High-efficiency spiral zone plates in sapphire

Here, we report embedded spiral zone plates (SZPs) processed by tightly focused femtosecond pulses, inducing a refractive index change in a sapphire crystal. The change in the index is investigated both experimentally and theoretically and found to be 0.93% lower than that before the laser irradiation. Benefiting from the investigation of the photorefraction, a high diffractive efficiency of up to 78% is achieved for the SZPs. In addition, the SZPs are embedded under the surface of the sapphire, which ensures that the components possess stable optical properties in the case of external refractive index changes. The high-performance elements having a high diffractive efficiency and stable optical performance will have tremendous applications in optical communication and optical manipulation.