  In the rapidly advancing landscape of 6G, characterized by ultra-high-speed
wideband transmission in millimeter-wave and terahertz bands, our paper
addresses the pivotal task of enhancing physical layer security (PLS) within
near-field wideband communications. We introduce true-time delayer
(TTD)-incorporated analog beamfocusing techniques designed to address the
interplay between near-field propagation and wideband beamsplit, an uncharted
domain in existing literature. Our approach to maximizing secrecy rates
involves formulating an optimization problem for joint power allocation and
analog beamformer design, employing a two-stage process encompassing a
semi-digital solution and analog approximation. This problem is efficiently
solved through a combination of alternating optimization, fractional
programming, and block successive upper-bound minimization techniques.
Additionally, we present a low-complexity beamsplit-aware beamfocusing
strategy, capitalizing on geometric insights from near-field wideband
propagation, which can also serve as a robust initial value for the
optimization-based approach. Numerical results substantiate the efficacy of the
proposed methods, clearly demonstrating their superiority over TTD-free
approaches in fortifying wideband PLS, as well as the advantageous secrecy
energy efficiency achieved by leveraging low-cost analog devices.
