Xiaorun Zang

Nonlinear microscopy using cylindrical vector beams: Applications to three-dimensional imaging of nanostructures

The three-dimensional (3D) optical fields that arise from the focusing of cylindrical vector beams (CVB) with radial and azimuthal polarizations provide new sources of contrast for optical microscopy of nano-objects. So far, these demonstrations have been restricted to two-dimensional transversal scanning, i.e., along the focal plane of interest, or use of point-like objects, i.e., single molecules and nanoparticles. Here, we demonstrate the first application of CVBs for 3D imaging of 3D nano-objects. This technique is done by acquiring 3D image scans of the second-harmonic generation signal from vertically-aligned semiconductor nanowires, whose second-order response is primarily driven by the longitudinal electric field, i.e., the field component along the nanowire axis. Our technique provides a new way to study individual nano-objects in three dimensions through the unique combination of nonlinear microscopy and CVBs.

Collective Effects in Second-Harmonic Generation from Plasmonic Oligomers

We investigate collective effects in plasmonic oligomers of different symmetries using second-harmonic generation (SHG) microscopy with cylindrical vector beams (CVBs). The oligomers consist 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 radially or azimuthally polarized CVBs in the focal plane. We observe that SHG from such rotationally symmetric oligomers is strongly modified by the interplay between the polarization properties of the CVB and interparticle coupling. We find that the oligomers with radially oriented nanorods exhibit small coupling effects. In contrast, we find that the oligomers with azimuthally oriented nanorods exhibit large coupling effects that lead to silencing of SHG from the whole structure. Our experimental results are in very good agreement with numerical calculations based on the boundary element method. The work describes a new route for studying coupling effects in complex arrangements of nano-objects and thereby for tailoring the efficiency of nonlinear optical effects in such structures.

Boosting nonlinearity of metasurfaces through decrease in number of particles (Conference Presentation)

Metal nanoparticles demonstrate unique optical properties that are mostly due to localized surface plasmon resonances (LSPRs). In addition, when nanoparticles are arranged in arrays (metasurfaces), their responses can be modified by the presence of the neighboring particles. As a result, sharp spectral features can be observed. Such features, called surface lattice resonances (SLRs), are related to the appearance of diffraction orders in the optical response. Both types of resonances can lead to local-field enhancement and thereby boost nonlinear optical effects. For the particular case of second-harmonic generation (SHG) the sample needs to be also non-centrosymmetric. This condition is fulfilled when, for example, V-shaped nanoparticles are used in the array. Increasing the number of particles typically increases the optical density, which should increase the nonlinear response with the square of the particle density. This approach, however, has its limitations because, when the particles are too close to each other, the quality of the LSPRs decreases leading to an effect opposite to the desired. Here, we will show the counterintuitive effect that the nonlinear response can be enhanced by reducing the number of particles in the array. In order to verify our idea, we use two arrays of V-shaped gold nanoparticles fabricated on a glass substrate by electron-beam lithography and lift-off methods. The particles are distributed in 500 x 500 nm2 square arrays in two configurations: i) all lattice points are filled with particles (V1) or ii) every other particle in the lattice is removed in a way that the remaining particles form a rotated (by 45°) square array with a pitch of 707 nm (V2). Both samples have two eigenpolarizations: one along the symmetry axis (y) of the V shape and other in the perpendicular direction (x). In the SHG experiments, the incident beam from an optical parametric oscillator was incident on the sample. Polarizers and a half-wave plate were used to control the polarization of the fundamental (1000 – 1300 nm) and second-harmonic beams. The SHG signal was collected by a photon counting system. The sample V2, that has reduced (by a factor of 2) density of particles in the array, shows the expected decrease in the strength of the resonance peak (1151 nm) and a slight redshift of the resonance wavelength with respect to the sample V1 (1081 nm). In order to achieve fair comparison of the nonlinear signals, we tuned the incident wavelength to the position of approximately equal losses for both samples (1135 nm). The sample V2 is found to have, by a factor of 7, stronger response than sample V1. Such enhancement in the nonlinearity is related to the improvement in the quality of the resonance for sample V2, for which the width of the resonance is reduced by ~30% compared to V1. This is due to SLRs that are present for sample V2. Our results are in good agreement with calculations by using an approach based on the discrete-dipole approximation.

Second-harmonic generation microscopy of plasmonic oligomers using cylindrical vector beams

Nonlinear optical effects in plasmonic metal nanostructures are attracting widespread interest [1]. The plasmonic resonances of individual nanoparticles can be modified by constructing oligomers of several particles, allowing the collective (simultaneous) excitation of several particles or the modification of the resonances of the overall structure by the interaction between the individual particles [2, 3]. The collective effects can be accessed through near-or far-field methods by modifying the oligomeric configuration and by using illumination schemes that depart from traditional excitation geometries based, e.g., plane waves. Recently, linear optical response of oligomers has been tailored using beams that exhibit inhomogeneous polarization states, such as cylindrical vector beams (e.g., azimuthal and radial) [4, 5]. Here, we design plasmonic oligomers whose structure is matched to the distribution of local polarization of tightly-focused cylindrical vector beams and demonstrate their potential in tailoring second-harmonic generation (SHG) on the nanoscale.

Collective nonlinear effects in metal oligomers using matched cylindrical vector beams

We prepare metal oligomers whose structure is matched to the local polarization of focused cylindrical vector beams. Significant collective second-harmonic signals prove the potential of such beams in engineering nonlinear effects on the nanoscale.