Towards an optical far-field measurement of higher-order multipole contributions to the scattering response of nanoparticles

Abstract

We experimentally show an all-optical multipolar decomposition of the lowest-order Eigenmodes of a single gold nanoprism using azimuthally and radially polarized cylindrical vector beams. By scanning the particle through these tailored field distributions, the multipolar character of the Eigenmodes gets encoded into 2D-scanning intensity maps even for higher-order contributions to the Eigenmode that are too weak to be discerned in the direct far-field scattering response. This method enables a detailed optical mode analysis of individual nanoparticles.Geometrically tailored nanostructures can be utilized to create strong local near-field enhancement for light harvesting [1,2] or higher-harmonic generation [3,4]. Furthermore, optimized directive emission of nanoparticles or antennas can be achieved by adapting their amplitude and phase response [5][6][7][8][9]. The underlying scattering processes are hereby governed by the Eigenmodes of the employed particle at the desired wavelength of operation. A typical example of such a metallic particle with a tailored optical response is a nanoprism. Its supported electromagnetic multipolar mode structure was shown on the single particle level via near-field measurements [10] and electron energy loss spectroscopy (EELS) [11]. It is used as a fundamental building block in many areas of nano-optics, plasmonics and sensing [3,[12][13][14] since its spectral response can be tuned easily by changing simple geometric parameters such as the thickness or base-length of the particle [15][16][17].For the investigation of the optical properties of such an individual nanostructure, an all-optical and extraordinarily versatile approach has been presented in literature. This approach relies on the utilization of spatially tailored electromagnetic field distributions at the nanoscale [18][19][20], such as tightly focused cylindrical vector beams (CVB; azimuthally and radially polarized [21][22][23], see Fig. 1(a) and (b)) in the focal plane of a focusing system to study the modal response of individual nanostructures. It was also shown that as a first step, the symmetry and geometry of a single metallic nanoprism can be retrieved from the scattering map recorded while scanning such a prism through the focus of tightly focused CVBs [24]. Each pixel of the two-dimensional scan image contains information about the power collected in transmission or reflection for each positon of the particle scanned through the focal plane. This is possible because the triangular symmetry of the particle of a size comparable to the focal spot gets encoded into the scanning map. For particle dimensions significantly smaller than the optical wavelength, at least asymmetric defor- * peter.banzer@mpl.mpg.de mations of the focal spot were experimentally observed in Ref. 24, allowing for an estimation of the particle's shape and orientation. While this trigonal shape of the scattering map seems to be an intuitive match with the geometry of the particle for particle dimensions comparable to the size of the focal s...

Towards polarization-based excitation tailoring for extended Raman spectroscopy

“…The stronger signal observed for off-axis in comparison to on-axis particle positions has multiple causes, including the excited modes in the particle as well as the collection geometry of the system as discusse din more detail below. This linear scan measurement already indicates that a position-dependent interaction of the focal fields with the nanostructure can be realized with the chosen excitation beam (similar to previously reported linear investigations [31,51,34]). However, the aforementioned scan only serves for alignment purposes in this study, and for illustrating the spatial dependence of the light-matter interaction.…”

Quantum Optics in Nanostructures

“…We also would like to point out fields of research which are relevant and which will undeniably be actively developed in the near future. Among them is the interaction of structured light with chiral nanostructures [ 174 , 175 , 176 , 177 , 178 ] and dielectric NPs with a high refractive index [ 179 , 180 , 181 , 182 ]. We also note experimental studies of interference effects that arise upon interaction of an NP and a QE.…”

Manipulation of Scattering Spectra with Topology of Light and Matter

“…These results demonstrate the role of the phase gradient and the illumination direction in the excitation of Mie resonances in the arbitrary shaped all‐dielectric meta‐atoms. Although the interactions of structured light with non‐spherical meta‐atoms such as a cluster of spheres, [ 94 ] split ring resonator, [ 95 ] rods, [ 96 ] and prisms [ 97 ] have been studied previously with AP/RP and Hermite Gaussian beams, the role of the phase gradient of the Laguerre–Gaussian beams in the excitation and suppression of higher‐order resonant modes inside all‐dielectric meta‐atoms of various shapes, aspect ratios, and orientations has not been explored to date. Complementary to recent experimental studies that addressed some aspects of the OAM beam interaction with spherical [ 98 ] and chiral nanostructures, [ 99 ] our study predicted the possibility of the excitation of various multipolar moments which are not accessible via unstructured light illumination and elucidated the role of the OAM in the scattering response of the all‐dielectric meta‐atoms with arbitrary shapes and orientations, enabling new ways of manipulating their optical responses on demand and/or in time‐modulated fashion.…”