... numerical method for simulation of focusing by VFZPs The 10 P Srisungsitthisunti, O K Ersoy, and X Xu, Volume Fresnel zone plates fabricated by femtosecond laser direct writing, ” Appl Phys Lett 90, ... and X Xu, Laser direct writing of volume modified Fresnel zone plates, ” J Opt Soc Am B 24, 2090–2096 (2007) Q Cao and J Jahns, “Comprehensive focusing analysis of various Fresnel zone plates, ” ... different number of layers up to 10 layers in fused silica by femtosecond laser direct writing The details of the experiments are explained in [2] The actual phase modulation induced by each FZP is...