Subwavelength structure (SWS) was formed by simple wet chemical etching using catalysis of gold (Au) nanoparticle. Single, nano-sized Au particle dispersion solution was coated onto silicon (Si)(111) substrate with polished surface. Then, the samples were soaked in an aqueous etching solution of h drofluoric acid and hydrogen peroxide The surface of 15-min-etched Si substrate appeared black. The reflectivity of the Si substrate was reduced to below 5% throughout the entire spectrum from 300 to 800 nm owing to SWS. The fractional area occupied by Si as a function of the depth across the textured layer showed a smooth increase of density up to a depth. The observed optical effects were explained by the formation of a subwavelength scale taper structure, representing an effective medium with a smooth transition of the refractive index from air to Si. (c) 2008 Elsevier B.V. All rights reserved.