Photothermal properties of gold nanorods and their application to five-dimensional optical recording (2009)

The development of controllable synthesis methods in the 1990s has sparked an enormous interest in gold nanorods, both fundamentally and application driven. Their anisotropic shape induces a splitting of the surface plasmon resonance (SPR) into a transverse and a longitudinal component, of which the latter occurs in the near-infrared wavelength range and is extremely sensitive to the wavelength and polarization of the impinging light. At the longitudinal SPR wavelength, gold nanorods exhibit strong light absorption, which makes them ideal photothermal energy converters. In this thesis we investigate the photothermal properties of gold nanorods, and apply them to achieve five-dimensional optical recording. In the past decades many optical recording techniques have been proposed which exploit the wavelength, polarization or the three spatial dimensions for multiplexed recording. However, the lack of a suitable recording medium has hampered the integration of these techniques to achieve a five-dimensional recording method, which could increase the information capacity by orders of magnitude. We used the photothermal properties of gold nanorods to achieve for the first time five-dimensional optical recording. To achieve this goal we developed a high-yield synthesis method which is viable at high temperatures. Elevating the reaction temperature increased the growth rate by almost three orders of magnitude, resulting in a synthesis protocol which enables rapid synthesis of gold nanorods on a large scale. We studied the photothermal properties of these nanorods using a combination of electron microscopy, white light spectroscopy, and ultrafast laser spectroscopy. To remove ensemble averaging in the experiments, we performed experiments on single gold nanorods. We measured the electron-phonon decay in single particles, and found that the electron-phonon coupling constant agrees well with the literature value. We also detected the acoustic vibrations of the nanorod and, in contrast to previous studies on ensembles of gold nanorods, we found that the acoustic mode frequencies compare well to theoretical predictions. Doping of the nanorods into a matrix is key to successful application in optical recording. We doped the nanorods into a polyvinyl alcohol (PVA) matrix, which provides a flexible and stable matrix for the nanorods. We investigated the thermal properties of the polymer nanocomposite, and found significant accumulation of heat caused by the high repetition rate of commercial femtosecond laser systems. We show that this heat accumulation can be prevented by employing single laser pulses, resulting in a thermally stable nanocomposite suitable for optical recording. Using a combination of white light spectroscopy and electron microscopy we studied the melting and reshaping of single gold nanorods in a PVA matrix. Unlike previous ensemble experiments, we find good agreement between the theoretical and experimental melting energy of a single nanorod. Higher aspect ratio particles were found to be thermodynamically less stable, leading to more pronounced partial reshaping at lower pulse energies. Finally, we employed this photothermal reshaping to achieve five-dimensional optical recording. The narrow longitudinal SPR linewidth combined with the dipolar optical response allowed us to optically address only a small subpopulation of nanorods in the laser irradiated region. Using this technique we achieved aspect ratio and orientation selective photothermal reshaping, which allowed us to incorporate three wavelength channels and two polarization channels. The distinct energy threshold required for the photothermal reshaping provided the axial selectivity required to record in multiple layers. We demonstrate that twophoton luminescence detection has an enhanced wavelength and angular selectivity compared to conventional linear readout mechanisms, which enabled cross-talk free recording and readout.