Abstract-Opal-based terahertz optical elements fabricated by self-assembly of porous SiO2 nanoparticles

V. E. Ulitko, G. M. Katyba, V. A. Zhelnov, I. M. Shmytko, G. A. Emelchenko, I. E. Spector, V. M. Masalov, V. N. Kurlov, K. I. Zaytsev, M. Skorobogatiy, 

 Fabrication of the cylindrical convex-plane lenses by mechanical processing of bulk pieces of nanoporous SiO2${}_2$. (a) Multirun grinding of the lenses using rotational holder, and Removal of lenses from the holder after grinding. (b) 3D schematic of a cylindrical lens. (c) Photo of the fabricated lenses made of two nanoporous SiO2${}_2$ materials, annealed at the temperatures of 900$900$ and 1200∘${1200}^{\circ }$C and, thus, featuring different refractive indices and focal distances. (d) SEM images of the two nanoporous SiO2${}_2$ pieces made of 300$300$-nm-diameter nanoparticles and annealed at the temperatures of 900$900$ and 1200∘${1200}^{\circ }$C, respectively.

https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-29-9-13764&id=450313

In this paper, we study artificial opals as a promising material platform for terahertz (THz) optics. Materials were synthesized using self-assembly of porous SiO2 nanoparticles and annealing at different temperatures to further tune their optical properties. Two distinct approaches for the fabrication of bulk THz optics from these novel materials were considered. First, THz cylindrical lenses of identical geometry but different refractive indices and focal lengths were produced using standard mechanical processing of opals, in order to highlight their compatibility with conventional technologies of bulk optics fabrication. Second, a THz axicone was made via direct sedimentation of aqueous colloidal suspension of SiO2 nanoparticles in the mold of geometry inverse to that of a desired optical shape, followed by annealing and polishing. The second approach has an advantage of being considerably less labor intensive, while capable of obtaining optical elements of complex geometries. Thus fabricated bulk THz optical elements were studied experimentally using continuous-wave THz imaging, and the results were compared with 2D and 3D numerical predictions based on the finite-difference time-domain and finite-element frequency-domain methods. Our findings highlight technological robustness of the developed THz optical material platform and, thus, open the door for creating a variety of bulk THz optical elements of complex shapes and widely-tunable optical performance.

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