Interaction between surface plasmon polaritons and molecules in strong coupling limit

Abstract

Miniaturization of optical elements and their integration to electronic circuits is limited by diffraction limit. It was realized that light being coupled to surface plasmons (SP) can overcome this limit. Employing also optically active molecules in combination with SPs can drive optical circuits to nm-scale and add functionalities. For efficient performance of plasmonic elements involving fluorescent dye molecules investigation of physics behind their interaction is of high priority. In this thesis interaction between surface plasmon polaritons (SPPs) and different dye molecules has been studied, especially within strong coupling limit, which brings in totally new physical properties in the form of hybrid SP-molecule polariton states. Strong coupling with SPPs was achieved altogether for Sulforhodamine 101 (SR101), Rhodamine 6G (R6G), TDBC and -carotene molecules. The measurements were done in Kretschmann geometry using two complementary detection techniques. In attenuated total internal reflection (ATR) experiments the signature of strong coupling, i.e., energy splittings in dispersion curve, was observed for samples having sufficient amount of molecules. The value of energy splitting is directly proportional to the square root of total molecular absorption, which is analogous to the strong coupling of dyes with photons inside optical microcavities. Collected scattered radiation also shows features of strong coupling. Moreover the energy gaps values are increased indicating role of interaction time. We performed also molecule excitation directly by laser in reverse Kretschmann configuration and analyzed the emission patterns revealing clear surface plasmon coupled fluorescence of -carotene. By increasing the concentration of -carotene we were able to collect also surface plasmon coupled Raman scattering signal. Light scattered out due to surface roughness during the SPP propagation is always p (TM)-polarized when detected on the direction perpendicular to the surface of silver film. However, we observe polarization conversion of scattered SPPmolecule polariton in the case of R6G and TDBC dyes. We assign this rather to the properties of strongly coupled molecular state then to conversion due to surface plasmon polariton scattering on metallic film imperfections. By comparing results of different dyes we can conclude that higher Stokes shift, leading to the faster decay of the absorption state of the molecule, results in the loss of the polarization conversion returning thus the pure SPP scattering behavior.