Optimizing plasmonic resonance properties in the near-IR

Abstract

Surface plasmon polaritons (SPPs) are electromagnetic waves that arise when a plasmon wave1–5 (i.e., a coordinated swarm of electrons) interacts strongly with a light wave of a similar frequency (!), and together they are confined to, and propagate along, an air/metal interface. SPPs are of substantial interest because it is possible to control and modify them with the use of normal circuit elements, to allow the SPP to be ejected as pure light. In addition, the dimensions of SPPs are much smaller in metals than in air, and so the usual diffraction limit of light in air does not apply. The confinement property of SPPs—which can lead to intense electric fields and enhanced light emission—is thus exploited in a number of applications, including surfaceenhanced Raman spectroscopy (which has a higher resolution than standard Raman spectroscopy). The metals that are generally used in plasmonic applications (gold and silver), however, are not ideal for all energy ranges. Although these materials have high electron concentration (n) values (mid1022cm 3), and work well in the UV (3–12eV) and visible (1.5– 3eV) ranges (because !p n1=2), they experience heavy losses in the near-IR region (0.5–1.5eV). This is because the loss is proportional to n/! (where is electron mobility), and the high value of n cannot be avoided. In response to this problem, it has previously been proposed1–3 that highly doped semiconductors, with smaller n values, may be better plasmonic materials (than the standard gold and silver) in the near-IR (NIR) region. Fortunately— mainly because of the need for transparent electrodes in LEDs, display circuits, and solar cells—the field of highly doped semiconductors is well developed.6 However, the goal in transparent electrode design is generally to have the highest possible n, whereas in plasmonic applications, it is to have the lowest possible n, but that is still high enough to produce the desired resonance wavelength ( res). The best possible material is thereFigure 1. Experimentally measured plasmonic resonant wavelength ( res) of gallium-doped zinc oxide (GZO) as a function of annealing temperature (TA).