Introduction

Today there is significant effort put into understanding the optical properties of molecules interacting with optical antennas. This interest is largely driven by many potential applications of such interactions, as well as a scientific curiosity for obtaining a detailed understanding of the complicated physics and chemistry arising in these unique systems. Establishing a detailed fundamental understanding of the optical properties in these mixed molecule–metal complexes will be essential in order to apply these materials to energy harvesting [435], nanoscale optical circuits [436] and ultra-sensitive chemical and biological sensors [437, 438].

The optical properties of molecules are characterized by localized excitations that reflect the electronic structure of the molecules. These localized electronic transitions can be engineered by introducing electron donating or electron withdrawing groups into the molecule by means of chemical synthesis. This allows for molecules to be designed with tailored optical properties. In contrast, the optical properties of metallic nanoantennas are dominated by the collective excitations of the conduction electrons, also known as SPPs. The excitation of a LSPR results in strong absorption in the UV–visible region and thus are responsible for NP's brilliant optical properties. The LSPR excitation is sensitive to the size, shape, material and surroundings of the NP, which provides significant opportunities for designing materials with optimum optical properties. This is possible due to significant advances in fabrication techniques as well as efficient classical electromagnetic simulation techniques. This feature makes plasmonic antennas uniquely suited for a wide range of applications in catalysis, optics, chemical and biological sensing and medical therapeutics.