Thermally Activated Variable Attenuation of Long-Range Surface Plasmon-Polariton Waves

A thermally activated variable attenuator for long-range (low-loss) surface plasmon-polariton (LRSPP) waves is discussed in this paper. The surface plasmon waveguide consists of a thin Au stripe on a thick layer of SiO₂, which is covered by a thick layer of index-matched polymer. The structure is optically symmetric near room temperature, supporting an LRSPP. Increased attenuation of the mode occurs upon heating of the Au stripe via the passage of current therein. Heating substantially changes the refractive index of the polymer near the stripe, rendering the structure highly optically asymmetric and cutting off the mode. Thermal modeling results that link the injected current density to the refractive-index perturbation are presented and discussed. Experimental results validate the thermal modeling and demonstrate the operation of the device at a free-space optical wavelength near 1550 nm. The resistivity of the stripe is also measured as a function of the drive current, and the simultaneous use of the stripe as a thermal monitor is discussed and demonstrated