The Effects Of External DC Electric Field On The Gain Coefficient And Response Time Of Two-wave Mixing In Photorefractive Crystal

Photorefractive crystals have great potential for optical storage,optical computing,optical information processing and other fields.Two-wave mixing(TWM)in photorefractive crystal is a basic process of photorefractive optics.The gain coefficient of TWM and response time of TWM are important parameters of the process of TWM.Different crystals have different gain and response time in the process of TWM.Although TWM has become popular as early as the 1990s,the part of research on TWM is lacked.The externally applied dc electric field has an important effect on the photorefractive effect in the photorefractive crystal and can significantly change the properties of the photorefractive material.In the current research,there are few analyses of the effect of the externally applied dc electric field on the gain and response time of TWM in oxide photorefractive crystal,and lack of deep theoretical research.In addition,the current research on semiconductor crystals is mainly based on small modulation research and the effect of the externally applied dc electric field on the TWM of semiconductor crystal under large modulation is not comprehensive enough.The main research work of this paper is as follows:(1)The expression of the space-charge field normalized to the weak light intensity in the TWM process of oxide photorefractive crystal is deduced by the Kukhtarev's one center-one band material equation,and the expression of the gain coefficient is obtained.In the process of theoretical derivation,an empirical correction function f(m)is introduced to replace the modulation depth m to explain the nonlinear relationship between the gain coefficient and the modulation depth,so that the traditional linear TWM response process of small modulation is extended to the one of large modulation.Based on this method,the expression of the gain coefficient of semiconductor crystals in the process of large TWM modulation is deduced,which makes up for the lack of current research on large TWM modulation of semiconductor crystals.The TWM experiment and theoretical analysis are more consistent under large modulation,which is of great significance for the practical application of photorefractive crystal TWM.(2)Using the derived expression of the gain coefficient of TWM in oxide photorefractive,the variations of space-charge field and gain coefficient with normalized diffusion field,normalized applied dc electric field,concentration ratio and normalized intensity are analyzed by simulation.Therefore,theoretical conditions for the maximum space-charge field and gain coefficient can be obtained.It can be obtained that when other conditions remain unchanged,increasing the applied dc electric field can significantly increase the space-charge field and the gain coefficient.When the diffusion field is low and the concentration ratio is relatively high,the applied dc electric field can significantly improve the space-charge field and gain coefficient,but the applied dc electric field cannot be increased infinitely.If the applied dc electric field is too large,the crystal will be broken down and the crystal will be damaged.(3)In addition of the research on the gain coefficient of TWM in oxide photorefractive crystals,the effect of applied dc electric field on the response time of oxide photorefractive crystal Li Nb O₃:Fe is also demonstrated.It is obtained through simulation that the response time could lengthen by the applied dc electric field,and the response time could be improved by increasing the incident intensity and donor concentration.Experimental results show that when the external electric field is increased from 0 to 8k V/cm,the response time of Li Nb O₃:Fe crystal is decreased from 87 s to 72 s.Furthermore,the response time decreases rapidly and then slowly as the incident intensity increases.When the external electric field is 10 k V/cm,the response time decreases from 180 s to 42s with the increase of the incident intensity.This experiment proves the rationality of the theoretical simulation.(4)By deriving the expression of the gain coefficient of TWM in the semiconductor crystal,taking In P:Fe crystal as an example,the space-charge field and gain coefficient of TWM under large modulation with parameters such as applied dc electric field,incident intensity and Fe²⁺concentration are obtained through simulation.The results show that when other conditions remain unchanged,the strong electric field can significantly improve the space-charge field and gain coefficient of TMW,while the weak electric field has little effect on the TWM in the semiconductor crystal.Only under the condition of strong electric field,there is an intensity-dependent resonant in In P:Fe crystal.It can also be found that the higher the modulation depth,the smaller the gain coefficient.Although the increase of the modulation depth does not theoretically obtain a high gain coefficient,the explanation of the large modulation is more consistent with the experimental process of TWM.