Analysis of waveguide-coupled directional emission for efficient collection of Fluorescence/Raman light from surface

• 2D radiation patterns of molecular dipole in layered structures were simulated using a simple method that combines optical reciprocity theorem and Fresnel theory.
• Kretschmann type plasmon waveguide and resonant mirror can be used to modulate radiation pattern of molecular dipole for high-efficiency signal collection.
• Waveguide coupled emission technique with TE and TM resonance modes can be used to identify orientation of the molecules adsorbed on the waveguide surface.

A theoretical method based on the optical reciprocity theorem combined with the Fresnel theory has been developed for the analysis of waveguide-coupled directional emission technique, which is useful for the surface Fluorescence/Raman spectroscopy. The Kretschmann-type waveguide with a molecular dipole located above or inside the core layer serves as the simulation model. The two-dimensional (2D) pattern of power density for the waveguide-coupled emission from the molecular dipole was calculated using the theoretical method. According to the results, with a given waveguide the 2D pattern of power density is highly dependent on both the orientation and position of the dipole. The maximum fraction of power occupied by the waveguide-coupled emission is 87% with the plasmon waveguide and 95% with the resonant mirror. Compared with the dipole emission in free space, the waveguide-coupled directional emission possesses easy collection, which is benefit for the detection of weak Fluorescence and Raman signals. From this point, the theoretical method used here is helpful for design and optimization of Kretschmann-type waveguide structures for high-sensitivity surface monitoring by Fluorescence/Raman spectroscopy.