Fig. 1. Schematic diagram of the vortex phase zone plate

Fig. 2. The simulation results of incident linearly polarized light with n=1 and NA=0.85. The axial shift distances of the vortex phase zone plate are: (a) |Δz|=10 μm; (c) |Δz|=15 μm; (e) |Δz| = 20 μm. (b), (d) and (f) are the corresponding yz cross-section distributions of the total intensity in (a), (b) and (c), respectively

Fig. 3. The simulation results of linearly polarized light (a)−(c) and circularly polarized light (d)−(f) with n=1, |Δz|=10 μm and NA=0.85. (a), (d) are the xy cross-section distributions of total intensity; (b), (e) are the normalized intensity along x and y axis directions; (c), (f) are the 3D iso-surface plots of the total intensity with I₁= 0.5I_max, I₂= 0.3I_max

Fig. 4. (a)−(d) are the phase distributions of vortex phase zone plate, (e)−(h) are the corresponding xy cross-section distributions of light intensity in the tight focusing field illuminated by circularly polarized light, in which NA=0.85, |Δz|=20 μm and the topology charge: (a), (e) n=1; (b), (f) n=2; (c), (g) n=3; (d), (h) n=4

Fig. 5. Schematic diagram of the optical path in experiment, where PHF is a pin hole filter, CL is a collimation lens, BS is a beam splitter, P is a polarizer, SLM is a spatial light modulator, PM is a plane mirror

Fig. 6. The experimental results with different axial shift distances. (a), (d) are the vortex phase distributions with |Δz|=10 μm and |Δz|=15 μm; (b), (c) are the total intensity distributions on xy cross-section at 10 μm and −10 μm, respectively; (e), (f) are the total intensity distributions on xy cross-section at 15 μm and −15 μm, respectively

Fig. 7. The experimental results of the focusing distribution on xy cross-section with different topology charges. (a), (b), (c), (d) are the phase distributions of the zone plate with n=1, n=2, n=3 and n=4, respectively; (e), (f), (g), (h) are the corresponding experimental results for (a), (b), (c) and (d) after zone plate modulation, respectively

Fig. 8. The curve of the size of the dark spot of doughnut-shaped spot as a function of the topology charge n