| When incident light irradiates a noble metal nanostructure,the free electrons on the surface of the nanostructure oscillate collectively driven by the incident light electric field,resulting in an electromagnetic mode,Localized Surface Plasmon(LSP).LSP offer beyond diffraction concentration and very high near-field enhancement intensity,making them promising for applications in optoelectronic devices,chemical sensing,biomedicine,and energy.The local optical response of these structures are controlled by their intrinsic modes.For bimetallic dimeric nanostructures,the mode coupling within the dimer can lead to a special optical response due to the difference in material composition,which has potential for applications in sensing and other fields,however,this research is still relatively insufficient,especially for the characterization of LSP dephasing times at different positions in bimetallic dimeric structures,which has hardly been carried out.In this paper,we study the local excitation field properties of Au-Ag bimetallic nanodimer structures with the help of Finite Difference Time Domain(FDTD)simulations,and investigate the dephasing times of LSP at different positions of the structures.1.The local response in the Au-Ag bimetallic dimer structure were investigated by FDTD simulation.It is found that due to the coupling effect,Au-Ag bimetallic dimer shows multiple absorption peaks,and its absorption is dominated by the gold disk.By changing the gap size of the dimer,it is found that the bimetallic coupling has different effects on different modes: for the mode near 600 nm wavelength,it shows an enhancement effect;for the mode at 725 nm wavelength,it shows an inhibition effect.By changing the size of the silver disk in the bimetallic dimer,it is found that the mode near 600 nm is not affected by the size of the silver disk,while the mode near 850 nm is directly controlled by the size of the silver disk.2.The dephasing times of hot spots at different positions of the dimer structure were investigated by using the quasi-normal mode and the resonant oscillator model,combined with the FDTD simulation results.It is found that there is no significant difference in the dephasing time at different positions of the homogeneous metal dimer structure.In the bimetallic dimer,the dephasing time at different positions in the high-frequency mode(～600 nm)is significantly different: the dephasing time at the Au-side hot spot is about 4.5fs,the gap is about 4.2 fs,the Ag-side hot spot is about 3.9 fs,and the dephasing time near the Au nanodisc hot spot is longer.In contrast,the dephasing time of the medium frequency mode(～750 nm)and the low frequency mode(～890 nm)hardly varies with the measured position.By varying the dimer gap,it is found that the difference in dephasing time at different positions decreases as the gap shrinks and the overall dephasing time is shortened.By comparing the dephasing times of different modes in the bimetallic dimer,it is found that the dephasing time corresponding to the high-frequency mode(quadrupole-dipole distribution)in the dimer(～4.5 fs)is longer than that of the medium-frequency mode(～2.9fs)and the low-frequency mode(dipole-dipole distribution,～2.0 fs).By comparing the dephasing time with that of the monometallic disc,it is found that the dephasing time corresponding to each mode in the bimetallic dimer is longer than that of the LSP mode in the monometallic disc due to the coupling effect,except for the low-frequency mode(～890nm). |
