(a) Schematic illustration of the computation approach developed in this work – the traditional beam propagation method combined with the quasi-discrete Hankel transform (BPM-QDHT) in three dimensions. (b) The optical wavefield was calculated by BPM-QDHT to obtain Φ(r, t) at the exit of the KZP lens. The wavefield Φ(r, f) at the focal plane is calculated using diffraction theory and QDHT in free space. (c) The topography of the binary Fresnel zone plate (FZP), kinoform zone plate (KZP) and trapezoid-kinoform zone plate (TKZP). (d) Partial 2D phase map of the wavefields inside the TKZP with diameter of 100 µm, outermost zone width of 500 nm and thickness of 1.8 µm, and wavefield intensity (red line) and phase (black line) at the exit of the lens. The white dashed line represents the zone profile.