A terahertz (THz) anti-reflection device, for example, a broadband tunable THz anti-reflection device, includes a silicon substrate having a plurality of recesses, each of the plurality of recesses having a plurality of cavities of decreasing dimension. The cavities may be nested polygonal cavities, for example, having a square or rectangular cross section. The recesses having the cavities may be positioned at regular periods, for example, periods ranging from 10 μm to 20 μm. The devices may be fabricated by conventional lithographic methods. Also disclosed are methods for modifying terahertz radiation and methods for fabricating anti-reflection devices.