ABSTRACT
By directing light, optical antennas can enhance light-matter interaction and improve the efficiency of nanophotonic devices. Here we exploit the interference among the electric dipole, quadrupole, and magnetic dipole moments of a split-ring resonator to experimentally realize a compact directional optical antenna. This single-element antenna design robustly directs emission even when covered with nanometric emitters at random positions, outperforming previously demonstrated nanoantennas with a bandwidth of 200 nm and a directivity of 10.1 dB from a subwavelength structure. The advantages of this approach bring directional optical antennas closer to practical applications.
ABSTRACT
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 0.00033lamda(0)(3). The corresponding Purcell factor is 900, making these ultrasmall disk resonators ideal candidates for studies of enhanced spontaneous emission and lasing.
Subject(s)
Nanostructures/chemistry , Nanostructures/ultrastructure , Nanotechnology/instrumentation , Surface Plasmon Resonance/instrumentation , Transducers , Equipment Design , Equipment Failure Analysis , Particle SizeABSTRACT
The reflection of surface plasmon polaritons by deep linear grooves structured into gold surfaces is investigated with numerical finite-difference-in-time-domain as well as boundary-element-method calculations. Groove widths of 25 and 100 nm are studied, with depths as large as 500 nm. The reflection depends strongly on wavelength, groove depth, and width. By systematically varying these parameters and studying the field profiles in the grooves as well as mode dispersion, we relate the resonances of the reflectivity to resonant coupling of propagating planar plasmon modes to cavity modes inside the grooves. By careful design of the groove width and depth the reflectivity can be tuned to values up to at least 30% for either a narrow or wide band of wavelengths.
Subject(s)
Models, Theoretical , Nanostructures/chemistry , Nanostructures/ultrastructure , Surface Plasmon Resonance/methods , Computer Simulation , Light , Scattering, Radiation , Surface PropertiesABSTRACT
We use cathodoluminescence imaging spectroscopy to excite and investigate plasmonic eigenmodes of Au nanowires with lengths of 500-1200 nm and approximately 100 nm width. We observe emission patterns along the Au nanowire axis that are symmetric and strongly wavelength dependent. Different patterns correspond to different resonant modes of the nanowire. From the observed patterns, we derive the spatial and spectral properties of the wire eigenmodes and determine the dispersion relation for plasmonic Au nanowire modes.