Ben Hopkins

Enhanced Third-Harmonic Generation in Silicon Nanoparticles Driven by Magnetic Response

We observe enhanced third-harmonic generation from silicon nanodisks exhibiting both electric and magnetic dipolar resonances. Experimental characterization of the nonlinear optical response through third-harmonic microscopy and spectroscopy reveals that the third-harmonic generation is significantly enhanced in the vicinity of the magnetic dipole resonances. The field localization at the magnetic resonance results in two orders of magnitude enhancement of the harmonic intensity with respect to unstructured bulk silicon with the conversion efficiency limited only by the two-photon absorption in the substrate.

Observation of Fano Resonances in All‐Dielectric Nanoparticle Oligomers

It is well-known that oligomers made of metallic nanoparticles are able to support sharp Fano resonances originating from the interference of two plasmonic resonant modes with different spectral width. While such plasmonic oligomers suffer from high dissipative losses, a new route for achieving Fano resonances in nanoparticle oligomers has opened up after the recent experimental observations of electric and magnetic resonances in low-loss dielectric nanoparticles. Here, light scattering by all-dielectric oligomers composed of silicon nanoparticles is studied experimentally for the first time. Pronounced Fano resonances are observed for a variety of lithographically-fabricated heptamer nanostructures consisting of a central particle of varying size, encircled by six nanoparticles of constant size. Based on a full collective mode analysis, the origin of the observed Fano resonances is revealed as a result of interference of the optically-induced magnetic dipole mode of the central particle with the collective mode of the nanoparticle structure. This allows for effective tuning of the Fano resonance to a desired spectral position by a controlled size variation of the central particle. Such optically-induced magnetic Fano resonances in all-dielectric oligomers offer new opportunities for sensing and nonlinear applications.

Invited Article: Broadband highly efficient dielectric metadevices for polarization control

Metadevices based on dielectric nanostructured surfaces with both electric and magnetic Mie-type resonances have resulted in the best efficiency to date for functional flat optics with only one disadvantage: a narrow operational bandwidth. Here we experimentally demonstrate broadband transparent all-dielectric metasurfaces for highly efficient polarization manipulation. We utilize the generalized Huygens principle, with a superposition of the scattering contributions from several electric and magnetic multipolar modes of the constituent meta-atoms, to achieve destructive interference in reflection over a large spectral bandwidth. By employing this novel concept, we demonstrate reflectionless (~90% transmission) half-wave plates, quarter-wave plates, and vector beam q-plates that can operate across multiple telecom bands with ~99% polarization conversion efficiency.

Multifold Enhancement of Third-Harmonic Generation in Dielectric Nanoparticles Driven by Magnetic Fano Resonances

Strong Mie-type magnetic dipole resonances in all-dielectric nanostructures provide novel opportunities for enhancing nonlinear effects at the nanoscale due to the intense electric and magnetic fields trapped within the individual nanoparticles. Here we study third-harmonic generation from quadrumers of silicon nanodisks supporting high-quality collective modes associated with the magnetic Fano resonance. We observe nontrivial wavelength and angular dependencies of the generated harmonic signal featuring a multifold enhancement of the nonlinear response in oligomeric systems.

Interplay of Magnetic Responses in All-Dielectric Oligomers To Realize Magnetic Fano Resonances

We study the interplay between collective and individual optically induced magnetic responses in quadrumers made of identical dielectric nanoparticles. Unlike their plasmonic counterparts, all-dielectric nanoparticle clusters are shown to exhibit multiple dimensions of resonant magnetic responses that can be employed for the realization of anomalous scattering signatures. We focus our analysis on symmetric quadrumers made from silicon nanoparticles and verify our theoretical results in proof-of-concept radio frequency experiments demonstrating the existence of a novel type of magnetic Fano resonance in nanophotonics.

Near-field mapping of Fano resonances in all-dielectric oligomers

We demonstrate experimentally Fano resonances in all-dielectric oligomers clusters of dielectric particles. We study two structures consisting of a ring of six ceramic spheres with and without a central particle and demonstrate that both structures exhibit resonant suppression of the forward scattering associated with the Fano resonance originated from the excitation of magnetic dipole modes. By employing the near-field measurement techniques, we establish the relation between near- and far-field properties of the Fano resonances and identify directly their origin. We support our findings by an analytical approach based on the discrete-dipole approximation and find an excellent agreement with the experimental data.

Optically isotropic responses induced by discrete rotational symmetry of nanoparticle clusters

Fostered by the recent progress of the fields of plasmonics and metamaterials, the seminal topic of light scattering by clusters of nanoparticles is attracting enormous renewed interest gaining more attention than ever before. Related studies have not only found various new applications in different branches of physics and chemistry, but also spread rapidly into other fields such as biology and medicine. Despite the significant achievements, there still exists unsolved but vitally important challenges of how to obtain robust polarisation-invariant responses of different types of scattering systems. In this paper, we demonstrate polarisation-independent responses of any scattering system with a rotational symmetry with respect to an axis parallel to the propagation direction of the incident wave. We demonstrate that the optical responses such as extinction, scattering, and absorption, can be made independent of the polarisation of the incident wave for all wavelengths. Such polarisation-independent responses are proven to be a robust and generic feature that is purely due to the rotational symmetry of the whole structure. We anticipate our finding will play a significant role in various applications involving light scattering such as sensing, nanoantennas, optical switches, and photovoltaic devices.

Circular dichroism induced by Fano resonances in planar chiral oligomers

We present a general theory of circular dichroism in planar chiral nanostructures with rotational symmetry. It is demonstrated, analytically, that the handedness of the incident fields polarization can control whether a nanostructure induces either absorption or scattering losses, even when the total optical loss (extinction) is polarization-independent. We show that this effect is a consequence of modal interference so that strong circular dichroism in absorption and scattering can be engineered by combining Fano resonances with planar chiral nanoparticle clusters.

Plasmonic Nanoclusters with Rotational Symmetry: Polarization-Invariant Far-Field Response vs Changing Near-Field Distribution

Flexible control over the near- and far-field properties of plasmonic nanostructures is important for many potential applications, such as surface-enhanced Raman scattering and biosensing. Generally, any change in the polarization of the incident light leads to a change in the nanoparticles near-field distribution and, consequently, in its far-field properties as well. Therefore, producing polarization-invariant optical responses in the far field from a changing near field remains a challenging issue. In this paper, we probe experimentally the optical properties of cruciform pentamer structures--as an example of plasmonic oligomers--and demonstrate that they exhibit such behavior due to their symmetric geometrical arrangement. We demonstrate direct control over hot spot positions in sub-20 nm gaps, between disks of 145 nm diameter at a wavelength of 850 nm, by means of scattering scanning near-field optical microscopy. In addition, we employ the coupled dipole approximation method to define a qualitative model revealing the relationship between the near and far field in such structures. The near-field profiles depend on particular mode superpositions excited by the incident field and, thus, are expected to vary with the polarization. Consequently, we prove analytically that the far-field optical properties of pentamers have to be polarization-independent due to their rotational symmetry.

Hybridization and the origin of Fano resonances in symmetric nanoparticle trimers

We study the light scattering by plasmonic and dielectric symmetric trimers to investigate the existence of polarization-independent Fano resonances. Plasmonic hybridization theory is revealed to hide simple physics, and we instead provide a simplified model for hybridization to derive a plasmonic trimers eigenmodes analytically. This approach is demonstrated to accurately recreate full wave simulations of plasmonic trimers and their Fano resonances. We are subsequently able to deduce the grounds for modal interference in plasmonic trimers and the related formation of Fano resonances. However, by generalizing our simplified hybridization approach, we are also able to investigate the eigenmodes of all-dielectric trimers. We show that bianisotropic coupling channels between high-index dielectric nanoparticles are able to increase the capacity for Fano resonances, even at normal incidence. We finally provide the first experimental measurements of sharp, polarization-independent Fano resonances from a symmetric all-dielectric trimer, with very good agreement with the predicted response from our simplified hybridization theory.