Long-range surface plasmon-polariton waveguides and devices in lithium niobate

Long-range surface plasmon-polariton waveguides and devices based on a thin narrow Au stripe buried in z-cut LiNbO3 claddings and designed for operation at free-space optical wavelengths near 1550nm are described and discussed. Parallel or antiparallel crystal orientations for the claddings lead to phase modulation or intensity modulation via mode cutoff, respectively. Theoretical results are given for straight waveguides, curved waveguides, and Bragg gratings in symmetric claddings, and for straight waveguides in electro-optically induced asymmetric claddings. The main theoretical findings are that 1dB∕mm of attenuation (or less) is achievable using Au stripes of reasonable dimensions (0.5–1μm wide, 20–40nm thick), that low overlap losses (<1dB) to large and small modes are achievable for the same stripe thickness, that radii of curvature in the range of 10–30mm are required for stripes having a moderate (10−3) confinement, that first order gratings having a reflectance of 0.9 and a bandwidth of 0.75nm are achievable and that they are electro-optically tunable over a 2.9nm range, and that an electro-optically induced index asymmetry of about 4×10−4 is sufficient to cut off weakly (10−4) confined modes. Structures were fabricated by direct wafer bonding and thinning to form the lithium niobate claddings. The measured optical insertion loss of 2mm long waveguides varied from 10to17dB, which are somewhat higher than theoretical expectations. Low frequency electro-optic mode cutoff measurements produced extinction ratios near 12dB and a linear transfer characteristic, thus demonstrating intensity modulation via this mechanism. The results suggest that bulk values for the optical and electro-optic properties of the LiNbO3 claddings have been retained.