The Ultrafast Dynamic Processes Of CdS Semiconductor And Phenothiazine Organic Molecule

Along with the great developments of the optical science and optical electronics in recent decades,the materials with excellent nonlinear optical properties have currently attracted considerable attention and show a variety of potential applications. Among the various classes of nonlinear optical materials,two-photon absorbing materials have been the subject of growing interest because of their potential application in many field,including two-photon fluorescence imaging,two-photon up-conversion laser,optical limiting and,three-dimensional data storage.With the wide utilization of the femtosecond laser system and the great achievement in the field of the ultrafast spectroscopy,ultrafast processes of the novel material can now be investigated in the picosecond and femtosecond domains.Because the ultrafast processes can reflect a lot of significant information about the interaction between light and matter,the research results of these processes are meaningful for the synthesizing and optimizing the new materials,as well as exploring the novel devices. Therefore,the study of nonlinear optical properties and ultrafast dynamics of the materials with intense two-photon absorption becomes one of the frontier and hot topics in the optical science.In this work,the ultrafast dynamics in CdS semiconductor crystal and phenothiazine organic molecule were studied using femtosecond laser technique. Different experimental methods were used to investigate different ultrafast processes. By using optical Kerr technique we got the ultrafast nonlinear optical property curve. Theχ⁽³⁾ of CdS crystal was calculated by comparing with standard sample CS₂. We also analyzed the reason,which caused the large nonlinearity of CdS.The purpose of the paper is to study the ultrafast response of optical nonlinearity of semiconductor. By using femtosecond pump-probe technique we studied the ultrafast excited state relaxation of CdS crystal and phenothiazine organic molecule.We also analyzed and explained the reason in order to study the excited state dynamics very and to provide the reliable theoretical and experimental basis for their practical application.The important result in the paper as following:1.The femtosecond ultrafast measurement system controlled by computer was set up,and it can measure optical Kerr effect,ultrafast pump-probe absorption spectroscopy,ultrafast pump-probe reflectivity spectroscopy.Through the technology of femtosecond pump-probe transient absorption,we can get the time-resolved spectroscopy of material that excited by laser.The basic theory of pump-probe technology is to get time-resolved by laser delay in space.The laser pulse is divided into pump pulse and probe pulse.The two pulses have a alterable time delay.The stronger pump pulse is able to excite the material leading the character to change,the probe pulse detects the changes coursed by pump pulse.The technology of optical Kerr effect is one of the method for measuring of material nonlinear optical response.The basic theory is using a strong linearly polarized light as pump pulse to excite the material to produce optical birefringence, and using the other pulse to detect.2.First,we studied the Kerr effect of CdS semiconductor crystal.There are two purposes.One is to study the third-order nonlinear of CdS,the other is to get the time zero point of femtosecond pump-probe experiment.Through comparing with standard sample CS₂ we obtain theχ⁽³⁾ of CdS crystal.The Kerr response time is about lps. The value ofχ⁽³⁾ is 7.9×10¹² esu by calculated.It is analyzed that the source of nonlinear coursed of local-field effect.3.Second,the ultrafast dynamics of CdS is studied by using femtosecond pump-probe method with laser pulse of 800 nm wavelength.The basic characteristics of CdS carrier relaxation contain three phases:a splitting upwards phase,a fast downwards phase and a slow downwards phase.CdS photogenic charge carrier relaxation contains several important aspects:carrier initial scattering,carrier-lattice energy redistributions,carrier recombination and transport.These are based on selective electron excitation on a time scale shorter than that of electron-electron and electron-lattice energy redistributions.The injected energy is subsequently redistributed among the electrons by e-e scattering eventually leading to the establishment of an electron temperature,and transferred to the lattice by electron-phonon interactions.4.About organic molecule,we studied phenothiazine which is a well known heterocyclic compound with electron-rich nitrogen and sulfur heteroatom.The method of experiment is femtosecond pump-probe technology with laser pulse of 800nm wavelength.The basic characteristics of phenothiazine excited state relaxation contain two phases:a splitting upwards and a slow downwards phase.After the energy transfer to the upper excitation state,the excitation energy experiences an ultrafast relaxation to the next lowest excitation state,and results in the transient population of this state and the thermalization of the temporally localized electron on this state.As a result,it was observed an ultra_fast process of the excited state absorption from the dynamic traces.By comparing with characteristic time,it is analyzed that the internal conversion time is related with branch structure.