Manipulation Of Second Order Nonlinear Process In Space And Frequency Domains Via Feedback-based Wavefront Shaping

Over the past decade,the research of focusing and imaging in complex media has become the focus of more and more people.But the traditional research focuses on the linear scattering medium.In order to compensate for scattering effects,achieve the higher signal noise ratio of focusing and smaller resolution imaging,etc.,the feedback-based wavefront shaping(FBWS)technology was proposed in 2007 and applied to strong linear scattering media.In addition to the linear scattering medium,the nonlinear medium will also be affected by scattering.Especially in the practical application of quantum communication,the standard source of entangledphoton pairs is nowadays the nonlinear optical process of spontaneous parameter down-conversion(SPDC)in optics.In reality,photon pairs generation and propagation with long distance in free space will be scattered by sample defect and air impurity,which influences the efficiency of propagation and even the rate of reception.Hence,we proposed and effectively applied FBWS to nonlinear processes.In this paper,we realized the manipulation of the second harmonic(SH)process and SPDC process in space and frequency domain via FBWS.Firstly,the basic model was built for the manipulation in space and frequency domain respectively.The complex nonlinear scattering process was simplified to provide theoretical technology for the experiment.Secondly,the experiment was designed for process.In space domain,single-point and multi-point focusing in random nonlinear media were realized.After iterative optimization,the focus size was Focus on a single pixel and the enhancement factor was estimated to be 27.In frequency domain,a multi-peak and a multi-space-angle optimization were also experimentally realized,indicating the flexible control of the emission spectra and directions.It provides theoretical and experimental proof for the application of the technique in random medium and ultrafast nonlinear optics.Finally,the technique was applied on the manipulation of photon pairs through SPDC process.In the experiment,the prediction efficiency was taken as the objective function of optimization and the ratio of coincidence counting and coincidence counting were measured during the optimization process.The combination of theory and experiment verified that FBWS could make up the defects in crystal and improve the quality of photon pair.This work proved the feasibility of manipulating nonlinear signals at quantum level with FBWS and also indicated applications in long distance quantum key distribution,quantum communications and quantum imaging,especially in complex environments.