Fabrication and characteristics of three-dimensionally buried porous silicon optical waveguides

A fabrication technology of three-dimensionally buried porous silicon (PS) optical waveguide with an extremely high refractive index contrast is presented, including its fundamental properties. The origins of attenuation losses are investigated by experimental and theoretical analyses in terms of microscopic observations, edge emission measurements, polarization mode determination of guided wave, and evaluation of bending loss. The results of these studies indicate that attenuation losses in PS waveguides are due to self-absorption by residual silicon, structural and optical inhomogeneities in the core region, and roughness at interfaces between the core and the cladding layers. Some possible ways for reducing these attenuation losses are discussed. It is also demonstrated that a buried bent PS waveguide with an extremely small curvature of 250 μm can be fabricated by simple planar processing, and that a visible optical wave propagates along it owing to a significantly high refractive index contrast between the core and the cladding layers.