Tunable plasmonic devices for active control of molecular coupling and optical trapping

P Lavanya, Devi and Gupta, Shourya Dutta (2019) Tunable plasmonic devices for active control of molecular coupling and optical trapping. Masters thesis, Indian institute of technology Hyderabad.

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Abstract

Plasmonics is the study of interaction of light with subwavelength metallic structures. Plasmonic nanostructures have been used for manipulating and enhancing light-matter interaction leading to numerous applications in sensing, non-linear signal enhancement and pho- tovoltaic efficiency enhancement. Typically noble metals like gold, silver and copper are the materials of choice due their low loss in the visible domain. The characteristics of plasmon resonances like the resonance wavelength and full width at half maximum can be controlled by altering the shape, size, material and assembly of the structure. In fact, spatial assembly of such subwavelength metal nano-structures in 2D planar arrays leads to the realization of the so called plasmonic metasurfaces that exhibit properties not found in nature. Furthermore, integration of single layer graphene (SLG) with plasmonic metasurfaces makes it possible to actively control the resonance wavelength by applying a gate voltage. Such electrically configurable devices have been used in earlier studies to actively control phase, amplitude and polarisation state of light in mid-IR wavelength regime. In the first part of this thesis, I extend the applications of these devices to demonstrate that it is possible to actively control the strong coupling between plasmonic and molecular resonances. Resonance tunability using both graphene and molecular integration has been exploited in this work. In particular, the Fermi energy of SLG, strength of molecule (in terms of concentration) and proximity of molecule to metasurface on the strong coupling are shown to control the resulting optical properties. Dispersion diagrams (plots of reflectivity versus Ef and λ) clearly illustrate anti-crossing of the resonance bands providing clear proof of control over strong coupling. Such devices can potentially be used for controlling and mediating molecular transitions at nanosecond time scales. In the second part of this thesis, I utilize graphene integrated devices for optical trapping of gold nano partciles. The high field confinement and the strong field gradients in the vicinity of the structure enables trapping of sub-100 nm particles. Note that it is difficult to trap such particles using convenstional optical tweezers. A multi-resonant structure is used for potentially realizing multiple trap positions depending on the illumination conditions. The stability of each of the equilibrium positions (where optical forces are zero) is analyzed using both the Maxwell’s stress tensor method and the gradient approximation. Control over the forces is shown by modifying the Fermi energy of SLG. It is observed that at equilibrium positions the surface averaged field on the nanoparticle’s surface is enhanced by a factor of almost 40 at resonance. Such enormous enhancement in the electric field magnitude can potentially be used for surface enhanced IR absorption spectroscopy of molecules present on the surface of the nanoparticle..

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