Directional Couplers Using Long-Range Surface Plasmon Polariton Waveguides

We present an experimental study of guiding and routing of electromagnetic radiation along the nanometer-thin and micrometer-wide gold stripes embedded in a polymer via excitation of long-range surface plasmon polaritons (LR-SPPs) in a very broad wavelength range from 1000 to 1650 nm. For straight 14-nm-thick stripes and a wavelength of 1550 nm, LR-SPP propagation loss is determined for the stripe widths varying from 2 to 12 mum and is found to be ~7 and 5 dB/cm for 10- and 4-mum-wide stripes, respectively. For the directional couplers based on 14-nm-thick and 8-mum-wide gold stripes and a wavelength of 1570 nm, the coupling lengths of 4.1, 1.9, and 0.8 mm are found for the respective waveguide separations of 8, 4, and 0 mum. We model the LR-SPP-based directional couplers using the effective-refractive-index method and obtain a good agreement with the experimental results. The transmission spectra of LR-SPP-based directional couplers are presented demonstrating an efficient (~30 dB) separation of different telecom wavelength bands. Various possibilities for dynamic control of wavelength division/multiplexing with LR-SPP-based directional couplers that utilize the thermo-optic effect in the polymer cladding are also discussed