Terahertz (THz) waves occupy a segment of the electromagnetic spectrum between the infrared and microwave bands. As such, they can be used for imaging and sensing in ways not possible with conventional technologies such as X-ray and microwave. Because THz radiation transmits through almost anything that is not metal or liquid, the waves can see through most materials that might be used to conceal explosives or other materials, such as packaging, corrugated cardboard, clothing, shoes, backpacks, and book bags. This invention is directed to an apparatus and method for generating high-energy THz radiation. Impinging optical radiation on the surface of a semiconductor substrate creates a photo-generated dipole emitting terahertz radiation. The surface of the semiconductor is modified to achieve maximum power of the terahertz radiation. Embodiments of surface modification include 1) a grating created in the top surface of a GaAs semiconductor substrate, 2) an InAs film formed on a Teflon base to create a grating structure on the semiconductor, and 3) a grating disposed in the surface of the semiconductor substrate such that the optical radiation engages the substrate at Brewsters angle.