Time-domain terahertz imaging and spectroscopy of x-ray blocking and scattering coatings

The terahertz (THz) band of the electromagnetic spectrum is the gap between the microwave/millimeter wave band and the far infrared band. This band has been historically defined to be around 0.1 to 10 THz, due to the long-standing difficulties in efficient generation and detection of these frequencies. Only recently have efficient and practical commercial THz systems become available. Imaging with THz waves offers several unique advantages. THz waves can penetrate several millimeters into non-conducting materials. This allows for imaging of features that are covered in an optically opaque coating. Since THz waves have a shorter wavelength than microwaves, they are able to resolve smaller features. Unlike the x-ray imaging that is often associated with imaging into materials, the low photon energy of THz waves cannot ionize materials. This negates the risk of irreversible material damage inherent with x-ray imaging.