NSOM Measurements of Plasmonic Standing Waves

Most optical guided modes use at least two interfaces. Good examples of this are fiber optics and silicon waveguides, in which total internal reflection bounces the light back and forth between the two sides of a higher index material.

In contrast, plasmonic surface modes are a special type of guided mode which exists on the surface of a medium interface. These waves occur when one of the mediums in plasmonic, meaning that its real permittivity is negative. Like all guided modes, its propagation constant is higher than the surrounding medium.

In the Engheta Group, we have a near-field scanning microscope. This is the optical equivalent of an RF scanning near-field probe. It consists of a sharpened fiber tip hovering just above a surface. It couples in light with a spatial resolution much less than the wavelength. It can image guided standing waves that would be invisible to any conventional microscope.

In a little experiment, I evaporated gold onto a microscope coverslip. I then used the Focused Ion Beam to etch a 6um diameter circle (200nm width) into the gold. I illuminated the underside of the slot ring with a 633nm fiber-coupled laser. Ideally, if the light were polarized (it wasn’t), on two of the sides (e.g. +x and -x), the light would couple to a plasmonic wave on the top surface of the gold disk. This wave is naturally focused to the middle of the circle, where the two waves pass through each other to form a standing wave pattern. The standing wave will have a peak-to-peak spacing of half the guided wavelength. By observing the spacing of the standing wave, I could verify that I was indeed observing plasmonic waves with a smaller than free-space wavelength, equivalent to an index of 1.1.

At the time of the experiment, 633nm was the longest wavelength we had. Gold has significant plasmonic losses at this wavelength. Additionally, the random polarization made the results less clean. However, it was very interesting to measure some SPPs.