Excitation of metal nanoparticles with sub-picosecond laser pulses causes a rapid increase in the lattice temperature, which can impulsively excite the phonon modes of the particle that correlate with the expansion co-ordinates. The vibrational periods depend on the size, shape and elastic constants of the particles. Thus, time-resolved spectroscopy can be used to examine the material properties of nanometer sized objects. This article will provide a brief overview of our recent work in this area of research, specifically, how the vibrational modes observed in the experiments are assigned and what information can be obtained from the measurements. Our work has mainly been concerned with noble metal particles (gold and silver) in aqueous solution. The different shapes that have been examined include spheres, rods and triangles, all with different sizes.
Ultrafast excitation of metal particles in solution coherently excites the phonon modes that correlate with the expansion coordinate of the particle. The period of the modulations yields information about the average size of the particles if their elastic constants and shape are known, or information about the elastic constants if the average size and shape is known. In this paper we describe recent experiments where we have used time-resolved spectroscopy to examine the elastic constants of: (i) gold nanorods with aspect ratios between 2 and 5; and (ii) spherical gold particles in aqueous solution at very high pump excitation levels. The first set of experiments shows that the elastic moduli (Young's modulus, and the bulk and shear modulus) of gold nanorods are significantly smaller than those of bulk gold. This is attributed to the structure of the nanorods, specifically, that they grown with a five-fold twinned structure. In the second set of experiments essentially the change in the elastic moduli with laser intensity is used to estimate the temperature of the particles. The results show that the particles can reach very high lattice temperatures (approaching the melting point of the metal). Examination of the transient absorption data suggests that the hot particles produce explosive boiling of the solvent in these experiments.
In this paper the dynamical response of cylindrical nanorods to ultrafast laser-induced heating is examined. Theoretical analysis predicts that both extensional and breathing vibrational modes of the rods should be excited by laser-induced heating. Analytical formulas for the frequencies of these modes are derived assuming that the length of the rods is much greater than their radii. Because the frequency of the fundamental extensional mode is much lower than that of the breathing mode, the extensional mode will dominate the response for a real experiment, i.e., for a finite-time heating/expansion process. The results of this model are compared to data from transient absorption experiments performed on gold nanorods with average aspect ratios (length / width) between 2.1 and 5.5, and widths on the order of 10-20 nm. The transient absorption traces show pronounced modulations with a period between 45 and 70 ps, which are only observed when the probe laser is tuned to the longitudinal plasmon band of the sample. The measured periods are in good agreement with the expected period for the extensional modes of the rods. The actual value of the measured period depends on the specific sample and probe laser wavelength. This occurs because the samples are polydisperse, and different length rods absorb in different regions of the spectrum. For rods with widths greater than 20 nm, the breathing mode can also be observed and, again, the measured periods are in good agreement with the theoretical calculations. The breathing mode is not observed for the thinner rods (~10 nm width) because in this case the period is comparable to the timescale for lattice heating in the experiment.
Ultrafast laser excitation of metal nanoparticles can create temperature increases of many hundreds of Kelvin. The aim of this paper is to provide an overview of our recent experimental studies of heat dissipation and the coherent generation of acoustic phonon modes in the particles. Our results show that the rate of heat dissipation depends on the surface area of the particles, and that both impulsive lattice heating and hot-electron pressure contribute to phonon excitation. The measured periods also depend on the pump laser intensity: higher intensities yield slower periods. This softening of the coherently excited phonon modes is due to the temperature dependence of the elastic constants of the particles.
The acoustic vibrational modes of Au nanorods with aspect ratios between 2.1 and 5.5 have been investigated by timeresolved
spectroscopy. The results show that laser excitation launches a coherent vibrational motion which has a period that depends linearly on the length of the rod. Due to polydispersity in the samples, the measured period also depends on the probe wavelength (i.e., for a given sample different probe wavelengths interrogate different length rods). Analysis of the data potentially provides information about the elastic properties of these unique materials.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.