Plasmonic gain in long-range surface plasmon polariton waveguides bounded symmetrically by dye-doped polymer

摘要

The plasmonic gain of a top-pumped active symmetric metal slab waveguide is investigated theoretically and experimentally. The structure consists of a thin Ag film cladded above and below by gain media (IR140-doped poly (methyl methacrylate)), and operating with long-range surface plasmon polaritons (LRSPPs) at near-infrared wavelengths. We consider the spatial distribution of the pump intensity and the position dependence of the dipole lifetime within the claddings when computing the LRSPP gain. We find that the bottom cladding provides significant gain to the LRSPP, despite the low pump transmittance through the Ag film, as long as the pump intensity is strong enough to saturate the gain material (～4MW/cm~2). In this situation, the LRSPP gain is doubled compared to the case where the top cladding only is active. The LRSPP gain was measured in a fabricated structure using the variable stripe length method, yielding g_(mod)= 16.7 cm~(-1) at a pump intensity of ～4 MW/cm~2. The measured LRSPP gain agrees very well with the computed value, implying that the bottom cladding provides significant gain to the mode. Active plasmonic devices based on the symmetric dielectric-metal-dielectric structure can be significantly more efficient by using gain layers as both the top and bottom claddings.