NOVELTY - Fabricating dimensional glass-based structure such as silica-based structure (100') comprising porous silicon layer (40), involves providing dimensional silica glass substrate (10); extracting oxygen from atomic elemental composition of substrate by reacting metallic gas with substrate in heated inert atmosphere to form metal-oxygen complex along surface of substrate; removing the metal-oxygen complex from surface of substrate to yield crystalline porous silicon surface portion of substrate; and optionally forming additional layer (50) over crystalline porous silicon surface.    USE - For fabricating dimensional silica-based substrate or structure comprising porous silicon layer (claimed) used for forming photovoltaic cell, thermoelectric cell, environmental interface device, or silicon-based optoelectric device, e.g. light emitting device, waveguide, photonic crystal or solar cell.    ADVANTAGE - The step of recrystallizing the overlayer by annealing at sufficient temperature and duration enhances crystallization and increases grain size in overlayer. The silica glass substrate is provided with topical film for controlling the thickness, porosity or crystalline character of resulting crystalline porous silicon surface portion of silica glass substrate. The silica glass substrate is pre-treated with gaseous reducing agent to infiltrate the surface of silica glass substrate with reducing agent and enhance the reaction of metallic gas and silica glass substrate.    DETAILED DESCRIPTION - Fabricating dimensional glass-based substrate or structure such as silica-based substrate or structure (100') comprising porous silicon layer (40), involves providing dimensional silica glass substrate (10); extracting oxygen from atomic elemental composition of substrate by reacting metallic gas with substrate in heated inert atmosphere to form metal-oxygen complex along surface of substrate, where the inert atmosphere is heated to reaction temperature sufficient to facilitate oxygen extraction; removing the metal-oxygen complex from surface of substrate to yield crystalline porous silicon surface portion of substrate; and optionally forming at least one additional layer (50) over the crystalline porous silicon surface portion of substrate.    DESCRIPTION OF DRAWING(S) - The figure shows view of dimensional silica-based porous silicon structure.    Silica glass substrate (10)    Porous silicon layer (40)    Additional layer (50)    Separation layer (60)    Dimensional silica-based structure (100')