Near-field beamfocusing with extremely large aperture arrays can effectively enhance physical layer security. Nevertheless, even small estimation errors of the eavesdropper's location may cause a pronounced focal shift, resulting in a severe degradation of the secrecy rate. In this letter, we propose a physics-informed robust beamforming strategy that leverages the electromagnetic (EM) caustic effect for near-field physical layer security provisioning, which can be implemented via phase shifts only. Specifically, we partition the transmit array into caustic and focusing subarrays to simultaneously bypass the potential eavesdropping region and illuminate the legitimate user, thereby significantly improving the robustness against the localization error of eavesdroppers. Moreover, by leveraging the connection between the phase gradient and the EM wave departing angle, we derive the corresponding piece-wise closed-form array phase profile for the subarrays. Simulation results demonstrate that the proposed scheme achieves up to an 80% reduction of the worst-case eavesdropping rate for a localization error of 0.25 m, highlighting its superiority for providing robust and secure communication.

翻译：通过超大孔径阵列实现近场波束聚焦可有效增强物理层安全性。然而，窃听者位置估计的微小误差可能引发显著的焦点偏移，导致保密速率严重恶化。本文提出一种基于物理感知的鲁棒波束成形策略，利用电磁焦散效应对近场物理层安全进行保障，该方案仅需相位调控即可实现。具体而言，我们将发射阵列划分为焦散子阵与聚焦子阵，从而同步绕过潜在窃听区域并照亮合法用户，显著提升对窃听者定位误差的鲁棒性。此外，通过建立相位梯度与电磁波出射角之间的关联，推导出子阵对应的分段闭式阵列相位分布。仿真结果表明，针对0.25米的定位误差，所提方案可实现最恶劣情况下窃听率降低80%的效果，充分展现了其在提供鲁棒安全通信方面的优越性。