# A Perspective on Terahertz Next-Generation Wireless Communications
## 3 Terahertz Devices  
The terahertz research world has commonly referred to the \"THz-gap\", when discussing the relative absence of technologies suitable for commercializing terahertz wave devices and systems, as compared to the neighboring microwave and photonic bands. Researchers continue to search for ways to fill this THz-gap, ranging from new materials studies (e.g., artificial materials [39; 40], metamaterials [41; 42; 43; 44; 45]), to new photonic and electronic devices for generating, detecting, and manipulating terahertz waves. From the perspective of next-generation terahertz communications, it currently appears that purely photonic sources and detectors will _not_ be the terahertz technology of choice. To justify this statement, we describe some modern photonic-based terahertz sources. These include quantum cascade lasers (QCL) [46], nonlinear optical-mixing sources [47; 48], and ultrafast laser-driven pulsed sources and detectors [49; 48]. The form factors and operational principles of these vary so widely that one-to-one comparisons in performance are very challenging. For example, terahertz QCLs can achieve admirable output powers (>100 mW [46]), but only at cryogenic temperatures, whereas mid-infrared QCLs, used in conjunction with nonlinear crystals can make microwatts of tunable continuous wave (CW) terahertz (1-5 THz) at room temperature [46]. On the other hand, pulsed nonlinear-mixing sources are completely different, having no CW mode at all, and thus no tunability, and rely on very high optical intensities to generate terahertz pulse streams with low average power (microwatts) [48]. All of these sources offer impressive performance in their own ways, but none so far are easily integrated into larger digital electronic systems, which is arguably their biggest downfall for communication systems. In most cases, they suffer from other debilitating physical requirements for operation. For example, ultrafast pulsed sources are generally far too bulky (> 5000 cm\({}^{3}\)) or inefficient (< 10\({}^{-5}\) efficiency) to implement in mobile devices and QCLs require cryogenic cooling or complex optical tuning schemes. One alternative is electronic-based terahertz sources, detectors, and mixers. Typical communication systems are composed of a front-end and a baseband circuit. Front-end circuits are responsible for up- and down-conversion of baseband signals to and from the carrier frequency. Front-end research therefore address both terahertz sources and detectors. To accomplish the mission of achieving multiple fold improvements in data-rates, terahertz communication systems will require both wideband front-ends at the carrier frequency and high-speed baseband circuits. Before beginning a discussion of these technologies we present Table 1 that summarizes some of the forefront technologies and their performance metrics. Complementary metal-oxide-semiconductor (CMOS) circuits are increasingly becoming a legitimate and competitive approach for implementing terahertz communication technologies up to about 300 GHz [61; 62; 63; 64], with the potential benefits of low cost and easy integrability into modern digital electronics. This is particularly true for mobile devices. One metric of interest in next-generation wireless is efficiency, an obvious necessity for battery-driven, mobile devices. Although evaluating the energy efficiency of wireless systems is very complicated, one can gauge the achievable energy efficiency in baseband circuits by examining that of wireline communication links. As of 2018, wireline communication transceiver efficiency is reaching to about 1 pJ/bit [65] while supporting about 100 Gbps data rate. Note that we adopt the Joule per bit definition of efficiency, since this is the most fundamental figure of merit for digital communication systems and is comparable in diverse system types. This indicates that it should be possible to implement high-speed baseband circuits for terahertz communications with reasonable energy efficiency.