Terahertz phase retrieval imaging in reflection

Abstract

Terahertz phase retrieval is a promising technique able to assess the complex diffracted wave properties through an iterative processing algorithm. In this letter, we demonstrate the implementation of this technique in reflection geometry with a continuous wave acquisition system working at 0.287 THz. To ensure a high signal-to-noise ratio in the measured dataset, we proposed a double parallel recording scheme with one detector and two lock-in amplification operating with the complimentary sensitivity setting. This provided a higher numerical aperture than conventional rasterscanning focal plane imaging. A specialized digital interferometric post-processing procedure was applied to obtain a surface heights map from the reconstructed phase distribution in the object's irradiated area.

Single-scan multiplane phase retrieval with a radiation of terahertz quantum cascade laser

“…1. Such a limited illumination diameter finds itself suitable when ranging up to 2.5 THz, with respect to lower frequencies implementations [53] that require larger inspection fields. Indeed, an obvious setup and sample rescale should be expected relative to the wavelength of operation.…”

“…Similar to the competing techniques, it also makes use of processing algorithms adapted from methodologies developed in the visible frequency range [42,43]. The most popular algorithms are single-beam multiple intensity reconstruction (SBMIR) [53] and the transport of intensity equation [48]. This multipleplane approach is characterized by the extreme simplicity of the object illumination setup and guarantees better convergence compared to single-plane phase retrieval algorithms, but the recording of the diffraction patterns often remains time-consuming.…”

“…Nevertheless, the limited sensitivity of the THz radiation matrix detectors, along with the recurrent mismatches between the immutable pixel pitches and wavelengths of the detected THz radiation, do not allow the adaptation of existing solutions for an arbitrary combination of source and detector. To illustrate this point, a previous experiment demonstrated a multiple-plane phase retrieval setup in a reflection configuration based on a sequential raster scan using a single-pixel Shottky diode sensor, aiming to assess the diffraction pattern generated from up-converted Gunn diode at the wavelength of ∼ 1 mm [53]. Other works, contrarily, made use of matrix detectors for the assessment of the set of intensity distributions in the diffraction field [48,59], coupled to a high-power optically pumped far-infrared gas laser as the radiation source.…”

Lensless Fourier-Transform Terahertz Digital Holography for Full-Field Reflective Imaging

“…Different from the transmission TDH, it has been used for imaging of the sample hidden behind the material, which is transparent to THz wave [11,12]. Reflection TDH can achieve highresolution imaging by recording holograms with double parallel recording planes; the inherent "twin image" problem is eliminated by using iterative phase restoration algorithm [13], but it needs to record several holograms. Fresnel off-axis reflection TDH imaging can yield the complex amplitude of the object by recording singleframe hologram, whereas the reconstruction algorithm includes Fresnel or angular spectrum propagation, which requires accurate reconstruction distance and multiple Fourier transform [14,15].…”

Enhanced Terahertz Phase Retrieval Imaging by Unequal Spaced Measurement

“…Without the need of the support constrain and the prior knowledge of the project, the method can obtain an optimal convergence and high-accuracy reconstruction by using different scanning strategies [ 19 ]. The scheme has been investigated in the THz range [ 20 , 21 ], in which, the multiple planes were recorded with ordered propagation and equal interval distance.…”