THz coherent lensless imaging

Abstract

Imaging with THz radiation has proved an important tool for both fundamental science and industrial use. Here we review a class of THz imaging implementations, named coherent lensless imaging, that reconstruct the coherent response of arbitrary samples with a minimized experimental setup based only on a coherent source and a camera. After discussing the appropriate sources and detectors to perform them, we detail the fundamental principles and implementations of THz digital holography and phase retrieval. These techniques owe a lot to imaging with different wavelengths, yet innovative concepts are also being developed in the THz range and are ready to be applied in other spectral ranges. This makes our review useful for both the THz and imaging communities, and we hope it will foster their interaction.

Assessment of cameras for continuous wave sub-terahertz imaging

“…Finally, we quantitatively compare camera operation under non-normal angles of incidence, as such layouts are rather typical in holographic and interferometric imaging setups. 13,14 As outlined in Figure 1, the cameras were rotated about the axis going through the centre of the sensor, to avoid shifting of the beam across the sensor area. Rotation was performed with 2 • steps, for angles ranging from −50 • to 50 • for the Tera-1024 camera and from −40 • to 40 • for the Pyrocam, as larger angles were not allowed by the diffraction on the entrance aperture.…”

“…Most well-known imaging techniques, providing either outstanding resolution, or high speed operation, or full wavefront resonstruction, such as near field imaging, 11 compressed sensing 12 or holographic phase retrieval 13 have been already demonstrated. For many of these techniques, requiring non-collinear beam alignment, a knowledge of the detector properties, especially, its angular, spectral and polarization sensitivity are essential for efficient imaging system design and implementation.…”

High-Power Lensless THz Imaging of Hidden Objects

“…Therefore, minuscule amounts of radiation reach the corresponding detectors, thus calling for high illumination power in combination with high detection sensitivity. In this respect, THz time-domain spectroscopy (THz-TDS) offers the advantage of highly sensitive fieldresolved detection [8], particularly based on optimized photoconductive antennas [9]. However, THz-TDS particularly suffers from a severe lack of high-average power, with typical sources in the microwatt regime (Fig.…”

Real-Time High Resolution THz Imaging with a Fiber-Coupled Photo Conductive Antenna and an Uncooled Microbolometer Camera

“…However, the achievable resolution and image quality, respectively, are limited by the irradiation wavelength, the numerical aperture (NA) of all optical components, as well as by the camera properties (pixel size, sensitivity etc.). To circumvent the limitations of the optical components, lens-less imaging [ 20 , 21 ] can be an option.…”

“…The FIR gas lasers [ 22 ] are based on a high power, mid-infrared CO 2 -laser pumping a THz cavity. Their THz emission can be continuous-wave (cw), with the output power exceeding at [ 21 ]. The output wavelength depends on the gas in the THz resonator.…”