Route to 4-Dimensional Terahertz Near-Field Tomography

Nonlinear Ghost Imaging (NGI) offers a versatile route to implementing time-domain terahertz imaging with a resolution that exceeds the typical diffraction limit [1], [2]. By transforming the detected field data, NGI can effectively “refocus” any chosen plane in the near-field region, thereby unveiling depth information in microscopic volumes that are relevant to both biological and technological applications [3]. Here, we develop a transformation-approach of NGI (Fig. 1) to retrieve the spectral response of two closely spaced subwavelength structures (λ/10) in a microscopic environment. We show how the contribution from the first plane (green curve in Fig. 1a) is progressively isolated with the separation in the sub-wavelength domain from the reconstructed spectrum of the second-plane feature (blue curve in Fig. 1c). Fig. 1b shows an example of our method's ability to resolve two metallic letters positioned on separate planes. Meanwhile, Figs. 1e and 1d compare the reconstructed images before and after addressing the spatio-temporal coupling introduced by near-field propagation. A full account of our hyperspectral tomography results will be discussed in detail.