Fig. 1. Schematics of UV exposure and subsequent heat treatment for PTR glass. (a) Schematic of 325 nm UV exposure; (b) schematic of nucleation during 450 ℃ heat treatment stage; (c) schematic of NaF crystal growth during 480 ℃ heat treatment stage

Fig. 2. Measurement results. (a) Relationship between refractive index and frequency; (b)relationship between absorption coefficient and frequency

Fig. 3. Metasurface element design. (a) Structural diagram of metasurface unit; (b) unit simulation model built in COMSOL

Fig. 4. Effects of θ1 and θ2 on phase and transmittance

Fig. 5. Electric field intensity distributions on xz plane under different metalens focal lengths when y=0. (a) 3000 μm; (b) 5000 μm;

Fig. 6. Normalized electric field intensity distribution in x direction at actual focal point

Fig. 7. Normalized electric field intensity distributions in z direction under different designed metalens focal lengths at x=0 and y=0.