Space-Time Metasurface Antenna Enabled High-Dimensional Electromagnetic Modulation for Intrinsic Physical Layer Security.

Physical-layer security (PLS) plays a critical role in modern wireless communication systems by exploiting the fundamental properties of the physical medium to safeguard data transmission. Classical spread-spectrum techniques enhance PLS through frequency-bandwidth expansion, yet they typically exploit only a single electromagnetic (EM) degree of freedom for information mapping. Moreover, the separation between information mapping and spectrum spreading processes further constrains their spectral efficiency, system flexibility, and intrinsic security. Here, we introduce a high-dimensional EM modulation paradigm for PLS communications enabled by a compact 1-bit waveguide-integrated space-time metasurface antenna (STMA). By combining direct modulation of the +1 harmonic with controllable phase, frequency, and pseudorandom modulation with adjustable time-delay, the STMA functions as an active information-processing platform that embeds information directly into the physical radiation process. In contrast to conventional spread-spectrum techniques, security enhancement in this design arises from expansion of the effective EM state space rather than bandwidth inflation alone, leading to improved spectral efficiency and resilience against both jamming and eavesdropping. These findings establish EM dimensionality engineering as a viable pathway toward secure and spectrally efficient wireless systems.