Optical tweezers combined with dark-field spectroscopy for nanoparticle dynamics characterisation

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Embargoed until 2017-04-30
Copyright: Andres Arroyo, Ana
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Abstract
The potential uses of contact free control of microscopic particles has maintained a sustained interest in optical tweezers research for almost three decades. Optical tweezers have been mostly associated with microscopic objects, however, there are considerable efforts aimed at extending trapping techniques to the nanoscale. Whilst direct imaging of nanoparticles below the diffraction limit presents only limited capability for dynamics characterisation, spectroscopic techniques have proven to be very useful in the study of particle size, geometry and orientation, inter-particle interactions, and the dielectric properties of the local environment. Metallic nanoparticles present an enhanced polarisability in the infra-red at typical trapping wavelengths, making them of great significance in optical trapping applications. Gold (Au) nanoparticles have been the focus of recent research, with spectroscopy being a favourable method of interrogating their trapping dynamics. Metallic nanoparticles experience localised heating; a robust method for characterizing the degree of heating of trapped Au nanospheres is presented in this work, based on the intrinsic temperature dependence of the nanoparticle’s localised surface plasmon resonance (LSPR). The temperature dependent refractive index of the ambient medium induces a change in the nanoparticle’s LSPR which can be monitored using spectroscopy. Additionally, asymmetrical particles present interesting properties; the orientation dependence of Au nanorods LSPR is exploited in this work to characterise their trapping dynamics. Furthermore, high dielectric constant nanoparticles also demonstrate geometric scattering resonances similar to those of plasmonic structures, however, they are associated with distinct magnetic and electric dipole moments and higher multipole modes. These resonances are achieved without the associated large extinction coefficient and therefore do not suffer from intrinsic optical loss and heating compared with their metal counterparts. Dark-field spectroscopy has been employed in this work to investigate the trapping dynamics of silicon (Si) nanoparticles of different dimensions and geometries. The unique combination of spectral and trapping properties make Si nanoparticles an ideal system for delivering directed nanoscale sensing in a range of potential applications.
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Author(s)
Andres Arroyo, Ana
Supervisor(s)
Reece, Peter
Gooding, Justin
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Publication Year
2016
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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