Research On The Nonlinear Optical Properties Of Several Non-spherical Gold Nanoparticles

In noble metal nanoparticles,the surface plasma resonance(SPR)results from collective oscillations of the free electrons in the conducting band when interacting with electromagnetic fields.The effect causes metal nanoparticles to have enhanced nonlinearity and fast response.Thus,metal nanoparticles may be used in many applications such as all-optical ultrafast switching,biological sensors,catalytic agents and medicine.The optical properties of metal nanoparticles are affected by factors such as the type,shape,and size of the material.In this paper,we have investigated the nonlinear absorption and ultrafast dynamics process of three different shapes of gold nanoparticles(gold nanocages,gold nanorods,and gold triangle nanoprisms),the specific work is as follows:The nonlinear absorption of gold nanocages were investigated using a broadband nanosecond Z-scan.It was found that at low intensities the sample shows saturation absorption(SA),while at higher intensities a switch from SA to reverse saturation absorption(RSA)occurs.Moreover,the nonlinear absorption of the sample is sensitively wavelength-dependent,and,in the resonant region,saturation intensity is the largest.The nonlinear absorption of gold nanorods were investigated by nanosecond Z-scan technique at 532 nm.It was found that the gold nanorods exhibit SA characteristics.The SA was ascribed to the bleaching of ground state plasmon.Besides,the ultrafast dynamics process of the gold nanorods were investigated by femtosecond pump-probe technology.It was found that the dynamics process includes a fast decay and a slow decay with time constant of 3ps and 105 ps,respectively.Theoretical results show that the fast decay process is due to the coupling of electron-phonon,and the slow decay process is due to the coupling of phonon-phonon.The nonlinear absorption of Au triangular nanoprisms were investigated by using broadband nanosecond Z-scan measurements.We found that Au triangular nanoprisms exhibit SA at low excitation intensities.With the increase of incident intensity,a switch from SA to RSA occurs.The ultrafast dynamics process of the Au triangular nanoprisms were investigated by femtosecond time-resolved transient absorption spectrum.Photo-dynamics process was found to be a double-exponential energy relaxation with a fast and a slow decay component.Interestingly,when probe wavelength is away from the plasma resonance peak,the decay of relaxation also shows the modulation due to the vibration mode of the coherent excitation.The comparison results of the nonlinear absorption characteristics of the three metal nanoparticles indicate that the gold nanocages exhibit the strongest SA,followed by the Au triangular nanoprisms and the gold nanorods.Ultrafast dynamics process results show that the shape has a certain effect on the dynamic process of metal nanoparticles,but this effect is not large.The relaxation times in Au triangular nanoprisms and gold nanorods are longer than that reported in Au nanospheres because they have higher initial temperatures of hot electrons.These nonlinear optical properties of these gold nanomaterials are indicative of the feasibility of their application in ultrafast optoelectronics.