Liquid crystal-integrated tunable metamaterials and metasurfaces: from microwave to optical frequencies

Conventional metamaterials (MMs) are fundamentally constrained by their static architectures, which lock electromagnetic properties at the time of fabrication and prevent real-time tunability. To overcome these limitations, this review systematically explores the integration of liquid crystals (LCs) with electromagnetic MMs and metasurfaces, focusing on dynamic and tunable devices spanning microwave, terahertz, and optical frequency regimes. The review outlines the theoretical foundations of MMs and provides a taxonomy of LC phases—nematic, smectic, cholesteric, blue phase, discotic, and lyotropic—distinguishing the present work from prior reviews addressing only nematic LCs. Four principal integration strategies (layered, embedded, microfluidic-based, and hybrid) are examined, followed by a systematic review of all major architecture families: split-ring resonator-based bulk MMs, fishnet structures, frequency selective surfaces, plasmonic metasurfaces, and chiral structures. Fabrication techniques including photolithography, EBL, FIB milling, nanoimprint lithography, and 3D printing are benchmarked against cost, throughput, and scalability metrics, with analysis of commercialization prospects. Dedicated sections address long-term stability and degradation mechanisms, electromagnetic loss sources and mitigation strategies, and a systematic comparison of LCs with competing tunable platforms including graphene, VO 2 , GST, MEMS, ferroelectrics, and transparent conducting oxides. Engineering trade-offs among tuning range, LC thickness, operating voltage, and switching speed are analysed quantitatively. Standardized figures of merit are proposed for microwave, THz, and optical platforms to enable cross-platform benchmarking. The convergence of artificial intelligence -assisted inverse design with LC-MM systems is discussed, including integration with Reconfigurable Intelligent Surfaces for 6 G communications. Applications in telecommunications, biomedical imaging, sensing, and display technologies are highlighted, and five key research bottlenecks are identified.