Physical-layer security uses the physics of signal propagation to prevent eavesdropping — shaping the electromagnetic field so that a legitimate receiver gets a strong signal while an eavesdropper gets a weak one. Near-field beamfocusing with large antenna arrays can concentrate energy at precise locations. But security depends on knowing where the eavesdropper is, and even small errors in that estimate cause the focused null — the quiet zone meant to exclude the eavesdropper — to shift, destroying the security guarantee.
Liu, Yu, and Schober (arXiv:2603.24077) exploit a different physical effect: electromagnetic caustics. A caustic is a curve of concentrated field intensity created when many rays converge tangentially — the bright curves at the bottom of a swimming pool, the focused light inside a coffee cup. By partitioning the transmit array into two subarrays with different phase profiles, the authors create a curved caustic beam that routes around the eavesdropper's estimated location while focusing on the legitimate user.
The caustic approach achieves roughly 80% reduction in worst-case eavesdropping rate even when the eavesdropper's position is known only to within 0.25 meters. The robustness comes from geometry: a caustic creates a extended region of field suppression along its curve, not a point null that shifts with estimation error. Mislocating the eavesdropper by a fraction of a wavelength moves the null off target in conventional beamforming. The caustic's suppression region is broad enough to absorb the error.
The design requires only phase shifts — no amplitude control, which simplifies hardware implementation. The closed-form solution for the phase profile comes directly from the connection between phase gradient and wave departure angle, making the caustic shape analytically controllable.
The structural lesson is that security degrades gracefully when the suppression mechanism is geometric (a curve) rather than focal (a point). Focal security is precise but brittle — exact when perfectly aimed, useless when slightly off. Caustic security trades peak suppression for robustness, using the physics of ray convergence to create a broad exclusion zone. The imprecise shield outperforms the precise one.