300-GHz in-line holography with high dynamic range

THz in-line holography, is a simple and effective way of three-dimensional (3D) imaging. In this work, a THz in-line holography system is presented to investigate the imaging performance. An electrical multiplier-chain emitter working at 300 GHz was utilized as illuminating source. A highly sensitive broad-band CMOS TeraFET detector mounted on a 2D mechanical translation stage for raster-scanning acted as a virtual camera to record the hologram. The data capture area was 60x60 mm² with a pixel size of 0.25 0.25 mm². The test objects were polyvinyl chloride (PVC) boards, one taped with 2-mm-wide straight aluminum stripes separated by 2-mm intervals, another one taped with 1.5-mm-wide aluminum stripes forming the word 'GUF' (size of each letter: 6x8 mm²), and the other two with the same patterns, but now realized as grooves in the material. The imaging results show that a resolution of at least 2 mm is achieved, with a large dynamic range of 60 dB due to the high sensitivity of the TeraFET detector. A property of in-line holography is its self-homodyning capability with the consequence that, in addition to the intensity, also the phase information is encoded in the hologram. The phase information of the transparent part of the object, and of the scattered and diffracted waves are retrieved with the same spatial resolution as the intensity, revealing the 3D imaging capability of the system. The quality and fidelity of the image results can be substantially improved in the future by enlarging the recording area and using an iterative phase recovery method. Furthermore, with a camera substituting the single-pixel scanning technique, the system will have the capability of real-time imaging. Combined with the 3D imaging ability, it will then have a wide application range for various topical areas.