Experimental observation of the short-range surface plasmon polariton mode and its longitudinal adiabatic compression in a metallic wedge

Abstract

In this study, we explore analytically and experimentally long- and short-range surface plasmon polariton (LR-SPP and SR-SPP, respectively) modes in gold wedges. Especially, we aim to observe the 2-dimensional confinement of the electromagnetic field in gold wedges as it could enhance the light-matter interaction by offering a local density of states which depends on the propagation constant, consequently on the wedge height. The LR-SPP mode can propagate over a long distance, but the real part of the propagation constant remains relatively insensitive to the decreasing wedge height. This mode also experiences cut-off at a wedge height of about 50 nm in our experimental condition. Meanwhile, the SR-SPP mode has a large propagation constant that increases further with decreasing wedge height. As a result, the effective wavelength of the mode shrinks confining the electromagnetic wave longitudinally along the propagation direction in addition to enhancing the transverse confinement of SR-SPP. In the experiment, we use gold wedges with different edge heights to excite each SPP mode individually and image the electromagnetic near field by using a pseudo-heterodyne scattering scanning near-field optical microscope. By imaging the LR-SPP mode field, we demonstrate that the theoretical and measured values of the effective wavelength agree quite well. By using short wedges, we measure the SR-SPP mode field and demonstrate that the effective wavelength decreases to 47% in about half a micrometer of propagation distance. This corresponds to a 3.5 times decrease of the vacuum wavelength or an effective index of 3.5. It is important to note that this value is, by no means, the limit of the electromagnetic field’s longitudinal confinement in a gold wedge. Rather, we were only able to measure the electromagnetic field up to this point due to our measurement limitations. The electromagnetic field will be propagating further, and the longitudinal confinement will increase as well. In conclusion, we measured the SR-SPP in a gold wedge and demonstrate the electromagnetic field confinement in the visible spectrum in gold wedges.