Terahertz diffractive structures for compact in-reflection inspection setup

Abstract

Two diffractive optical elements are used to create a compact raster THz scanning setup in reflective configuration. The first one focuses the radiation into the small focal spot on the sample, while the second one collects reflected radiation and focuses it on the detector. To assure small size of the setup and large apertures of optical elements, structures work in the off-axis geometry. Thus, the focal spot is formed 100 mm after and 60 mm below the optical axis of the element, which measures 75 mm in diameter. The designed iterative algorithm allows further minimization of these values.

Off-Axis Diffractive Optics for Compact Terahertz Detection Setup

“…The comparison of the 1st, the 2nd and the 4th order kinoforms as well as corresponding focal spots are presented in Figure 5. The scans of the analogous structures performed along the optical axis can be found in our previous paper [10]. Their character remained the same.…”

“…Using off-axis DOEs is advantageous due to the possibility of sustaining a small size of the focal point while significantly reducing a minimal focusing distance (unlike on-axis diffractive lenses and refractive lenses [9]). In the recent publication [10] we have already presented in-reflection inspection setup with plane wave-to-point "emitter lens" and point-to-point "detector lens". The whole setup has been designed for the frequency ν = 520 GHz.…”

The Magic of Optics—An Overview of Recent Advanced Terahertz Diffractive Optical Elements

“…The most typical polymers are polyethylene (PE) and high density polyethylene (HDPE), Teflon (PTFE), polyolefine based on poly 4 methyl pentene-1 (TPX), Tsurupica. There is also a whole variety of 3D printed materials [37] like for example polyamide PA12 [6], high-impact polystyrene-HIPS [37], cyclic olefin copolymer-TOPAS [4,38] which are not only relatively transparent for terahertz radiation but also easy in manufacturing and thus commonly used to manufacture DOEs.…”

“…The structure with similar idea of functioning can be designed and described by iterative algorithms based on the convolution approach of light propagation. Iterations are necessary to increase the efficiency of the designed structure that can be understood as a kind of synthetic hologram (here using an iterative algorithm resembling ping-pong or Iterative Fourier Transform Algorithm-IFTA-approaches) [6]. The depicted geometry and design can be used for minimizing the in-reflection imaging setups thanks to the remarkable possibility of increasing NA of the system by diffractive design approach.…”

“…Depending on the functioning of the element (focusing, redirecting, splitting, etc.) [1][2][3][4][5][6][7][8] such structures have different shapes. They can look like Fresnel lenses, spirals, maps of mountains, peacock eyes, hyperbolas or some other shapes, also looking like totally arbitrary random patterns [9][10][11][12].…”