SU-8 grating assisted intermodal interference in surface plasmon polariton waveguide

The interference between the long range surface plasmon (LRSP) mode and cladding mode in a gold stripe waveguide is theoretically investigated and experimentally demonstrated. Epoxy polymer SU-8 is used as the dielectric cladding. Long period relief gratings are formed on the SU-8 top surface by ultraviolet light bleaching. The cladding mode, which is excited due to the field mismatch between the LRSP mode and the lead-in fiber mode, interferes with the guiding mode supported by the surface plasmon waveguide. A sinusoidal interference pattern with a contrast ratio of over 12 dB is experimentally observed. Because of the diffraction of introduced SU-8 gratings at the leading edge of waveguide, dips in the transmission spectrum shift continuously with the input fiber position deviation. This waveguide mode interferometer has potential applications in plasmonic waveguide sensors.