Optical properties of chiral nanostructures embedded in planar media

Narodowe Centrum Nauki, PRELUDIUM

Project type: Research project

Affiliation: Institute of Geophysics, Information Optics Department

Project realisation: April 1, 2021 - March 31, 2022

mgr Krzysztof Czajkowski Project leader

Chirality is a property assigned to various objects, or indeed fields, if the object cannot be superimposed with its mirror image by a sequence of rotations and displacements. It is prominently present in building blocks of life such as amminoacids or DNA/RNA helices. Immense progress in nanostructure fabrication in recent years has enabled realization of artificial chiral nanomaterials in a variety of shapes, made of both dielectric and plasmonic materials. The optical response of a chiral object is distinct for each circular polarization of light leading to so-called chiroptical effects such as circular dichroism (difference in absorption of each circular polarization) or circular birefringence (difference in refractive index for each circular polarization).
The goal of the project is to elucidate the role of coupling between the scattered field of chiral nanostructures and optical modes of planar media in the chiroptical response of the system. We hypothesize that including planar medium as an environment in the analysis of chiral nanostructures is of paramount importance as it modifies the symmetry of the system, which is one of key properties determining chiroptical effects. Furthermore, it is omnipresent in realistic nanostructures. Also, recent studies render coupling of nanoresonators to optical modes of the planar cavities an efficient way to enhance and control light-matter interactions in nanocavities formed with nanoresonators.
To realize this goal, we plan to develop a novel semi-analytical T-matrix framework which will be suitable for analysis of the optical response of chiral nanostructures embedded in planar media. The advantage of the T-matrix method is that it provides far more insight into physical mechanisms behind chiroptical effects and it is more efficient than traditional modelling tools such as FDTD and FEM. It is also a suitable method for studying large-scale arrays of nanoresonators, which are commonly obtained experimentally, but are difficult to simulate their optical response with FDTD/FEM. The key novelty of our approach is that it enables to efficiently study both periodic and random arrays of arbitrarily shaped nanoparticles embedded in planar media. To that end, we propose a film of multipoles approach, which reduces the problem of finding the scattered field for all particles in an array to a single-particle problem, in which all interaction within the system is accounted (semi-)analytically. By analyzing the effective T-matrix of the system, we will scrutinize the key factors determining the chiroptical effects and exemplify them by studying random and periodic nanoparticle arrays placed on a substrate or in a Fabry-Perot cavity.
The chirality of the nanostructures studied in the project is provided either by the geometrical properties of the nanoresonators or by the presence of a chiral molecule layer in the vicinity of achiral nanoresonators. The chiroptical response of the chiral molecular layer could be exploited to distinguish between enantiomers or separate them by selective photoabsorption, which is, however, hindered by the fact that the intrinsic electromagnetic chirality of biomolecules is relatively low. A long-standing goal of chiral nanophotonics is to enhance chiroptical response by utilizing nanostructures. Recent, theoretical studies are focused on freespace structures (mostly single particles), which limits their capability to explain experimental results. Here, we would like to provide further understanding of nanophotonic enhancement of chiroptical response of biomolecules by studying substrate-mediated effects. We also propose a novel concept of simultaneous sensing of refractive index and enantiomer discrimination to extend the capabilities of such nanostructurebased devices.
We anticipate that the project will be an important step in understanding light-chiral matter interactions in experimentally attainable nanoscale environments. The expected results and theoretical tools developed upon completion of the project will be useful for modelling and design for future nanophotonics devices exploiting chiroptical effects.