Investigation On Linear And Nonlinear Optical Properties Of Pt Nanoparticles And Pd Nanoparticles

Surface plasmon resonance（SPR）is a collective oscillation phenomenon when the free electron air mass on the surface of metal nanoparticles is in the external electromagnetic field.The local electric field enhancement of SPR will affect the optical Kerr polarizabilityχ^（3）,which is the main index of nonlinear optics.In the visible light band,gold,silver and copper nanoparticles are typical examples of nonlinear metals due to SPR.However,some metals have good nonlinear optical properties in the visible light band although their plasmon resonance absorption peaks are in the ultraviolet band.The representative metals are platinum and palladium.In order to further understand the optical properties of Pt nanoparticles（Pt NPs）and Pd nanoparticles（Pd NPs）,the linear optical properties of Pt NPs and Pd NPs are studied by FDTD.The nonlinear absorption and ultrafast dynamics of Pt NPs and Pd NPs were studied by Z-scan and pump probe techniques.The specific work is as follows:1.Linear optical properties of Pt NPs and Pd NPsThe optical properties of sphere structure and rod structure of Pt NPs and Pd NPs were simulated by the finite-difference time-domain（FDTD）method.The influence of radius,axial length and aspect ratio of Pt nanorods and Pd nanorods on SPR were analyzed.The influence of axial length of Pd nanorods and Au nanorods on SPR was compared under the same size,and it was found that Pd nanorods had better broadband tuning SPR.Comparing the rod structure and sphere structure of Pt NPs and Pd NPs,it is found that the rod structure has more advantages in broadband tuning SPR.A method for improving the sensitivity of optical fiber SPR sensor is proposed.Broadband tuning of SPR using Pt nanorods and Pd nanorods provides a new idea for the design of filters,biosensors and hydrogen production.The simulation work can provide theoretical guidance for the preparation and nonlinear optical properties of Pt NPs and Pd NPs.2.Nonlinear optical properties and ultrafast dynamics of Pt NPsThe nonlinear absorption properties of Pt NPs whose SPR in the ultraviolet band were studied by using multi wavelength（500-700 nm）multi excitation nanosecond Z-scan technique.Pt NPs were prepared by ablation method,whose diameter is 25 nm±2 nm.The effect of off-resonant wavelength on the nonlinear absorption was observed from the distance to SPR at three excitation energies of 752μJ,1088μJ and 1388μJ.SPR of Pt NPs is 226 nm.The results show that the excitation wavelength is far away from SPR,and the phenomena of saturated absorption（SA）and reverse saturated absorption（RSA）still exist.The nonlinear absorption depends on the excitation wavelength and energy.When the excitation wavelength is constant,SA transforms to RSA with the increase of excitation energy.When the excitation wavelength approachs to SPR,SA transforms to RSA with the increase of excitation energy.The energy relaxation dynamics of Pt NPs excited by 400 nm femtosecond laser was studied by femtosecond pump probe technique.The results show that the kinetic process consists of a fast decay with a time constant of 850 ps and a slow decay with a time constant of 9 ps.The fast decay is the result of the electron phonon coupling process,and the slow decay is the result of the phonon phonon coupling process.3.Nonlinear optical properties of Pd NPsA nanosecond Z-scan technique with multi wavelength（500-650nm）excitation was used to study the nonlinear absorption properties of Pd NPs at off-resonant wavelength.Pd NPs were prepared by water bath method,whose diameter is 16 nm±2 nm.The effects of off-resonant wavelength and SPR on the nonlinear absorption were observed at 630μJ,730μJ and 752μJ excitation energies.SPR of Pt NPs is 286 nm.The results show that Pd NPs and Pt NPs have similar experimental results.SA and the conversion from SA to RSA depend on the excitation wavelength and energy.Both Pt and Pd are transition metals.Compared with Pt NPs,when SA and RSA are generated in the off-resonant region under multi-wavelength and-multi excitation,Pd NPs need less excitation energy.The nonlinear absorption properties of Pt NPs and Pd NPs at off-resonant wavelengths provide a supplement for the systematic study of the nonlinear optical properties of Pt NPs and Pd NPs.