Fig. 2: Numerically estimated and experimentally obtained intensity distributions of the nonparaxial Airy beam in the transverse (xy) (at the focal point) and in the longitudinal (xz) planes at 0.6 THz. S denotes a THz source, L is a lens, A stands for the Airy phase mask, ZP is a nonparaxial zone plate, D labels THz detector.

Panel a – using Airy phase mask with an additional zone plate. The distance between the Airy lens and the ZP is 10 mm, and for the transverse plane scan the detector is placed in 7 mm from the ZP. The starting scanning point along the z-axis is 1 mm away from the ZP; panel b – using the Airy lens only. The starting scanning point along the z-axis is 1 mm away from the Airy lens. Distributions are normalized to the maximum value of the signal. A dashed blue line in bottom panels – guide for an eye to accentuate Airy beam which exhibits parabolic trajectory. A light blue shaded area in theoretical plot enlightens the reciprocal trajectories. Note that in case of nonparaxial Airy generation with mask without zone plate (panel b), the Airy profile is not distorted as in the case with zone plate (panel a) although the beam increases its dimensions as it propagates along the optical axis proportionally to z². Reciprocal caustic trajectories following x∼1/z law are also nicely pronounced both in theory and experiment. As it is seen the experiments fit well the modelling results (panel b)