Opal-like matrices based on hollow SiO2 nanoparticles
Protopopova A.-E.P.1,*, Masalov V.M.1, Sukhinina N.S.1, Kurlov V.N.1, Zaytsev K.1.2, Katyba G.M.1; 10sipyan Institute of Solid State Physics of the Russian Academy of Sciences, Chernogolovka, Russia, 2Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow, Russia
Abstract
At present, the number of materials suited for fabricating optical elements for the terahertz (THz) frequency range is significantly limited, and the development of artificial materials with tunable optical properties combining low losses and dispersion with stability under a variety of external conditions that are relevant for real applications remains an important task. Opal matrices based on solid submicron silicon dioxide (SiO₂) particles prove to be such a material, with optical characteristics (such as a low absorption coefficient and refractive index) tunable by varying the annealing temperature. However, such artificial opals have certain flaws: the refractive index can be smoothly tuned only within a comparatively narrow range at high annealing temperatures, and at low annealing temperatures, the opals significantly adsorb moisture from the atmosphere.
In this work, opal‑like matrices composed of hollow SiO₂ nanoparticles are proposed as such a material, in which the refractive index is controlled through a predetermined particle geometry: the SiO₂ shell thickness and the size of the internal air cavity. Hollow SiO₂ particles are obtained by a template method consisting of the sequential synthesis of polymer template particles, core‑shell PMMA–SiO₂ hybrid particles, and hollow SiO₂ particles. Opal‑like matrices are formed by natural sedimentation into a structure close to a close‑packed fcc lattice.
The resulting structures are characterized by THz time‑domain spectroscopy (THz‑TDS) and Fourier‑transform infrared (FTIR) spectroscopy. The influence of water molecules on THz absorption is minimized by high‑temperature annealing of the hollow nanoparticles, as a result of which the SiO₂ shell becomes impermeable to water molecules. The samples exhibit a low absorption coefficient (by intensity) and very low refractive‑index dispersion over the 0.2–2.0 THz range. By varying the particle geometry, the refractive index can be tuned smoothly and predictably over a wide range; tuning from ~1.07 to ~1.48 is demonstrated. The experimental refractive‑index values agree well with the effective‑medium approximation. The work was supported by the Russian Science Foundation, grant № 25–79–10280.
Speaker
Protopopova Aisiena-Ekaterina
Osipyan Institute of Solid State Physics of the Russian Academy of Sciences
Russia
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