Amorphous silica microstructured optical fibers with micro/nanoscale arrays of pores designed in virtually any desired pattern are an exemplary platform for the manipulation of photons in cylindrical geometries, while, in contrast, crystalline semiconductors are ideal hosts for the control of electrons in planargeometries. The integration of semiconductors into the pores of microstructured optical fibers merges these two conflicting frameworks into one entity to result in a novel class of fiber geometry optoelectronic materials. The realization of such structures is a challenge in materials synthesis because conventional nanofabrication techniques cannot conformally coat such extreme aspect ratio pores with films of uniform thickness. The focus of this dissertation was to develop an innovative, high pressure chemical deposition technique that removes the mass transport constraints in such intricate structures. The meter long, ultra-high aspect ratio pores of microstructured optical fibers are treated as high pressure chemical reactors for the deposition ofGroup IV and II-VI semiconductors from organometallic precursors. The behavior of molecules compressed to high pressures and confined to the small dimensions of the micro/nanoreactors is dramatically altered such that nearly every aspect of the pathway from molecular precursor to reaction product, including reactant flow, surface chemistry, chemical kinetics and thermodynamics, and nucleation and growth, differs from that under conventional conditions. Taking such factors into account, the necessary chemical principles have been developed to fabricate high quality Group IV and II-VI semiconductor core optical fibers by identifying and inventing high pressure chemical pathways that are suitable for the large aspect ratio, confined geometry capillary templates.