Ultrasmall Mode Volume Plasmonic

We study the resonant modes of nanoscale disk resonators sustaining metal-insulator-metal (MIM) plasmons and demonstrate the versatility of these cavities to achieve ultrasmall cavity mode volume. Ag/SiO2/Ag MIM structures were made by thin-film deposition and focused ion beam milling with cavity diameters that ranged from d) 65-2000 nm. High-resolution two-dimensional cavity-mode field distributions were determined using cathodoluminescence imaging spectroscopy and are in good agreement with boundary element calculations. For the smallest cavities (d) 65-140 nm), the lowest order mode (m) 1, n) 1) is observed in the visible spectral range. This mode is of similar nature as the one in plasmonic particle dimers, establishing a natural connection between localized and traveling plasmon cavities. A cavity quality factor of Q) 16 is observed for the 105 nm diameter cavity, accompanied by a mode volume as small as 0.00033λ03. The corresponding Purcell factor is 900, making these ultrasmall disk resonators ideal candidates for studies of enhanced spontaneous emission and lasing. Optical microcavities have been central in enabling control of the light emission in LEDs and lasers for application in quantum optics and telecommunication.1-3 These applications rely on the effective confinement of light in a small cavity mode volume. While most of the cavity designs so far are based on dielectric material assemblies, the confinement o