Optical properties of natural or designed materials are determined by the electromagnetic multipole moments that light can excite in the constituent particles. In this paper, we present an approach to calculating the multipole excitations in arbitrary arrays of nanoscatterers in a dielectric host medium. We introduce a simple and illustrative multipole decomposition of the electric currents excited in the scatterers and connect this decomposition to the classical multipole expansion of the scattered field. In particular, we find that completely different multipoles can produce identical scattered fields. The presented multipole theory can be used as a basis for the design and characterization of optical nanomaterials.

Halogen bonding is arguably the least exploited among the many non-covalent interactions used in dictating molecular self-assembly. However, its directionality renders it unique compared to ubiquitous hydrogen bonding. Here, the role of this directionality in controlling the performance of light-responsive supramolecular polymers is highlighted. In particular, it is shown that light-induced surface patterning, a unique phenomenon occurring in azobenzene-containing polymers, is more efficient in halogen-bonded polymer–azobenzene complexes than in the analogous hydrogen-bonded complexes. A systematic study is performed on a series of azo dyes containing different halogen or hydrogen bonding donor moieties, complexed to poly(4-vinylpyridine) backbone. Through single-atom substitution of the bond-donor, control of both the strength and the nature of the noncovalent interaction between the azobenzene units and the polymer backbone is achieved. Importantly, such substitution does not significantly alter the electronic properties of the azobenzene units, hence providing us with unique tools in studying the structure–performance relationships in the light-induced surface deformation process. The results represent the first demonstration of light-responsive halogen-bonded polymer systems and also highlight the remarkable potential of halogen bonding in fundamental studies of photoresponsive azobenzene-containing polymers

We investigate an extension to the concept of degree of polarization that applies to arbitrary electromagnetic fields, i.e., fields whose wave fronts are not necessarily planar. The approach makes use of generalized spectral Stokes parameters that appear as coefficients, when the full 3 x 3 spectral coherence matrix is expanded in terms of the Gell-Mann matrices. By defining the degree of polarization in terms of these parameters in a manner analogous to the conventional planar-field case, we are led to a formula that consists of scalar invariants of the spectral coherence matrix only. We show that attractive physical insight is gained by expressing the three-dimensional degree of polarization explicitly with the help of the correlations between the three orthogonal spectral components of the electric field. Furthermore, we discuss the fundamental differences in characterizing the polarization state of a field by employing either the two- or the three-dimensional coherence-matrix formalism. The extension of the concept of the degree of polarization to include electromagnetic fields having structures of arbitrary form is expected to be particularly useful, for example, in near-field optics.

We study the photoresponsive behavior of thin films of supramolecular 4-nitro-4′-hydroxyazobenzenepoly(4-vinylpyridine) complexes. Hydrogen bonding between the phenol and pyridine moieties allows attaching a chromophore to essentially each repeat unit of the polymer, thereby suppressing macroscopic phase separation and crystallization. Moreover, the noncooperative nature of hydrogen bonding leads to random complexation of the chromophores to the polymer backbone, which enables a systematic study of the effect of chromophore concentration on the photo-orientation of the complexes. Two regimes are observed: Photoinduced birefringence increases linearly with the chromophore concentration until nominally every third polymer repeat unit is occupied. Beyond that concentration, a new regime is observed with a drastically steeper slope of increase. The latter regime is connected to the interplay between the formation of a hydrogen-bonded supramolecular complex and intermolecular interactions between the mesogenic chromophores. Such a behavior significantly enhances the birefringence at high concentrations and also leads to high remnant birefringence. Hence, the supramolecular approach yields a superior optical performance compared to guest-host polymers and even surpasses the properties of many covalently functionalized polymers, while still allowing modular tunability of the materials properties.