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

Area of Science:

• Materials Science
• Nanotechnology
• Optics

Background:

• Plasma etching of silicon (Si) at specific angles produces self-organized conical needles, termed black silicon (b-Si).
• These b-Si structures exhibit form birefringence, a phenomenon where the physical texture creates optical anisotropy.
• The needle height (∼200 nm) is comparable to the skin depth of silicon for visible light.

Purpose of the Study:

• To characterize the form birefringence of black silicon (b-Si) produced by plasma etching.
• To propose and demonstrate a method for measuring the retardance of b-Si nanostructures.
• To investigate the potential for replicating b-Si nanopatterns for material imprinting.

Main Methods:

• Fabrication of b-Si using plasma etching with Si oriented at angles θ_i < 50-70° relative to the plasma E-field.
• Characterization of optical retardance and birefringence using polariscopy.
• Development of a reflection-type polariscope with a voltage-controlled liquid-crystal retarder for direct measurement.
• Replication of the b-Si surface into a nickel (Ni) shim.

Main Results:

• Quantified form birefringence (Δn) of -0.45 over the 400-700 nm spectral range.
• Measured retardance (Δn × d/λ) of approximately 0.15 for the b-Si needle region.
• Demonstrated high-fidelity replication of the b-Si nanopattern into Ni.
• Achieved form birefringence values comparable to highly birefringent materials like calcite and barium borate.

Conclusions:

• Black silicon exhibits significant form birefringence at visible wavelengths, rivaling established optical materials.
• The developed polariscope method effectively quantifies the retardance of these nanostructured surfaces.
• Replication of b-Si into Ni enables its use for imprinting nanopatterns onto diverse substrates.