We report the design and experimental realization of a deep subwavelength-engineered slotted photonic crystal fabricated using a commercial monolithic silicon photonics process with a minimum feature size near 40 nm. The deep subwavelength design includes a corrugated, slotted unit cell shape that leverages electromagnetic interface conditions to localize optical energy in low refractive index regions, achieving a four-fold enhancement of the electric field energy compared to an equivalent slotted photonic crystal without the nanoscale corrugations. This demonstration establishes a basis for future study of commercially fabricated, subwavelength-engineered photonic structures where intense light-matter interaction and manipulation of optical properties on-chip is critical, including biosensing and optical trapping applications.