Study of third-order optical nonlinear devices using semiconductor microcrystals

本研究的目的是研究CdS_xSe_1-x&gt；掺杂玻璃中三阶光学非线性的增强效应，并证明其在构建光学功能器件方面的有效性。本文研究了在不同波长的光泵浦下，Cd_xSe_&lt；1-x&gt；掺杂玻璃在同一信号波长下的三阶非线性极化率，并用旋转波近似的密度矩阵方法计算了三阶非线性极化率。计算中考虑了微晶尺寸和成分的分布。结果表明，与在比带隙波长更长的信号波长处泵浦的情况相比，当在接近带隙的波长处泵浦时，获得了更大的强度依赖的折射率变化，通常为10^3倍。讨论了考虑非线性吸收的材料的优值系数，它与非线性极化率的虚部有关，在低吸收波长区获得较大的折射率变化，使我们在设计各种全光功能器件时具有灵活性。设计了近带隙抽运光控制的光选通开关，并证明了其有效性。结果表明，这种方法可以显著降低开关能量。此外，还可以利用低于信号光强度的泵浦光来控制信号光波。

The purpose of this study is to investigate the enhancement of third-order optical nonlinearity in CdS_xSe_<1-x> doped glass and to demonstrate its effectiveness for constructing optical functional devices. In this study, third-order nonlinear susceptibility observed at a signal wavelength is examined in the CdS_xSe_<1-x> doped glass pumped by light of different wavelength.We calculated the third-order nonlinear susceptibility using a density matrix method with rotating wave approximation. The distributions of microcrystal size and composition are taken into account in the calculation. It is shown that, comparing to the case of being pumped at the signal wavelength, which is longer that the band gap wavelength, larger intensity dependent refractive index change, typically by a factor of 10^3, is obtained when being pumped at the wavelength close to the band gap. A figure of merit of the material under consideration is discussed considering the nonlinear absorption, which is associated with the imaginary part of nonlinear susceptibility.The large refractive index change obtained at low absorption wavelength region gives us flexibility in designing a variety of all-optical functional devices. We designed the optical gate switch controlled with pump beam close to band gap and showed its ellectiveness. It is shown that the switching energy can be remarkably reduced. Also, the signal lightwave can be controlled with the pump light of which intensity is lower than that of the signal light.