Long-range surface plasmon-polariton mode cutoff and radiation in embedded strip waveguides

Using a fully vectorial formulation, the normal mode analysis method is used to model the radiative spreading of surface plasmon-polariton mode input into regions where the bound surface mode is cut off or radiative. The analysis was performed at λ0=1.55μm for embedded finite width strip metal waveguides, where the metal was Au and the surrounding dielectrics were SiO2 and LiNbO3. Mode cutoff is induced by an asymmetry between the index of refraction of the top clad and the bottom clad. Comparisons between predictions and experimental results show very good agreement. The analysis was performed for a range of waveguide dimensions, and results show that a change in insertion loss of better than 20dB is possible for an appropriate waveguide and dielectric asymmetry. Practical applications to make use of this cutoff effect are outlined.