Figure 2.

Volumetric reconstruction of a semitransparent object with subwavelength metallic features. (a) Optical image of the sample: metallic letters “U” and “S” on plastic substrates. (b) Near-field imaging scheme and 3D structure of the sample. The sample is placed in close proximity to the nonlinear crystal and is composed of two metallic letters, each deposited on a 50 μm-thick substrate (Kapton) and separated by a 300 μm-thick plastic slab, leading to an overall distance between the two letters of ∼350 μm. (c) Temporal response of the sample, corresponding to the spatial average of the collected function, m^GI(x, y, t). (d) Fixed-time image m^GI(x, y, t_max) of the sample in correspondence with the peak time t_max. (e) Hyperspectral image M^GI(x, y, ω), averaged between 1.6 and 2.3 THz, without any inverse propagation operator. In this scenario, the TNGI directly reconstructs the parts of the sample closer to the generating crystal (f) same as panel (e) but following the application of the inverse operator with z₀ = 390 μm. In all the panels, the total field of view was 2 mm × 2 mm with a 32 × 32 spatial sampling. Supplementary videos are reported for the raw data (Videos S1 and S2).