Photonic nanohelix generated by a binary spiral axicon.

We demonstrate the possibility of generating a photonic nanojet with a helix shape (photonic nanohelix) at the diffraction of a Gaussian beam on a binary spiral axicon. We investigate the influence of the illuminating beam parameters on the shape and size of the generated photonic nanohelix by solving the Helmholtz equation using the finite element method. The simulated photonic nanohelix has depth of focus (DOF) of 2.21 μm (3.49λ) and FWHM of 304.5 nm (0.48λ), and the intensity maximum is 11.59 times higher than the incident beam maximum intensity. Using a near-field scanning optical microscope with a metal tip, it has been demonstrated that when we illuminate a binary spiral axicon with NA=0.5 and depth of etching of 500 nm by a linearly polarized plane wave (λ=633  nm), the output beam is in the form of a photonic nanohelix of DOF 2.00±0.25  μm (3.16±0.39λ) with FWHM 339±5  nm (0.54±0.01λ).