Toon Coenen

Directional Emission from Plasmonic Yagi–Uda Antennas Probed by Angle-Resolved Cathodoluminescence Spectroscopy

Optical nanoantennas mediate optical coupling between single emitters and the far field, making both light emission and reception more effective. Probing the response of a nanoantenna as a function of position requires accurate positioning of a subwavelength sized emitter with known orientation. Here we present a novel experimental technique that uses a high-energy electron beam as broad band point dipole source of visible radiation, to study the emission properties of a Yagi-Uda antenna composed of a linear array of Au nanoparticles. We show angle-resolved emission spectra for different wavelengths and find evidence for directional emission of light that depends strongly on where the antenna is excited. We demonstrate that the experimental results can be explained by a coupled point dipole model which includes the effect of the dielectric substrate. This work establishes angle-resolved cathodoluminescence spectroscopy as a powerful technique tool to characterize single optical nanoantennas.

Directional emission from a single plasmonic scatterer

Directing light emission is key for many applications in photonics and biology. Optical antennas made from nanostructured plasmonic metals are suitable candidates for this purpose but designing antennas with good directional characteristics can be challenging, especially when they consist of multiple elements. Here we show that strongly directional emission can also be obtained from a simple individual gold nanodisk, utilizing the far-field interference of resonant electric and magnetic modes. Using angle-resolved cathodoluminescence spectroscopy, we find that the spectral and angular response strongly depends on excitation position. For excitation at the nanodisk edge, interference between in-plane and out-of-plane dipole components leads to strong beaming of light. For large nanodisks, higher-order multipole components contribute significantly to the scattered field, leading to enhanced directionality. Using a combination of full-wave simulations and analytical point scattering theory we are able to decompose the calculated and measured scattered fields into dipolar and quadrupolar contributions.

Signature of a Fano Resonance in a Plasmonic Metamolecule's Local Density of Optical States

We present measurements on plasmonic metamolecules under local excitation using cathodoluminescence which show a spatial redistribution of the local density of optical states at the same frequency where a sharp spectral Fano feature in extinction has been observed. Our analytical model shows that both near- and far-field effects arise due to interference of the same two eigenmodes of the system. We present quantitative insights both in a bare state, and in a dressed state picture that describe Fano interference either as near-field amplitude transfer between coupled bare states, or as interference of uncoupled eigenmodes in the far field. We identify the same eigenmode causing a dip in extinction to strongly enhance the radiative local density of optical states, making it a promising candidate for spontaneous emission control.

Deep-subwavelength imaging of the modal dispersion of light.

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.

Nanoscale optical tomography with cathodoluminescence spectroscopy

Tomography has enabled the characterization of the Earths interior, visualization of the inner workings of the human brain, and three-dimensional reconstruction of matter at the atomic scale. However, tomographic techniques that rely on optical excitation or detection are generally limited in their resolution by diffraction. Here, we introduce a tomographic technique--cathodoluminescence spectroscopic tomography--to probe optical properties in three dimensions with nanometre-scale spatial and spectral resolution. We first obtain two-dimensional cathodoluminescence maps of a three-dimensional nanostructure at various orientations. We then use the method of filtered back-projection to reconstruct the cathodoluminescence intensity at each wavelength. The resulting tomograms allow us to locate regions of efficient cathodoluminescence in three dimensions across visible and near-infrared wavelengths, with contributions from material luminescence and radiative decay of electromagnetic eigenmodes. The experimental signal can be further correlated with the radiative local density of optical states in particular regions of the reconstruction. We demonstrate how cathodoluminescence tomography can be used to achieve nanoscale three-dimensional visualization of light-matter interactions by reconstructing a three-dimensional metal-dielectric nanoresonator.

Angle-resolved cathodoluminescence spectroscopy

We present a cathodoluminescence spectroscopy technique which combines deep subwavelength excitation resolution with angle-resolved detection capabilities. The cathodoluminescence emission is collected by a paraboloid mirror (effective NA = 0.96) and is projected onto a 2D CCD array. The azimuthal and polar emission pattern is directly deduced from the image. As proof of principle, we use the technique to measure the angular distribution of transition radiation from a single crystalline gold surface under 30 keV electron irradiation. We find that the experiment matches very well with theory, illustrating the potential of this technique for the characterization of photonic structures with deep subwavelength dimensions.

Imaging the hidden modes of ultrathin plasmonic strip antennas by cathodoluminescence.

We perform spectrally resolved cathodoluminescence (CL) imaging nanoscopy using a 30 keV electron beam to identify the resonant modes of an ultrathin (20 nm), laterally tapered plasmonic Ag nanostrip antenna. We resolve with deep-subwavelength resolution four antenna resonances (resonance orders m = 2-5) that are ascribed to surface plasmon polariton standing waves that are confined on the strip. We map the local density of states on the strip surface and show that it has contributions from symmetric and antisymmetric surface plasmon polariton modes, each with a very different mode index. This work illustrates the power of CL experiments that can visualize hidden modes that for symmetry reasons have been elusive in optical light scattering experiments.

Resonant Modes of Single Silicon Nanocavities Excited by Electron Irradiation

High-index dielectric or semiconductor nanoparticles support strong Mie-like geometrical resonances in the visible spectral range. We use 30 keV angle-resolved cathodoluminescence imaging spectroscopy to excite and detect these resonant modes in single silicon nanocylinders with diameters ranging from 60 to 350 nm. Resonances are observed with wavelengths in the range of 400-700 nm, with quality factors in the range Q = 9-77, and show a strong red shift with increasing cylinder diameter. The photonic wave function of all modes is determined at deep-subwavelength resolution and shows good correspondence with numerical simulations. An analytical model is developed that describes the resonant Mie-like optical eigenmodes in the silicon cylinders using an effective index of a slab waveguide mode. It shows good overall agreement with the experimental results and enables qualification of all resonances with azimuthal (m = 0-4) and radial (q = 1-4) quantum numbers. The single resonant Si nanocylinders show characteristic angular radiation distributions in agreement with the modal symmetry.

Gallium plasmonics: deep subwavelength spectroscopic imaging of single and interacting gallium nanoparticles.

Gallium has recently been demonstrated as a phase-change plasmonic material offering UV tunability, facile synthesis, and a remarkable stability due to its thin, self-terminating native oxide. However, the dense irregular nanoparticle (NP) ensembles fabricated by molecular-beam epitaxy make optical measurements of individual particles challenging. Here we employ hyperspectral cathodoluminescence (CL) microscopy to characterize the response of single Ga NPs of various sizes within an irregular ensemble by spatially and spectrally resolving both in-plane and out-of-plane plasmonic modes. These modes, which include hybridized dipolar and higher-order terms due to phase retardation and substrate interactions, are correlated with finite difference time domain (FDTD) electrodynamics calculations that consider the Ga NP contact angle, substrate, and native Ga/Si surface oxidation. This study experimentally confirms previous theoretical predictions of plasmonic size-tunability in single Ga NPs and demonstrates that the plasmonic modes of interacting Ga nanoparticles can hybridize to produce strong hot spots in the ultraviolet. The controlled, robust UV plasmonic resonances of gallium nanoparticles are applicable to energy- and phase-specific applications such as optical memory, environmental remediation, and simultaneous fluorescence and surface-enhanced Raman spectroscopies.

Deep Subwavelength Spatial Characterization of Angular Emission from Single-Crystal Au Plasmonic Ridge Nanoantennas

We use spatially and angle-resolved cathodoluminescence imaging spectroscopy to study, with deep subwavelength resolution, the radiation mechanism of single plasmonic ridge antennas with lengths ranging from 100 to 2000 nm. We measure the antennas standing wave resonances up to the fifth order and measure the dispersion of the strongly confined guided plasmon mode. By directly detecting the emitted antenna radiation with a 2D CCD camera, we are able to measure the angular emission patterns associated with each individual antenna resonance. We demonstrate that the shortest ridges can be modeled as a single point-dipole emitter oriented either upward (m = 0) or in-plane (m = 1). The far-field emission pattern for longer antennas (m > 2) is well described by two interfering in-plane point dipoles at the end facets, giving rise to an angular fringe pattern, where the number of fringes increases as the antenna becomes longer. Taking advantage of the deep subwavelength excitation resolution of the cathodoluminescence technique, we are able to determine the antenna radiation pattern as a function of excitation position. By including the phase of the radiating dipoles into our simple dipole model, we completely reproduce this effect. This work demonstrates how angle-resolved cathodoluminescence spectroscopy can be used to fully determine the emission properties of subwavelength ridge antennas, which ultimately can be used for the design of more complex and efficient antenna structures.