Formation of optical needles using ring gratings of variable height on a quantized substrate

Subject of study. This study analyzes the influence of variations in the relief height of subwavelength ring gratings with standard and quantized substrates on the formation of optical needles in the near-diffraction zone. Aim of study. The aim of this study is to form optical needles using variable-height ring gratings on a quantized substrate. Method. The finite-difference time-domain method was employed for numerical simulation. Main results. Features of optical needle formation by variable-height ring gratings on a quantized substrate in the near-diffraction zone were investigated. Gaussian beams and Laguerre–Gaussian modes (0, 1) with different types of polarization (circular, radial, and azimuthal) were used as the input radiation. Numerical simulations showed that a ring array that combines two axicons and is complemented by a reverse quantized substrate can generate a light spike exceeding 11 wavelengths in length for a circularly polarized Gaussian beam. The smallest focal-spot size was obtained for a radially polarized Gaussian beam using a direct quantized substrate. Practical significance. The findings of the study can be applied in material processing, nanolithography, and nanoparticle micromanipulation.