Design and Fabrication of Al_2O_3/SiO_2 Double-Layer Antireflection Coatings on 4H-SiC Substrate

在4H-SiC基底上设计并制备了Al2O3/SiO2紫外双层减反射膜,通过扫描电镜(SEM)和实测反射率谱来验证理论设计的正确性。利用编程计算得到Al2O3和SiO2的最优物理膜厚分别为42.0nm和96.1nm以及参考波长λ=280nm处最小反射率为0.09%。由误差分析可知,实际镀膜时保持双层膜厚度之和与理论值一致有利于降低膜系反射率。实验中应当准确控制SiO2折射率并使Al2O3折射率接近1.715。用电子束蒸发法在4H-SiC基底上淀积Al2O3/SiO2双层膜,厚度分别为42nm和96nm。SEM截面图表明淀积的薄膜和基底间具有较强的附着力。实测反射率极小值为0.33%,对应λ=276nm,与理论结果吻合较好。与传统SiO2单层膜相比,Al2O3/SiO2双层膜具有反射率小,波长选择性好等优点,从而论证了其在4H-SiC基紫外光电器件减反射膜上具有较好的应用前景。Al2O3/SiO2 double-layer UV antireflection coatings were designed and fabricated on 4H-SiC substrate,and the validity of theoretical design was further verified by scanning electron microscope (SEM) and reflection spectrum. The optimal physical thickness of Al2O3 and SiO2 is 42.0 nm and 96.1 nm respectively by programming calculation. And then the minimum reflectance of 0.09% is obtained at reference wavelength λ=280 nm. According to error analysis,keeping the sum of double-layer thickness consistent with theoretical value is helpful to reduce the reflectance. In addition,the refractive index of SiO2 should more accurate and the refractive index of Al2O3 should be controlled close to 1.715 in the experiment. Al2O3/SiO2 double-layer coatings were deposited on 4H-SiC substrate by electron beam evaporation and the physical thickness is 42 nm and 96 nm respectively. SEM images show that the deposited layers and the substrate perform good adhesion to each other. The practical minimum reflectance is 0.33% at λ=276 nm which is close to theoretical value. Compared with conventional SiO2 single layer,Al2O3/SiO2 double-layer coatings show low reflectance and better wavelength selectivity. These results make the possibility for 4H-SiC based UV optoelectronic devices with Al2O3/SiO2 films as antireflection coatings

Thermal-mechanical modeling of nodular defect embedded within multilayer coatings

The initiation of laser damage within optical coatings can be better understood by thermal-mechanical modeling of coating defects. The result of this modeling shows that a high-temperature rise and thermal stress can be seen just inside the nodular defect compared to surrounding coating layers. The temperature rise and thermal stress tend to increase with seed diameter. Shallower seed tend to cause higher temperature rise and greater thermal stress. There is a critical seed depth at which thermal stress is largest. The composition of the seed resulting from different coating-material emission during evaporation can affect the temperature rise and thermal stress distribution

effect of alumina interlayers on the microstructure and optical properties of ag films deposited on glasses

Optical properties and microstructure of Ag films with Al2O3 interlayer of different thickness on glass substrates were investigated by using spectrophotometer and XRD. The spectra results showed that the reflectivity of Ag films increased with the thickness of Al2O3 interlayer increasing from 9 nm to 24 nm and then decreased with increasing Al2O3 film thickness to 24 nm. Compared with pure Ag film on glass substrate, the reflectivity of Ag films with the Al2O3 interlayer of 24 nm was higher and that of Ag films with 9-18 nm and 30-36 nm Al2O3 interlayer was lower. Thus, the reflectivity of Ag film can be promoted by introducing a certain thickness of Al2O3 interlayer. The XRD analysis showed that with a certain thickness of Al2O3 interlayer, the crystalline grains increased, scattering decreased, and the strain increased

Robust and Fast Initialization for Intensity-Based 2D/3D Registration

Intensity-based 2D/3D registration is a key technique using digitally reconstructed radiographs (DRRs) to register the preoperative volume to the patient setup during the operation. Although DRR-based method provides a high accuracy, the small capture range hinders its clinical use. In this paper, such problem was addressed by a robust and fast initialization method using a two-level scheme including automatic tracking-based initialization (Level I) and multiresolution estimation based on central-slice theorem and phase correlation (Level II). It provided almost optimal transformation parameters for intensity-based registration. Experiments using a public gold standard data set and a spinal phantom have been conducted. The mean target registration error (mTRE) was limited in the range from 2.12 mm to 22.57 mm after tracking-based initialization. The capture range based on level II only was 20.1 mm and the mTRE in this capture range was 2.92 +/- 2.21 mm. The intensity-based 2D/3D registration using proposed two-level initialization achieved the successful rate of 84.8% with the average error of 2.36 mm. The experimental results showed that the proposed method yielded the robust and fast initialization for intensity-based registration methods. In a similar way, it can be applied to other registration methods to enable a larger capture range and robust implementation.</p

Robust and Fast Initialization for Intensity-Based 2D/3D Registration

Intensity-based 2D/3D registration is a key technique using digitally reconstructed radiographs (DRRs) to register the preoperative volume to the patient setup during the operation. Although DRR-based method provides a high accuracy, the small capture range hinders its clinical use. In this paper, such problem was addressed by a robust and fast initialization method using a two-level scheme including automatic tracking-based initialization (Level I) and multiresolution estimation based on central-slice theorem and phase correlation (Level II). It provided almost optimal transformation parameters for intensity-based registration. Experiments using a public gold standard data set and a spinal phantom have been conducted. The mean target registration error (mTRE) was limited in the range from 2.12 mm to 22.57 mm after tracking-based initialization. The capture range based on level II only was 20.1 mm and the mTRE in this capture range was 2.92 +/- 2.21 mm. The intensity-based 2D/3D registration using proposed two-level initialization achieved the successful rate of 84.8% with the average error of 2.36 mm. The experimental results showed that the proposed method yielded the robust and fast initialization for intensity-based registration methods. In a similar way, it can be applied to other registration methods to enable a larger capture range and robust implementation.</p