Aplanatic Zone Plate Embedded in Sapphire

In this letter, we report the design and fabrication of an aplanatic zone plate, whose phase distribution is described by a fourth-order non-spherical function. Its optimal coefficients are determined by an iterative calculation based on the ray tracing method. The special zone plate, with a diameter of 10 mm, can significantly reduce the spherical aberration. The focal spot diameter is 43.4 μm, which is less than that of spherical lens, i.e., 153.6 μm. In addition, the zone plate is buried inside a z-cut sapphire crystal by femtosecond laser direct writing technology. This ensures that the zone plate possesses stable optical properties in complex and harsh environments. The novel embedded zone plate has an important application in optical aberration balance system. It improves the system's integration and stabilization in a high temperature environment, especially in a refractive index changing environment.