Dear Colleagues,

We are pleased to present our Special Issue focused on the increasing implementation of metamaterials (MMs) with smart materials that can introduce tunability to their exotic properties. The interest in this research field is growing quite rapidly with the effect of replacing bulky and thick components throughout the electromagnetic spectrum, from optics to the microwave regime passing from terahertz to visible frequencies. This interest arises from the outstanding properties of tunable metasurfaces and metamaterials which are an ultrathin planar medium composed of subwavelength dielectric or metallic elements able to drastically change their optical response. On the other hand, a paramount role is played by the fundamental dispersion relations that are found at each metal–dielectric interface. An example is surface plasmon polaritons (SPPs) or the gap surface plasmons (GSPs) generated in these devices: the hybridization of MMs with active media such as Liquid Crystals (LCs) can drastically modify the way SPPs and GSPs arise, and the high birefringence of LCs enables the control and tunability of plasmonic interfaces. Indeed, the key to realizing novel breakthrough applications and systems today is the introduction of active controls and reconfigurability of MMs features. This is possible, for instance, by means of external stimuli that change the dielectric response of the medium surrounding the MM. In the case of LCs, an externally applied electric field can modify the molecular director  orientation. A variety of materials modify their optical properties upon thermal (e.g., VO₂) or magnetic stimuli (magnetoplasmonic particles). Finally, the optical response of phase-change GST materials (GeSbTe) is determined by their degree of crystallization. In this scenario, the proposed Special Issue aims to connect researchers with expertise in different technological areas, focusing their attention on recent theoretical and experimental results.

This Special Issue addresses the following main topics, but it is not strictly limited to:

• General aspect of tunable metasurfaces and metamaterials;
• Metamaterial properties tunable by means of liquid crystals;
• Metamaterial properties tunable by external stimuli (electric, thermal, magnetic, etc.);
• Surface plasmon polaritons and gap surface polaritons at the plasmonic interface through an active medium;
• Phase change materials (e.g., GeSbTe);
• UV-Vis-NIR, Terahertz, microwave liquid crystal metamaterials/metasurfaces;
• New frontiers on liquid crystals;
• Numerical simulations focused on tunable materials exploiting liquid crystals;
• Semiconductor 2D metamaterials combined with liquid crystals and/or active media;
• 2D materials such as crystals or photonic crystals;
• Nonlinear optical properties induced by LCs, doped LCs, and metamaterials;
• High refractive index single layer materials (such as MoS₂, WS₂, etc.) or exotic bulky metasurfaces (a-Si:H, SiN_x:H, SiC_x:H, etc.).

Dr. Giuseppe Emanuele Lio
Prof. Dr. Roberto Caputo
Dr. Francesco Riboli
Guest Editors

Manuscript Submission Information

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• tunable metamaterials
• birefringence
• smart metamaterials
• plasmonic resonances
• metacrystals