The Nonlinear Effects At The Low Light Levels

Recently, much attention has been attracted to the theory and experiment about nonlinearity at the low light levels in the quantum optics and laser physics fields. In this thesis, by means of quantum optics theory, we have investigated the problem theoretically and obtained a series of meaningful results.The nonlinear effect at the low light levels breaks the limitation of damage medium, frequency restraint and low efficiency in conventional nonlinear optics, and exhibits new features with quantum entanglement and exhibits new features with quantum information, so it can be used for detecting weak signal, making photon device and processing quantum information. Since many experimental researches about this subject are based on the quantum interference among multiple levels system, then we study mainly the nonlinearity of two kinds of four-level system interacting with laser fields. The key points of this thesis are presented as follows:1. A general idea about nonlinearity at the low light levels and study tendency.2. The theory and method about nonlinearity and several important phenomena. Such as electromagnetically induced transparency (EIT), population inversion, coherent population trapping (CPT), etc.3. Making an analysis of Y-type four-level system interacting with three fields, discussing gain and dispersion effects of this system on different conditions.4. Making an analysis of Quasi-∧-type four-level system interacting with two fields, discussing gain and dispersion effects of this system on different conditions.5. Possible applications of the nonlinearity at the low light levels and a sample of which in quantum information.