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.

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.

The surface plasmon polariton ͑SPP͒ field intensity in the vicinity of gratings patterned in an otherwise planar gold surface is spatially resolved using cathodoluminescence ͑CL͒. A detailed theoretical analysis is presented that successfully explains the measured CL signal based upon interference of transition radiation directly generated by electron impact and SPPs launched by the electron and outcoupled by the grating. The measured spectral dependence of the SPP yield per incoming electron is in excellent agreement with rigorous electromagnetic calculations. The CL emission is shown to be similar to that of a dipole oriented perpendicular to the surface and situated at the point of electron impact, which allows us to establish a solid connection between the CL signal and the photonic local density of states associated to the SPPs. DOI: 10.1103/PhysRevB.79.113405 PACS number͑s͒: 73.20.Mf, 41.60.Ϫm, 78.60.Hk Surface plasmon polaritons ͑SPPs͒ are electromagnetic waves bound to the interface between a metal and a dielectric.1 The strong coupling between optical radiation and the collective plasmon oscillations of the conduction electrons near the metal surface leads to complex SPP behavior that can give rise to large field enhancements, 2 negative refraction, 3 and many other interesting phenomena resulting from sub-100 nm optics intrinsic to SPPs at visible and nearinfrared frequencies.A major bottleneck in nearly all studies on the fundamental properties of SPPs is the limited spatial resolution by which plasmonic phenomena can be measured. Optical microscopy suffers from the diffraction limit, whereas nearfield microscopy has a resolution limited by the tip aperture to typically 100 nm. In contrast, SPPs can also be excited using high-energy electron irradiation, with the electron beam focused to a nanometer size spot, thus enabling the excitation of SPPs with nanoscale resolution. Only a few studies of electron-beam irradiation of plasmonic structures have been reported, mainly focusing on measurements of the mode distribution of plasmons in nanoparticles 4-6 or plasmon losses in planar surfaces. 7,8 However, no detailed analysis of the different emission components and their interaction has been presented and no connection of the emission to the plasmonic density of states has been established.In this Brief Report, we use electron-beam irradiation to study fundamental properties of SPPs propagating on a twodimensional substrate. In particular, we use the electron beam of a scanning electron microscope ͑SEM͒ impinging on a single-crystalline Au substrate as a nanoscale source of SPPs with a broad spectral range. Our key findings are as follows. ͑1͒ We have developed a model of cathodoluminescence ͑CL͒ emission which includes the excitation of SPPs, eventually outcoupled from the Au surface, and transition radiation ͑TR͒, 9,10 as well as the interference of the two components; ͑2͒ extensive CL measurements performed over the visible spectrum and at distances up to a few microns from the grating are well re...

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.

Numerous optical technologies and quantum optical devices rely on the controlled coupling of a local emitter to its photonic environment, which is governed by the local density of optical states (LDOS). Although precise knowledge of the LDOS is crucial, classical optical techniques fail to measure it in all of its frequency and spatial components. Here, we use a scanning electron beam as a point source to probe the LDOS. Through angular and spectral detection of the electron-induced light emission, we spatially and spectrally resolve the light wave vector and determine the LDOS of Bloch modes in a photonic crystal membrane at an unprecedented deep-subwavelength resolution (30-40 nm) over a large spectral range. We present a first look inside photonic crystal cavities revealing subwavelength details of the resonant modes. Our results provide direct guidelines for the optimum location of emitters to control their emission, and key fundamental insights into light-matter coupling at the nanoscale.