Tilted black-Si: ∼0.45 form-birefringence from sub-wavelength needles.

The self-organised conical needles produced by plasma etching of silicon (Si), known as black silicon (b-Si), create a form-birefringent surface texture when etching of Si orientated at angles of θ < 50 - 70 (angle between the Si surface and vertical plasma E-field). The height of the needles in the form-birefringent region following 15 min etching was d ∼ 200 nm and had a 100 μm width of the optical retardance/birefringence, characterised using polariscopy. The height of the b-Si needles corresponds closely to the skin-depth of Si ∼λ/4 for the visible spectral range. Reflection-type polariscope with a voltage-controlled liquid-crystal retarder is proposed to directly measure the retardance Δn × d/λ ≈ 0.15 of the region with tilted b-Si needles. The quantified form birefringence of Δn = -0.45 over λ = 400 - 700 nm spectral window was obtained. Such high values of Δn at visible wavelengths can only be observed in the most birefringence calcite or barium borate as well as in liquid crystals. The replication of b-Si into Ni-shim with high fidelity was also demonstrated and can be used for imprinting of the b-Si nanopattern into other materials.