Spectroscopic investigations of rotating gold nanorods in an optical trap

Abstract

Due to their extraordinary plasmonic properties, gold nanorods are prime candidates for a variety of sensory, biomedical, spectroscopic, and data storage applications. Recent studies have demonstrated that gold nanorods trapped by optical tweezers can function as nanoscopic rotary motors driven by scattering induced optical torques. The system holds promise for applications within numerous fields, ranging from nano/microfluidic flow control and nanorobotics to sensing and DNA manipulation. However, for many proposed applications, understanding and controlling the photothermal effects associated with laser trapping is crucial. The reason being that they could lead to a number of unwanted effects, but could also be interesting and useful in their own right. To deduce how photothermal effects such as heating, reshaping, and possibly bubble formation affect the particle, spectroscopic investigations on trapped rotating gold nanorods were performed. An optical setup was constructed which included circularly polarized laser tweezers, dark-field illumination, photon correlation spectroscopy, video microscopy, and a free-space coupled grating spectrometer. The setup allowed continuous real-time measurements of scattering spectra and rotational dynamics of individual monocrystalline gold nanorods at high resolution over extended periods of time. Through continuous tracking of plasmon resonance shifts and rotational dynamics of trapped particles, a number of important effects were observed. These included changes in the refractive index of the water surrounding, nanoparticle reshaping effects and bubble generation. Moreover, it was found that by using a small collection fiber information about translational Brownian motion of the trapped particle is detectable in the autocorrelation signal. The results presented in this thesis shed light on thermal processes on the nanoscale and will be useful in evaluating the applicability and performance of nanorod motors for possible future applications.