Session: Nanoscience:

The interaction of plain light with a fluorescent molecule at the nanometer scale is inefficient due to the size mismatch between light wavelength and excitons, thereby resulting in poor energy conversion. Plasmon-exciton coupling (PEC) has emerged as one of the leading contenders for fluorescent enhancement at the nanometer scale. By careful design of plasmonic nanostructures, a fluorophore can exhibit high fluorescence intensity; a property that is desirable for application in single-molecule sensing, high-resolution devices, and energy storage devices. Here, we report a novel plasmonic gold nanoslit array design on a flexible substrate created via pattern transfer technique and provided evidence on the mechanism of fluorescence enhancement due to plasmon-exciton interaction in a gold nanoslit array. The nano slits with 15 nm Au film at the bottom of the slit was employed for dual reasons: its increased surface area for exciton binding and its advantage of exciting intense field due to both localized and surface plasmon polariton modes. Theoretical studies using a validated semi-analytical model of nanoplasmonics revealed a strong dependence of surface plasmon generation efficiency (SPGE) on slit width. Since SPGE has a proven relationship with an optical signal enhancement for PEC systems as validated by numerical finite-difference time-domain (FDTD) simulation, nanoslit array of 100nm width, 600nm periodicity, and 150 nm depth was used for experimental investigations. For that, Acridine (Ac), Propidium iodide (PI), and Dihydroethidium (DHE) dyes were used to determine the excitation/emission wavelength requirement necessary for optimum coupling with the plasmonic fields. Optical analysis using Cytoviva hyperspectral imaging microscopy and Axio Z2M imaging system reveal that Ac, PI, and DHE dyes embedded in the nanostructure array showed florescent enhancement by a factor of 56.0, 20.9, and 1.7, respectively. For our plasmonic nanostructure design, we found that the extent of plasmon-induced fluorescent enhancement is more a function of fluorophores emission wavelength overlap with plasmonic energy than absorption wavelength. These findings stand to provide foundational understanding and guidance for the development of high-performance optoelectronics.

Jianjun Wei

Appearances