Terahertz Digital Holographic Imaging of Voids Within Visibly Opaque Dielectrics

Terahertz digital off-axis holography (THzDH) has been demonstrated as a non-destructive tool for imaging voids within visually opaque dielectrics. Using a raster scanning heterodyne detector, the imager captures lensless transmission holograms formed by the interaction of a highly coherent, monochromatic beam with 3-D printed structures. Digital hologram reconstructions from two structures were used to measure the imager's modulation transfer function and to show that terahertz digital holography can provide sub-millimeter resolution images of voids within visually opaque printed structures. As a demonstration we imaged embedded air- and lossy dielectric filled-voids whose refractive indices differ from the host material.