Dynamic terahertz focal planar scanning via cascaded metasurfaces

Abstract

Terahertz (THz) planar focal spot scanning is critical for imaging, sensing, and optical routing, but traditional approaches suffer from bulkiness or limited tunability. Herein, we propose a cascaded metasurface device with two mechanically rotatable dielectric layers (M1 and M2) to achieve dynamic focal scanning. The core mechanism relies on rotation-induced phase superposition: tuning angles α1 and α2 modulates the combined phase distribution, enabling continuous in-plane focal displacement. Simulations demonstrate controllable scanning under M2 rotation (α1 = 0°, α2: −75° to 75°) and opposite co-rotation (α1 = −α2: −60° to 60°), with stable focal length and efficiency. Experimental characterization via a THz near-field system confirms close agreement between simulated and measured scanning trajectories, verifying the device's predictability and reliability. This passive scheme offers simplicity, large tuning range, and broadband compatibility, avoiding external energy supplies. The proposed cascaded platform enriches THz optical manipulation tools and facilitates integration into dynamic imaging, sensing, and beam steering systems.