Broadside coupling to long-range surface plasmons in metal stripes using prisms, particles, and an atomic force microscope probe.

Techniques for broadside coupling to long-range surface plasmon waves propagating along metal stripes are investigated. The baseline technique consists of evanescently coupling an optical input beam originating from a polarization maintaining fiber to the plasmon wave via a right-angle prism positioned above the metal stripe, and providing an optical output some distance away through a mirror arrangement of identical elements. The technique is modeled theoretically using plane waves and implemented to measure the attenuation of the long-range plasmon wave propagating along a metal stripe supported by a thin freestanding dielectric membrane. An alternative technique for providing an output is proposed, whereby a tipless atomic force microscope probe physically contacts the metal stripe to generate out-of-plane scattering and a multimode fiber positioned nearby is used to capture a portion of the scattered light. This technique is easier to implement than the baseline technique, resulting in attenuation measurements of significantly better quality. The goodness of fit of the best fitting linear models to the measurements was significantly improved using this technique (0.93 and 0.99), and the measured attenuations were in very good agreement with the theoretical ones (6.01% and 0.27% error). This simple technique for optical probing and coupling could be applied to other surface plasmon waveguides and possibly to dielectric waveguides with modes having sufficient field strength in their evanescent tail. Output scattering using micron-sized particles located on the metal stripe was also investigated. The stability of the experimental setup was assessed and found to be about 0.01 dB peak to peak over a few minutes at constant temperature using a reference optical signal.