Volume zone plates consisting of a number of Fresnel zone plate layers are designed and fabricated inside a fused silica by the femtosecond laser direct writing method. Light diffracted from different layers of a volume zone plate constructively interfere at the focal spot to increase diffraction efficiency. The technique is experimentally verified to be effective for both low-numerical-aperture (NA) and high-NA zone plates, resulting in a significant increase in overall diffraction efficiency. The spatial resolution of a high NA volume zone plate is .
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