Designed Terahertz Metalenses Based on Photo-Thermo-Refractive Glass

Objective Terahertz metalenses have broad prospects in the future; however, currently, common metalens preparation methods such as lithography, electron beam lithography (EBL), and focused ion beam (FIB) usually require the use of a photoresist, which is cumbersome. Using photothermo-refractive (PTR) glass as a substrate eliminates the need for a photoresist during the preparation process. PTR glass is a UV-sensitive silicate glass material doped with photosensitive elements, such as Ce, Ag, and Sn. After UV exposure and thermal treatment, small NaF microcrystals precipitate. Owing to the different etching rates of hydrofluoric acid on the PTR glass substrate and NaF microcrystals, selective exposure of the area to be etched can be achieved, resulting in the desired groove structure. To obtain a complete metalens unit structure, metal thin films can be prepared in the grooves using methods such as evaporation, magnetron sputtering, atomic layer deposition, and chemical vapor deposition. To verify whether PTR glass can be applied to metalenses, THz metalenses are designed using PTR glass as a substrate. Methods We measure and analyze the absorption coefficient and refractive index of PTR glass in the terahertz band by scanning its terahertz transmission spectrum (Fig. 2) and obtain the relative dielectric constant of the material. A cell simulation model is constructed in COMSOL at a working frequency of 0.4 THz [Fig. 3(a)], and the cell structure is simulated to obtain the cell transmittance and phase. Results and Discussions Using the control-variable method, we simulate the ring width, opening angle, and rotation angle of the resonant ring, obtaining nine structures (Table 1). According to Formula (5), we arrange the units to obtain metalenses with focal lengths of 3000, 5000, 8000, and 10000 μm with numerical apertures of 0.6042, 0.4819, 0.3960, and 0.3593, respectively. After analyzing the electric-field intensity of the metalens (Fig. 5), we obtain the normalized electric-field intensity along the x-direction at the focus (Fig. 6). We calculate that the focusing efficiencies of these metalenses are 22.89%, 16.223%, 15.58%, and 17.51%, respectively, which are higher than those of metal-structured metalenses using other substrates. Conclusions Based on transmission and Panchaiatnam-Berry (PB) phase theories, we design nine types of metalens units with PTR glass as the substrate, resulting in nine types of metalens units. At a frequency of 0.4 THz, the phase can change from 0 to 2π, and the transmittance is stably higher than 0.27. At the same time, by arranging the units, terahertz metalenses with focal points of 3000, 5000, 8000, and 10000 μm are designed. The focusing efficiency of this structure can be higher than 15% with good focusing performance. The possibility of preparing metalenses using PTR glass as the substrate is verified through simulations. An alternative method for preparing metalenses without using a photoresist is found, which is expected to reduce the manufacturing cost in the future metalens industry and provide a new development direction for materials with photothermal sensitivity, such as PTR glass. © 2024 Science Press. All rights reserved.