## 3 Emerging and State-of-the-Art Technologies
### Techniques for Reconfigurable RF Front-End Systems and Antennas
Consequently, graphene-based optical nanoantennas are widely integrated into optoelectronic devices for plasmonic applications and have become a focal point of research worldwide [184; 185]. Furthermore, integrating graphene metal hybrid antennas [186; 187] into optoelectronic platforms enables the realization of versatile photonic systems. Various techniques for frequency tuning [188] and active modulation [189] of optical properties particularly via in situ control using external electric fields have been explored. Additionally, multiple approaches for beam scanning and reconfigurable beam control have been proposed, including reflectarrays [190] and leaky-wave graphene antennas [191]. However, the relatively low conductivity of graphene introduces significant losses, which degrades device efficiency. Therefore, innovative methods are required to fully harness the potential of multiferroic and graphene-based structures for RF applications. To mitigate losses at mmWave frequencies, particularly for RIS applications, novel approaches utilizing the mechanical control of individual array elements have emerged [192; 193; 194; 195]. These methods employ rotary actuators [192; 193], electromagnets [194] (as shown in Figure 10c), or electric motors [196] for actuation. Similar to MEMS, such mechanical elements often function as switches with discrete states [195]. One major advantage is their simplified biasing networks compared to semiconductor-based solutions. Nonetheless, these systems face inherent limitations due to the physical response times of mechanical actuators and electromagnetic mechanisms [193; 194; 196]. Additionally, the incorporation of larger mechanical components, as demonstrated in [194; 196], can lead to bulky structures, which restrict their integration in compact base stations or large-scale RIS implementations. Ongoing and future research aims to overcome these constraints and enhance the practicality and scalability of mechanically reconfigurable RF systems. As noted, mechanical RF components currently deliver exceptional performance and reliability. However, they are often bulky, expensive, and not easily scalable despite outperforming semiconductor-based switches in terms of RF characteristics due to reduced leakage and parasitic effects at mmWave frequencies [200]. While RF-MEMS offer superior RF performance, they typically require high DC actuation voltages and specialized packaging, which limits their integration into compact systems. To address the growing demand for miniaturized mmWave devices, particularly for handheld and IoT applications, phase change materials (PCMs) have emerged as a promising alternative for RF switching [201; 202]. PCMs can exhibit resistance changes spanning five orders of magnitude when subjected to nanosecond-scale voltage pulses, making them ideal for low-loss, high-speed switching at microwave and mmWave frequencies [201]. The two most widely studied PCMs are vanadium dioxide (VO\({}_{2}\)) and germanium telluride (GeTe) [202]. Their operation relies on reversible changes in the crystal structure of chalcogenide materials between amorphous (high-resistivity) and crystalline (low-resistivity) states, typically triggered by thermal pulses. Figure 10: Examples of reconfigurable devices with (**a**) PIN diodes at Jerusalem cross unit cell [148], (**b**) MEMS as mm-waves switches [158], (**c**) an electromagnet-controlled element with varying height for RIS [194], (**d**) a PCM RF switch using coplanar waveguide technology [197], (**e**) a liquid metal double-layer FSS [198], and (**f**) a liquid crystal-based antenna [199]. This switching mechanism mirrors that of conventional electronic switches, enabling the development of fundamental PCM-based RF components such as SPDT, SP4T, and SP8T switches [203; 204; 205]. An optical image of a PCM-based RF SPDT switch fabricated on coplanar waveguide technology is shown in Figure 10d.