Abstract
Light can be totally internally reflected inside a sphere made of dielectric material with refractive index higher than the environment. At specific wavelengths, standing waves are established, forming the photonic whispering-gallery modes (WGMs). On the interface of air and gold, surface plasmons (SPs) can be excited by photons if the momentum matching condition is fulfilled. When the spherical WGM resonator is placed on the gold substrate, the evanescent near fields of the WGMs can interact with the gold substrate. Depending on the polarization and the orbiting orientation of the WGMs, the interaction can result in spectral shift and intensity variation. The influence of the gold substrate on the WGM is of fundamental interest and practical importance when the sphere is to be connected to metallic electrodes for photoelectric effects. In this thesis, we investigate the influence of single-crystalline gold substrate on the WGMs of self-assembled semiconducting fluorescent π-conjugated polymer microspheres. In particular, we focus on the effects on WGM peak intensity and spectral shift. For peak intensity, we found that all peaks and the broadband fluorescence background in the emission spectrum are enhanced by the gold substrate. However, the intensity of transverse magnetic (TM) WGMs is usually lower than the corresponding transverse electric (TE) modes. This is found to stem from the WGM-to-SP coupling of the TM modes via Otto configuration. As for the spectral shift, we have observed that gold substrate results in blue shift of TE modes and red shift of TM modes. While for ITO substrate, both TE and TM show red shift. With theoretical and numerical analysis, we attribute the red and blue shift of the TM and TE mode to the negative and positive Goos-Hänchen shift on the gold substrate, respectively. The negative and positive Goos-Hänchen shifts correspond to effective enlargement and shrinkage of the sphere circumference and thus the red-shifted and blue-shifted peak positions, respectively. Typically, the assignment of TE and TM modes relies on theoretical analysis. This study shows that by identifying the metal surface plasmon coupling effect and the Goos-Hänchen shifts, the assignment of TE and TM mode can also be done experimentally. Finally, the influence of the graphene substrate on the whispering gallery mode is discussed. Graphene is known to be an efficient quencher and the quenching effect depends on the dipole orientation with respect to the graphene surface. Dipoles parallel to the graphene plane are quenched much more effectively than those perpendicular to the surface. Therefore, it is interesting to know if graphene substrate has different quenching effects on the TE and TM modes. The preliminary results show that there is no observable difference. This might be due to the fact that the coupling of WGMs to graphene substrate is different from that of dipole sources to graphene. The details of the influence of graphene on WGMs require further experimental and theoretical works.