Design and fabrication of 3-D printed conductive polymer structures for THz polarization control

In this paper, we numerically and experimentally demonstrate the inverse polarization effect in three-dimensional (3-D) printed polarizers for the frequency range of 0.5 2.7 THz. The polarizers simply consist of 3-D printed strip lines of conductive polylactic acid (CPLA, Proto-Pasta) and do not require a substrate or any further metallic deposition. The experimental and numerical results show that the proposed structure acts as a broadband polarizer between the range of 0.3 THz to 2.7 THz, in which the inverse polarization effect is clearly seen for frequencies above 0.5 THz. In the inverse polarization effect, the transmission of the transverse electric (TE) component exceeds that of the TM component, in contrast to the behavior of a typical wire-grid polarizer. We show how the performance of the polarizers depends on the spacing and thickness of the CPLA structure; extinction ratios higher than 20 dB are achieved. This is the first report using CPLA to fabricate THz polarizers, demonstrating the potential of using conductive polymers to design THz components efficiently and robustly. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
Terahertz (THz) radiation has attracted the attention in a wide range of fields, including, but not limited to, in vivo biomedical characterisation [1,2], the automotive industry [3], botanics [4], cultural heritage [5], and materials characterization [6].Thus, devices able to manipulate THz radiation are in high demand.Unfortunately, the materials used for optical components, for example glass, are opaque at terahertz frequencies.To address this problem, researchers have turned their attention to 3-D printed technology given the low attenuation of some printable plastics in this frequency range [7].Additionally, the resolution of conventional three-dimensional (3-D) printers is high enough to fabricate components for the THz range (0.4 mm aprox.).Several devices built using this technology have been reported in recent years.Examples include anti-reflective structures [8][9][10], tunable prisms [11], Bragg fibers [12].In addition, some new materials have been explored, for example, conductive filaments, used for printed electronic circuits [13].The conductive properties of these materials are attributed to the mixture of a polymer host and either graphene (Black Magic 3-D) or carbon black (Proto-pasta) [13].Production of several 3-D printed polarizers has been reported recently [14][15][16].Some of these structures printed in plastic are subsequently coated with metallic thin films to improve the performance of the device [14,15].Super ink-jet printers are able to print complicated metallic structures with comparable accuracy to photolithography [16].Thus, by printing wire grid patterns on THz transparent substrates such as silicon, super ink-jet printing can be used to fabricate broadband polarizers.In this paper we report, to the best of our knowledge, the first 3-D printed polarizers made of    gure, it is cle a distance no transmitted polarizer.Usin e effective abso the bulk mate below 5 cm −1 how an inversi d but the TM s in the air gap se formed insid ental charact erimental char was used.Thi range from 0 geometry [7] ngle between th of 10°.This an f Fig. 3    Figure 5(a for the TE c refractive ind frequencies ab propagate ins frequency com results in n TM theory [22].In e, the TE comp ses.In addition izers.From thi nction of the th olarizers given extinction ratio between transm er [21].As expected all frequencies arrive at the same time, around 9 ps.Comparing this arrival time with the arrival time for frequencies in the TE component (Fig. 5(a)), we can conclude that the refractive index for the TE components above 0.5 THz is around 1.This value is again confirmed using effective-medium-theory equations [22] and is in agreement with the FEM simulation results at 2.5 THz (Fig. 2).Finally, a typical waveform for the TM component is shown in Fig. 5(d).The rapid oscillations of the THz pulse between 5 and 10 ps in Fig. 5(d) are due to the high frequency components that leaked through the polarizer.These high frequency components are shown in Fig. 5(b) as a slightly red area around 10ps for frequencies from 0.5THz to 2.7 THz (circled in white).From Fig. 5(d), it is clear that the amplitude of the THz pulse near to 11ps is zero, consequently, when the Gaussian window (1ps standard deviation) is multiplied by that region the resulting Gabor transform vanishes.This explains the black spot in Fig. 5(b) in the TM frequency-time distribution (circled in blue).By taking a wider Gaussian window, more non-zeros amplitude values will enclosed by the window eliminating this black region, but with the cost of losing certainty in the arrival time of every frequency.

Conclusion
In conclusion, we have shown that it is possible to fabricate functional polarizers for a wide range of frequencies in the THz range by printing strip lines of conductive PLA (proto-pasta).
In this work we printed 3cm diameter polarizers made of 0.3 mm strip depositions of conductive PLA separated by 0.3 mm air gaps.We printed five different polarizers with different thickness from 1 mm to 5 mm.The best performance was found with the 5 mm thick polarizer.In addition, we demonstrated the inverse polarization effect in a 3-D printed polarizer for frequencies above 0.5 THz.This frequency matches with the cut-off frequency calculated for a metallic rectangular waveguide with the same dimensions.This type of printed device is quick and easy to fabricate, mechanically robust and additionally low cost, making it an invaluable addition for future THz components for communication and imaging applications.
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