2021 8th NAFOSTED Conference on Information and Computer Science (NICS) Advanced Terahertz Devices and Systems Toward 6G and Beyond Masayuki Fujita Graduate School of Engineering Science Osaka University Toyonaka, Japan fujita@ee.es.osaka-u.ac.jp Abstract—Recent progress in terahertz devices and systems based on terahertz silicon photonics with resonant tunneling diodes for next-generation information communication technology, 6G and beyond, is reviewed Index Terms—6G, imaging, information communication technology, integration, photonic crystal, ranging, resonant tunneling diode, sensing, silicon photonics, terahertz, wireless communications I INTRODUCTION A wide untapped region exists between radio waves and light in the electromagnetic spectrum: terahertz (THz) waves with frequencies of approximately 100 GHz to 10 THz THz frequencies combine the penetration of radio waves and the large bandwidth of light, which make them excellent candidates for next-generation information communication technology such as ultra-broadband wireless communications, spectroscopic sensing, nondestructive imaging, and high-resolution ranging [1-9] Since 1979, commercial wireless communication systems have been evolving into a new generation every around 10 years With each advance in generation, the data rate, which is an important performance indicator for communication systems, also increased In the fourthgeneration system, the maximum data rate reached over Gbit/s In 2020, the fifth-generation (5G) system was launched, and it is expected that a data rate of over 10 Gbit/s will be developed in futuristic 5G systems [10] At present, the development of next-generation communication systems beyond 5G or 6G toward Society 5.0 has been actively discussed [10-12] In general, according to the Shannon– Hartley theorem, the utilization of a higher frequency can provide a broader bandwidth of the system and lead to a higher data rate [13] The data rate of the 5G system approaches the theoretical Shannon limit in conventional GHz bands Therefore, THz communications with frequencies over 100 GHz are expected to achieve data rates of over 100 Gbit/s for the 6G system Such a very highcapacity system could realize extremely low-latency wireless communications for advanced remote control applications such as automated driving, autonomous operation, drones, and robotics, which can be remotely controlled by artificial intelligence (AI) systems (Fig 1) Such 6G systems will allow wireless transmission of uncompressed ultrahigh definition (UHD) 8K video or holographic display This will improve the quality of cybernetic avatars, distant education and inspection, e-sports, telemedicine, telework, and devices for augmented reality (AR) and virtual reality (VR) Moreover, this will lead to the advancement of cyber–physical fusion systems (CPSs) by utilizing the big data of UHD video and Internet of Things 978-1-6654-1001-4/21/$31.00 ©2021 IEEE (IoT) sensors In addition to data transmission, sensing functions using electromagnetic waves, such as the positioning of terminals, detection of terminal users, and their surrounding objects will play an important role in the realization of CPS To this end, communications and sensing functions converge in the 6G system THz waves could enhance the precision and resolution of sensing owing to their shorter wavelength nature compared with microwaves for the 6G system and beyond Despite the promise of THz waves for 6G applications, THz frequencies are at the upper limit of the capabilities of conventional electronics, and the development of THz devices and systems is a challenging field of interdisciplinary research [1-9] Specifically, it is difficult to generate a significant amount of power from THz sources At present, most THz systems employ inefficient frequency conversion sources using optical-to-THz converters (photomixing) [5] or frequency multipliers from microwave to THz waves [7] Such THz-wave generators are complex, cumbersome, large, and require high power consumption Therefore, THz devices must be as efficient as possible to conserve limited power for 6G evolution and beyond Resonant tunneling diodes (RTDs) are a major candidate for both THz transmitters and receivers because of their simple and low-power electronic devices [14-17] Furthermore, a low-loss platform for integrating THz devices is essential for various practical applications However, the propagation loss of transmission lines based on conventional electronics is high in the THz region, mainly owing to the high ohmic loss in metals [18-23] Thus, an alternative metal-free integrated platform is necessary for manipulating THz waves In this paper, we review the recent progress in the field of “THz silicon photonics” based on RTDs and metalfree silicon photonic crystals for THz applications II RESONANT TUNNELING DIODES AND PHOTONIC CRYSTALS FOR ADVANCED TERAHERTZ APPLICATIONS Compound-semiconductor-based RTDs are potential THz-active devices because of their capabilities in fundamental THz oscillation without frequency conversion and highly sensitive THz-wave detection [14-17] In contrast to normal electronics, there is a specific resonant voltage in such devices that yields peak current owing to the quantum effect Thus, there exists a region in which the current decreases with increasing voltage, which is called the negative differential conductance (NDC) region This nonlinear behavior makes it possible to generate the fundamental oscillation in the THz band at room temperature and to synchronize the received THz signals with the oscillation in the device as a coherent detector [24] A compact and highly sensitive wireless communication 348 2021 8th NAFOSTED Conference on Information and Computer Science (NICS) system using coherent detection has been developed This system displayed its ability to generate error-free transmission of 30 Gbit/s using all-electronic systems without error correction [24] An uncompressed 8K video (equivalent to 48 Gbit/s) was successfully transmitted wirelessly using THz waves based on two-channel THz transmitters using an ultrafast photodiode and sensitive THz coherent receivers using RTDs [25, 26] Recently, amplified detection of THz waves was made possible in RTDs [27], which could further enhance the data rate and transmission distance Additionally, the feature of the self-oscillating mixer in RTD demonstrates ranging and imaging applications in a system composed of a single RTD device [8] Suzuki for their fruitful continuous collaborations The author would like to express his sincere thanks to the collaborators at the University of Adelaide, including Prof Withawat Withayachumnankul, at Nanyang Technological University, including Prof Ranjan Singh, and at Université Lille, including Prof Ducournau Guillaume, for their support and collaboration This work was supported in part by CREST, JST (JPMJCR1534, JPMJCR21C4), KAKENHI, Japan (20H00249), and is the commissioned research by National Institute of Information and Communications Technology (NICT), Japan (03001) Photonic crystals are metal-free dielectric microstructures with periodic refractive index distributions on scales comparable to wavelengths [28-32] Relevant photonic crystal designs exhibit a photonic bandgap (PBG) where optical modes cannot exist, making it possible to produce compact and low-loss components using the PBG effect [33] Various advanced THz devices based on silicon photonic crystals and microstructures [34-36] have been developed, including ultralow-loss waveguides [37-40], compact frequency selectors (diplexers or (de)multiplexers) [41-43], compact [44] and broadband antennas [45, 46], and high-Q cavities [47] Another advance is an efficient interface for a THz hollow fiber developed through the integration of a tapered coupling structure and a photonic crystal waveguide [48] Analogous to the topological phase of matter, the topological phase of a THz wave is achieved using a photonic crystal platform as a robust waveguide [49, 50] To overcome the problem of the small operating bandwidth of a photonic crystal, effective medium cladding [51, 52] and unclad silicon THz waveguides [53] were developed Active functions, including THz-signal generation, detection, frequency conversion, and modulation, can be integrated with the THz silicon photonics platform (Fig 2) An efficient mode converter to the deepsubwavelength region was developed using a photonic crystal waveguide for integrating microscopic-THz electronic active devices, including RTDs [54-56] Such THz silicon photonics technology can miniaturize advanced communication systems [56], spectroscopic sensing systems [47], and non-destructive imaging systems [8] [2] REFERENCES [1] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] III CONCLUSION THz silicon photonics technology based on RTDs and metal-free silicon integrated platforms for manipulating THz waves can go a long way toward 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Koala, J Kim, M Fujita, and T Nagatsuma, “Hybrid integration between resonant tunneling diodes and unclad microphotonic diplexer for dual-channel coherent terahertz receiver,” IEEE J Sel Top Quant Electron., (Early Access) https://doi.org/10.1109/JSTQE.2021.3130813 2021 8th NAFOSTED Conference on Information and Computer Science (NICS) Fig Concept map of 6G and beyond with various applications toward Society 5.0 Fig Fusion of silicon photonic crystal-related technologies and RTD explores the field of “Terahertz Silicon Photonics” 351 ... active devices, including RTDs [54-56] Such THz silicon photonics technology can miniaturize advanced communication systems [56], spectroscopic sensing systems [47], and non-destructive imaging systems. .. RTDs and metal-free silicon integrated platforms for manipulating THz waves can go a long way toward achieving 6G and beyond ACKNOWLEDGMENTS The author would like to sincerely thank the former and. .. Information and Computer Science (NICS) Fig Concept map of 6G and beyond with various applications toward Society 5.0 Fig Fusion of silicon photonic crystal-related technologies and RTD explores