Three-dimensional (3D) metamaterials show the potential for realizing efficient nonlinear nanoscale devices. Despite the recent progress, the nonlinear metamaterials lack in terms of conversion efficiencies when compared against conventional nonlinear materials that rely on phase-matching techniques. Here, we demonstrate how the nonlinear responses of 3D metamaterials can be improved by stacking metasurfaces on top of each other and by applying phase-matching techniques. We demonstrate this by successfully fabricating phase-matched metamaterials consisting of stacked metasurfaces. Especially, we observe a 25-fold enhancement of second harmonic generation emission from a device consisting of five metasurfaces.
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.
The optical responses of metal nanoparticles are associated with their localized surface-plasmon resonances. Such resonances give rise to strong local fields near the particles, which are particularly advantageous for nonlinear interactions. Here, we present an overview of our results on second-harmonic generation (SHG) from metasurfaces consisting of metal nanoparticles and discuss factors that affect its efficiency. Our metasurfaces consist mainly of ordered arrays of nanoparticles. L or T shaped particle appear non-centrosymmetric also at normal incidence, a requirement for SHG. The quality of the particles is crucial for homogeneous plasmon resonances and high overall SHG efficiency. Beyond this, subtle details of sample structure strongly influence the response. Furthermore, the response from SHG-active non-centrosymmetric particles can be enhanced by SHG-passive centrosymmetric particles. Both effects arise from lattice interactions between the members of the array. By extending these concepts further, we have shown the importance of surface lattice resonances for SHG efficiency, allowing the response to be enhanced even for reduced particle density. The nonlinear responses are assumed to depend mainly on the resonance at the fundamental wavelength. However, this is not sufficient as such, because the details of the local-field distributions, which are closely associated with particle geometry, are also crucial. We have also extended our work to random metal nanoisland films, where a dielectric overlayer shifts the plasmon resonance away from the resonance at the SHG wavelength. Rather surprisingly, the SHG response is enhanced because of strongly enhanced local fields at the fundamental wavelength as the dielectric loading is increased.
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.
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.
The emerging field of on-chip integration of nanophotonic devices and cold atoms offers extremely strong and pure light-matter interaction schemes, which may have profound impact on quantum information science. In this context, a long-standing obstacle is to achieve strong interaction between single atoms and single photons, while at the same time trap atoms in vacuum at large separation distances from dielectric surfaces. In this work, we study new waveguide geometries that challenge these conflicting objectives. The designed photonic crystal waveguides are expected to offer a good compromise, which additionally allow for easy manipulation of atomic clouds around the structure.