Physical Layer Security Techniques for Terahertz Communications Above 100 GHz: A Review

Terahertz (THz) communication above 100 GHz is widely recognized as a key enabler for sixth-generation (6G) networks because of its ultra-broad bandwidth and strong directionality. Meanwhile, the rapid evolution of artificial intelligence has considerably weakened conventional cryptographic methods at the network layer, making THz physical layer security increasingly critical. THz links are inherently susceptible to jamming and eavesdropping, which calls for dedicated security mechanisms that integrate physical structures with advanced signal processing. This review summarizes recent advances in two complementary directions. The first addresses signal domain strategies, including frequency hopping spread spectrum techniques, channel modeling, and artificial noise injection, to strengthen confidentiality and robustness against intentional interference. The second focuses on spatial domain strategies, where intelligent reflecting surfaces and beam steering architectures leverage topological diversity to reduce interception risks. This review also discusses the practical challenges these techniques may face in future 6G scenarios and identifies potential directions for further research.