Study Of The Ultrafast Electron-phonon Dynamics In Au Nanorods

Considerable work has been done on metal nanoparticles recently because of their many interesting properties like enhanced near electromagnetic fields, wavelength tunable light absorption, and strong photothermal effect as well as potential technological applications including biological sensing, imaging, and nanoelectronic. One extensively studied system is the Au nanoparticle. For Au nanodots, the SPR absorption spectrum contains an intense absorption band in the visible region, while the SPR of Au nanorods splits into two modes: a longitudinal mode parallel to the long axis of the nanorod and a transverse mode perpendicular to it. The absorption peak of the transverse mode is essentially the same with SPR absorption of nanodots, while the longitudinal mode absorbs at lower energy and can be tuned by the aspect ratio. In recent years, there has been a great deal of interest in investigating the electron dynamics in Au nanoparticles after excitation with femtosecond laser pulses. Because of the anisotropy of Au nanorods with two perpendicular resonant modes, Au nanorods have special properties and application relative to the isotropic nanodots, and study of its ultrafast electron dynamics is always a research focus.In this thesis, the ultrafast electron-phonon (e-ph) coupling dynamics in Au nanorods are studied with femtosecond transient depolarization experiments. The main purpose is to explore whether the ultrafast e-ph coupling dynamics of Au nanorods will present special properties or not relative to that of Au nanodots and bulk Au. The main contents include following three parts:Firstly, a high-sensitivity femtosecond pump-probe (transient absorption spectroscopy) system has been designed, which consists of Ti:sapphire femtosecond pulse laser, pump-probe part, spectrometer (Avantes) and computer. After optimization of the system by improving the stability of femtosecond laser and the continuum white light, the signal to noise ratio (SNR) of the system is 10-4.Secondly, the ultrafast electron-phonon (e-ph) coupling dynamics in Au nanorods of two perpendicular probe polarization directions are studied with femtosecond transient depolarization experiments. For one thing, the transient spectra are strongly dependent on the probe polarization directions at all the pump energies. The bleach signal of the longitudinal mode has a larger magnitude when the polarization of the probe beam is parallel to that of the excitation beam. The bleach signals of the transverse mode at two probe directions show reversed trend. In the same probe polarization direction, the transient spectra are also strongly dependent on the pump energy, with larger magnitude at higher pump energy. For another thing, an unexpected anisotropic e-ph coupling dynamics for two probe directions, which differ not only in the magnitudes but also in the relaxation times. When probing at the perpendicular direction, the relaxation time is always short at the same pump power. Additionally, the e-ph relaxation time constants increase with the pump energy in both polarization directions. At low pump power, previous experimental results have indicated that the e-ph relaxation times are proportional to the pump power in Au nanodots and also in nanorods.Thirdly, analysis and discussion of the ultrafast e-ph coupling dynamics in Au nanorods based on the experimental results, and a theoretical model proposed to further prove our results. Actually, Au nanorod has strong anisotropic character: its longitudinal mode is parallel to the long axis of the nanorod. The effective light intensity to excite the longitudinal mode is the projection of pump intensity to the long axis of Au nanorod. If the polarization direction of probe light has been changed, it will result in the different weights of pump energy assigned to two directions. This may be the reason for the unexpected anisotropic e-ph coupling dynamics. And then we proposed a simple theoretical model to further discuss these anisotropic e-ph coupling dynamics and also pump power dependent relaxation times by fitting the e-ph coupling dynamics in Au nanorods.Above all, the experimental results and theoretical analysis show that anisotropic e-ph coupling dynamics exists in Au nanorods, which differs not only in the magnitudes but also in the relaxation times. However, this anisotropic e-ph coupling dynamics is not due to an intrinsic process but is the result of different weights of pump energy assigned to two directions because of the anisotropy of Au nanorods. The e-ph coupling kinetics measured by transient experiments cannot be simply described by a single exponential function but by the integral of components contributed by random distribution of Au nanorods at all directions in aqueous solution. The relationship between electron-phonon relaxation times and pump power is linear even in relatively high power. The intrinsic electron-phonon relaxation time is 0.75 ps which is similar to the characteristic electron-phonon coupling time for nanodots and bulk Au.