Long-Range Surface Plasmons Along Membrane-Supported Metal Stripes

Long-range surface plasmon-polaritons propagating along a thin metal stripe on a supporting ultrathin freestanding dielectric membrane (the ldquo<i>membrane-supported metal stripe</i> rdquo) have been investigated in detail theoretically and experimentally. The materials considered for the structure are Au on a thin Cr or Ti adhesion layer as the stripe, Si₃N₄ or SiO₂ as the membrane, and vacuum (air) or H₂O as the background. Three operating wavelengths were considered: lambda₀ = 632.8, 1310, and 1550 nm. Theoretical results reveal that the long-range mode in this structure has a moderate mode power attenuation (MPA ~ &nbsp;2-12 dB/mm at lambda₀ = 1310 nm), good confinement [ <i>n</i>_eff - <i>n</i>₁&nbsp; ~ (1-3) &times;10^-3 ], a good <i>M</i>₂ figure of merit (10-25), and a reasonable range extension factor (<i>R</i>_<i>e</i>&nbsp; ~ &nbsp;3-25), when operated in vacuum or in H₂O and for reasonable dimensions. The adhesion layer was found to modestly impact the attenuation. Structures were fabricated as Au/Cr stripes (25 or 20 nm thick) on freestanding (20 or 30 nm thick) large-area Si₃N₄ membranes clamped around their perimeter, and propagation of the long-range mode was observed and characterized in air and in H₂O (and similar optical fluids). The measurements reported include an optical streak, mode outputs, output polarization, and MPA. Errors between measured and theoretical attenuations were below about 7% for all of the structures tested.