# Off-Axis Diffractive Optics for Compact Terahertz Detection Setup
## 4 Experimental Evaluation  
The performance of all manufactured structures has been experimentally verified in the setup, shown in the Figure 4.  
All marked dimensions have been chosen empirically before designing optical elements taking into account dimensions of available sources and detectors as well as total size and usability of a  
Figure 4: The measurement setup for off-axis diffractive lenses with frequency multipliers with Schottky diodes as a source and a detector of THz radiation. \(x_{s}=100\) mm is the distance from the source of radiation to the thin structure, \(x_{d}=100\) mm corresponds to the distance along optical axis from the structure to the image plane and \(y_{d}=60\) mm reflects the perpendicular shift of the detector (and focal point) from the optical axis. future scanner head. This optical setup consists of two frequency multipliers with Schottky diodes to emit and detect THz radiation with the structure under test in the middle. The comparison of the 1 st, the 2 nd and the 4 th 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. It can be noticed that in subsequent HOKs respective zones are indeed wider proportionally to their order. The designed structures generally resemble Fresnel lenses. Additional discontinuities on the borders of zones (regions where phase drops from 2\(\pi\) to 0) result from the iterative optimization algorithm and enhance geometric parameters of focal spots. All intensity distributions have been separately normalized for the sake of visibility but measured peak values have been denoted on images as \(P_{max}\) (in relation to the best performing structure). It can be seen that for kinoforms of higher-order intensity of focal spot significantly decreases (by 85% for the 2 nd order and by almost 97% for the 4 th order kinoform). It is clearly connected with the thickness of the structures. The absorption coefficient of PA12 is already relatively high at 520 GHz (around 2.2 cm\({}^{-1}\)) [21] and starts to play a significant role when the thickness of the structures is doubled or quadrupled. The transparency issue can be partially solved by application of less attenuating media. However, the number of materials accessible for 3D printing is rather limited. Therefore, another manufacturing method has been proposed based on the additive manufacturing of the molds. These molds correspond to the designed DOEs and then are filled with melted paraffin. Thus, the mold corresponding to the fourth order kinoform has been manufactured and three kinds of paraffin from different manufacturers have been tested. These kinds of paraffin have been marked as b, c and d where b refers to transparent paraffin, c to white paraffin and d to reused paraffin. The experimental results together with reference PA12 lens are shown in Figure 6.  
Here, all power distributions have also been normalized separately and the ratios of recorded maxima to the peak power obtained from reference lens are denoted as \(P_{max}\). The intensities of focal spots obtained for all three paraffin HOKs are significantly higher than the one from PA12 lens (by 42%, 76% and 36%, respectively). On the other hand, differences between very similar paraffin samples obtained from different distributors are also far from being negligible. Finding the reasons for that would require more in-depth study, especially of the differences between materials. Figure 5: The comparison of phase delay maps (**top** panels) of (**a**). 1 st (\(p=2\)), (**b**). 2 nd (\(p=4\)) and (**c**). 4 th (\(p=8\)) order kinoforms with normalized intensity distributions in focal planes (**bottom** panels). All intensity images are normalized to unity, while the maximum power detected for each structure in respect to the value registered for the first order kinoform (**a**) is denoted as \(P_{max}\).