Elements of photonic devices with high aspect ratio patterns are fabricated. A stabilizing catalyst that forms a stable metal-semiconductor alloy allows to etch a substrate in vertical direction even at very low oxidant concentration without external bias or magnetic field. A metal layer on the substrate reacts with the oxidant contained in air and catalyzes the semiconductor etching by the etchant. Air in continuous flow at the metal layer allows to maintain constant the oxidant concentration in proximity of the metal layer. The process can continue for a long time in order to form very high aspect ratio structures in the order of 10,000:1. Once the etched semiconductor structure is formed, the continuous air flow supports the reactant species diffusing through the etched semiconductor structure to maintain a uniform etching rate. The continuous air flow supports the diffusion of reaction by-products to avoid poisoning of the etching reaction.