Nonlinear Coherent Dynamics Of Gold Nanoparticles

Because of its significant quantum confinement effect and excellent photoelectric properties(such as high specific surface area,strong absorption,single photon emission,etc.),nanomaterials have a wide range of applications in the fields of quantum science,material science,and life science.In particular,noble metal nanoparticles,such as gold and silver nanoparticles,have a surface plasmon resonance effect caused by the collective oscillation of conduction band electrons,which will have a significant local confinement effect(reaching a nanometer-scale spatial limit)to the incident light field.The local plasmon resonance can be further adjusted and optimized by the size,shape,spacing and surrounding medium environment of the nanoparticles.Moreover,compared with single molecules,quantum dots and other luminous bodies,gold nanoparticles(AuNPs)not only eliminate the defects of photobleaching and photoblinking,but also have extremely strong multiphoton emission intensity.This makes AuNPs widely used in near-infrared in vivo microscopy imaging,drug delivery,cancer treatment and nonlinear optics.However,due to the different aggregation states of AuNPs,it will also cause a lot of uncertainty in the application of ensemble AuNPs;direct observation and research on a single AuNP can effectively break through these limitations.In particular,after eliminating the ensemble averaging effect,single AuNP exhibited good nonlinear coherent dynamics characteristics.However,limited by the requirements of high sensitivity and high time resolution,the current research on the ultrafast coherent dynamics of AuNPs is mainly focused on the aspect of photoelectron emission,and the study on the ultrafast coherent dynamics exhibited in the process of multiphoton emission not yet reported.On the other hand,there is also a lack of effective physical models to illustrate the nonlinear coherent dynamic evolution behavior of a single AuNP.Therefore,it is urgent to develop the corresponding physical model and carry out the study of the nonlinear coherent dynamics of a single AuNP with high sensitivity and high time resolution,which makes the all-optical coherent manipulation of AuNPs possible and greatly expands its use in super-resolution imaging,quantum device preparation and infrared sensing application potential.In this paper,we proposed a three-level AuNPs coherent manipulation model,and developed a double-pulse excitation scheme with high time resolution(<1fs).Which allows us to study the the ultrafast nonlinear coherent dynamics characteristics effect of a single AuNP in the two-photon photoluminescence(TPPL).The thesis first introduces the research progress and application of AuNPs functionalization scheme and nonlinear photodynamic process.Based on the difficulties in current applications,the research goal of this thesis is determined,that is to study and explain the nonlinear coherent dynamics characteristics of a single AuNP in the TPPL.Then,we proposed a three-level model based on broadband resonance,and constructed a numerical equation for the evolution of the nonlinear coherent dynamics of AuNPs.We performed numerical simulation and analysis of the system,and compared with the nonlinear dynamics of general high-order harmonics,which confirmed the model's greatly enhanced photon emission and phase-sensitive characteristics in the TPPL.In the experimental part,we proposed a double-pulse coherent excitation scheme,and studied the evolution behavior of the TPPL intensity with the change of double-pulse time-delay.Based on its special coherence enhancement,excitation polarization dependence and power dependence,the three-energy broadband resonance was determined.And the experimental results obtained are in good agreement with the numerical simulation.Finally,using the non-linear coherence characteristics of AuNPs,we invented the coherent modulation microscopic technology,which achieved a four-order improvement in the signal background ratio.At the same time,using AuNPs' excellent excitation polarization-dependent and wavelength-dependent characteristics,the coherent modulation imaging method has been further extended.The coherent modulation imaging technology of amplitude projection and the wavelengthdependent tomography technology have been developed.The main innovations of the thesis work:1.A three-level physical model based on broadband resonance was proposed,and an ultrafast nonlinear coherent evolution equation was constructed.The nonlinear coherent evolution process of AuNP was studied by numerical simulation,and the strange photoluminescence enhancement in the coherence time was found.By comparing with nonlinear processes such as high-order harmonics,the characteristics of the nonlinear coherent evolution of a single AuNP are clarified.2.An ultra-fast two-pulse two-photon excitation scheme is proposed,and which is a new method for studying the nonlinear coherent dynamics process by measuring the photoluminescence intensity of a single AuNP.The dynamic process of AuNP was measured on the flobal time-scale(sub-femtosecond to tens of picoseconds),and the characteristics of the nonlinear coherent dynamics of AuNPs were explained through the laser power-dependent and polarization-dependent characteristics.Combined with the numerical simulation of three-level model,the corresponding physical parameters in the nonlinear coherent dynamics process(such as decoherence time,intermediate state lifetime,population transfer rate,etc.)are obtained.3.Based on the phase sensitivity of a single AuNP in ultra-fast doublepulse excitation,we invented the coherent modulation microscopy imaging technology.Under the same total power excitation,the photoluminescence intensity obtained by double-pulse coherent excitation is 50 times higher than that of single-pulse excitation.Based on coherent modulation microscopy imaging,we effectively suppress the influence of strong background light and other noise on imaging,and increase the signal-to-background ratio of AuNPs imaging by four orders of magnitude.4.Based on the dependence of a single AuNP on excitation polarization and wavelength,we developed amplitude projection coherent modulation imaging and wavelength-dependent coherent modulation tomography.While obtaining the fluorescence imaging of a single AuNP,the orientation characteristics of AuNP are obtained,and the imaging effect that breaks the optical diffraction limit is achieved.And the wavelength-dependent information increases the lateral resolution of imaging,enabling imaging to describe environmental dynamics in more dimensions.