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The art of light engineering unveils a world of possibilities through the meticulous manipulation of photonic properties such as intensity, phase, and polarization. The precise control over these optical properties under various conditions finds application in fields spanning communication, light-matter interactions, laser direct writing, and imaging, enriching our technological landscape. In this study, we embark on a journey to establish a rational framework for the design and assembly of nonparaxial THz imaging systems. Our focus centers on a lensless photonic system composed solely of flat-silicon diffractive optics. These elements include the high-resistivity silicon-based nonparaxial Fresnel zone plate, the Fibonacci lens, the Bessel axicon, and the Airy zone plate, all meticulously crafted using laser ablation technology. A systematic exploration of these flat elements in various combinations sheds light on their strengths and weaknesses. Our endeavor extends to the practical application of these optical components, where they illuminate samples and capture the light scattered from these raster-scanned samples using single-pixel detectors. Through a comprehensive examination, we evaluate imaging systems across diverse metrics that include contrast, resolution, depth of field and focus. This multifaceted approach allows us to distill rational design principles for the optimal assembly of THz imaging setups. The findings of this research chart an exciting course toward the development of compact, user-friendly THz imaging systems where sensors and passive optical elements seamlessly integrate into a single chip. These innovations not only enhance capabilities in THz imaging but also pave the way for novel applications, ushering in a new encouraging era of advanced THz photonic technology., Comment: 26 pages

Terahertz coloured digital holography ranging from 1.39 THz up to 4.25 THz is demonstrated. It is shown that it can be applied for the investigation of low-absorbing objects, and it is illustrated via inspection of stacked graphene layers placed on high-resistivity silicon substrate. Holograms are recorded using an optically-pumped molecular THz laser operating at discrete emission lines of 1.39 THz, 2.52 THz, 3.11 THz, and 4.25 THz frequencies and it unveiled that phase-shifting methods allow to qualitatively reconstruct coloured THz holograms with improved quality achieved by removing unwanted information related with so-called DC term and conjugated beam forming virtual image., Comment: Submitted to "Scientific Reports" on 30th June 2023

It was demonstrated that optical modulation together with simultaneous terahertz (THz) imaging application enables an increase in contrast by an order of magnitude illustrating hence the technique as convenient contactless tool for characterization of graphene deposited on high-resistivity silicon substrates. It was shown that the single- and double-layer graphene can be discriminated and characterized via variation of THz image contrast using a discrete frequency in a continuous wave mode. Modulation depth of 45 % has been reached, the contrast variation from 0.16 up to 0.23 is exposed under laser illumination for the single and double layer graphene, respectively. The technique was applied in the development and investigation of graphene-based optical diffractive elements for THz imaging systems., Comment: 8 pages, 9 figures