入射光在AlGaAs窗口层界面散射对透射式GaAs光电阴极分辨力的影响

Effect of Incident Light Scattering at AlGaAs Window Layer Interface on Resolution of Transmission-mode GaAs Photocathode

为研究透射式GaAs光电阴极在不同波长入射光环境下的图像分辨能力,提出了一种基于散射传递函数的AlGaAs窗口层界面散射理论模型。制作了透射式GaAs光电阴极,利用不同波长入射光散射传递函数和点扩散函数变化,基于有参峰信噪比拟合对AlGaAs窗口层界面散射引起的入射光学图像退化程度进行了定量分析。结果表明,在相同的粗糙度下随着入射光波长的增加,入射光学图像的能量损失越小,成像质量越高,而三代微光像增强器不同波长入射光测试条件下极限分辨力变化趋势与仿真计算结果一致,可为后续提高透射式GaAs光电阴极分辨力提供了技术支撑。

Negative Electron Affinity(NEA) GaAs photocathode,as the most promising Ⅲ-Ⅴsemiconductor photocathode for low-light image intensifier,has wide applications in underwater target detection,range gated imaging,and night vision.According to the photoemission theory of transmissionmode GaAs photocathode,the incident light needs to pass through AlGaAs window layer before it can be absorbed and converted into photogenerated carriers by GaAs emission layer.The performance of image resolution on GaAs photocathode could result from the resolution of optoelectronic image and optical image.The resolution of optoelectronic image is determined by the longitudinal diffusion of photoelectrons in the emission layer.The resolution of optical image is mainly affected by the scattering of incident photons on the rough surface of the window layer.Most research on GaAs photocathode has focused on obtaining a higher optoelectronic image resolution,but there is rare research concentrated on improving the optical image resolution.However,the resolution of transmission-mode GaAs photocathode is hard to improve due to the ignorance of the optical image resolution.In order to study the image resolution of the transmission-mode GaAs photocathode under different wavelengths of incident light,a theoretical model based on the Scattering Transfer Function(STF) for the analysis of interfacial scattering from the AlGaAs window layer is proposed.A transmission-mode GaAs photocathode was fabricated,and the degradation of the incident optical image caused by the interfacial scattering of the AlGaAs window layer was quantitatively analyzed based on the signal-to-noise ratio fitting with reference peaks by using the variation of the STF and the Point Spread Function(PSF).The STF of the AlGaAs window layer rough surface under different incident light wavelengths was calculated and the incident light wavelength was set as 530 nm,700 nm,and 830 nm respectively.The obvious characteristic is that the value of STF is improved when the incident light wavelength increases.Therefore,When the wavelength of incident light is reduced from 830 nm to 530 nm,the reduction of STF means that the area or integral value enclosed by the surface transfer function is smaller,resulting in the smaller the capacity of optical image information and the more blurred the imaging.The PSF of the AlGaAs window layer rough surface can be obtained by Fourier quasitransform based on the STF.The results show that the central value of the PSF increases with the increase of the incident wavelength,which means the central energy of the optical image plane becomes more concentrated.In order to analyze the imaging quality of the third generation low light level image intensifier due to the scattering of incident light with different wavelengths on the rough surface of AlGaAs window layer,the PSF is convoluted with the original image of the given standard resolution target to simulate image degradation.When the incident light wavelength increases from 530 nm to 830 nm,the peak signalto-noise ratio after image degradation increases from 26.31 to 27.65,increasing by 5.1%.Computational results indicate that under the same surface roughness,compared with the incident light with the wavelength of 530 nm,the incident light with the wavelength of 830 nm is more beneficial to obtain a larger central value of the scattering transfer function and point spread function,which means that it is more conducive to obtain a sharper incident optical image.Furthermore,when the incident light wavelength is 850 nm,the signal-to-noise ratio of the image after scattering degradation is the highest,which also shows that the energy loss of the incident optical image is the smallest,the information capacity of the optical image is the highest,and the imaging quality is the best.This work will provide a reference for developing high-performance GaAs photocathodes.