Fig. 1. Multi-order refractive index thin-film flat lens based on phase change material. (a) Schematic diagram of lens focusing; (b) structural diagram of lens; (c) principle diagram of lens focusing; (d) refractive index of GST material in infrared band; (e) extinction coefficient of GST material in infrared band

Fig. 2. Flow chart of multi-order refractive index thin-film flat lens design

Fig. 3. FDTD simulation results of focusing properties of multi-order refractive index thin-film flat lens. (a) Field intensity distribution curve of focal plane of lens with NA=0.60; (b) field intensity distribution of focal plane of lens with NA=0.60; (c) field intensity distribution curve along normal direction when NA=0.60; (d) field intensity distribution along normal direction when NA=0.60; (e) field intensity distribution curve of focal plane of lens with NA=0.53; (f) field intensity distribution of focal plane of lens with NA=0.53; (g) field intensity distribution curve along normal direction when NA=0.53; (h) field intensity distribution along normal direction when NA=0.53

Fig. 4. Imaging results of multi-order refractive index thin-film flat lens. (a) Schematic diagram of imaging optical path; (b) imaging result of lens with NA=0.60; (c) discernible minimum line pairs and their normalized intensity distribution curves when NA=0.60; (d) imaging result of lens with NA=0.53; (e) discernible minimum line pairs and their normalized intensity distribution curves when NA=0.53

Fig. 5. Comparison of focusing performance of lens designed with different refractive index orders. (a) Error between simulated and designed focal lengths; (b) FWHMs of focal points

Table 1. Refractive index and extinction coefficient of GST material with different crystallization degree