A novel fabrication method of Si photonic slabs based on the selective formation of porous silicon is reported. Free-standing square lattices of cylindrical air holes embedded in a Si matrix can be achieved by proton beam irradiation followed by electrochemical etching of Si wafers. The photonic band structures of these slabs show several gaps for the two symmetry directions for reflection through the z-plane. The flexibility of the fabrication method for tuning the frequency range of the gaps over the near- and mid-infrared ranges is demonstrated. This tunability can be achieved by simply adjusting the main parameters in the fabrication process such as the proton beam line spacing, proton fluence, or anodization current density. Thus, the reported method opens a promising route towards the fabrication of Si-based photonic slabs, with high flexibility and compatible with the current microelectronics industry.