Programmable Anisotropy in Low‐Symmetry van der Waals Crystals via Moiré and Symmetry Engineering

ABSTRACT Anisotropy underpins polarization‐sensitive optoelectronics, yet deterministic control of anisotropic responses in low‐symmetry van der Waals (vdW) materials remains challenging. Moiré superlattices offer a route to reconfigure material physical properties, but their capacity to regulate anisotropy in low‐symmetry systems is largely unexplored. Here, we realize programmable in‐plane anisotropy in pentagonal PdSe 2 by introducing interlayer twist and layer‐number asymmetry, and quantify the response using polarization‐resolved Raman and nonlinear optical spectroscopy. Varying either parameter reshapes the polar patterns, with Raman scattering and second‐harmonic generation (SHG) evolving in opposite ways as the twist angle is tuned. At large twist, SHG undergoes a transition to pronounced dichroism with a sharply enhanced polarization ratio and a critical angle of 44°, attributed to the symmetry breaking that drives cancellation and reinforcement among specific components of the second‐order susceptibility tensor. Exploiting this control, we demonstrate optical encoding for light‐communication schemes and polarization‐resolved photocurrent generation for sensitive photodetection and imaging. First‐principles calculations further reveal twist‐induced reorganization of the electronic structure, including indirect–direct bandgap conversions and the emergence of flat bands at small angles. Our results establish moiré and symmetry engineering as powerful levers for anisotropy control in low‐symmetry vdW materials, and provide a versatile platform for exploring moiré physics and related quantum phenomena.