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This Review covers the state-of-the-art technologies on photonics-based terahertz communications, which are compared with competing technologies based on electronics and free-space optical communications. Future prospects and challenges are also discussed. Almost 15 years have passed since the initial demonstrations of terahertz (THz) wireless communications were made using both pulsed and continuous waves. THz technologies are attracting great interest and are expected to meet the ever-increasing demand for high-capacity wireless communications. Here, we review the latest trends in THz communications research, focusing on how photonics technologies have played a key role in the development of first-age THz communication systems. We also provide a comparison with other competitive technologies, such as THz transceivers enabled by electronic devices as well as free-space lightwave communications.

The realization of integrated, low-cost and efficient solutions for high-speed, on-chip communication requires terahertz-frequency waveguides and has great potential for information and communication technologies, including sixth-generation (6G) wireless communication, terahertz integrated circuits, and interconnects for intrachip and interchip communication. However, conventional approaches to terahertz waveguiding suffer from sensitivity to defects and sharp bends. Here, building on the topological phase of light, we experimentally demonstrate robust terahertz topological valley transport through several sharp bends on the all-silicon chip. The valley kink states are excellent information carriers owing to their robustness, single-mode propagation and linear dispersion. By leveraging such states, we demonstrate error-free communication through a highly twisted domain wall at an unprecedented data transfer rate (exceeding ten gigabits per second) that enables real-time transmission of uncompressed 4K high-definition video (that is, with a horizontal display resolution of approximately 4,000 pixels). Terahertz communication with topological devices opens a route towards terabit-per-second datalinks that could enable artificial intelligence and cloud-based technologies, including autonomous driving, healthcare, precision manufacturing and holographic communication.Robust terahertz wave transport is demonstrated on a silicon chip using the valley Hall topological phase. Error-free communication is achieved at a data rate of 11 Gbit s−1, enabling real-time transmission of uncompressed 4K high-definition video.

Optical frequency division via optical frequency combs has enabled a leap in microwave metrology, leading to noise performance never explored before. Extending this method to the millimetre-wave and terahertz-wave domains is of great interest. Dissipative Kerr solitons in integrated photonic chips offer the unique feature of delivering optical frequency combs with ultrahigh repetition rates from 10 GHz to 1 THz, making them relevant gears for performing optical frequency division in the millimetre-wave and terahertz-wave domains. We experimentally demonstrate the optical frequency division of an optically carried 3.6 THz reference down to 300 GHz through a dissipative Kerr soliton, photodetected with an ultrafast uni-travelling-carrier photodiode. A new measurement system, based on the characterization of a microwave reference phase locked to the 300 GHz signal under test, yields attosecond-level timing-noise sensitivity, overcoming conventional technical limitations. This work places dissipative Kerr solitons as a leading technology in the millimetre-wave and terahertz-wave field, promising breakthroughs in fundamental and civilian applications.A 300 GHz signal is generated by the combination of a low-noise stimulated Brillouin scattering process, dissipative Kerr soliton comb and optical-to-electrical conversion. A phase noise of −100 dBc Hz−1 is achieved at a Fourier frequency of 10 kHz.

The increasing demand of unoccupied and unregulated bandwidth for wireless communication systems will inevitably lead to the extension of operation frequencies toward the lower THz frequency range. Higher carrier frequencies will allow for fast transmission of huge amounts of data as needed for new emerging applications. Despite the tremendous hurdles that have to be overcome with regard to sources and detectors, circuit and antenna technology and system architecture to realize ultrafast data transmission in a scenario with extensive transmission loss, a new area of research is beginning to form. In this article we give an overview of emerging technologies and system research that might lead to ubiquitous THz communication systems in the future.

With many potential applications in mind, great effort is being applied to develop a terahertz-wave technology platform on which waves can be manipulated with sufficient confinement and efficient interaction for the development of smart components. Here, we utilize the in-plane resonance of a thin, planar photonic-crystal slab with negligible absorption loss to successfully demonstrate and visualize terahertz-wave trapping. We artificially introduce free carriers, which interact with the trapped waves, and capture them in the slab by absorption. Our system exhibits an experimental absorptivity (interaction efficiency) of ∼99% and a broad bandwidth (absorptivity of ≥90%) that covers 17% of the centre frequency. We also demonstrate its application to the stabilization of terahertz wireless communication systems. Our study shows the capability of photonic crystals as a terahertz-wave platform, the application of which may be extended to other components including filters, couplers, antennas, detectors, modulators, switches and emitters. Trapping of a terahertz wave in a photonic-crystal slab and subsequent ‘capture’ through absorption are demonstrated. Over 90% of the wave lying within 17% of the centre frequency is absorbed. Application to the stabilization of terahertz wireless communication systems is shown.

We report for the first time the successful wavelength stabilization of two hybrid integrated InP/Polymer DBR lasers through optical injection. The two InP/Polymer DBR lasers are integrated into a photonic integrated circuit, providing an ideal source for millimeter and Terahertz wave generation by optical heterodyne technique. These lasers offer the widest tuning range of the carrier wave demonstrated to date up into the Terahertz range, about 20 nm (2.5 THz) on a single photonic integrated circuit. We demonstrate the application of this source to generate a carrier wave at 330 GHz to establish a wireless data transmission link at a data rate up to 18 Gbit/s. Using a coherent detection scheme we increase the sensitivity by more than 10 dB over direct detection.

In this paper, we report on a terahertz (THz) sensing system. Compared to previously reported systems, it has increased system sensitivity and reduced size. Both are achieved by using a photonic crystal (PC) cavity as a resonator and compact resonant tunneling diodes (RTDs) as signal source and as detector. The measured quality factor of the PC cavity is higher than 10,000, and its resonant frequency is 318 GHz. To demonstrate the operation of the refractive index sensing system, dielectric tapes of various thicknesses are attached to the PC cavity and the change in the resonator’s refractive index is measured. The figure of merit of refractive index sensing using the developed system is one order higher than that of previous studies, which used metallic metamaterial resonators. The frequency of the RTD-based source can be swept from 316 to 321 GHz by varying the RTD direct current voltage. This effect is used to realize a compact frequency tunable signal source. Measurements using a commercial signal source and detector are carried out to verify the accuracy of the data obtained using RTDs as a signal source and as a detector.

The authors report on a terahertz band wireless communication experiment employing a resonant tunneling diode receiver (Rx) and uni‐travelling carrier photodiode transmitter. An intermediate‐frequency based transmission facilitates a low‐complexity envelope‐detection type Rx architecture for complex‐modulated signals. Forward error correction recoverable data‐rates using quadrature phase shift keying modulation and 16‐level quadrature amplitude modulation are 60 and 68 Gbit/s, respectively.  

Resonant tunnelling diode (RTD) oscillators used for high‐speed terahertz wireless communication systems have garnered significant attention in recent years. In these systems, RTD devices are directly on‐off modulated through baseband (BB) circuits, which limit the data rate. This letter investigates an efficient approach to increase the bandwidth of the BB circuits, by addressing the broadband connection between the two‐RTD oscillator chip and the radiofrequency circuit board. As a result, an error‐free wireless communication has been achieved with a data rate of 25 Gbit/s in the 300‐GHz band using the RTD transmitter and receiver.

Recent advances in silicon (Si) microphotonics have enabled novel devices for the terahertz (THz) range based on dielectric waveguides. In the past couple of years, dielectric waveguides have become commonplace for THz systems to mitigate issues in efficiency, size, and cost of integration and packaging using metal-based waveguides. Therefore, THz systems have progressively evolved from cumbersome collections of discreet components to THz-wave integrated circuits. This gradual transition of THz systems from numerous components to compact integrated circuits has been facilitated at each step by incredible advances in all-Si waveguides allowing low-loss, low dispersion, and single-mode waveguiding operation. As such, all-Si waveguides position themselves as highly efficient interconnects to realize THz integrated circuits and further large-scale integration in the THz range. This review article intends to reevaluate the evolution stages of THz integrated circuits and systems based on all-Si waveguides.