Spontaneous Emission Enhancement of Fluorophore by Plasmonic Nanoantenna

Optical nanoantennas are very efficient for manipulating and controlling light. These nanoantennas support plasmonic oscillations and enhance the local field. This enhancement can be used in different applications. In this thesis, I have designed and fabricated metallic (plasmonic) nanodisks (NDs) array to enhance the emission of fluorescence dye molecules. For emission enhancement, a fluorescence dye (LDS750) with an emission band (emission peak 650 nm) close to plasmon resonance of plasmonic nanoantenna is selected. Localized surface plasmon resonance of NDs depends on the size, shape as well as dielectric properties of the materials. The plasmonic nanodisks are designed to observe their interaction with LDS750 dye in the range of 600-700 nm wavelength. The proper geometry of the nanodisks is designed to achieve plasmon resonance, which spectrally overlaps with the emission spectrum of LDS750. Diameter and period of the nanodisk array have been varied to find out the best spectral overlap. The effect of these parameters on the optical response has been investigated numerically and experimentally. Simulations are performed using the Finite-Difference Time-Domain (FDTD) method. The fabrication process of NDs is done by using electron beam lithography, electron beam evaporation process followed by development and lift-off. Investigated plasmonic NDs arrays are made of gold (Au) with periods of 360 nm, 400 nm and 440 nm while diameter range is in between 105-195 nm for each period. LDS750 fluorescence dye molecules are mixed with Ploymethyl methacrylate 2% solution in anisole (PMMA A2) as dielectric host medium. A layer of PMMA and LDS750 mix is added on top of the NDs to investigate the emission enhancement of LDS750. The numerical and experimental study of the NDs and LDS750 hybrid system, provide how the parameters of the plasmonic nanodisks such as diameter, period and substrate, play significant roles in the enhancement of the emission of LDS750 dye. The time-resolved fluorescence spectroscopy results and Fluorescence Lifetime Imaging (FLIM) results are strong evidence of emission enhancement of LDS750 dye in the presence of NDs. The interaction between the enhanced localize electric field of NDs and LDS750 modified the spontaneous emission by increasing the rate of LDS750 excitation. Consequently, the decay rate change of LDS750 due to the Purcell enhancement. The achieved results from this research would guide the design and fabricate plasmon hybrid system for practical applications