Chalcogenide phase-change material advances programmable terahertz metamaterials: a non-volatile perspective for reconfigurable intelligent surfaces

1 Introduction

As the global deployment of 5G technology advances, the telecommunications sector is steadily transitioning to 6G [1], [2], which is expected to dramatically enhance the key performances, such as data rates, energy efficiency, latency, and network density. This cutting-edge technology stands out for its promise of ultra-high data transmission rate up to several hundred Gbps and requires 10–100 times better energy efficiency than 5G [3]. It aims to harness a broader spectrum, focusing on terahertz (THz) frequencies, to realize unprecedented data transmission speeds and interconnected intelligence [4], [5]. THz waves, ranging from 0.1 to 10 THz, can offer expansive bandwidth and inherent robustness against interference, which is pivotal for achieving terabit-level data speeds in 6G networks [6]. However, integrating these technologies poses substantial challenges, particularly the high cost and power consumption associated with adapting multiple input multiple output (MIMO) systems from 5G to 6G, which could significantly escalate the price for each gigabit data. This highlights the urgent need for novel solutions that are both cost-effective and energy-efficient.

Reconfigurable intelligent surface (RIS) represents a promising technology with tremendous potential to address these challenges. By using digitally programmable metamaterials (MTMs), RIS dynamically controls electromagnetic waves, facilitating a versatile and manageable wireless transmission environment [7]. This innovative approach effectively reduces the hardware and financial demands of the traditional transmitters and receivers. Currently, RIS has been frequently investigated with positive-intrinsic-negative (PIN) and varactor diodes, however, which typically work well below 15 GHz. In addition, multilayer circuits are desired to bias these diodes individually [8]. Recently, optically controlled diodes, eliminating the need for multilayer circuits, have also demonstrated promising potential in RIS applications [9].

To enable THz RIS, several active control materials have been investigated, including graphene, liquid crystals, and phase-change materials [10]. Ge₂Sb₂Te₅ (GST), a chalcogenide phase-change material, is notable for its vast conductivity and permittivity variation and picosecond switching speed between amorphous and crystalline states [11]–[13], as shown in Figure 1(a) and (b). A comparison of the modulation performances for THz MTMs with graphene, and vanadium dioxide (VO₂), is methodically expounded in Table. 1 [14]–[16], which highlights the advancement of GST in terms of modulation deep and speed. Furthermore, its non-volatile characteristics show significant potential for developing advanced THz RIS systems with integrated memory functions and low power consumption.

Figure 1:

Active modulation of THz wave with phase-change GST films. (a) Phase states change with thermal, electrical, and optical excitations and the corresponding THz responses. (b) Measured and simulated transmission of a GST film on a p-type silicon substrate at amorphous and crystalline states. The film thickness is 300 nm. (c) Schematic diagram of GST-based THz RIS.

This work briefly scrutinizes the recent advancements of active THz MTMs with GST in the last three years, spotlights the external stimuli facilitating GST phase transitions, discusses their constraints and unsolved problems, and forecasts a progressive trajectory toward the non-volatile, multifunctional GST-based THz RIS in the rising 6G communication systems.

2 Active THz MTMs with GST

GST was initially proposed for actively modulating THz MTMs in 2019 [16], as illustrated in Figure 2(a). In this configuration, GST films, sputter-deposited onto quartz substrates, enable non-volatile control of Fano and dipole resonant modes in split-ring resonators (SRRs), achieving spatial and temporal selectivity. Various phase-change approaches have been investigated in this work, including thermal annealing, current pulse, and optical pumping. Since then, GST has attracted considerable interest in the active MTMs at THz frequencies.

Figure 2:

Active THz MTMs with GST. (a) Active modulation of THz SRRs with GST film [16]. (b) Thermally switchable bi-functional metasurface [17]. (c) Electrically controlled multilevel modulation [18]. (d) EIT metasurface [19]. (a) Reproduced with permission [16]. Copyright 2019, Wiley-VCH. (b) Reproduced with permission [17]. Copyright 2022, Wiley-VCH. (c) Reproduced with permission [18]. Copyright 2022, American Chemical Society. (d) Reproduced with permission [19]. Copyright 2022, Wiley-VCH.

3 Programmable THz MTMs with GST for RIS

In 2022, programmable THz metasurfaces with GeTe films were reported [25], which highlight the versatility and potential of chalcogenide phase change materials in digital THz wave manipulation, i.e., a key capability of RIS. In 2023, theoretical simulations of GST-based digital surfaces were reported, demonstrating the potential of GST for precisely controlling beam direction and modulation amplitude [26]. The proposed models and programmable concepts contribute to advance the GST-based RIS implementation in the future 6G communications.

4 Conclusion and prospective

The progression towards 6G MIMO communication heralds a significant shift in wireless network capabilities, emphasizing ultra-fast, reliable, and energy-efficient operations. RIS utilizes digitally controlled MTMs to dynamically manipulate electromagnetic waves, achieving a more flexible and controllable wireless transmission environment. However, current RIS systems are typically designed with PIN and varactor diodes with commercial packaging, which cannot directly mitigate to THz frequencies due to their package size and cut-off frequencies. Their beam switching time is expected to be below 500 ns for most application scenarios. With the benefit of non-volatile phase transition properties, rapid state-switching capabilities, and multi-level modulation, GST shows significant advantages over the other materials in the THz spectrum. The incorporation of GST into RIS is instrumental in overcoming the cost and energy problems in the advanced THz communication systems. However, several challenges warrant careful scrutiny in the thermal, electrical, and optical approaches. Thermal annealing exhibits intrinsic limits in phase transition rate and modulation speed. Electrical methods are pivotal for compact circuit integration and multilevel control, however, which may suffer from filamentation and heating uniformity. Optical pumping offers rapid phase transition in the focal spot. However, the laser sources are bulky, complex, and expensive for practical applications.

Despite the aforementioned challenges, GST-tuned THz MTMs show great promise in the MIMO systems of the 6G communication due to their large modulation range, high switching speed, and non-volatile and continuous modulation. Compared to the other active stimuli at THz frequencies, such as VO₂ (it requires 1.125 W of sustained electrical power or a switching optical power of 5 mJ/cm² for ultrafast phase changes within 30 ps [15]), GST stands out with its much lower switching power, i.e., 0.6366 mJ/cm² for effective non-volatile phase changes within 19 ps [16]. Micro-heater and current pulses could be directly integrated with field programmable gate arrays (FPGAs), which could be functional digitally and programmably, as shown in Figure 1(c). In this case, GST may enable THz RIS as the substitute for the classic PIN diodes at microwave frequency. Furthermore, optically addressed GST RIS can be manipulated with laser beams or fiber arrays instead of complicated multilayer bias networks. Most notably, the non-volatile GST approach addresses low power consumption, which is one of the key constraints for 6G commercialization. Further research directions may include hybrid modulation strategies with thermal, electrical, and optical methods, dynamic beamforming with memories, and ultra-high-speed THz communication with GST RIS.