Visualization of internal structure and internal stress in visibly opaque objects using full-field phase-shifting terahertz digital holography

We demonstrate full-filed phase-shifting terahertz digital holography (PS-THz-DH) by use of a THz quantum cascade laser and uncooled, bolometer-type 2D focal plane arrays. The proposed method enables us to separate the necessary diffraction-light from unnecessary diffraction-light images without the need for spatial Fourier filtering, leading to suppress the decrease of spatial resolution. 3D shape of a visibly opaque object is visualized with a sub-millimeter lateral resolution and a sub-µm axial resolution. Furthermore, the digital focusing of amplitude image enables the visualization of internal structure with the millimeter-order axial selectivity. The demonstrated results imply a possibility for non-destructive inspection of visibly opaque non-metal materials.