Nanocrystalline porous silicon is created by etching Si in fluoride solutions. The reaction is initiated by valence band holes that are provided by laser irradiation or suitable oxidants. The composition of the fluoride solution can be used both to elucidate the mechanism of etching as well as to control properties of the resulting microporous film. Several new classes of stain etchants - containing some combination of HF, NH@sub 4@HF@sub 2@, HCl, HNO@sub 3@, Fe(III), Mn(VII) and water - have been investigated. Once porosified, the films exhibit visible photoluminescence (PL). The peak PL wavelength depends on the etchant composition. Properties of the film, such as morphology, porosity and the PL maximum wavelength, respond to the etchant composition. Of particular interest is the observation of a blue shift in PL, which correlates with an increasingly positive electrochemical potential (E@sub 0@) of the oxidant. It is argued that E@sub 0@ plays a role much like wavelength in photoelectrochemical etching and that smaller nanocrystals are produced with more positive values of E@sub 0@. Micrometer scale Si pillars are formed by chemically enhanced laser ablation using nanosecond excimer laser irradiation (308 nm, ~3 J cm@super -2@) of Si in the presence of SF@sub 6@. Smaller pillars are formed using femtosecond irradiation (790 or 390 nm, ~ 1 J cm@super -2@). We can control the initiation of precursor holes by ruling a grating into the Si substrate prior to irradiation. Near-field amplification of the laser intensity enhances the formation of the precursor holes and aligns them parallel to the rule. Continued irradiation leads to the break up of the holes into pillars. The pillars can be thinned and eventually removed by wet chemical etching in aqueous KOH resulting first in ordered arrays of pillars with aspect ratios approaching 10,000 (e.g. tens of microns in length, with ~10 nm tips) and then macropores. Macropore shape is determined by crystallography and the anisotropy of the wet etchant.