Development of 3D Microstructures for the Formation of a Set of Optical Traps on the Optical Axis

Abstract

Three-dimensional (3D) microstructures used in the formation of optical traps on the optical axis in the near diffraction zone are calculated and studied. Subwavelength, variable-height annular gratings (a lattice period of 1.05λ) with a standard and graded-index (GRIN) substrate are considered as microstructures. Two scenarios are examined for changing the refractive index n of the GRIN substrate: from a maximum n in the center to a minimum n at the edges (direct GRIN) and, conversely, from a minimum n in the center to a maximum n at the edges (reverse GRIN). The propagation of light through the proposed 3D microstructures is simulated using the finite-difference time-domain (FDTD) method. The possibility of obtaining not only single but also a set of optical traps on the optical axis is demonstrated. It is also shown that compared to the results obtained with a diffractive axicon, the size of the focal spot can be reduced by 21.6% when use is made of the proposed 3D microstructures and the light needle is increased by 2.86 times.