Softening of the symmetric breathing mode in gold particles by laser-induced heating

The symmetric breathing mode in spherical gold particles has been examined by time-resolved spectroscopy using different intensity pump laser pulses. The results show that the period of the breathing mode increases as the pump laser power increases, up to pump laser powers of 2-3 muJ/pulse. This is attributed to softening of the elastic properties of the particles due to laser-induced heating. At pump laser powers greater than similar to3uJ/pulse the period versus intensity data flatten off. This most likely arises from saturation of the sample absorption at high pump intensities. The particles studied in these experiments were relatively large-between 50 and 100 nm in diameter. Large particles were chosen because they have slower heat dissipation times, which means that the temperature in the particles is better defined during the course of the experiment. The particle temperatures were estimated from the laser power density, the heat capacity of gold, and the absorption at the pump wavelength-assuming that the samples obey Beer's law. This allows us to compare the experimental results to calculations of the period versus temperature, which are based on the known temperature-dependent elastic constants of gold. The experimental and calculated periods are in excellent agreement up to the melting point of gold, which is predicted to occur at similar to3 muJ/pulse (approximately the same point where the period versus intensity data flatten off). At higher powers the measured periods are significantly shorter than those predicted for molten gold particles. This implies that we can approach the melting point of gold, but we cannot completely melt the particles. Analysis of the damping of the beat signal indicates that we may form solid-core/liquid-shell particles at high laser powers.