Subwavelength full-field terahertz ptychography via longitudinal shifts

What is it about?

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. Attribute 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 biological material, a polymer sample, and a resolution target. The uneven illumination, as well as the diffraction effects associated with detector edges, are 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 (0.74λ).

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Photo by Michael Dziedzic on Unsplash

Photo by Michael Dziedzic on Unsplash

Why is it important?

It is a challenge to take into account the lateral resolution, imaging quality, and imaging speed at the same time in the THz domain. In this work, we demonstrate subwavelength resolution, full-field, lensless, and complex-amplitude THz ptychography. To suppress the incoherent infrared background, a digital phase-locked treatment based on an optical chopper is proposed to simplify the traditional way. By combining the ideas of dual-plane phase retrieval and ptychographic reconstruction, we propose an algorithm to simultaneously reconstruct the object and the probe. Different from the typical ptychography, the diversity of the diffraction pattern is provided by longitudinal shifts of the sample along the optical path instead of its lateral movements, and an extended illumination rather than a localized probe is adopted to realize full-field recording. Attribute to the lensless single-beam mode and full-field illumination, the sample can be placed as close to the detector as possible, and the aperture of the whole sensor can be utilized to achieve the highest NA. We verify it on the extended samples, and the experiment on the USAF 1951 shows that the lateral resolution achieves 88 μm (0.74λ) at 2.52 THz. With these features, this method is more practical than previous solutions and helps to promote the application of THz imaging.

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