directly and clearly in the process of integrating measured slope data. Analyzing results show that the method of cubic spline is better than trapezoidal, Simpson and 3/8 Simpson rules.
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