Organic Mixed Conductors in Fully Direct-Written THz Metaoptics

In the past two decades, the interest towards low-cost and portable terahertz (THz) metadevice technologies has surged thanks to the considerable promise that these electronic and hybrid electronic–photonic systems will likely play in upcoming sensing, imaging, and telecom applications¹. Indeed, a wealth of metadevice platforms based on inorganic semiconductors (e.g. III-V compounds and silicon), 2D materials (e.g. graphene and transition metal dichalcogenides) have been proposed, along with those that rely on superconductors, metal oxides and liquid crystals^2,3. Interestingly, the use of organic electronic materials in tuneable metastructures has not yet been pursued^4–6, and their THz properties are largely unexplored⁷. A possible explanation of this gap in both the scientific literature and technology is possibly due to the relatively low charge carrier mobilities and poor charge injection of organic semiconductors with respect to inorganic and layered materials⁸. This limitation has possibly originated the preconception that organic materials have no utility in high-frequency applications⁹.<br/>In this work, we challenge this point of view by demonstrating for the first time the use of organic electronic materials as tuning layers within reconfigurable THz metaoptics. Indeed, we show that the transmission of metallic metasurfaces resonating in the 0.5-0.75 THz range can be effectively modulated by an organic mixed ion-electron conductor (OMIEC) through a charge carrier screening effect, as previously demonstrated with inorganic semiconductors and layered nanomaterials^10,11. In particular, we exploit the polymer p(g2T-TT), the prototype material of an emerging class of OMIECs characterized by conjugated polymer backbones that exhibit high hole (or electron) mobilities, while also displaying liquid-like ionic conduction thanks to their hydrophilic glycolated sidechains^12,13. By optimizing the dissolution of p(g2T-TT) in high boiling point solvents, we are able to fabricate THz metadevices where both the metastructure and the active materials are processed solely by cost-effective and mass-scalable direct-writing techniques, namely inkjet printing and femtosecond laser writing, which are also compatible with large-area and plastic substrates¹⁴. Moreover, through time-domain terahertz spectroscopy performed on bare OMIEC thin-films, we show that the large conductivity modulations of these polymers, until now probed only at very low frequencies, are effectively preserved in the terahertz range, leading to optimal metadevice reconfigurability.<br/>These results point to a new spectrum of applications for organic electronic materials in terahertz technologies, where their unique characteristics may lead to groundbreaking wireless bioelectronic and neuromorphic technologies^15,16.<br/><br/><b>References</b><br/>1. Leitenstorfer, A. <i>et al.</i> <i>J. Phys. Appl. Phys.</i> <b>56</b>, 223001 (2023).<br/>2. Wang, L. <i>et al.</i> <i>Nanomaterials</i> <b>9</b>, 965 (2019).<br/>3. Degl’Innocenti, R., Lin, H. & Navarro-Cía, M. <i>Nanophotonics</i> <b>11</b>, 1485–1514 (2022).<br/>4. Bonacchini, G. E. & Omenetto, F. G. <i>Nat. Electron.</i> <b>4</b>, 424–428 (2021).<br/>5. Chen, S. <i>et al.</i> <i>Nat. Nanotechnol.</i> <b>15</b>, 35–40 (2020).<br/>6. Tan, S. T. M. <i>et al.</i> <i>Adv. Mater.</i> <b>34</b>, 2202994 (2022).<br/>7. Tsokkou, D., Cavassin, P., Rebetez, G. & Banerji, N. <i>Mater. Horiz.</i> <b>9</b>, 482–491 (2022).<br/>8. Natali, D. & Caironi, M. <i>Adv. Mater.</i> <b>24</b>, 1357–1387 (2012).<br/>9. Zschieschang, U. <i>et al.</i> <i>Adv. Funct. Mater.</i> <b>30</b>, 1903812 (2020).<br/>10. Chen, H. T. <i>et al.</i> <i>Nature</i> <b>444</b>, 597–600 (2006).<br/>11. Tassin, P., Koschny, T. & Soukoulis, C. M. <i>Science</i> <b>341</b>, 620–621 (2013).<br/>12. Quill, T. J. <i>et al.</i> <i>Nat. Mater.</i> <b>22</b>, 362–368 (2023).<br/>13. Giovannitti, A. <i>et al.</i> <i>Proc. Natl. Acad. Sci. U. S. A.</i> <b>113</b>, 12017–12022 (2016).<br/>14. Perinot, A., Passarella, B., Giorgio, M. & Caironi, M. <i>Adv. Funct. Mater.</i> <b>30</b>, 1907641 (2020).<br/>15. Inal, S., Rivnay, J., Suiu, A.-O., Malliaras, G. G. & McCulloch, I. <i>Acc. Chem. Res.</i> 7b00624 (2018).<br/>16. Van De Burgt, Y., Melianas, A., Keene, S. T., Malliaras, G. & Salleo, A. <i>Nat. Electron.</i> <b>1</b>, 386–397 (2018).