Narrow-Band Near-Infrared Photocurrent Enhancement via Toroidal Dipole Resonance in Si₁₋ₓGeₓ Nanodisk Array

Abstract

Enhancement of light absorption and photocurrent by toroidal dipole resonances in a Si nanodisk array in the near-infrared (NIR) spectral range by controlling both the structural and material parameters have been investigated. To optimize absorption, we introduced Si₁₋ₓGeₓ alloying (x < 0.375) to tune the material loss while maintaining nearly constant refractive index. Simulations revealed that absorptance does not increase monotonically with Ge content but reaches a maximum at a specific composition, corresponding to the critical coupling condition. Experimental fabrication and characterization confirmed this behavior, showing that a small Ge incorporation (x ≈ 0.125) enhances absorptance more than threefold and increases photocurrent up to 3.4 times compared with pure Si metasurfaces. This study demonstrates that alloy engineering provides a practical route to achieve critical coupling and maximize photocurrent in Si-based metasurfaces operating in the NIR region.