Detection of Acoustic Oscillations of Single Gold Nanospheres by Time-Resolved Interferometry

We measure the transient absorption of single gold particles with a common-path interferometer. The prompt electronic part of the signal provides images for diameters as small as 10 nm. Mechanical vibrations of single particles appear on a longer time scale (period of 16 ps for 50 nm diameter). They reveal the full heterogeneity of the ensemble, and the intrinsic damping of the vibration. We also observe a lower-frequency mode involving shear. Ultrafast pump-probe spectroscopy of individual particles opens new insight into mechanical properties of nanometer-sized objects.

Figure 1

Sketch of the pump-probe interferometer. A pump pulse and a pair of reference and probe pulses are focused on the sample in a microscope. The reference-probe pulses arise from a single pulse, split in time (10 ps delay) and polarization by a properly oriented calcite crystal. The delay between the pump pulse and the reference-probe pair can be scanned with a delay line. After the microscope, probe and reference are recombined by a second crystal, and their interference monitors pump-induced changes in the optical properties of the sample. A quarter-wave plate ($\lambda /4$) and a polarizer (pol) are used to set the working point of the interferometer.

Figure 2

Raster-scanned images of single gold nanoparticles and corresponding histograms of field changes $\mathrm{Re}(\Delta \zeta )$ for 20 nm (a),(b) and 10 nm diameter (c),(d). The integration times were 100 and $200\mathrm{ms}/\mathrm{\text{pixel}}$, respectively. A minimum signal level was required to start the fit to a Gaussian spot, indicated by a dashed line in the histograms. The relative width of the distributions, as deduced from rough Gaussian fits, is 34% (20 nm) and 11% (10 nm). The noise level in (c) is $7.1\times {10}^{-5}$, less than 3 times the shot-noise limit.

Figure 3

(a) Two delay scans of one single gold particle, with the probe on the red side (black trace) and on the blue side (gray trace) of the surface plasmon resonance (see inset). The period of the oscillations is 17.6 ps for both traces, corresponding to a particle size of 53 nm [3]. (b) Full complex modification of the field by the particle, calculated from two traces with the interferometer set to either pure amplitude or pure phase sensitivity ($\lambda =520\mathrm{nm}$). The arrows indicate the temporal progression. The inset shows a $7\times$ zoom of the center part of the plot.

Figure 4

(a) Delay scan of a single gold nanoparticle. The oscillation pattern shows a complex modulation. (b) Power spectra of this particle’s oscillation (top spectrum) and of two other particles. Frequencies and amplitudes are normalized to those of the $(0,0)$ mode (the absolute frequencies were 67, 59, and 63 GHz, from top to bottom). The low-frequency peak (top spectrum: 28 GHz) lies between calculated frequencies of the $(0,2)$ mode for free boundary ($F$, thick line on bar) and for rigid boundary ($R$, end of bar). The ratio of its frequency to that of the breathing mode is constant.