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

• Published in: Optics express 2020-05, Vol.28 (11), p.16012-16026
• Main Authors: Gailevičius, Darius, Ryu, Meguya, Honda, Reo, Lundgaard, Stefan, Suzuki, Tai, Maksimovic, Jovan, Hu, Jingwen, Linklater, Denver P., Ivanova, Elena P., Katkus, Tomas, Anand, Vijayakumar, Malinauskas, Mangirdas, Nishijima, Yoshiaki, Hock Ng, Soon, Staliūnas, Kȩstutis, Morikawa, Junko, Juodkazis, Saulius
• Format: Article
• Language: English
• ISSN: 1094-4087; 1094-4087
• DOI: 10.1364/OE.392646

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 i < 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 μ m 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 Δ n × d / λ ≈ 0.15 of the region with tilted b-Si needles. The quantified form birefringence of Δ n = −0.45 over λ = 400 − 700 nm spectral window was obtained. Such high values of Δ 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.