Terahertz band (0.1 to 10 THz) with high carrier frequency and large available bandwidth has become a promising candidate to meet the 100 Gbit/s or even 1 Tbit/s data rate required by the future six-generation (6G) mobile communication networks. Compared with the all-electrical methods to generate terahertz signals, the photon-assisted technology can break the bottleneck of the bandwidth limit of the electronics devices, and generate the terahertz signal with high frequency, large bandwidth, flexible tunability and easy integration with the large capacity fiber link. In this paper, we introduce the typical methods to generate terahertz signals based on the photon-assisted technology, and review the representative achievements in different areas of terahertz communication, such as the large capacity terahertz transmission, the long distance terahertz transmission, the real-time terahertz communication, and the integrated terahertz sensing and communication. Based on the photon-assisted technology and various key techniques, devices and advanced digital signal processing (DSP) algorithms, we have obtained many great achievements in broadband terahertz communication and sensing, and the experimental setups and results have also been demonstrated in detail.

From the perspective of wireless transmission distance in the terahertz-band (THz-band), the 10-100 m, 100-1000 m, and ≥1000 m ranges are recommended for indoor communications, medium-distance communications, and long-distance communications, respectively. However, most existing photonic terahertz communication demonstrations reported in previous studies have a distance of less than 100 m, with some being even shorter at 10 m or 1 m. In this study, by combining high-gain and high-sensitivity terahertz modules, as well as signal processing technology, we have successfully demonstrated transmission over 850 m with a net rate of 50 Gbit/s at a 320 GHz carrier frequency through outdoor field experimental verification. This particular demonstration represents the most significant distance-rate product, as well as the longest wireless distance achieved so far for photonic terahertz wireless communication. The results demonstrate that the integration of terahertz photonics, electronics, and cutting-edge DSP algorithms, has the potential to enable long-distance and high-speed wireless communication for frequencies ≥300 GHz. Our work represents another significant stride towards achieving our goal of realizing the world's first photonic terahertz-wave communication over kilometer-level distances.

To meet the upcoming 6G communications challenges, terahertz-wave (THz-wave) wireless data transmission over 100 Gb/s is urgently needed in the short term. However, advanced digital signal processing (DSP) technologies suitable for millimeter-wave (mm-wave) bands cannot satisfy the requirements; increasing the carrier frequencies for more spectra is essential. Photonics-aided THz-wave generation (PTG) technology has become a valid way to solve the bandwidth bottleneck. New progress has been made in PTG by using optical frequency combs and optical modulators. The application of DSP technology for heterodyne detection can not only overcome the nonlinear effect in photoelectric devices but also enhance the receiver sensitivity and spectral efficiency. Multidimensional multiplexing technology can reduce the baud rate of each subchannel to realize the long-distance transmission of THz-wave signals. In this tutorial, we introduce the key technologies and principles of ultrahigh speed and large capacity THz-wave signal transmission. These technologies can effectively improve the transmission capacity and distance and reduce the bandwidth required by optical and electronic equipment.

Terahertz (THz) photonics technique can break through the bandwidth limitation of electronic mixing method and be seamless integrated with optical access networks, which has bright prospects in future 6G broadband communication. However, due to the bottleneck of photoelectric conversion efficiency of uni-travelling photodiode (UTC-PD), the power of the generated THz-wave signal is usually only in the microwatt level, and the THz-wave wireless transmission distance is short. In this paper, we summarize the key techniques we use to extend the wireless range for photonics-aided THz-wave communication, which includes THz-wave amplifiers and antennas, heterodyne detection, probabilistic shaping (PS) technique as well as advanced digital signal processing (DSP). Meanwhile, we report our recent research including 104\200\400-m photonics-aided THz-wave wireless transmission records.

This paper proposed a plasmonic resonator system, consisting of a metal-insulator-metal structure and two stubs, and a Fano resonance arose in its transmittance, which resulted from the coupling between the two stubs. On the basis of the proposed structure, a circle and a ring cavity are separately added above the stubs to create different coupled plasmonic structures, providing triple and quadruple Fano resonances, respectively. Additionally, by adjusting the geometric parameters of the system, multiple Fano Resonances obtained can be tuned. The proposed structure can be served as a high efficient refractive index sensor, yielding a sensitivity of 2000 nm/RIU and figure of merit (FOM) of $4.05\times {10}^4$ and performing better than most of the similar structures. It is believed that the proposed structure may support substantial applications for on-chip sensors, slow light and nonlinear devices in highly integrated photonic circuits.

Photon-assisted generation of terahertz (THz) signals can support larger modulation bandwidths in future ultrahigh-speed wireless communication systems. Single-input–multiple-output (SIMO) or multiple-input–multiple-output (MIMO) technology effectively uses the spatial dimension to improve the quality of the received signal, such as signal-to-noise ratio (SNR) and bit error rate (BER) ratio. Therefore, we experimentally demonstrate $D$ -band photon-assisted THz 1 $\times$ 2 SIMO 4.6-km wireless communication. We use a photon-assisted THz technology to generate a 124.37-GHz $D$ -band signal, which is received according to the maximum ratio combining (MRC) technology after 1 $\times$ 2 SIMO wireless transmission and uses fully blind digital signal processing (DSP) for carrier recovery algorithm without any training sequence. According to the experimental results, the maximum SNR diversity gain between 1 $\times$ 1 and 1 $\times$ 2 SIMO can be up to 2.8 dB. After the MRC algorithm, the BER can be reduced by one order of magnitude, which is less than 15% of soft decision forward error correction (SD-FEC). This is the first experimental demonstration of high gain and long range in a $D$ -band THz 1 $\times$ 2 SIMO system with a line rate of 43.5 Gb/s. These results will provide important assistance to our future efforts in photon-assisted THz wireless communications.