Chiral plasmonic metasurfaces are intriguing platforms to achieve large nonlinear chiroptical effects, which could be desirable for sensitive chiral molecular analysis. The combination of a superlinear response with high plasmonic nearfield enhancement leads to large nonlinear optical activity, which can be much stronger than its linear counterpart. A complex mixture of multiresonances, structural symmetry, and anisotropy in 3D chiral plasmonic nanostructures as well as experimental configurations has up to now hampered understanding of the underlying physics and quantitative modeling of the relevant nonlinear chiroptical effects. Here we study third-harmonic circular dichroism of archetypical Born-Kuhn type bilayer chiral metasurfaces made of corner-stacked orthogonal gold nanorods arranged in C 4 symmetry, which avoids most of the aforementioned issues. With a coupled anharmonic oscillator model built upon chirally coupled electric dipole moments, we are able to retrieve the nonlinear chiroptical response of such chiral nanostructures. The comparison of nonlinear spectroscopic measurements with our model as well as nonlinear simulations enables interpretation of the large nonlinear circular dichroism. Our research might pave the way toward flexible control over nonlinear susceptibility tensors of artificial chiral meta-molecules at will and could allow for highly sensitive nonlinear chiral sensing.

In several publications, it has been shown how to calculate the near- or far-field properties for a given source or incident field using the resonant states, also known as quasi-normal modes. As previously noted, this pole expansion is not unique, and there exist many equivalent formulations with dispersive expansion coefficients. Here, we approach the pole expansion of the electromagnetic fields using the Mittag-Leffler theorem and obtain another set of formulations with constant weight factors for each pole. We compare the performance and applicability of these formulations using analytical and numerical examples. It turns out that the accuracy of all approaches is rather comparable with a slightly better global convergence of the approach based on a formulation with dispersive expansion coefficients. However, other expansions can be superior locally and are typically faster. Our work will help with selecting appropriate formulations for an efficient description of the electromagnetic response in terms of the resonant states.

The aim of Moosh is to provide a complete set of tools to compute all the optical properties of any multilayered structure: reflection, transmission, absorption spectra, as well as gaussian beam propagation or guided modes. It can be seen as a semi-analytic (making it light and fast) solver for Maxwell's equations in multilayers. It is written in Octave/Matlab, available on Github and based on scattering matrices, making it perfectly stable. This software is meant to be extremely easy to (re)use, and could prove useful in many research areas like photovoltaics, plasmonics and nanophotonics, as well as for educational purposes for the large number of physical phenomena it can illustrate.

Tailoring not only the linear but also the nonlinear properties of plasmonic structures has been a longstanding idea. The plasmonic dolmen structure with its many degrees of freedom in design has been of particular interest. We are investigating this system in detail in the retarded weak-coupling regime, the so-called plasmonic analog of electromagnetically induced absorption. While it is generally expected that the enhanced absorbance leads to an increased nonlinear generation, we find that the details are more complex. A thorough wavelength and polarization resolved study reveals two distinct nonlinear contributions. Our nonlinear spectroscopy method exhibits a surprisingly high sensitivity to minute structural asymmetries. Our experimental results are corroborated by finite-element simulation. We envision that our findings will stimulate further research into phase tuning, structural symmetries, and manipulation in nonlinear plasmonic systems in order to fully exploit the ability to tailor the linear and specifically the nonlinear optical properties of the nanostructured matter.

We present an advanced formulation of the Fourier modal method for analyzing the second-harmonic generation in multilayers of periodic arrays of nanostructures. In our method, we solve Maxwell's equations in a curvilinear coordinate system, in which the interfaces are defined by surfaces of constant coordinates. Thus, we can apply the correct Fourier factorization rules as well as adaptive spatial resolution to nanostructures with complex cross sections. We extend here the factorization rules to the second-harmonic susceptibility tensor expressed in the curvilinear coordinates. The combination of adaptive curvilinear coordinates and factorization rules allows for efficient calculation of the second-harmonic intensity, as demonstrated for one- and two-dimensional periodic nanostructures.