Nonlinear optical processes provide completely new contrast mechanisms for microscopy. The polarization vector of a focused field is three-dimensional (3D) and spatially inhomogeneous, thereby opening up new opportunities for the characterization of complex nano-objects. The 3D control of focal fields further benefits from the use of unconventional states of polarization, e.g., radially (RP) and azimuthally polarized (AP) incident beams. Of particular importance is the fact that focused RP beam gives rise to a strong longitudinal electric-field component at the focus. In contrast, focused AP beam maintains a strictly transverse electric-field distribution in the focal volume, mimicking the structure of the incident beam before focusing. In this Paper, we summarize several new capabilities and additional benefits made possible by vector beams in nonlinear microscopy of various types of nano-objects. As one of the first demonstrations, we have shown that second-harmonic generation microscopy with vector beams has superior sensitivity to the morphology of individual metal nanoparticles. We have also shown that efficient coupling of incident light to metal nano-objects requires tailored focal fields matching the modes of individual particles and even their assemblies (or so-called oligomers). We also used vector beams to characterize the crystal structure of semiconductor nanodisks and to couple light to vertically-aligned semiconductor nanowires. In addition, nanowires have been used to probe the longitudinal fields of advanced polarization states in three dimensions.
Plasmonic oligomers allow new ways to manipulate nonlinear optical effects such as second-harmonic generation (SHG) through collective resonances. However, earlier techniques to probe such effects have relied mostly on the use of plane waves or focused beam excitations with homogenous states-of-polarization (e.g., linear) that obviously do not match the spatial symmetries of the oligomer. Here, we investigate collective effects in the SHG from individual plasmonic oligomers using microscopy with cylindrical vector beams such as radial or azimuthal polarizations. The oligomers were prepared by electron-beam lithography. The oligomers consisted of gold nanorods that have a longitudinal plasmon resonance close to the fundamental wavelength that is used for SHG excitation and whose long axes are arranged locally such that they follow the distribution of the transverse component of the electric field of radial or azimuthal polarizations. We found that SHG from such oligomers is strongly modified by the interplay between the properties of the incident cylindrical vector beam and interparticle coupling. We find that the oligomers with radially-oriented nanorods exhibit small coupling effects. In contrast, we observed that the oligomers with azimuthally-oriented nanorods exhibit large coupling effects that lead to silencing of SHG from the whole structure. We found good qualitative agreement between our experimental findings and calculations using the method of moments. The work describes a new route to investigate coupling effects in arrangements of nanostructures and thereby to control the efficiency of nonlinear effects in these structures.
We show that surface second-harmonic generation (SHG) with focused Gaussian vector beams can be described in terms
of effective Mie-type multipolar contributions to the SHG signal even in the electric dipole approximation of constitutive
relations. Traditionally, Mie-type multipoles arise from field retardation across nanoparticles. In our case, the multipolar
light-matter interaction is due to excitation with Gaussian vector beams and the tensorial properties of the SH response.
As different multipoles have different radiative properties, we demonstrate the presence of multipoles by measuring
strongly asymmetric SH emission into reflected and transmitted directions from a nonlinear thin film with isotropic
surface symmetry, where symmetric emission is expected using traditional formalisms based on plane-wave excitation.
The proposed multipole approach provides a convenient way to explain the measured asymmetric emission. Secondly,
we generalize the treatment beyond the electric dipole approximation and propose that analogous vector excitation-induced
multipolar effects could also occur in the microscopic light-matter interaction. Our results may allow new
possibilities to designing confined and thin nonlinear sources with desired radiation patterns.
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