Figure 1.

(a) Schematic of the non-diffractive Bessel beam generated by an axicon. The beam profile consists of a high intensity central core surrounded by a series of low intensity concentric lobes. The beam is defined by its half conical angle $\theta$, depth of focus (Bessel length) $z_b$, and diameter of the central core 2${\omega }_b$ at 1/$e^2$. (b) The Gaussian beam generated by a focusing lens, with a depth of focus $z_g$ = 2 × Rayleigh range, and a beam diameter 2${\omega }_g$ at 1/$e^2$. (c) The setup for investigating propagation of the beams through obstruction by metallic particles immobilised on a glass slide (inset-microscope image). With $\Delta$z being the distance between the obstacle and the starting point of a beam focus (the reference plane).