Highly efficient generation of entangled photon pair by spontaneous parametric downconversion in defective photonic crystals
We study the process of spontaneous parametric downconversion in a one-dimensional defective quadratic nonlinear photonic crystal. It is shown that the strong confinements of both the pump and signal waves around the defective layer result in a giant enhancement of the entangled photon-pair generation. An enhancement factor as high as 3.4×106 is obtained in our defective structure based on the dual-localized modes. Furthermore, the linewidth of the downconverted fields is only 0.1 nm. Such a photonic crystal structure can be applied as a highly efficient source of entangled photon pairs for highly integrated all-optical circuits.
Image segmentation is an important problem of digital image processing and also a common difficult problem. In this paper, a new image thresholding method based on parzen-window estimation and Renyi entropy is proposed. The method used Parzen-window technology to estimate the spatial probability distribution of image gray level values, then combined with the Renyi entropy to construct a new criterion function, and at last searched the optimal global threshold in the scope of gray level to maximum the criterion function. This new method has some advantages, such as high accuracy to image segmentation, fine stability comparing with the traditional Renyi entropy method.