In recent years there has been considerable interest in nanoscale metal devices that take advantage of surface plasmon polaritons (SPPs) and localized surface plasmons to concentrate optical energy to sub-wavelength length scales. Of particular interest for investigations into quantum electrodynamics are metal-based cavities that concentrate optical energy in the vicinity of optical emitters. Although metal losses limit the maximum cavity quality (Q) factors attainable in metal-based cavities, the much smaller mode volumes (V) achievable in these structures provide a Q/V that can be commensurate with those characteristic of dielectric cavities.
An important advantage of these metal cavities is the wide choice of optically active layers that may be incorporated into the device. Our group has recently demonstrated a metal-based plasmonic cavity containing coupled colloidal PbS nanocrystal optical emitters that showed strong modification of the nanocrystal luminescence spectrum. In the work described here, a similar plasmonic nanocavity design is used, consisting of a silver nanowire lying parallel to a silver substrate. In this case, however, the coupled gain medium is a self-assembled monolayer of organic dye in the gap between the nanowire and the substrate.
A schematic of the cavity design is shown in Fig. 1. Surface plasmon modes of the nanowire and substrate hybridize across the gap, yielding gap-mode states of long wave-vector confined in the region under the nanowire. The sub-nm roughness of the Ag substrate is formed through a ‘template stripping’ technique, using metal deposition onto an atomically-smooth Si wafer and subsequent transfer of the metal film to another substrate. A thin layer (2-5nm) of aluminum oxide is deposited onto the silver by atomic layer deposition. A thiol-reacting dye, Alexa Fluor 532 maleimide, is assembled in the next layer by covalently binding to a self-assembled monolayer of (3-mercaptopropyl) oxysilane on the oxide. Another thin layer of aluminum oxide is then deposited on top of the dye. The result is a smooth and covalently bound monolayer of emitter, sandwiched between two thin layers of dielectric, which act as spacers from the metal surfaces. Cavity fabrication is completed by depositing silver nanowires onto the sample in a droplet of ethanol that is allowed to dry.
Fluorescence from dye molecules within the gap couples to the modes of the cavity, resulting in clear modifications of the dye emission spectrum, as shown in Fig. 2. Polarization measurements indicate that the emission is highly polarized along the axis of the cavity, in agreement with the behavior of nanocrystal-based cavities.
The gap size between the silver substrate and the nanowire is an essential variable to understanding the limits of our system. Placing the dye too close to the metal surface quenches its fluorescence, but placing it too far prevents it from coupling its emission into the cavity modes. Organic dyes give us the capability to explore these limits by controlling the gap thicknesses with layer-by-layer growth or by binding it to molecules self-assembled on thickness controlled atomic layer deposition of dielectrics.