Research On The Nonlinear Optics In Asymmetrical Quantum Well

Nonlinear optics has become one of the most active fields in modern science has developed into a new subject with the development of the laser technology. After several decades, the basic principles on the nonlinear optics, the research on the new materials, and the discovery and applications of new effects have been developed greatly. The nonlinear optics has been one of the most active and important branches of optics.After the essence of all kinds of nonlinear optical phenomena has been known through long-range theoretical and experimental studies, nowadays our researches are being focused on searching and composing the nonlinear optical materials with special function. The ideal nonlinear optical materials require very large nonlinear optical susceptibility, small threshold power and very fast response time. A large number of researches have shown that the low dimensional semiconductor materials are one of the ideal nonlinear optical materials. With the progress of crystal growth technology, many low dimensional quantum systems with different dimensions and sizes can be fabricated. So studying of the nonlinear optical effects in the low dimensional semiconductor materials, such as quantum wells, quantum wires and quantum dots have gained more popularity.In the first chapter of the dissertation, the research background, the fundamental conception and the method have been introduced. The recent research achievements of nonlinear optics in the low dimensional semiconductor materials have been summarized also.In the second chapter, the second-order susceptibility coefficients for asymmetrical quantum well are investigated. Under the effective-mass approximation, we get the expressions of the wave functions and eigenenergy of the electron by solving the Schrodinger equations. Within the framework of the compact-density-matrix approach and an iterative method, the second-order susceptibility coefficients have been derived. Numerical calculations are performed for the typical GaAs/AlGaAs quantum well. The results showed that the second order susceptibility increase as the parameter "a" increasing, it is well known the asymmetry of the system has an obvious impact on the nonlinear susceptibility. The greater the asymmetry of the system is, the stronger of the second-order effect will be. While the parameter "a" increased, the asymmetry of the system becomes weak, which induces the decrease of the second-order susceptibility, Therefore an appropriate adjustment of the parameters "a" and "U₀" can results in larger second-order susceptibility.In the third chapter, the third-harmonic generations in the asymmetry quantum wells are investigated. The analytic expression of the third-harmonic generation coefficient is derived by using the compact-density-matrix approach and iterative method. And numerical calculations are performed for the typical GaAs/AlGaAs quantum well. The results show that the third-harmonic generations become stronger with the increase of the parameter "a", but decrease with the increase of the parameter Uo, which weaken the asymmetry of the system. Therefore, the asymmetry of the quantum system also has a large impact on the third-order susceptibility.In the fourth chapter, the refractive index changes have been investigated in an asymmetric quantum potential well with the same method above. The results show that the linear refractive index change has nothing to do with the incident light intensity, but third-order nonlinear refractive index changes strongly depend on the incident light intensity. The stronger of the incident light intensity is, the larger the third-order nonlinear refractive index change will be. Moreover the third-order nonlinear refractive index change also has relations with the asymmetry of the system. The larger the asymmetry is, the stronger the third -order nonlinear refractive index change will be. Therefore a large nonlinear refractive index will be obtained by adjusting the asymmetry of the system and the incident light intensity.In the last chapter, the main contents and conclusions of my dissertation have been summarized.