It has been shown that metallic nanoparticles enable the generation of surface plasmons in the visible wavelength ranges. Although Ga nanoparticles are not traditionally used for plasmonic applications, the size-dependence of the Ga droplet plasmon energy was recently reported. In particular, for ensembles of randomly-distributed Ga droplets with sizes ranging from 10 to 100 nm, the plasmon energy ranges from 3.3 to 2.5 eV . Furthermore, a comparison of literature reports for the optical constants of liquid (amorphous) Ga and Ag nanoparticles reveals conductivity values which are of the same order of magnitude . Thus, both the plasmon resonance tuning and the dissipation losses for Ga nanoparticles are promising for active plasmonics. On III-V semiconductor surfaces, nanometer-sized metallic amorphous liquids (i.e. droplets) or crystalline/amorphous solids (i.e. islands), mostly consisting of group III elements, often form during thermal annealing, exposure to a group III element, and ion irradiation. In the case of focused-ion beam (FIB) irradiation of III-V semiconductor surfaces, group V elements are preferentially sputtered, forming a group III-rich surface region. With continued FIB irradiation beyond a threshold ion dose, we observed group III-rich droplets or islands, as shown in Fig. 1 [3,4]. To date, we have fabricated highly-ordered 2D square and 1D chain arrays of Ga nanodroplets via FIB irradiation of pre-patterned holes on GaN surfaces. The droplet (chain) sizes and interdroplet (interchain) spacings were controlled by the ion dose and the spacing of pre-patterned holes, respectively. We have examined the influence of interdroplet (interchain) spacing and droplet (chain) diameter on the plasmon resonance energy. Evidently, the resonances are blue-shifted to higher energy (shorter wavelength) as the droplet (chain) diameter decreases as shown in Fig. 2 . These resonances may be due to droplet diameter-dependent dipole oscillations, similar to literature reports of size-dependent surface plasmon resonances of randomly-distributed Ga nanodroplets .
This work was supported by the AFOSR under contract FA9550–06–1–0279 through the MURI program, monitored by Dr. Harold Weinstock.
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