Tilted black-Si: ~0.45 form-birefringence from sub-wavelength needles
Abstract
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 θi < 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 n× d/λ≈ 0.15 of the region with tilted b-Si needles. The quantified form birefringence of n = - 0.45 over λ = 400-700~nm spectral window was obtained. Such high values of n 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.