Subwavelength full-field terahertz ptychography via longitudinal shifts

Abstract

We demonstrate subwavelength resolution, full-field, and lensless terahertz ptychography via longitudinal shifts of the sample. An extended illumination is adopted to realize full-field recording. Compared to a typical scheme, this implementation requires fewer diffraction patterns, which improves the imaging throughput. To suppress the incoherent infrared background, an optical chopper is introduced and a preprocessing algorithm based on digital phase-locked is proposed. By combining the ideas of multi-plane phase retrieval and ptychographic reconstruction, a reconstruction algorithm is proposed to simultaneously retrieve the object and the probe. Due to the lensless single-beam mode and full-field illumination, the sample is placed as close to the detector as possible, and the aperture of the whole sensor is utilized to achieve the highest numerical aperture. Note that the proposed approach could be applied to other wavebands as well. We apply this method to a biological material, a polymer sample, and a resolution target. The uneven illumination, as well as the diffraction effects associated with detector edges, is separated from the object reconstruction. The secondary veins of the dragonfly forewing, as small as ∼20 µm, are observed, and the lateral resolution achieves 88 µm (0.74 λ).