Optimization and experimental validation of a bi-focal lens in the microwave domain

The design of photonic devices is usually done through analytical modeling or variation in geometry and material parameters to obtain the required functionalities. Here, we report the use of topology optimization to obtain a bi-focal lens that concentrates the electromagnetic field at different spatial positions depending on the wavelength. Numerical inverse design is carried out to obtain the permittivity layout, satisfying this objective. The resulting device is then 3D printed using two low-loss dielectrics, and experimental field mapping at microwaves demonstrates the ability to enhance the field locally at distinct locations for two separate frequencies.