Resolution limits of terahertz ptychography

Abstract

We present a study on the resolution limits and resolution factors of terahertz (THz) ptychography. Simulations of a binary amplitude object show that ptychography shares the same intrinsic resolution factors with digital holography, i.e. it is diffraction-limited. Reconstructions of amplitude and phase objects obtained from holographic and ptychographic experiments are comparable. A lateral resolution of around one wavelength λ is achieved on an amplitude object, while a depth resolution of around λ/5 is reported on a weakly diffracting phase object. THz ptychography is expected to complement THz holography for imaging biological samples and THz transparent specimens.

Coherent terahertz imaging of a textile and a hidden object

“…The reconstructions in the top row were obtained applying the known shifts, while in the middle row the shifts were retrieved via cross-correlation of the exit waves. The lateral resolution of the reconstructed hidden object, calculated from the modulation transfer function along concentric rings centered with the Siemens Star [5], amounts to ≈ 140 µm (≈ 1.5λ ). The bottom row shows that the reconstruction of the hidden object is feasible even without knowing the shifts.…”

Coherent reconstruction of a textile and a hidden object with terahertz radiation

“…For instance, a holographic approach, where an off-axis reference beam is superimposed to the exit wave, would obviate the need for the longitudinal scan and reduce the measurement time. As a drawback, the setup would not allow reconstructing objects with size similar to the detector size with high resolution, since the off-axis reference beam must not interact with the object [19].…”

“…Although originally demonstrated in the visible range, the SBMIR algorithm can be migrated to the THz regime without fundamental changes. However, as already pointed out for THz ptychography [18,19], the increase of the wavelength by almost three orders of magnitude implies Fresnel numbers on the order of ∼10, making the far-field approximation not applicable. This also plays a role in the choice of the recording distances, which is further detailed in Supplement 1 along with a study on the lateral resolution of the reconstructions.…”

THz coherent lensless imaging

“…Through the ePIE algorithm, Valzania et al reconstructed both amplitude and phase objects, reaching a lateral resolution of about 2λ and a depth resolution of λ/30 [86], and demonstrated that THz ptychography shares the same intrinsic resolution factors as THz off-axis digital holography [172]. Figure 9 summarizes some reconstruction steps.…”