1. INTRODUCTION

Light-matter interaction in complex photonics systems can be traced back to disordered media. When light interacts with disordered or aperiodic strongly scattering media, a variety of interesting optical phenomena take place, ranging from random lasing [1] and Anderson localization to light transport [2]. Research in this area is still very active, as there is still much work to be done in terms of both fundamental physics and potential applications. For example, imaging through strongly scattering media [3] may find applications in several everyday situations like navigation, medical imaging, and rescue operation.

The advent of nanofabrication has enabled us to consider light-matter interaction in ordered photonics structures, like photonic crystals and metamaterials. Photonic crystals were proposed in 1987 by John and Yablonovich for light-trapping and cavity QED, respectively [4,5]. A major challenge was to achieve a photonics band gap in all directions, which is required for complete control over the local density of photonics states. Soukoulis and coworkers proposed structures which combine manufacturability with a large omnidirectional band gap [6,7]. Nowadays, we witness commercial applications of photonic crystals, such as photonic-crystal fibers [8], and they are still a subject of intense fundamental research, such as in quantum photonics [9] and optomechanics [10].

Optical metamaterials originate from the seminal work of Pendry on perfect imaging by negative refraction [11]. Since a negative refractive index requires a magnetic response at optical frequencies, one of the major endeavors was to design nanostructures with a negative effective permeability in the visible and near-infrared range. Soukoulis and coworkers were among the first to propose metamaterial designs with such a feature [12,13].

Because absorption losses in optical metamaterials remain a challenge, as is the nanofabrication of bulky structures, the device concept is now exceeded by the so-called metasurfaces or flat optics, employing ultra-thin structured layers. Proposed by Capasso and coworkers [14], two-dimensional optics became one of the top ten emerging technologies of 2019.

These topics are intimately related to the field of plasmonics, which exploded after the seminal work of Ebbesen and coworkers [15]. Their work has shown how surface plasmon polaritons in nanostructures facilitate light transport through subwavelength holes; it is now a unique resource applied to very different contexts, ranging from biosensing to quantum photonics [16].

This feature issue contains original work focused on the above-mentioned research topics, and it shows how broad and prominent this field has become.

With this feature issue, the guest editors and the authors of the contributed articles would like to celebrate the 70th birthday of Professor Costas Soukoulis, one of the leading scientists in the field of light-matter interactions in complex photonics systems.